Chapter_2

2. Air Conditioning System 2.1

Consider the small single-story office building in Fig. 2-33. Lay out an all-air central system using an air handler with two zones. There is space between the ceiling and roof for ducts. The air handler is equipped with a direct expansion cooling coil and a hot water heating coil. Show all associated equipment schematically. Describe how the system might be controlled.

Fig. 2-33:

Answer:

Lay Out

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2. Air Conditioning System Schematic

Description of control: In this particular system each zone thermostat (T6 and T7) maintains the desired temperature of each zone by controlling the temperature of the air being supplied to the zones by mixing hot air and cold air. The mixing dampers takes a signal from each zone thermostat (T6 and T7) and open or close for mixing to maintain the desired room temperature. Because the heating value is normally open (NO) and direct acting (DA) and the outdoor thermostat is direct acting (DA), an increase in outdoor temperature will cause the hot water valve (V1) to close to a lower flow condition. The heating coil outlet thermostat (T3) will take a signal from the outdoor thermostat (T5) then send to heating valve (V1). Also, simultaneously, refrigerant valve (V2) as normally closed (NC) and direct acting (DA) and the coil outlet thermostat (T4) is direct acting (DA), an increase in coil temperature will cause the refrigerant valve (V2) to open to a higher flow condition. In this case, where the air delivered by the fan is constant, the rate of outside air intake is determined by the setting of the dampers. The schematic shows an economizer arrangement where the outdoor air used to provide cooling when outdoor temperatures are sufficiently low. Sensor (T1) set at high limit determines the damper positions and thus the outdoor air intake up to the low limit as determined by sensor (T2). Also signal from the supply fan starter will set the damper to its initial position as the supply fan starts. Low temperature safety switch (LLT) set at temperature below low limit (T2) will signal to supply fan to stop. In case of fire. Fire safety switch (FS) will signal also the supply fan to stop as soon as fire enters the return duct.

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2. Air Conditioning System 2.2

Suppose the building of Problem 2.1 is to use a combination air-water system where fan-coil units in each room are used for heating. Schematically lay out this part of the system with related equipment. Discuss the general method of control for (a) the supplied air and (b) the fan-coil units.

Answer: Layout

Schematic

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2. Air Conditioning System (a)

General method of control for the supplied air.

In this system, supply air discharge thermostat (T2) maintains the desired supply air temperature in supply duct by controlling the cooling coil and outdoor air damper. Refrigerant valve (V2) as normally closed (NC) and direct acting (DA), an increase in coil temperature will cause the refrigerant valve (V2) to open to a higher flow condition. In this case, where the air delivered by the fan is constant, the rate of outside air intake is determined by the setting of the dampers. The schematic shows an economizer arrangement where the outdoor air used to provide cooling when outdoor temperatures are sufficiently low. Sensor (T1) set at high limit determines the damper positions and thus the outdoor air intake up to the low limit as determined by sensor (T2). (b)

General method of control for the fan-coil units.

For the fan coil units, each room thermostat (T3) will send signal to heating valve (V1). Because the heating valve is normally open (NO) and direct acting (DA) and the room thermostat 9T3) is direct acting, an increase in room temperature will cause the hot water valve (V1) to close to a lower flow condition. Also simultaneously, refrigerant cooling coil control and damper control is in operation. When maximum zone temperature read by the analyzer, the cooling coil valve would open.

2.3

Lay out a year-round all-water system for the building of Problem 2.1. Show all equipment schematically. Discuss the control and operation of the system in the summer, in the winter and between seasons.

Answer:

Layout

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2. Air Conditioning System Schematic

Control and Operation of the System

In summer: Direct-acting room thermostat will maintain the desired room temperature during summer by controlling the chilled water valve C1, normally closed, direct acting while the heating coil is not in operation. In winter: Direct-acting room thermostat will maintain the desired room temperature during winter by controlling the heating coil valve C2, normally open, direct acting while the cooling coil is not in operation. In between seasons: Direct-acting room thermostat will maintain the desired room temperature in between seasons by controlling simultaneously the heating and cooling coil.

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2. Air Conditioning System

2.4

Apply single-package year-round rooftop type unit(s) to the single-story building in Fig. 2.33.

Answer:

Layout

Schematic

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2. Air Conditioning System 2.5

Suppose a variable-air-volume (VAV) all-air system is to be used to condition the space shown in Fig. 2-34. Assume that the space is the ground floor of a multistory office building. Describe the system using a schematic diagram. The lighting and occupant load are variable. Discuss the general operation of the system during (a) the colder months and (b) the warmer months.

Fig. 2-34

Schematic of a single-duct VAV system

General Operation of the System

Colder months: In colder months, each zone thermostat (T) maintains the desired temperature of each zone by controlling the temperature of the air being supplied to the zones by operating zone volume damper. The supply air thermostat (T3) will take a signal from the discriminator relay then send to the heating valve (V1) to operate heating coil.

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2. Air Conditioning System Warmer months: In warmer months, each zone thermostat (T) maintains the desired temperature of each zone by controlling the temperature of the air being supplied to the zones by operating zone volume damper. The supply air thermostat (T3) will take a signal from the discriminator relay then send to the cooling valve (V2) to operate cooling coil.

2.6

Devise a central equipment arrangement for the system of Problem 2-5 that will save energy during the winter months. Sketch the system schematically.

Fig. 2-34

Schematic diagram

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2. Air Conditioning System

2.7

Suppose an air-to-water heat pump is used to condition each space of Fig. 2-34, where the water side of each heat pump is connected to a common water circuit. Sketch this system schematically, showing all necessary additional equipment. Discuss the operation of this system during the (a) colder months, (b) warmer months, and (c) intermediate months.

Fig. 2-34

Schematic diagram

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2. Air Conditioning System

(a) The air coil will serve as air heating coil. The water coil will be the evaporator. (b) The air coil will serve as air-cooling coil. The water coil will be the condenser. (c) The air coil will serve as air heating coil when the room temperature is below the setting point and as air cooling coil when the room is above the setting point.

2.8

A building such as that shown in Fig. 2-34 requires some outdoor air. Explain and show schematically how this may be done with the system of Problem 2-5. Incorporate some sort of heat recovery device in the system. What controls would be necessary?

Fig. 2-34

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2. Air Conditioning System Schematic diagram

Use air-to-water heat recovery system to incorporate into the system of Problem 2-5. Pump control is necessary for the heat recovery system to operate.

2.9

How can an economizer be used to advantage during (a) the winter months, (b) summer months, and (c) intermediate seasons?

Answer: (a) During the winter months economizer can be used to advantage by using outdoor air to provide the cooling as the outdoor temperature is sufficiently low. (b) During the summer months economizer can be used to advantage by setting the outdoor damper open sufficiently to provide the minimum outdoor air required for maintaining good indoor air quality. (c) During the intermediate season economizer can be used to advantage by providing an outdoor sensor which determines the damper position and thus the outdoor air intake.

2.10

The system proposed in Problem 2-7 requires the distribution of outdoor ventilation air to each space. Sketch a central air-handler system for this purpose that has energy recovery equipment and an economizer. Do not sketch the air distribution system. Discuss the control of this system, assuming that the air will always be delivered at 72 F.

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2. Air Conditioning System Schematic diagram

Control:

Assuming that the air will always be delivered at 72 F. When T2 > 72 F, at low outdoor air temperature, economizer is in operation while there is no heat recovery, sensor T1 determines the damper position and thus the outdoor air intake. When T2 < 72 F, at low outdoor air temperature, economizer is not in operation while the heat recovery is in operation to maintain the air delivered at 72 F.

2.11

Make a single-line block diagram of an all-water cooling system. The system has unit ventilators in each room with a packaged water chiller, and pumps. Explain how the system will be controlled.

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2. Air Conditioning System

Schematic diagram:

Opening and closing the control valve C1 of each cooling coil for each unit ventilator will control the system. Packaged chiller and pump will maintain the highest temperature read by load analyzer to design temperature.

2.12

Sketch a diagram of a air-water system that uses fan-coils around the perimeter and an overhead air distribution system from a central air handler. Show a hot water boiler, chiller, and water distribution pumps. Explain the operation of the system in the summer and in the winter. What kind of controls does the system need?

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2. Air Conditioning System Schematic diagram:

Other than the fan-coil control and central air handler control, a pump control and, boiler control and chiller control are necessary for the system. 2.13

Make a sketch of a variable-volume system with a secondary perimeter heating system for a perimeter zone. Discuss the operation and control of the system for the different seasons of the year.

Schematic diagram:

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2. Air Conditioning System Operation and Control:

Variable-volume system will operate continuously only on summer seasons while simultaneously with perimeter heating during the winter season. Perimeter heating will operate when the zone perimeter temperature is below the room-desired temperature. 2.14

Diagram a combination air-to-air heat recovery and economizer system. Describe the operation and control of the system for various times of the year.

Schematic Diagram:

Control: Energy recovery system will continuously operate while economizer system will control the supply air temperature during summer and not operate during the winter season.

2.15

A large manufacturing facility requires hot and chilled water and electricity in its operation. Describe how internal combustion engines operating on natural gas could provide part or all of these needs, using heat recovery and generating electricity. The objective would be to save energy.

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2. Air Conditioning System Schematic Diagram;

Operation: The internal combustion engines on operating on natural gas could provide part or full heating through a heat recovery system using the exhaust gas to provide hot water for heating coils and provide part of full cooling through its electricity produce by running the chiller to provide chilled water for cooling coils of the system.

2.16

Thermal storage is often used to smooth the demand for cooling in large buildings. Imagine that the chiller can also make ice during the night time hours for use later when the peak cooling demand is high. Make a sketch of such a central plant, and describe its operation for a typical daily cycle. How would this system benefit the building owner? Describe the control system.

Schematic Diagram:

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2. Air Conditioning System Operation: For a typical daily cycle, thermal storage method utilizes chillers by running them off peak making ice, storing the ice, and utilizing its cooling capability during the next peak period. Benefit to the Building Owner: The benefit is the reduce cost in HVAC system which is the demand charge of the electric utility for the maximum rate of use of electricity during certain specific hours of the normal business day, usually during summer months. It also permits installation of less chiller capacity, and this means less frond end (capital) investment in chillers. Control: During night time hours the chiller will make ice by sending chilled water to storage tank then back to chiller by passing air handling unit. During high peak cooling demand or day time the chilled water will by pass storage tank to supply the air handling units for the system.

2.17

Make a sketch of a single-zone system for a small building that uses a groundcoupled heat pump. Show all the major parts of the system, including the ground heat exchanger. Discuss operation of the system in summer and winter.

Sketch:

Operation: During summer, coil will cool the air, while in winter will heat the air by reversing the valve of the refrigeration system.

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2. Air Conditioning System 2.18

Sketch a variable-air-volume reheat system that has four zones. Discuss the operation of a typical zone.

Sketch:

General Operation of the System For a typical zone, the air is throttled to some predetermined ratio and then reheat takes over to temper the air. The control thermostat activates the reheat unit when the zone temperature falls below the upper limit of the controlling instrument’s setting.

2.19

Sketch a dual-duct VAV system. Show the fans and a typical zone. Describe a way to control the speed of the fans as the terminal devices reduce the airflow to the various zones.

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2. Air Conditioning System Sketch:

As the terminal devices reduce the airflow to the various zones the speed of the fans can be controlled by adjustable frequency control system. The fan speed is normally controlled to maintain a constant static pressure at some location in the duct system. 2.20

It is desirable for the water leaving a cooling coil to be at a fixed temperature for return to the chiller. Sketch a coil, control valve, and so on, to accomplish this action, and describe the operation of the system.

Sketch:

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2. Air Conditioning System

Operation: When the water leaving the cooling coil is lower than the desired temperature, the control valve will lower the chilled water flow rate. When the water leaving the cooling coil is higher than the desired temperature, the control valve will raise the chilled water flow rate. -

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