Department of Chemical & Biomolecular Engineering

THE NATIONAL UNIVERSITY

of SINGAPORE

Chemical Engineering Process Laboratory I

Experiment T4

Air Conditioning Study

Name :

Matric No. :

Group :

Date of Expt. :

GRADE :

Objectives: x To examine the refrigeration cycle of the air conditioner. x To evaluate the cooling capacity and power consumption under different operating conditions. Apparatus: Computer linked air conditioner unit, stop watch and measuring cylinder. Theory The basic configuration of the air conditioner unit is shown in Figure1. Air is drawn into a variable speed centrifugal fan and discharge into a glass fibre duct. Steam can be added at the fan discharge to provide humidification. The air that can be preheated by two finned electric heating coils flows through the evaporator of the refrigeration unit and finally passes over two finned electric heating coils for reheating. Water condensate can be collected and measured. Air conditions at different stages are determined by wet and dry thermocouples and the mass flow rate is measured by a sharp edged orifice and differential pressure transducer. The steam is provided by an atmospheric boiler with three heating elements that can be switched to create various rates of steam production. The water level is controlled by a float switch and solenoid valve and observed through a sight glass. The refrigerator unit consists essentially of a compressor, a condenser, an evaporator and a thermal expansion valve. A propeller fan blows air across the condenser coil. The air is cooled and dehumidified at the evaporator, where the refrigerant R-134a is allowed to evaporate creating a cooling effect. An amount of heat equivalent to the heat absorbed by the evaporator and the work supplied to the compressor is rejected at the condenser to the atmosphere. The process undergone by the refrigerant is as follows (Figure 2): High pressure liquid refrigerant flows from the condenser to the evaporator through the thermal expansion valve. The low pressure refrigerant then evaporates in the evaporator providing the required cooling effect. The vapour refrigerant thus generated is received by the compressor where, the vapour pressure is raised and superheated vapour is delivered to the condenser. The vapour is air cooled at the condenser and liquefied for the beginning of the next cycle. The saturation temperature of the refrigerant in the evaporator is below the dew point of the air. When air is forced to flow across the evaporator coil, heat is transferred from the air to the refrigerant which then evaporates. This causes the air temperature and the humidity to be reduced. The heat absorbed by the evaporator can be calculated as follows: Sensible heat transfer Qs = ma Cpa (Tai - Tao) and Latent heat transfer Ql = ma hfg (Wai - Wao) The sum Qs + Ql is known as the capacity of the air conditioner. The performance of an air conditioner is measured by

Coefficient of Performance (C.O.P.) = consumedPower

Capacity

When capacity is expressed Btu/h and the power in kW, this ratio is called the Energy Efficiency Ratio (EER)

Experimental Procedure (AC575) Switch on the main supply to the system and turn on the main switch on the panel. Use deionised water to fill the reservoirs of the wet bulb sensors and set the fan speed to a moderate speed. Switch on the compressor on the panel and turn on the computer. Type CHAIN“575COLJ” to run the software and input R134a flowmeter calibration factor, atmospheric pressure and heater resistances to the computer. Select no 1 “Display schematic diagram and system parameters at 60 seconds intervals” under “Menu of Routines”. After system has reached steady state, take the following readings:

(i) Condenser and evaporator pressures. (ii) Temperatures of the Refrigerant entering the compressor T1, leaving the compressor

T2, leaving the condenser T3 and entering the evaporator T4. (iii) Dry and wet bulb temperatures of air before and after the evaporator coil. (iv) Power consumed, refrigerant and air flow rates. (v) Flow rate of water condensate from the evaporator using stop watch and measuring

cylinder. Note: an appreciable time elapses before condensate is discharged from the drain at a

constant rate due to the large surface area of the evaporator.

Repeat above procedures for another fan speed setting. Switch off the computer and air conditioning unit at the end of the experiment. (When boiler and air heaters are used in the experiment, set the fan to the maximum speed and allow it to run for at least 5 minutes before switching off and turn off the water supply to the boiler.) For each measurement (temperature, pressure, power input) note down carefully the accuracy of the readings e.g. for thermocouple read-out 'T = r 1oC. Experimental Procedure (A660) Check the wet bulb reservoir is filled to the level mark. Switch on the main supply to the system and turn on the main switch on the panel. Set the fan speed to a moderate speed. Switch on the compressor on the panel and start the data logging program. Take the following readings when system has reached steady state.

(i) Evaporator outlet pressure P1, condenser inlet pressure P2 and condenser outlet pressure P3.

(ii) Temperatures of the refrigerant at evaporator outlet t13, condenser inlet t14 and condenser outlet t15.

(iii) Dry and wet bulb temperatures of air before and after the evaporator coil. (iv) Supply voltage and current of the compressor. (v) Refrigerant and air flow rates. (vi) Flow rate of water condensate from the evaporator using stop watch and measuring

cylinder. Note: an appreciable time elapses before condensate is discharged from the drain at a

constant rate due to the large surface area of the evaporator.

Repeat above procedures for another fan speed setting. Switch off the computer and air conditioning unit at the end of the experiment.

(When boiler and air heaters are used in the experiment, set the fan to the maximum speed and allow it to run for at least 5 minutes before switching off and turn off the water supply to the boiler.) For each measurement (temperature, pressure, power input) note down carefully the accuracy of the readings e.g. for thermocouple read-out 'T = r 1oC. Tabulation and Calculations 1) To determine the capacity, the coefficient of performance and EER at each fan speed.

(The properties of moist air can be determined from the psychrometric chart attached). 2) For each of the derived quantities, estimate the uncertainty in the calculated quantity by a

simple error analysis using the instrument's reading accuracies. e.g. Qs = ma Cpa (Tai - Tao)

'

s

s

QQ

a

a

mm'

+ pa

pa

CC'

+ > @> @aoai

aoai

TTTT

��'

where 'Qs is the uncertainty in Qs when those in ma, Cpa and (Tai - Tao) are 'ma, 'Cpa and '[Tai - Tao] respectively.

3) Indicate the measured pressures and temperatures on a P-h Chart for the refrigerant R134a. 4) Check the energy and mass balance for the air and refrigerant flowing through the evaporator. Results and Discussions: References: 1. Van Wylen, G. J., and Sonntag, R. E., Fundamentals of Classical Thermodynamics, SI Version, 2

nd

Edition; John Wiley & Sons, Inc.; 1978.

2. Smith, J. M., Van Ness, H. C., Introduction to Chemical Engineering Thermodynamics, McGraw-Hill Book Company, 5th edition 1996.

LIST OF DEFINITIONS AND SYMBOLS ma air flow rate kg dry air/s T Temperature oC W absolute humidity of air kg moisture/kg dry air Cpa specific heat of moist air kJ/kg dry air oC hfg latent heat of water kJ/kg (= 2465 kJ/kg) v specific volume m3 /kg Subscripts: ai air inlet ao air outlet

Figure 1. Schematic diagram of air conditioner unit (AC575)

Figure 1. Schematic diagram of air conditioner unit (A660)

Figure 2. Refrigeration cycle

Compressor

high pressure low pressure

Condenser

Evaporator

Expansion valve

Work

Heat rejected

Heat absorbed