An ISO 9001 Company
INSTRUCTION MANUAL RA2 ISSUE 2 September 2009
Air Conditioning Unit RA2
RA2 Air Conditioning
IMPORTANT SAFETY INFORMATION All practical work areas and laboratories should be covered by local safety regulations which must be followed at all times. It is the responsibility of the owner to ensure that all users are made aware of relevant local regulations, and that the apparatus is operated in accordance with those regulations. If requested then Armfield can supply a typical set of standard laboratory safety rules, but these are guidelines only and should be modified as required. Supervision of users should be provided whenever appropriate. Your Air Conditioning Unit RA2 has been designed to be safe in use when installed, operated and maintained in accordance with the instructions in this manual. As with any piece of sophisticated equipment, dangers may exist if the equipment is misused, mishandled or badly maintained.
Electrical Safety The equipment described in this Instruction Manual operates from a mains voltage electrical supply. It must be connected to a supply of the same frequency and voltage as marked on the equipment or the mains lead. If in doubt, consult a qualified electrician or contact Armfield. The equipment must not be operated with any of the panels removed. To give increased operator protection, the unit incorporates a Residual Current Device (RCD), alternatively called an Earth Leakage Circuit Breaker, as an integral part of this equipment. If through misuse or accident the equipment becomes electrically dangerous, the RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person. At least once each month, check that the RCD is operating correctly by pressing the TEST button. The circuit breaker MUST trip when the button is pressed. Failure to trip means that the operator is not protected and the equipment must be checked and repaired by a competent electrician before it is used.
Heavy Equipment This apparatus is heavy. •
The apparatus should be placed in a location that is sufficiently strong to support its weight, as described in the Installation section of the manual.
1
RA2 Air Conditioning •
When lifting is required, two or more people will be required for safety. All should be made aware of safe lifting techniques to avoid strained backs, crushed toes, and similar injuries.
•
Safety shoes and/or gloves should be worn as appropriate.
Hot Liquids and Steam This apparatus contains steam and hot water at temperatures capable of causing scalds. •
Always allow time for the apparatus to cool before disassembly.
•
Avoid skin contact with hot water and steam. Take particular care if refilling the apparatus during use. Be aware that the flow of hot steam can extend for some distance and may not be visible.
•
Ensure that the outlet is directed away from anything that could be harmed by raised temperatures or damp air.
•
Always operate the apparatus according to the Operational Procedures described in this manual.
•
Use only those fluids described in this manual when setting up and operating this equipment.
Hot Surfaces This apparatus is capable of producing temperatures that could cause burns. •
The apparatus should not be left unattended while switched on.
•
Do not touch any surfaces with a ‘Hot Surfaces’ warning label.
•
Do not allow the apparatus to come into contact with flammable materials or liquids.
•
Allow enough time for the equipment to cool before handling any of the components.
•
Do not cover or store the equipment until it has cooled.
•
Any safety guards or insulated covers are there for operator protection- they must not be removed except as described in this manual,.
•
Always operate the apparatus according to the Operational Procedures described in this manual.
Moving and Rotating Components This apparatus has a rotating fan. •
Do not remove any protective guards while the equipment is in operation. 2
RA2 Air Conditioning •
Long hair should be tied back out of the way and dangling items such as necklaces, scarves or neckties should be removed or secured so that they cannot become entangled in the equipment.
•
Do not touch or insert any object into any moving component while the apparatus is in use.
•
Ensure that the apparatus is switched off and that all moving parts have come to rest before handling the equipment, except as described in the Operational Procedures section of this manual.
High Pressures A component within this apparatus (the refrigeration unit) is designed to operate with internal pressures greater than that of the surrounding atmosphere. •
Do not attempt to pierce or open any part of the refrigeration unit.
•
Ensure the unit is positioned so the pressure relief valve is pointed in a safe direction.
•
Keep the external temperature above 0°C and below 40°C.*
•
Protect the unit from damage.
*Temperatures for storage only. The operational range is described in the Operation section of this manual.
Water Borne Hazards The equipment described in this instruction manual involves the use of water, which under certain conditions can create a health hazard due to infection by harmful micro-organisms. For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal. Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness. Under the COSHH regulations, the following precautions must be observed:•
Any water contained within the product must not be allowed to stagnate, i.e. the water must be changed regularly and drained if the equipment will not be in use for some time.
•
Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly.
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RA2 Air Conditioning •
Where practicable the water should be maintained at a temperature below 20°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water.
•
A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above.
Further details on preventing infection are contained in the UK publication “The Control of Legionellosis including Legionnaires Disease” - Health and Safety Series booklet HS (G) 70.
Refrigerant R134a This equipment incorporates a sealed unit containing refrigerant R134a (Also known as: HFC-134a; 1,1,1-2 Tetrafluoroethane; Norflurane; Norfluran). This is a common refrigerant introduced to replace CFC (chloro-fluoro-carbon) refrigerants such as R-12. R134a is colourless, nonflammable and noncorrosive with a very faint odour. In the RA2 it is contained within a completely sealed unit, and is safe under normal use as described in this manual. It is the responsibility of the owner to check local regulations regarding R134a and ensure that these are complied with. R134a can reach temperatures capable of causing cold burns (frostbite). This may specifically constitute a hazard if R134a has been cooled and pressurised into liquid form and then escapes as a liquid through a leak, or experiences sudden expansion (as may happen if the sealed unit is pierced) forming a jet of cold vapour. R134a vapour may cause irritation of the eyes and mild irritation of the skin. It is relatively non-toxic if inhaled, but may cause asphyxiation if inhaled in sufficient concentration. In the event of exposure to flames or high temperatures (over 50°C), R134a may break down into toxic components. •
Do not attempt to open or pierce the sealed unit containing the refrigerant.
•
Always operate the equipment within the safe temperature limits described in this manual.
•
In the event that the sealed unit is ruptured, follow local regulations and take appropriate steps to reduce the potential hazard. As a suggestion only, procedure may be as follows (local requirements will vary): o Remove all personnel from the immediate area o Avoid skin and eye contact with any escaped refrigerant o Extinguish nearby flames o Increase ventilation so that vaporised refrigerant can dissipate harmlessly
4
RA2 Air Conditioning In the event of damage to the refrigeration unit, the unit must only be repaired or replaced by a suitably qualified engineer. Contact Armfield or your local agent for advice.
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RA2 Air Conditioning
AIR CONDITIONING UNIT RA2 Contents 1
2
3
4
5
Introduction to the Equipment ......................................................................................................................... 8 1.1
Diagram 1: Front View of Apparatus ........................................................................................................ 9
1.2
Diagram 2: Top View of Apparatus......................................................................................................... 10
Description ....................................................................................................................................................... 11 2.1
Overview .................................................................................................................................................. 11
2.2
Control Box.............................................................................................................................................. 11
2.3
Axial Fan.................................................................................................................................................. 12
2.4
Pre-heater and Re-heater ........................................................................................................................ 12
2.5
Evaporator ............................................................................................................................................... 13
2.6
Condensing Unit ...................................................................................................................................... 14
2.7
Humidifier ................................................................................................................................................ 15
2.8
Air Velocity Sensor .................................................................................................................................. 16
2.9
Temperature / Relative Humidity Sensor ................................................................................................. 17
2.10
Data Logger/Equipment Controller and Software................................................................................... 17
Operation and Software.................................................................................................................................. 18 3.1
Safety........................................................................................................................................................ 18
3.2
Using the Software................................................................................................................................... 18
3.3
Operation of the Humidifier..................................................................................................................... 19
3.4
Operation of Remote Controller/Data Logger and Software................................................................... 19
3.5
Data Logging Facilities ........................................................................................................................... 21
3.6
USB Interface........................................................................................................................................... 22
Specifications.................................................................................................................................................... 23 4.1
Overall Dimensions.................................................................................................................................. 23
4.2
Electrical Supply...................................................................................................................................... 23
4.3
Ventilation................................................................................................................................................ 23
4.4
Refrigerant ............................................................................................................................................... 23
4.5
USB Channel Numbers ............................................................................................................................ 24
4.6
Operating Conditions............................................................................................................................... 25
Routine Maintenance ...................................................................................................................................... 26
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RA2 Air Conditioning
6
7
8
9
5.1
General.....................................................................................................................................................26
5.2
RCD Test ..................................................................................................................................................26
5.3
Calibration of Relative Humidity Sensors ................................................................................................26
5.4
Calibration of Temperature and Air Flow Sensors ..................................................................................27
5.5
Cleaning Procedure .................................................................................................................................27
Background and Theory..................................................................................................................................30 6.1
Background ..............................................................................................................................................30
6.2
Nomenclature ...........................................................................................................................................31
6.3
Psychrometric chart and Glossary ...........................................................................................................32
6.4
Using Calculations instead of the Psychrometric Chart to Determine the Air State................................34
6.5
Other Calculations Required....................................................................................................................38
Laboratory Teaching Exercises ......................................................................................................................41 7.1
Exercise A: Psychrometric Charts ...........................................................................................................41
7.2
Exercise B: Sensible Heating ...................................................................................................................44
7.3
Exercise C: Humidification ......................................................................................................................48
7.4
Exercise D: Cooling with Dehumidification ............................................................................................51
7.5
Exercise E: Enthalpy ................................................................................................................................55
Installation Guide................................................................................................................................................i 8.1
Connection to an electricity supply .............................................................................................................i
8.2
Data logger/remote controller and software...............................................................................................i
Electrical Wiring Diagram................................................................................................................................ii
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RA2 Air Conditioning
1 Introduction to the Equipment The Armfield RA2 Unit represents a model of an Air Conditioning system by demonstrating the effects of essential Air Conditioning processes: cooling, heating, humidifying and dehumidifying. The effect and relationships of the primary processes involved in air handling systems can be investigated. The RA2 Unit is designed so that the student can simulate different environments and perform measurements to allow psychrometric data analysis. The unit is totally self-contained and is supplied with software and a computer interface device to allow remote control, on-line monitoring and logging of results. The software also includes an online Help Text detailing each of the exercises defined in this manual.
RA2 Air Conditioning Unit
8
1.1
Evaporator
Pre-heaters
Fan assembly
Boiler outlet
RH/T sensor
RH/T sensor
Air velocity sensor
Louver assembly
9 RH/T sensor
Diagram 1: Front View of Apparatus
Re-heaters
RH/T sensor
RA2 Air Conditioning
LH duct assembly
RH duct assembly
Pre-heaters
Evaporator
RH/T sensor
Air velocity sensor
Louver assembly
10 Fan assembly
Steam lance
LH duct assembly
Re-heaters
RH duct assembly
RH/T sensor
Diagram 2: Top View of Apparatus
RH/T sensor
RA2 Air Conditioning
1.2
RH/T sensor
RA2 Air Conditioning
2 Description Where necessary, refer to the drawings on pages 9 and 10. 2.1
Overview The RA2 is a bench-top unit which comprises of a square ventilation duct mounted on a mild steel support frame. The duct is made of clear acrylic so all components are clearly visible: air fan, air preheater, humidifier tube, chiller/dehumidifier heat exchanger and air reheater. The duct consists of 4 main parts: Left-Hand (LH) assembly, Right-Hand (RH) assembly, Fan assembly and Louvre assembly. An axial fan moves the air to be conditioned through the duct. Heating elements are used to heat the air. Humidification is provided by steam delivered through a tube from a boiler. The refrigerating capacity is generated by an evaporator (heat exchanger) which is connected to the refrigeration unit. The refrigeration unit and boiler are located underneath the duct. Temperature and humidity sensors record the temperature and relative humidity at every stage of operation. The air flow rate is determined using an air velocity transmitter. An acrylic louvre is located at the end of the duct. The equipment needs to be connected to a suitable PC (not included) to allow remote control and data acquisition with the RA2 software. Additional USB drivers are included to allow students to create their own control software, for example using LabView.
2.2
Control Box The control box is located beneath the louvre assembly. Accessible from the front of this are the On/Off power switch for the whole unit, the RCD switch and test button, and the USB socket for connection to a PC. The signals accessible via the USB interface include the On/Off remote compressor switch, fan speed control, air velocity display, preheater, reheater and boiler heater control, temperature sensor displays and Relative Humidity sensor display. RCD
USB Interface Connection
USB Status Indicator Lights Power On/Off Figure 1. RA2 electrical control panel
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RA2 Air Conditioning 2.3
Axial Fan The axial fan moves the air through the duct. The speed of the fan may be controlled to give different air flow rates. The fan must be on when both the preheater and re-heater are on to avoid heat damage to the acrylic duct during operation. The fan is protected with a guard, which prevents objects from reaching the blades.
Axial fan
Flow straightener
Figure 2. Front view of fan assembly 2.4
Pre-heater and Re-heater The pre-heater comprises two electric elements of 200W each, for a total power of 400W. It is located downstream of the fan in order to preheat the air flowing through the evaporator. In the second part of the duct, after the evaporator, there is a re-heater (200W) which can be used to reheat the cooled or cooled and dehumidified air. The elements are arranged at an angle to give efficient heat transfer to the air stream. Air sensing thermostats are incorporated in the duct above the heater elements to provide overheat protection.
Figure 3. Heating coils
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RA2 Air Conditioning 2.5
Evaporator The refrigerating capacity of approximately 500W at 20°C is generated by an evaporator, which is part of a compact refrigeration system. The refrigeration unit is used to cool and dehumidify the air stream. The evaporator consists of a directexpansion coil operated with a thermostatic expansion valve. The evaporator is clearly visible within the ventilation duct, and the rest of the refrigeration unit- the condensing unit- is placed just underneath the duct. The refrigerant used is R134a. Air passing across the evaporator fins is cooled as the refrigerant flowing through the tubes absorbs heat and is boiled (evaporated). Refrigerant flowing through the coil tubes is controlled by a thermostatic expansion valve mounted at the inlet to the evaporator coil. This valve automatically feeds just enough refrigerant into the coil for the refrigerant to be completely converted (boiled) from liquid to gas. The valve is controlled by a temperature-sensing bulb mounted on the coil outlet (suction) connection. The evaporator itself is complete with an angled draining tray at the bottom. During the dehumidification experiment, condensate can be collected and measured with a graduated cylinder.
Evaporator fins
Condensate outlet
Figure 4. Evaporator assembly
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RA2 Air Conditioning 2.6
Condensing Unit
The Condensing Unit, located below the ventilation duct, incorporates a compressor and a condenser. The compressor is used to compress gaseous refrigerant leaving the evaporator, and in the fan cooled condenser the refrigerant gives away the heat gained in the evaporator. The Condensing Unit also incorporates a refrigerant collector, filter/dryer, sight glass and high/low pressure cut-out for safety purposes.
Exp. valve thermocouple
Receiver
Expansion valve
Compressor
Sight glass Condenser
High pressure line
Filter/Drier Low pressure line
High/Low Pressure
Figure 5. Refrigeration unit assembly
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RA2 Air Conditioning 2.7
Humidifier
Humidification is provided by a water boiler of 5L total volume. Steam is generated when the water is boiled using the electric element, (2kW). The boiler is made of plastic and includes a tube which delivers steam to the air duct. It also includes a drain valve, and can be refilled manually through the filler cap and refill lance. Distilled water is recommended in order to avoid scaling of the vessel and duct. The boiler incorporates a cut-out switch, which prevents the electrical element from overheating if the water level falls too low. If this occurs, wait 2 minutes and refill boiler, the cut off will self reset and steam can be produced again with 5 -12 minutes. Power to the boiler heaters can be remotely controlled and monitored using the Armfield RA2 Software.
Vapour Lance Refill Cap
Boiler vessel
Figure 6. Boiler assembly
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RA2 Air Conditioning 2.8
Air Velocity Sensor
The air velocity in the duct is measured by the air velocity transmitter. This operates on the hot film anemometer principle, using special thin film. It has very good accuracy at low air velocities. The working range is 0–10m/s and the response time can be up to 4 seconds at constant temperature. Therefore it is important to obtain steady conditions in order to have stable velocity measurement. Steady state in the system is usually obtained after about 15 minutes. The velocity transmitter is mounted in the duct in the best position to measure the average air velocity. Care should be taken to ensure the correct angle between the sensor head and the air flow.
Air velocity sensor tip with thin film
Figure 7. Air velocity sensor
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RA2 Air Conditioning 2.9
Temperature / Relative Humidity Sensor
Temperature and Relative Humidity (T/RH) sensors are located at every stage of operation. There are 4 T/RH sensors in total: at the duct inlet, before the evaporator, after the evaporator and at the duct outlet. Temperature and Relative Humidity is measured by the sensor. The RH sensor is a water resistant type so that it can operate in the range from 10 to 100% Relative Humidity.
Relative Humidity sensor
Temperature sensor
Holding Assembly
Figure8. Temperature/Relative Humidity (T/RH) sensor block For improved accuracy, each RH sensor is provided with a manufacturers’ calibration certificate. The values on this certificate should be entered into the software, see the Routine Maintenance section.
2.10 Data Logger/Equipment Controller and Software The Armfield RA2 Air Conditioning Unit is designed to be operated using the RA2-304 software supplied with the equipment. The RA2 Air Conditioning Unit must therefore be connected to a suitable PC running the RA2-304 software (or an equivalent program created by the student). The RA2 software also allows data logging of experimental results, and performs some standard calculations on the data.
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RA2 Air Conditioning
3 Operation and Software Where necessary, refer to the drawings on pages 2 to 10. 3.1
Safety The RA2 unit contains a highly volatile fluid under pressure, but it is completely safe provided the instructions in this manual are followed correctly. Safety devices have been incorporated into the unit to prevent accidents. Moreover the working fluid is relatively harmless in the gas or liquid state. It is neither inflammable nor toxic, but it must be not allowed to enter the eyes.
3.2
Using the Software The CM12-304 Software is powerful Educational and Data Logging Software with a wide range of features. Some of the major features are highlighted below, but the full details on the software and how to use it are provided in the presentations and Help texts provided with the Software Check that the USB connection is made between the RA2 unit and the PC, and that the RA2 software is installed and running. Check that the circuit breakers and RCD device at the rear of the unit are in the on (up) position. Turn the unit on by pressing the ON/OFF switch on the unit, then click on the Power On switch on the RA2 software mimic diagram. On starting the software, the user is met by a simple presentation which gives them an overview of the capabilities of the software and explains in simple terms how to navigate around the software and summarises the major facilities complete with direct links to detailed context sensitive ‘help’ texts. A toolbar is displayed at all times, so users can jump immediately to the facility they require. The toolbar shows a selection of icons (standard for all Armfield Software) and pop-up text naming the icon when the cursor is placed over it.
Example of Standard Toolbar
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RA2 Air Conditioning 3.3
Operation of the Humidifier The humidifier boiler should be filled with water before use, and drained after use if the equipment is not to be used again for some time. Distilled water is recommended for filling, in order to avoid scaling of the boiler vessel and duct interior. The equipment is filled through the filling cap using the filling lance. The sight glass in the front of the unit allows the water level to be viewed during filling. Humidification is controlled from a PC via the RA2 software. A PID controller within the software maintains the boiler setting based on the temperature measured by temperature sensor T5. The temperature Set Point, Proportional Band, and the Integral and Derivative times may be adjusted by the user. Alternatively the boiler power setting may be entered manually as a percentage value, using the same controller window as for the PID settings. The water level must be monitored during use, and the boiler refilled as necessary to maintain the level. Great care must be taken to avoid scalding from steam if refilling the boiler during use. Do not look directly into the filling lance and wear insulating gloves if available. Allow time for the water to cool before draining the boiler vessel.
3.4
Operation of Remote Controller/Data Logger and Software The Armfield RA2 Air Conditioning Unit is controlled using the RA2 software supplied, which allows real-time monitoring and data logging of all sensor outputs and control of the heaters and refrigeration unit. Recorded results can be displayed in tabular and graph format. The software runs on a Windows PC which connects to the RA2 using a USB interface. Installation of the software is described in the Installation Guide, and the software must be installed before connecting the PC to the RA2. The software may then be run from the Start menu (Start > Programs > Armfield Refrigeration and Air Conditioning > RA2). Operation of the software is described in a walkthrough presentation within the software, and also in the online Help Text accessible via the software Help menu. Operation and setting of specific controls is also provided within the experiments described in this manual.
3.4.1
Mimic Diagram The equipment is usually controlled from the Mimic Diagram screen in the software. This shows all the sensor outputs, and includes controls for the fan, the heaters, the humidifier and the Chiller. The software also automatically generates a series of ‘Watchdog’ pulses, required by the plc, ensuring that the hardware shuts down safely in case of a software or communications failure.
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RA2 Air Conditioning
Figure 9. Screenshot of mimic diagram 3.4.2
Controlling the Heaters The heaters are controlled by controllers in the software. Click on the appropriate PID symbol to open the controller window.
Figure 10. Screenshot of the controller window 20
RA2 Air Conditioning
Control can be either closed loop (Automatic) which uses the temperature sensor immediately following the heater as the process variable in a PID loop, or open loop (Manual) where the user defines the percentage time the heaters are ‘ON’ for, and hence the output power. When performing humidity measurements and investigations it is best to use automatic control as this produces stable temperatures most rapidly, and maintains these conditions by varying the heater power. However when doing quantitative heater power investigations it is better to use Manual control. This allows an accurate measurement of heater power to be made, but does take longer to stabilise. 3.4.3
Controlling the Fan The Fan is controlled from the software using the up/down buttons. The associated air velocity is displayed on the sensor reading box..
3.5 Data Logging Facilities Sampling can be Automatic or Manual. In automatic sampling, samples are taken regularly at the requested interval. In manual sampling, single samples are taken at operator request (useful when conditions have to be changed and the equipment let stabilise in the new condition). The RA2 software defaults to manual data sampling, allowing the operator to take a reading once the equipment has stabilised. As the data is sampled, it is stored in spreadsheet format, updated as the data is sampled. The table also contains columns for the calculated values. New sheets can be added to the spreadsheet for different data runs. Sheets can be renamed. Extremely and powerful graph plotting tools are available on the software, allowing the user full choice over what is displayed, including dual y axes, points or lines, displaying data from different runs, etc. The automatic formatting and scaling is normally used but can be overridden.
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RA2 Air Conditioning
Other powerful operator facilities on the software include: -
-
Data scaling and multiple point calibration of the sensor readings Control of data update rates and digital filtering Graphical display of sensor readings over the last two minutes (useful for ascertaining when the equipment has stabilised after a change). Bar graph display of sensor values Manual Data Entry Addition of user notes
Data can be saved as Armfield format data files (for reading back into the Software at a later date) or exported as Microsoft Excel files.
3.6 USB Interface The RA2 interfaces to the computer using a USB interface, built into the sensor and instrumentation enclosure. This interface is sometimes referred to as the IFD5 interface. The use of USB means that any current or projected Windows based PC can be used. There is no need to open the PC or fit anything inside. Even if all the USB ports are full, expanders are very cheap and readily available. The hardware and software are fully compatible with Windows 98, ME, 2000 and XP operating systems.
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RA2 Air Conditioning
4 Specifications 4.1
4.2
Overall Dimensions Length:
-
1700mm
Depth:
-
440mm
Height
-
605mm
Electrical Supply PRODUCT-A
4.3
PRODUCT-B
PRODUCT-G
Green/yellow lead
Earth (Ground) Earth (Ground) Earth (Ground)
Brown lead
Live (Hot)
Live (Hot)
Live (Hot)
Blue lead
Neutral
Neutral
Neutral
Fuse rating
13A
15A
13A
Voltage
220-240V
110-120V
220V
Frequency
50Hz
60Hz
60Hz
Ventilation The equipment must be situated in a well ventilated environment or in a large room. The laboratory should be a minimum 50m³ in order for the RA2 not to affect the lab air conditions, consequently altering the results.
4.4
Refrigerant This equipment includes a sealed unit containing refrigerant R134a (Also known as: HFC-134a; 1,1,1-2 Tetrafluoroethane; Norflurane; Norfluran). This is a common refrigerant introduced to replace CFC (chloro-fluoro-carbon) refrigerants such as R12. R134a is colourless, non-flammable and non-corrosive with a very faint odour, and is safe under normal use as described in this manual. See the safety section at the front of this manual for additional information.
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RA2 Air Conditioning 4.5
USB Channel Numbers The Armfield Windows™-compatible software allows data logging of the sensor outputs and operation of the fan. However, users may prefer to write their own software for control and data logging, and for the convenience of those wishing to do so, Armfield has provided additional USB drivers allowing operation of the equipment via the USB socket on RA2. The relevant channel numbers are as follows:CHANNEL NO
SIGNAL FUNCTION
Analog Outputs (0-5 V dc exported from socket): Ch 0 signal RH1 Ch 1 signal T1 Ch 2 signal RH2 Ch 3 return T2 Ch 4 signal RH3 Ch 5 signal T3 Ch 6 signal RH4 Ch 7 signal T4 Ch 8 signal RH5 (Not used on RA2) Ch 9 signal T5 (Not used on RA2) Ch 10 signal RH6 (Not used on RA2) Ch 11 signal T6 (Not used on RA2) Ch 12 signal Air flow 1 Ch 13 signal Air flow 2 (Not used on RA2) Ch 14 signal Mains Voltage Ch 15 signal Not used Analog Inputs (0-5 V dc input to socket): DAC0 signal Fan speed Digital Outputs ( 0-5 V dc): DC 0 Duct over-temperature DC 1 Boiler low level Digital Inputs (0-5 V dc): Ch 0 ON signal Ch 1 Watch dog Ch 2 Preheat PWM Ch 3 Reheat PWM Ch 4 Boiler PWM Ch 5 Compressor ON/OFF
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RA2 Air Conditioning 4.6
Operating Conditions Operating conditions for RA2 are enclosed by the air conditions envelope (refer to psychrometric chart) as follows:
Temperature Operating Range When operating the RA2 the ambient temperature and humidity must be taken into consideration for the experiments to work effectively. Below is a table outlining the operating conditions for the various components of the RA2:
Temperature [˚C]
Relative Humidity [%]
10 20 30 40 -
PreHeater
Humidifier / Boiler
-
• • •
• • • •
10 20 30 40 50 60 70 80 90 100
• • • • • • • • • •
• • • • • • • • •
Dehumidifier / Chiller
ReHeater
• • •
• • • • • •
• The dot symbolises where that particular component could be used.
25
• • •
• • • • • • • • • •
RA2 Air Conditioning
5 Routine Maintenance To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment. 5.1
General The equipment should be disconnected from the electrical supply when not in use. Water should be drained from the boiler before storage and whenever the unit is not to be used for several days.
5.2
RCD Test Test the RCD by pressing the TEST button at least once a month. If the RCD does not trip when the Test button is pressed then the equipment must not be used and should be checked by a competent electrician.
5.3
Calibration of Relative Humidity Sensors The humidity sensors are supplied with basic calibration already performed, but greater accuracy can be achieved using the calibration data provided by the sensor manufacturer. From the Software select ‘Options’ then ‘Calibrate IFD Channels’ which opens the calibration window. From the drop down menu select the sensor to be calibrated (e.g. RH1) and press the button for ‘Direct’ Calibration. This should display a calibration graph and table similar to that shown. Each sensor has a number marked on it, and calibration certificates are provided for each number defining the Zero Offset and the Slope for the sensor.
Enter: Zero Offset Value Enter: (Slope * 100) + zero Offset
26
RA2 Air Conditioning The table shows the Engineering Units to be displayed corresponding to the voltage input from the sensor. Up to 20 calibration points may be entered for precise calibration. To calibrate the RA2 RH Sensors enter the voltage from the sensor at 0%RH (the zero offset value on the certificate) as indicated. Then calculate the voltage at 100% RH (Slope (V/%) * 100 (%) + Zero Offset (V)) and enter in the table against 100% RH as shown. Calibration of each sensor should be completed before calibration of the next sensor is started. New calibration values will take effect after the software is restarted, and will remain saved within the software on the PC used for the calibration. The original calibration supplied with the software may be recovered by reinstalling the software; any modified calibration will be lost if the software is reinstalled for any reason. Calibration must be performed separately for every PC that will be used with the RA2. 5.4
Calibration of Temperature and Air Flow Sensors The temperature sensors used are highly accurate thermistors and should never need recalibration. The thermistors themselves can be physically replaced without recalibration. Similarly the air flow sensor is delivered with a calibrated voltage output. However, if required these sensors can be calibrated in the same way as described for the RH sensors above. Up to 20 calibration points can be accommodated in the table. If required, the ‘Manual’ calibration mode allows points to be added into the table by inputting actual engineering values (measured on a reference sensor) at different levels. The values entered into the table can still be viewed and altered using the ‘Direct’ mode.
5.5
Cleaning Procedure Cleaning the ventilation duct The duct must be dismantled to clean the internal parts. The duct consists of 4 parts: Left-Hand assembly, Right-Hand assembly, Fan assembly and Louvre assembly. The fan assembly comes off after undoing the 2 screws. The same applies to the louver assembly on the opposite duct end. It is also possible to disconnect the two main duct parts (LH and RH assemblies) from the evaporator by undoing the screws holding them in place. Care should be taken while cleaning the duct so that the sensors and heaters are not damaged. Retain all screws for reassembly after cleaning. Use a soft, lint-free brush, sponge or cloth for cleaning, with cold or warm water and a small quantity of mild detergent if required. Cleaners designed for use with acrylic baths are generally suitable; avoid the use of abrasives and solvents. Deposits of scale may be cleaned with the application of a mild descaler suitable for use on acrylic. It may be possible to remove shallow scratches and rubs on the clear acrylic surfaces by using a specialist plastic polish. Dry all sections thoroughly before re-assembling the duct. 27
RA2 Air Conditioning
Cleaning the velocity transmitter The sensor element may be cleaned using blown air or a soft brush, or with gentle application of isopropyl alcohol. Cleaning the boiler If it becomes necessary to descale the boiler, this will require the use of a proprietary descaling solution. Always read the manufacturer’s label carefully when using any descaling chemicals and follow the instructions properly. Check that the product is suitable for use with all types of material it may come into contact with during the descaling process. Flush the boiler thoroughly after descaling using clean water, ensuring all traces of descaler are removed, and then rinse with distilled or demineralised water. If the unit is not to be used immediately then dry the boiler with a lint-free cloth.
28
RA2 Air Conditioning
5.6
Inverter Settings (Only on RA2-B and RA2-G)
An inverter is fitted to versions RA2-B and RA2-G to convert the 60Hz electrical supply to 50 Hz to suit the compressor in the refrigeration system. The following inverter settings are included for information in the event that the settings need to be restored. Channel P-01 P-02 P-03 P-04 P-05 P-06 P-07 P-08 P-09 P-10 P-11 P-12 P-13 P-14 P-15 P-16 P-17 P-18 P-19 P-20 P-21 P-22 P-23 P-24 P-25 P-26 P-27 P-28 P-29 P-30 P-31 P-32 P-33 P-34 P-35 P-36 P-37 P-38 P-39 P-40 P-41 P-42 P-43 P-44 P-45 P-46 P-47
Value 50 50 0.5 0.5 0 0 240 7 50 0 3 0 Read only 0 0 0..10V 4 1 100 0 0 0 0 0 8 0 0 0 0.0 Auto-0 1 P-09 5 0 100 1 0P-buS t 3000 (3 second trip) 101 0 0 0 1.0 1.0 0 0 0 0 0..10
Parameter Maximum speed in Hertz Minimum Speed in Hertz Acceleration ramp time in seconds Deceleration ramp time in seconds Stop mode select Reserved Motor rated voltage in Volts Drive rating in Amps Motor rated frequency in Hertz Motor rated speed Boost start voltage as a percentage Drive control mode selection Trip log Extended menu access Digital input function select Analogue input format in Volts Effective switching frequency in Hertz User relay output select User relay output limit as a percentage Preset speed 1 in Hertz Preset speed 2 in Hertz Preset speed 3 in Hertz Preset speed 4 in Hertz 2nd decal ramp time in Seconds Analogue output function select Skip frequency hysteresis band in Hertz Skip frequency in Hertz V/F characteristics adjustment voltage in Volts V/F characteristics frequency adjust in Hertz Terminal mode restart function Keypad mode restart function Boost frequency Boost period duration in Seconds Brake chopper enable (not S1) Analogue input scaling as a Percentage Serial communications address MODBUS enable / baudrate select Trip enable / delay Access code deffinition Parameter access lock Analogue input offset as a Percentage Display speed scaling factor (0 is disabled) User PI proportional gain User PI intergral time constant in Seconds User PI operating mode User PI reference select User PI digital reference as a Percentage User PI feedback select 2nd analogue input format in Volts
29
RA2 Air Conditioning
6 Background and Theory 6.1
Background The changes of air condition that may be investigated with the RA2 are: •
Heating of air
•
Cooling of air
•
Humidification of air
•
Dehumidification of air with cooling
The properties of air that may be measured directly by the RA2 sensors and controls are: •
Air velocity
•
Relative humidity
•
Temperature (at multiple locations)
•
Power input (electrical) to each heater unit (preheat, reheat and boiler)
The constants assumed by the software for calculations are: •
Heat capacity ratio (γ or κ) for air:
1.41 @20°C
[ratio, dimensionless]
•
Heat capacity ratio (γ or κ) for water: 1.33 @20°C
[ratio, dimensionless]
•
Acceleration due to gravity (g):
9.81
[m/s²]
•
Ideal gas constant (R):
8.314472
[J K-1 mol-1]
•
Constant pressure specific heat (cp): 1.005 @20°C [kJ kg-1 K-1]
•
Constant volume specific heat (cv): 0.715?
[kJ kg-1 K-1]
Variables that cannot be measured by the RA2 and must be input from additional measurements are: •
Ambient (atmospheric) pressure
30
RA2 Air Conditioning 6.2
Nomenclature
Name
Symbol
Units
Temperature (Dry Bulb)
T, DBT
°C
Vapour pressure
Pw
Pa
Saturation pressure
Ps
Pa
Relative Humidity
RH
%
Mixed air velocity
ν0
m/s
Recirculate air velocity
ν1
m/s
x or ω
kg/kg dry air
Q&
Watts
Enthalpy change rate
∆H&
kJ/kg
Work transfer rate
W&
kJ/kg
Compressor work
W& comp
kJ/kg
Work transfer from fan motor
W& fan
kJ/kg
Cross-sectional area of duct
A
m2
Specific air volume
α
m3/kg dry air
Air mass flow rate
m& a
kg/s dry air
Vapour mass flow rate
m& w
kg/s dry air
Condensate mass flow rate
m& cond
kg/s dry air
Refrigerant mass flow rate
m& ref
kg/s
hA,B,C,D
kJ/kg
Heat transfer rate at reheater
Q& reh
kJ/kg
Heat transfer rate at preheater
Q& preh
kJ/kg
Humidity Ratio Heat transfer rate
Air enthalpy
31
Notes Measured by temperature sensor
RH = Pw / Ps * 100 [%]
Many psychrometric chart read g/Kg
0.04m2
Measured at points A, B, C, D etc.
RA2 Air Conditioning 6.3 6.3.1
Psychrometric chart and Glossary Introduction to the Psychrometric Chart
A simple Psychrometric chart. (A larger Psychrometric chart is located with Exercise A) A psychrometric chart is a graph of the physical properties of moist air at a constant pressure or often equated to an elevation relative to sea-level. The chart graphically expresses how various properties relate to each other, and is thus a graphical 'equation of state'. The versatility of the psychrometric chart lies in the fact that by knowing two independent properties of some moist air (at a constant known pressure), the other properties can be determined. Changes in state, such as when two air streams mix, can easily be graphically modeled using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. For locations at or below 2000 ft (600 m), a common assumption is to use the sea level psychrometric chart. The most common chart is the "ω-t" (omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as the abscissa and the humidity ratios (ω) appear as the ordinates. This is the type of chart shown above and provided with the RA2. In order to use a particular chart, for a given air pressure or elevation, at least two of the six independent properties must be known (DBT, WBT, RH, Humidity Ratio, Specific Enthalpy, and Specific Volume). The RA2 measures the Temperature (dry bulb temperature) and the relative humidity at various places along the duct. Thus the ‘state’ of the humid air can be determined for each of the four measurement points by plotting the T and RH measurements on the psychrometric chart. From the chart it is then possible to determine the Humidity Ratio (x), the Enthalpy (h) and the Specific Volume (v) at each of the measurement points.
32
RA2 Air Conditioning
6.3.2
Glossary of Terms Dry Bulb Temperature, DBT or T (oC) is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. On the standard psychrometric chart this is shown horizontally along the abscissa. Wet Bulb Temperature or Saturation Temperature, WBT, (oC) is that of an air sample after it has passed through a constant-pressure, ideal adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. Note: the Wet Bulb Temperature has been omitted from the Psychrometric chart provided with the RA2 for clarity. It would normally be displayed on the 100% RH line, with gridlines approximately parallel to those of Enthalpy. Relative Humidity,ϕ or RH, (%) is the ratio between the actual water vapour pressure and the saturation vapour pressure (the vapour pressure of saturated air at the same temperature). As the actual vapour pressure cannot exceed the saturation pressure, the maximum value for relative humidity (RH) is 100%. It is sometimes considered to be the amount of water in the air compared with the amount of water that the air could contain (at the same temperature) if saturated (100% RH). Humidity Ratio, w (ω) or x, (kg/kg). The humidity of air, expressed as a percentage mass of water vapour in a unit mass of dry air. Also sometimes called the mixing ratio. Specific Enthalpy, h (kJ/kg) also called heat content per unit mass, is the sum of the internal energy of a thermodynamic system. It is a measure of the useful work that may be done by the air. Specific Volume, v (m3/kg) also called Inverse Density. Volume per unit mass of dry air. Dew Point Temperature, DP (oC) is that at which a moist air sample at the same pressure would reach water vapour saturation, i.e. at which water will begin to condense out of air during cooling. This will vary according to the moisture content of the air. At this saturation point, water vapour would begin to condense into liquid water fog or (if below freezing) solid hoarfrost, as heat is removed. The dew point temperature is measured easily and provides useful information, but is normally not considered an independent property. It duplicates information available via other humidity properties and the saturation curve. The dew point temperature has been omitted from the Psychrometric chart provided with the RA2 for clarity. Saturation Vapour Pressure Ps (N/m2, Pa) The pressure at which the vapour phase of a material is in equilibrium with the liquid phase of the same material. The saturation vapour pressure varies with temperature. In the case of saturated air (air saturated with water vapour), the saturation vapour pressure is the pressure (at a specific temperature) when the rate of evaporation of water equals the rate of condensation of water, and is also the point at which the relative humidity is 100%.
33
RA2 Air Conditioning 6.4
Using Calculations instead of the Psychrometric Chart to Determine the Air State The standard method of determining the parameters required to analyse HVAC systems is to use the psychrometric chart as described above. However these parameters can also be calculated. This section describes the formulae used in the RA2 software to determine the air state.
6.4.1
Saturation Pressure and Partial Pressure of the Water Vapour The maximum saturation pressure of the water vapor in moist air varies with the temperature of the air vapor mixture and can be expressed as:
pws = e(77.3450 + 0.0057 T - 7,235 / T) / T8.2
(1)
where
pws = water vapor saturation pressure (Pa) e = the constant 2.718....... T = temperature of the moist air (K) Equation (1) represents the curve on the psychrometric chart at 100% RH. Relative Humidity (RH) is defined as the partial pressure of the water vapour, divided by the partial pressure of saturated air at the same temperature. RH = pw / pws x 100%
(2)
From equations (1) and (2) the partial pressure of the water vapour can be calculated if the temperature and RH are known.
6.4.2
Humidity Ratio The humidity ratio can be determined from the partial pressure of water vapor and air:
x = 0.62198 pw / (pa - pw)
(3)
where
pw = partial pressure of water vapor in the moist air (Pa) pa = atmospheric pressure of the moist air (Pa) Thus from equations (1), (2) and (3), the humidity ratio (x) (i.e. the abscissa of the psychrometric chart) can be determined from the temperature and RH measurements.
34
RA2 Air Conditioning 6.4.3
Calculating Enthalpy The enthalpy of moist air can be expressed as: h = cpa t + x [cpw t + hwe]
(4)
where
h
= specific enthalpy of moist air (kJ/kg)
cpa = specific heat capacity of air at constant pressure (kJ/kg.oC,) = 1.01 t
= air temperature (oC)
x
= humidity ratio (kg/kg)
cpw
= specific heat capacity of water vapour, (kJ/kg.oC) = 1.84
hwe 6.4.4
= 2,502 - evaporation heat of water at 0oC (kJ/kg)
Specific Volume of Moist Air per Mass Unit of Dry Air Specific volume is defined as the total volume of dry air and water vapor mixture per kg of dry air (SI-units). The specific volume can be expressed as: vda = V / ma
(6)
where
vda = specific volume of moist air per mass unit of dry air (m3/kg) V = total volume of moist air (m3) ma = mass of dry air (kg) When dry air and water vapor with the same temperature occupies the same volume the equation for an ideal gas can be applied. pa V = ma Ra T
(7)
where
pa
= partial pressure air (Pa)
Ra
= the individual gas constant air (J/kg.K) = 286.9
T
= temperature of the moist air (K)
Combining (6) and (7):
35
RA2 Air Conditioning vda = Ra T / pa
(8)
The partial pressure of air can be expressed as: pa = p - pw
(9)
where
p = pressure in the humid air (Pa) pw = partial pressure water vapour (Pa) Combining (8) and (9): vda = Ra T / (p - pw)
(10)
The ideal gas law can also be applied for the water vapor: pw V = mw Rw T
(11)
where
pw
= partial pressure water vapor (Pa)
Rw
= the individual gas constant water vapor (J/kg.K) = 455
T
= temperature of the moist air (K)
The mass of water vapor can be expressed by the humidity ratio and the mass of air: mw = x ma
(12)
where
x = humidity ratio (kg/kg) Combining (11) and (12): pw V = x ma Rw T Therefore, from (6): vda = x Rw T / pw i.e.
pw = x Rw T / vda
Therefore, from (10): vda = Ra T / (p - (x Rw T / vda)) Which can be re-arranged to express the specific volume of moist air per unit mass of dry air as: 36
RA2 Air Conditioning
vda = (1 + x Rw / Ra) Ra T / p
6.4.5
(13)
Specific Volume of Moist Air per unit Mass of Dry Air and Water Vapour To calculate the total mass flow from the air speed, we need to know the density of the moist air. The specific volume, v, of the moist air can be expressed as: v = V / ma + mw where v = specific volume of moist air per mass unit of dry air and water vapor (m3/kg) Therefore, from (12): v = V / ma (1 + x) From (6) v = vda / (1 + x) Combining this with (13) and re-arranging, the specific volume of moist air per unit mass of dry air and water vapor can be expressed as: v = (Ra T / p) [(1 + x Rw / Ra)/ (1 + x)]
(14)
For the low humidity ratios found in an air conditioning system such as RA2 there will only be very small differences between the specific volume of moist air per unit mass of dry air (vda) and the specific volume of moist air per unit mass of dry air and water vapour (v).
37
RA2 Air Conditioning
6.5 6.5.1
Other Calculations Required Calculating Mass Flow Rate From the continuity equation: .
.
.
m aA = m aB = m a
where A and B are two points along the duct. In the experiments that follow, the letter subscripts refer to the positions along the duct as shown below:
Inlet
Preheat
Refrigerator
A
B
C
Reheat
Outlet
D
Thus, for a simple duct, the mass flow rate is constant through the duct. The air flow rate (F) is measured by the air speed sensor at position D. The volume flow rate can be calculated to be F.A m3/s, where A is the cross section area of the duct. Therefore the mass flow rate can be expressed as: .
m a = F . A. / v
(15)
where v = specific volume of moist air per mass unit of dry air and water vapor (m3/kg) F = Flow rate of the air (m/s) A = Area of the duct (m2)
The Specific Volume, v, (Inverse Density) of the air can be calculated from equation (14) or determined from the psychrometric chart, by plotting the dry bulb temperature and measured RH at the air flow sensor position.
38
RA2 Air Conditioning Note: most standard psychrometric charts show vda, not v, but the difference will be small when calculating flow rates, see section 6.6.5 for more details. 6.5.2
Sensible Air Heating Sensible Heating is heating that does not involve a change of phase (e.g. evaporation) of any of the materials involved. Similarly sensible cooling of air does not involve any condensation. The sensible heat of a material is the heat energy of the air that may be gained or lost through convection and conduction. The sensible heat is a result of the material’s specific heat capacity, its mass, and its temperature compared to some defined datum or reference temperature (e.g. measured using a standard scale of temperature such as Kelvin, Fahrenheit or Celsius, all of which use fixed reference points). The term ‘sensible heat’ rather than simply ‘heat’ is used in order to distinguish it from latent heat. From first law of thermodynamics:
& = ∆H & −W & ∆Q AB W (work transfer rate) is zero Therefore the effective heating (or cooling) of the air between positions A and B can be expressed as: & = m (h − h ) ∆Q AB a B A
(16)
Individual enthalpy can be determined from the psychrometric chart or calculated from equation (4) Alternatively, the change in enthalpy may be calculated as cpa(TB – TA) + x cpw (TB – TA) where TA is the initial temperature of the air TB is the temperature of the air after heating cpa is the specific heat capacity of air at constant pressure cpw is the specific heat capacity of water vapour at constant pressure x is the humidity ratio Note on Latent Heat: Latent heat is the heat energy required for a material (e.g. water) to undergo a change of phase (e.g. evaporation from liquid to vapour). For example, a mass of liquid water will not immediately and completely change phase to water vapour as it reaches the evaporation temperature of 100°C, but requires additional heat input for the entire mass to evaporate. The temperature of the water
39
RA2 Air Conditioning will remain at 100°C (the temperature of the phase change) until the change is complete. The heat that must be added to enable the phase change, which does not result in a change of temperature, is the latent heat. The RA2 provides the facilities to investigate latent heat as the input power to the humidifier is measured. Also it is possible to collect the condensate from the chiller over a period of time. However detailed analysis of this non-sensible heating and cooling is beyond the scope of the standard experiments for the RA2. This would make an ideal topic for project work. 6.5.3
Energy Balance and Heating Efficiency Electrical Heater power = V2/R watts x mark space ratio Efficiency = sensible air heating/ Electrical Heater power Note: In an HVAC system it is quite possible to obtain ‘efficiencies’ of >100% as heat may be gained from the surroundings as well as lost. It is more correct to term efficiency investigations as an ‘Energy Balance’.
40
RA2 Air Conditioning
7 Laboratory Teaching Exercises 7.1
Exercise A: Psychrometric Charts
Objective To investigate and understand the use of psychrometric charts, understand relative humidity (RH) measurements and the effect of temperature on RH and understand the Humidity Ratio. Method To change the conditions of the air entering a duct and looking at the changes in RH, temperature and humidity ratio by using a psychrometric chart and computerised calculations. Equipment Required RA2 Air Conditioning Unit Compatible PC (not supplied by Armfield) RA2-304 Software Optional Equipment Barometer for measuring local ambient pressure (if not available then some alternative is required, such as a local weather report or an appropriate default value). Equipment Set Up The boiler is not required for this exercise and need not be filled. Ensure that the equipment and PC have been set up as described in the installation guide, and that the PC is connected and switched on with the RA2-304 software running. The software should indicate ‘IFD: OK’ in the bottom right of the software window, and the red and green USB indicator lights on the electrical console should be illuminated. Check that the RCCD (circuit breaker) on the electrical console is in the up (OFF) position. Check that the sensor readings in the software indicate reasonable values. Procedure Switch on the fan to 40%, set PID1 to control T2 at 28oC. Set PID3 to control T4 at 20oC. Switch the chiller on. (These values are suitable for typical ambient temperatures in the laboratory around 21oC. They can be raised or lowered to suit the actual laboratory conditions.) 41
RA2 Air Conditioning Check that the preheat element on the mimic diagram should change between grey and red to indicate the times during which power is being supplied to the heater. Check that the preheat temperature sensor rises then stabilises at approximately the set temperature. Check that the velocity sensor reading in the software increases. Adjust the sampling configuration by selecting “Sample” from the top menu and the “configure…”, select the sampling operation as manual. Allow the system to stabilise for approximately 15 minutes. Select the
icon to record the sensor readings in the results table.
Results From the results table record T and RH at each of the four positions. From the psychrometric chart, estimate the Humidity Ratio (x), the Enthalpy (h) and the Specific Volume (v) at each of the positions. Compare the estimates with the values of x, h and v in the table produced by the software. Discussion Describe what happens to the Humidity Ratio as the air proceeds down the duct and how it is related to the Relative Humidity. What would happen to the Humidity Ratio if the Humidifier was set to 50%?
42
RA2 Air Conditioning
43
RA2 Air Conditioning 7.2
Exercise B: Sensible Heating
Objective To investigate sensible heating of air in a duct. Method To change the condition of the air entering a duct by increasing the preheat temperature. To investigate the effect of temperature change on heating power and electrical power. Equipment Required RA2 Air Conditioning Unit Compatible PC (not supplied by Armfield) RA2-304 Software Optional Equipment Barometer for measuring local ambient pressure (if not available then some alternative is required, such as a local weather report or an appropriate default value). Theory Heating of Air The air is heated without adding any additional moisture, so the humidity ratio remains constant. The vapour pressure of saturated air increases with increasing temperature. Hence the relative humidity of the heated air decreases. The heating of air in the duct using the preheater can be represented in the following diagram:
hA
hB
wA
wB
maA
maB
The relevant equations have already been covered in Common Theory and Calculations.
44
RA2 Air Conditioning
Mass balance From continuity equation .
.
.
m aA = m aB = m a .
.
.
m aB = m aC = m a where .
m aA
= mass flow rate at inlet to duct,
.
m aB
= mass flow rate at inlet to refrigerator, and
.
m aC
= mass flow rate at outlet of refrigerator
Energy balance
From first law of thermodynamics:
& = ∆H & −W & ∆Q AB where
& ∆Q AB = change in energy between duct inlet and entrance to refrigerator, & ∆H
= change in enthalpy
& W
= rate of work done by the air
& (work transfer rate) is zero Within the duct, W
∴
& = m (h − h ) ∆Q AB a B A
where hA
= enthalpy at duct inlet, and
hB
= enthalpy at entrance to refrigerator
A similar energy balance can be performed across the refrigerator: & = m (h − h ) ∆Q BC a C B
The energy balance across the reheater is illustrated below: hC
hD
wC
wD
maC
maD 45
RA2 Air Conditioning
which may be expressed as: & = m (h − h ) ∆Q CD a D C
Heat Transfer
The heat transfer between two points may be calculated as in the following equations
& a Cpa (TB − TA ) m
(between duct inlet and refrigerator inlet)
& a Cpa (TC − TB ) m
(between refrigerator inlet and refrigerator outlet)
where Cpa
= Constant pressure specific heat capacity of dry air at constant
pressure = 1.0035 kJ/kga When apparatus runs at nearly ambient temperatures, external losses or gains are very small and close agreements should be achieved between the enthalpy change and heat transfer. Equipment Set Up
The boiler is not required for this exercise and need not be filled. Ensure that the equipment and PC have been set up as described in the installation guide, and that the PC is connected and switched on with the RA2-304 software running. The software should indicate ‘IFD: OK’ in the bottom right of the software window, and the red and green USB indicator lights on the electrical console should be illuminated. Check that the RCCD (circuit breaker) on the electrical console is in the up (OFF) position. Check that the sensor readings in the software indicate reasonable values. Procedure
Set the fan to 60%. Set the Preheat control to manual and set to 30%. Let the system stabilise. Adjust the sampling configuration by selecting “Sample” from the top menu and the “configure…” select the sampling operation as manual. Once the system has stabilised select the results table
icon to record the sensor readings in the
Increase the Preheat control in steps of 10%, allowing stabilising and repeat experiment.
46
RA2 Air Conditioning Check that the preheat element on the mimic diagram changes to red to indicate that the heater is in operation. Check that the preheat temperature sensor rises then stabilises at approximately the set temperature. The heater element on the mimic diagram should change between grey and red to indicate the times during which power is being supplied to the heater. Results
Using the data collected in the experiment calculate the following for each of the conditions: •
the mass flow rate
•
the heat transferred into the air
•
electrical power input to the heater
Now plot the following graphs: •
heat transfer Vs Change in Temperature (∆T)
•
Electrical Power Vs Change in Temperature (∆T)
Discussion
The heat transfer graph should be a straight line, why? The electrical power input of the heater may not be, comment. Does the mass flow rate affect the heat transferred to the air? Does varying the power supplied to the heat have an affect?
47
RA2 Air Conditioning 7.3
Exercise C: Humidification
Objective
To investigate the humidification of air. To investigate the effect of vapour content and temperature on relative humidity. Method
To humidify air by the introduction of water vapour using a supply of steam. To heat the air stream in order to allow investigation of the effect of heating. To perform mass and energy balances for this humidification system. Equipment Required
RA2 Air Conditioning Unit Compatible PC (not supplied by Armfield) RA2-304 Software Optional Equipment
Barometer for measuring local ambient pressure (if not available then some alternative is required, such as a local weather report or an appropriate default value). Theory
Humidification of the air flow can be represented as in the following diagram: hA
hB
wA
wB
maA
maB
Equipment Set Up
The boiler is required for this exercise, and should be filled before use to MAX LEVEL (as indicated on the sight glass) with clean, preferably distilled or de-ionised (demineralised), water. The filling procedure is described in the Operational Procedures section of this manual (see 3.3). Ensure that the equipment and PC have been set up as described in the installation guide, and that the PC is connected and switched on with the RA2-304 software 48
RA2 Air Conditioning running. The software should indicate ‘IFD: OK’ in the bottom right of the software window, and the red and green USB indicator lights on the electrical console should be illuminated. Check that the RCCD (circuit breaker) on the electrical console is in the up (OFF) position. Check that the sensor readings in the software indicate reasonable values. Procedure
In the software, set the fan to 40%. Select the boiler controller (PID). Set the controller to Manual Control and the Manual Output to 100% to run the boiler heater at full power. Observe the equipment as the boiler heats the water. As soon as steam appears at the steam lance outlet, decrease the boiler setting to 40%. Allow the system to stabilise (this will take approximately 10 minutes). Select the
icon to record the sensor data on the results sheet.
Set the boiler to 30%. Allow the system to stabilise (approx. 10 minutes) and select the icon. Set the boiler to 20%, allow the system to stabilise and select the
icon.
Remember to refill the boiler as necessary.
Investigate the effect of temperature on relative humidity hA
hB
wA
wB
maA
maB
Set the boiler to 40%. Open the preheat controller window and set the preheater to Manual. Set the Manual Output to 50%. Allow the system to stabilise, then select the
icon.
Set the preheater to 100%. Allow the system to stabilise then select the
icon.
If time permits, the experiment may be repeated at a lower air flow rate by adjusting the fan speed setting. Ensure that the flow rate is never less than 82 m³/hr.
49
RA2 Air Conditioning If draining the boiler after use, remember to first allow sufficient time for the water to cool. Results
The software logs the output from the relevant flow, temperature and humidity sensors. The software calculates the mass flow rate from the flow velocity. Check the calculation for one set of sensor readings. The software calculates the heat transfer. Check the calculation for one set of sensor readings. For each set of data in turn, enter the values of temperature and humidity on the h-x diagram and identify the change of state (dew point). For each set of data, determine the enthalpy, h, and relative humidity, ϕ, from the diagram. For the first set of data, compare the enthalpy to the heat transfer. Compare the results obtained at different boiler power settings, including the first set of data. Compare the results obtained at different preheat temperatures, including the first set of data. If the experiment was performed at different flow rates then compare the results obtained from this, including the first set of data. Discussion
What effect would you expect decreasing the boiler setting to have on the relative humidity and on the humidity ratio of the air stream? Was this reflected in the results obtained? Is there any observable relationship between the relative humidity and the humidity ratio? How is this affected by the boiler setting? What is the effect of heating the air with the preheater? Describe any change in the energy balance with different boiler settings. Contrast this with any change in the energy balance resulting from the preheater setting. If the experiment was performed at different flow rates, include a discussion of the effect of changing the air flow rate. What are the implications of your findings on the use of steam to humidify air? In what situations might both heating and humidification be required? Are there any additional considerations in air conditioning systems that might arise from the use of water vapour and heating?
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RA2 Air Conditioning 7.4
Exercise D: Cooling with Dehumidification
Objective
To investigate wet surface cooling and dehumidification of air. Method
By humidifying air using the introduction of steam to the air stream. By cooling this humidified air using a compression-based refrigeration (cooling) unit. By performing mass and energy balances on the system. By varying the moisture content of the air to investigate the effect this has on the results obtained. Equipment Required
RA2 Air Conditioning Unit Compatible PC (not supplied by Armfield) RA2-304 Software Optional Equipment
Barometer for measuring local ambient pressure (if not available then some alternative is required, such as a local weather report or an appropriate default value). Theory
Cooling with dehumidification can be represented in diagram form as follows: hB wB
hC wC
maB
maC
mcond h4
h1
hcond
Mass and energy balances may be performed as described in earlier experiments. The electrical power input into the compressor is logged by the software. This is taken to be approximately equal to the energy consumed by the compressor in doing work.
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RA2 Air Conditioning Mass balance
From continuity equation:
& aC = m & aB = m &a m
From this, the mass flow rate of steam can be calculated as:
& cond = m & a ⋅ ∆w = m & a (w B − w C ) m
Hence total mass flow of moist air [kg/s] is:
& a −m & cond = m & a (1 − ∆w ) m
Energy balance
& = ∆H & −W & ∆Q BC comp & W comp = Compressor work ≈ Compressor power input .
& & a (h B − h C ) − m & cond h cond ] − W ∆Q BC = [m comp
Air enthalpy change can be calculated as: & a (h B − h C ) m
Condensate enthalpy can be calculated as: & cond h cond m
(assume hcond = 419.04kJ/kg )
Equipment Set Up
The boiler is required for this exercise, and should be filled before use to MAX LEVEL (as indicated on the sight glass) with clean, preferably distilled or de-ionised (demineralised), water. The filling procedure is described in the Operational Procedures section of this manual (see 3.3). Ensure that the equipment and PC have been set up as described in the installation guide, and that the PC is connected and switched on with the RA2-304 software running. The software should indicate ‘IFD: OK’ in the bottom right of the software window, and the red and green USB indicator lights on the electrical console should be illuminated. Check that the RCCD (circuit breaker) on the electrical console is in the up (OFF) position. Check that the sensor readings in the software indicate reasonable values.
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RA2 Air Conditioning
Procedure
NOTE: Where the ambient temperature of the laboratory is low, better results may be obtained with the use of the preheater. Exact requirements will vary as the ambient humidity will also affect the results, and so are left to the discretion of the individual user. In the event of extremely high ambient humidity (relative humidity close to 100%), it may not be possible to investigate cooling over a sufficiently wide range of humidity values, and in such cases it may be preferable to postpone the experiment until ambient conditions are more favourable. However, in high ambient humidity conditions it should be possible to investigate dehumidification without needing to humidify first (i.e. without using the boiler). In the software, set the fan speed to 40% and turn the cooler on. In low to moderate humidity conditions it will be necessary to use the boiler to humidify the air. Set the boiler to full power until steam appears at the exit to the steam lance, then switch off full power and set the boiler Manual Output to 25%. Allow the system to stabilise (approximately 25 minutes), then perform a condensate collection by collecting the condensate from the compressor condensate outlet tube over a timed period of 20 minutes. Record the volume collected and the time. Select the
icon and collect data for 15 minutes every 10 seconds.
Repeat for a boiler setting of 50% and 75%. If time permits, the experiment may be repeated at a lower air flow rate by adjusting the fan speed setting. Ensure that the flow rate is never less than 82 m³/hr. Switch off the boiler (set Mode of Operation to Off). Switch off the preheater, if used. Allow a few minutes for the temperatures to stabilise, and then switch off the fan. If draining the boiler after use, remember to first allow sufficient time for the water to cool. Results
The software logs the output from the flow, temperature and humidity sensors. Calculate the mass flow rate of dry air from the flow velocity. Calculate the mass flow rate of steam from the continuity equation. Calculate the mass flow rate of moist air by performing a mass balance calculation. Calculate the vapour condensation rate from the times condensate collection values. The software calculates the heat transfer. Check the calculation for one set of sensor readings. Compare the flow rate of moist air with the vapour condensation rate.
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RA2 Air Conditioning For each set of data in turn, enter the values of temperature and humidity on the h-x diagram and identify the change of state (dew point). For each set of data, determine the enthalpy, h, and relative humidity, ϕ, from the diagram. Determine the moisture content drop, ∆w. Compare the results obtained at different boiler power settings, including the first set of data. If the experiment was performed at different flow rates then compare the results obtained from this, including the first set of data. Discussion
What was the effect on the relative humidity and humidity ratio? What was the effect of water vapour content on air cooling, as shown by the experiment? What is the significance of the results when considering air conditioning system design? Discuss the positioning of the humidifier within a system when cooling is to be employed: what would be the effect on the energy balance and efficiency if the air was humidified after cooling and not before as in the RA2? Discuss methods for improving the accuracy of results. As project work, students may devise a method of determining compressor efficiency.
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RA2 Air Conditioning 7.5
Exercise E: Enthalpy
Objective
To investigate and understand the affect on enthalpy, understand relative humidity (RH) measurements and the effect of temperature on RH and understand the Humidity Ratio. Method
Looking at the changes in enthalpy as the temperature and humidity are changed. Equipment Required
RA2 Air Conditioning Unit Compatible PC (not supplied by Armfield) RA2-304 Software Optional Equipment
Barometer for measuring local ambient pressure (if not available then some alternative is required, such as a local weather report or an appropriate default value). Theory
The enthalpy of moist air can be expressed as: h = cpa t + x [cpw t + hwe]
(4)
where h
= specific enthalpy of moist air (kJ/kg)
cpa
= specific heat capacity of air at constant pressure (kJ/kg.oC,) = 1.01
t
= air temperature (oC)
x
= humidity ratio (kg/kg)
cpw
= specific heat capacity of water vapour, (kJ/kg.oC) = 1.84
hwe
= 2,502 - evaporation heat of water at 0oC (kJ/kg)
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RA2 Air Conditioning Equipment Set Up
The boiler is required for this exercise and needs to be filled. Ensure that the equipment and PC have been set up as described in the installation guide, and that the PC is connected and switched on with the RA2-304 software running. The software should indicate ‘IFD: OK’ in the bottom right of the software window, and the red and green USB indicator lights on the electrical console should be illuminated. Check that the RCCD (circuit breaker) on the electrical console is in the up (OFF) position. Check that the sensor readings in the software indicate reasonable values.
Procedure
Switch on the fan to 40%, set PID1 to control T2 at 23oC. (These values are suitable for typical ambient temperatures in the laboratory around 21oC. They can be raised or lowered to suit the actual laboratory conditions.) Check that the velocity sensor reading in the software increases. Allow the system to stabilise. Select the icon to record the sensor readings in the results table every 10 seconds for 15 minutes. Turn the boiler on to 40% and allow the system to stabilise. Select the icon to record the sensor readings in the results table every 10 seconds for 15 minutes. Turn the boiler off and the cooler on. Make sure dehumidification is occurring before selecting the icon to record the sensor readings in the results table every 10 seconds for 15 minutes. Results
From the results table calculate the average enthalpy at each of the four positions and plot these in a bar graph. Conclusion
Describe what happens to the enthalpy as the air proceeds down the duct and how it is related to the Relative Humidity and temperature. If the experiment was performed at different flow rates, include a discussion of the effect of changing the air flow rate.
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RA2 Air Conditioning
8 Installation Guide Before operating the equipment, it must be unpacked, assembled and installed as described in this Installation Guide. Safe use of the equipment depends on following the correct installation procedure.
The RA2 Air Conditioning Unit is supplied fully assembled and ready for use. It is a benchtop unit. Due to its weight it should be carried by two people. The following procedures must be carried out prior to operation 8.1
Connection to an electricity supply
Check that the voltage specified on the equipment matches the supply voltage. Note: this unit must be earthed. Ensure that the miniature circuit breakers are in the off (down) position. The mains electrical connection is on the rear of the unit. For details about the electricity supply refer to the Electrical Supply in the Specifications section of this manual. 8.2
Data logger/remote controller and software
The RA2 unit is controlled via computer software. A USB port is located on the electrical control panel at the right hand end of the RA2. The USB lead from the RA2 should be connected to the USB port on a suitable computer running the Armfield RA2 Software.
The basic operation of the Air Conditioning Unit has been confirmed. Refer to the Operational Procedures section in the product manual for further information.
i
RA2 Air Conditioning
9 Electrical Wiring Diagram
Electrical Wiring Diagram ADM31712
ii
RA2 Air Conditioning
RA2 Air Conditioning
Main Office:
US Office:
Armfield Limited Bridge House West Street Ringwood Hampshire England BH24 1DY
Armfield Inc. 436 West Commodore Blvd (#2) Jackson, NJ 08527
Tel: +44 (0)1425 478781 Fax: +44 (0)1425 470916 Email:
[email protected] Web: http://www.armfield.co.uk
Tel:
(732) 928 3332
Fax:
(732) 928 3542
Email:
[email protected]
