Air Conditioning

Department Of Mechanical & Industrial Engineering Qatar University Energy Systems Laboratory Air Conditioning System Experiment Aim: 1. 2. 3. 4.

5.

The aims of the experiment are mainly to allow the students to: Recognize and get familiarized with the components of large central air conditioning systems. Conduct experiments showing the integration of different air conditioning operations such as pre-heating, humidification, dehumidification, reheating, mixing, etc. Analyse all processes by plotting the measured data on psychrometric charts with and without recirculation. Compare measured and calculated data such as: a. Boiler power in the humidifier section. b. Electric power consumption in the pre-heater section. c. % recirculation of air when the system is set for recirculation. Discuss the reasons of any differences in point 4

Apparatus: Recirculating air conditioning unit A770 as shown in the schematic diagram.

Experimental procedure: The experiment involves different air conditioning processes. In order to study each individual process while keeping the inlet condition of air almost the same, the experiment is done in stages. The condition of the air is observed by noting down the dry and wet bulb temperatures after each stage. The electric currents used by all equipment in the entire process are also taken down to facilitate for further studies of energy balance on each section. The whole set of processes given below is to be carried out using a fixed fresh air flow rate (controlling the blower) Experiment without recirculation 1. The vane in the duct is adjusted for 0% recirculation. 2. Switch on the blowers. This allow the flow of air in the system. 3. Switch on the electric heaters to preheat the air. After it reaches a steady state, the condition of air is noted down by noting down the dry and wet bulb temperatures of state 1(section A), state 2 (section B) and state 3( section C) 4. Switch on the boiler, compressor of the refrigerating system and the reheater. 5. The preheated air is now humidified by the steam coming out of the boiler. Boiler power is adjusted to avoid introducing steam, more than the maximum capacity the air can carry. State 4( section C) denotes the condition of air after humidification. 6. At the evaporator of the refrigerator which has been switched on earlier, cooling and dehumidification take place. Record state 5( section D) which is after the refrigerating system. 7. The electric heaters after the refrigerating system, reheats the cooled and dehumidified air. State 6( section E) denotes the condition of air after reheating. Experiment with Recirculation 1. Now the system is adjusted for some recirculation and all the state temperatures and electric currents are taken. Note that state 3 cannot be measured in this case. NOTE: Make sure that the system has reached steady state conditions before recording your data.

Calculation: 

Air flow rate:

There are two inclined manometers, one at the inlet of the apparatus to measure the flow rate of fresh air and the other in the duct to measure the total air flow rate in the system. The empirical relation for airflow rate is: Mass flow rate = 0.0757 where

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kg/s

z= differential pressure in mm of water on the inclined scale and, va = specific volume of the air at the orifice in m3/kg ( obtained from the psychrometric chart)

Amount of water added in the boiler section 

Power input to the boiler. Latent heat of vaporization at atmospheric pressure.

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If on a psychrometric chart, a line is drawn joining the states of fresh air and recirculated air, the state of the air after mixing should come on this line. The ratio of % of fresh air and recirculated air is inversely proportional to the length ratio in which this line is divided by the the point after mixing.

Observations: Temperatures Without Recirculation Dry Bulb Station

State

A

State 1

B

State 2

C

State 3

C

State 4

D

State 5

E

State 6

z

mm water

Orifice Differential (In Duct) z

mm water

Condition of Air

With Recirculation

Wet Bulb Dry Bulb

Wet Bulb

Degree Centigrade

Other Data Orifice Differential (Intake)

Voltage

Vm

V

Pre-heater Current

Ip

A

Boiler Current

Ib

A

Reheater Current

Ir

A

Compressor Current

Ic

A

Results: 

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For each setting (without and with recirculation), locate the condition of air on the psychrometric chart from the measured dry and wet bulb temperatures, at each section. Join the states of air before and after each process using a line to show the process direction. Note: Plot processes with recirulation using red colour and for the one without recirculation use blue colour and plot both on the same psychrometric chart. Calculate the rate of heat addition in the preheater section using the measured air flow rate and enthalpies from the Psychrometric chart. Compare the answer with the measured heater power. Calculate the amount of water added during humidification (from the psychrometric charts), with the setting at ‘part 7’ of the procedure section (without recirculation). From this calculate the boiler power by knowing the enthalpy of evaporation of water at atmospheric pressures and Compare it with the actual power consumption in the boiler. For the recirculating air conditioning process find the actual ratio of fresh air intake to recirculated air flow rate from the two manometers. Find the same ratio from the psychrometric chart using the plotted recirculating processes. Compare.

Discussion: Refer to the plot on the Psychrometric chart. Discuss about the various processes happening between the states 1 to 6. Discussion should be separate for each set of readings(with and without recirculation). Relate it to the ideal air-conditioning processes. For heat addition, humudification and recirculation processes, discuss possible reasons for any differences between calculated results from psychrometric charts and the measured results using the measuring instruments.