PERFORMANCE OF ANAIR-CONDITIONING SYSTEM

ChBE 343Department of Chemical and Biomolecular Engineering

Rice University

Revised 11 March 2013

A. PURPOSE OF EXPERIMENTTo carry out heat and material balances on a refrigerant evaporator, determine the coefficient of perfomance of anair-conditioning system and how it is influenced by various parameters, and to study the characteristics of a typicalexpansion valve.

B. SIMPLIFIED DESCRIPTION OF EXPERIMENTIn essence a (compression type) air-conditioning system comprises a compressor, a (refrigerant) evaporator and a re-frigerant condenser. See Figure 1. This unit uses R-22 which boils (or condenses) at 42◦F at 71.5 psig and at 110◦F at226.4 psig. The liquid boiling point in the evaporator varies, but 42◦F is typical. In this refrigerant evaporator, typically23,000 BTU/hr of heat is transferred from air to the refrigerant. The vaporized refrigerant is piped to the compressorwhich discharges it at a pressure which varies (mostly with the outside air temperature) but which commonly lies inthe range 220-260 psig. This hot gas is condensed and sub-cooled in a condenser in which about 32,000 BTU./hr aretransferred from the refrigerant to the outside air.

Figure 1: A Simplified Vapor Compression Air Conditioning Unit

The refrigerant circuit pressure jumps about 170 psi in passing through the compressor and drops an equal amountin passing through the rest of the equipment. The major pressure drop of around 140 psi or so occurs across the ex-pansion valve. This device, which is described in detail elsewhere (and in fact is the subject of a separate part of thisexperiment) serves the important function of maintaining the refrigerant leaving the evaporator in a slightly (5-12◦F)superheated state. In this way the fluid entering the compressor will be vapor. (Liquids, being incompressible, moreor less, tend to damage compressors, so theirpressence is limited.

A more detailed drawing of the refrigerant circuit is shown in the Appendix. Pressure tapping points are shown, aswell as a drier, filter, turbine flow meter and rotameter. The latter is used to measure the refrigerant flowrate.

C. REFRIGERANT CIRCUITThe refrigerant is chiorodifluoromethane (CHCIF2) also known as R-22, and by trade names such as Freon-22. Adiagram showing the ideal thermodynamic cyclic path taken by the refrigerant is shown in Figure 1. Thermodynamicdata for this refrigerant are widely available and may be obtained from any reliable source, including the NIST Chem-istry Webbook1 may be used.

Figure 2: Ideal Thermodynamic Paths for Air Conditioning Unit

Saturated liquid refrigerant (point 1 on the diagram) passes through the expansion valve where it flashes to a mixture ofvapor and liquid at the evaporator pressure (point 2 on the diagram). The refrigerant then passes through the evaporator(horizontal finned-tube exchanger) where it is completely vaporized by heat transfer with the air that is being cooled.The refrigerant leaving the evaporator is at a condition of 5-12◦F superheat (point 3 on the diagram). The refrigerantthen enters a compressor (reciprocating type) where it is brought up to the condenser pressure (point 4 on the digram).Finally, the refrigerant is passes through the condenser (another finned-tube exchanger) where heat is exhausted to theoutside air. This returns the refrigerant to the saturated liquid state (point 1 on the diagram).

Details about the thermodynamics can be found in your thermodynamics textbook2 or Perry’s Chemical Engineers’Hanbook.3 The ideal thermodynamic paths for refrigerant in the condenser and evaporator are isobaric. The ideal pathfor refrigerant passing through the compressor is isentropic. and the ideal path for the valve is isenthalpic.

D. VALVES, SWITCHES, AND INSTRUMENTATIONYou must never turn any of the valves on this experiment, except the drain for the condensation vessel.

There are five switches that you will be using with this apparatus. The main power switch, the fan switch, the com-pressor switch, and the meter switch are all turned to the “on” position at the beginning of the experiment. There is arotary switch to select among the thermocouple temperature readouts.

The flowrate of the refrigerant is measured by two devices, operating in series. The first is turbine flowmeter. Thesecond device is a high pressure rotameter with magnetic coupling between the internal float and the external indicator.The turbine flowmeter is considered accurate, but not reliable. The rotameter is reliable, but cannot be read accurately.Generally, you should find the readings from the turbine flowmeter adequate for your purposes, but you should reportreadings from both devices.

The pulses from the turbine flowmeter go to a pulse counter which counts for 10 seconds, displaying the previous 10second count. 11235.8 pulses correspond to a US gallon. If Z is the pulse count, the refrigerant flow rate is given by

Refrigerant Flow Rate =3600

10· 1

11235.8·Z = 0.03204 ·Z US GPH

All essential temperatures are measured by copper-constantan thermocouples. The temperature probe locations are in-dicated in the diagrams in the appendix. A rotary switch is provided to switch among the temperature probes. Proper

wet-bulb temperature readings require that the water reservoirs (inverted funnels on the back of the unit) are filled tothe line with de-ionized water.

Several measuring devices are associated with the inside are circuit. Inside air is drawn in through a filter, passesthrough a reducer and a honeycomb straightener, and flows through an 11” x 11” (inside dimensions) wooden duct toa test section where the velocity profile can be measured with an anemometer whose range is (0-25 ft/sec) and wet anddry thermocouples provide temperature and humidity information. The air then flows across the refrigerant evaporatorwhere some water drops out. Next comes another measuring zone where wet and dry thermocouples are located.Finally, the air is moved through a blower and exhausted into the room. The air flow rate is changed by adjusting thedamper at the exit of the blower.

Upstream of the anemometer in the test section is another anemometer (with range 0-50 ft/sec). This anemometer en-ables the center line velocity to be maintained constant (by adjusting the damper if necessary) while velocity profilesare being measured with the other.

The anemometers are EXPENSIVE and DELICATE. Adequate velocity profile information is already available. Pleasedo not adjust the position of these devices. To obtain the average air velocity from the measured centerline velocity,use

v =Cvo

with C = 0.8 unless instructed otherwise by the teaching staff.

There are wet and dry thermocouples (all referenced to 32◦F) which register air properties before and after passagethrough the refrigerant evaporator.

The condensate water which leaves the evaporator should be measured, recorded and discarded about every 15 min-utes, simultaneously with other measurements.

There are two meters that measure power. Each has a digital gauge, but generally you should rely on the reading fromthe rotating disk power meters. The smaller meter on the left reads the power from the compressor motor and the con-densing unit fan. For your calculations you should neglect the fan and assume that the compressor power dominates.The meter on the right measures the power to the inside blower motor.

The dials (designed to be read periodically by meter-readers) rotate much too slowly to be useful in this application.The main disk is used. Every revolution of the disk corresponds to 3.6 watt-hours for the compressor meter and 14.4watt-hours for the blower power meter. The power consumed over a small time period may be determined by timinga few revolutions, or, in the case of the small Kw-H meter by reading the associated pulse counter. Each revolution ofthe main disk gives rise to 8 pulses being sent to a pulse counter. This counter counts for one minute and then displays.The power consumption corresponding to a reading ν on the pulse counter is therefore,

Power =(60)(3.6)

8ν = 27ν watts

E. GENERAL OPERATING INSTRUCTIONSThere are two parts of this experiment. The main experiment is the air conditioning experiment discussed in this doc-ument. The other experiment involves a separate apparatus to explore the behavior of an expansion valve. The teammembers should split their time so they experience both experiments. You should be able to complete the expansionvalve experiment in an hour or two. The main experiment should run for about three hours total (half for each air flowrate). You should be running both parts at the same time.

You will be performing two different runs, corresponding to different air velocities. One of the runs should be made at20 ft/s and a second run at about 16 ft/s. The actual velocities you choose are not critical, as long as you choose valuesnear the maximum flow and a velocity reduced from the maximum by about 20 percent. You should be recording

measurements from all of the temperature ports and all of the flows mentioned in the previous section. This includesthe power measurements for the compressor and the air blower. You also need to record the pressure of the condenserand evaporator. As mentioned above, you should check and measure the condensate accumulation at about fifteenminute intervals. Each run should be about 90 minutes long to allow for accurate condensation rate measurements. Itwill take the unit a while to reach steady-state operation.

F. REQUIRED CALCULATIONS AND DISCUSSIONThe most critical part of the calculations is determining the water concentration in the air entering the evaporator andproperly account for the enthalpy change of the damp air entering to produced cool, condensed water and a cool,saturated air stream. This requires the use of theory or a psychrometeric chart to interprete the wet and dry bulb tem-peratures. See Perry’s Handbook3 or McCabe and Smith4 for details.

In the end, you will do a complete energy balance around the evaporator unit. You will also provide the heat dutiesof the two exchangers and the power required for the compressor. At minimum, your report also needs to include thefollowing information (some by observation and some by calculation):

• relative humidity of the air entering the evaporator

• relative humidity of the air leaving the evaporator

• enthalpy flows of the air and water, including a statement of heat loss

• enthalpy change of the refrigerant in the evaporator (plus cooling duty)

• duty of the condenser

• power required for the compressor

• overall heat balance calculation for the evaporator

• coefficient of performance of the refrigeration unit

• heat balance around the blower motor

• temperature, pressure, enthalpy, and enthalpy values at all four numbered points in Figure 1

All measured values must be included in the report. You need to sure that you have taken full advantage of all of theinstrumentation and gauges reported in the previous sections. Be sure to discuss how you approached your calculationsand any deviations you observe in the heat balance.

The results of the expansion valve experiment can be included as an appendix section in your report.

G. HELPFUL HINTS• VERY IMPORTANT: You must not open or close any valves on the air conditioner unit, with the exception of

the drain on the condesate collector.

• Keep in mind that it is an honor code violaton to refer to or use any materials from previous years’ courses. Forthe case of this lab, it will result in some very serious (and obvious) mistakes.

• Each team of three is responsible for completing both the Expansion Valve Experiment and the Air ConditioningExperiment and include both in your report.

• You should complete sample calculations of your expectations for a typical humidity level before you come tothe lab.

• Additional hints can be found in the Appendix.

Bibliography

[1] P. J. Linstrom and W. G. Mallard, editors. NIST Chemistry WebBook, NIST Standard Reference Database Number69. National Institute of Standards and Technology, Gaithersburg MD, 20899, June 2005.

[2] J.M. Smith, H.C. Van Ness, and M.M. Abbott. Introduction to chemical engineering thermodynamics. McGraw-Hill chemical engineering series. McGraw-Hill, 2005.

[3] R.H. Perry and D.W. Greeen. Perry’s Chemical Engineers Handbook, Eighth Edition. McGraw-Hill Education,2007.

[4] W.L. McCabe, J.C. Smith, and P. Harriott. Unit Operations of Chemical Engineering. McGraw-Hill ChemicalEngineering Series. McGraw-Hill, 2005.

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