Objective To assess the performance of the vapour compression cycle as a refrigerator and as a heat pump and its dependence on various parameters. To learn how to use the equipment to measure temperatures at various test points and the flow rates for

Refrigeration Experiment

ENCH 371 Chemical Engineering Laboratories

Objective

To assess the performance of the vapour compression cycle as a refrigerator and as a heat pump and its dependence on various parameters. To learn how to use the equipment to measure temperatures at various test points and the flow rates for liquids and gases.

Introduction

Refrigerators and heat pumps are devices that absorb heat at a low temperature and reject heat at a higher temperature. Both refrigerators and heat pumps operate in the same way using a reversed heat engine cycle, but are labeled separately according to their purpose. Devices that are used to maintain a temperature below ambient are known as refrigerators (or air conditioners) while devices that are used to supply heat at a higher temperature than ambient are known as heat pumps. Some devices, in particular those used for space cooling, may be used as both a heat pump and a refrigerator (as is the case with the Christchurch Town Hall heating/cooling system).

The vapour compression cycle is the most commonly used refrigeration/heat pump cycle and involves the same four processes as a heat engine cycle but in the reverse order (i.e. evaporation – compression – condensation – expansion/throttling). Figure 1 gives a schematic representation of the four essential mechanical components in this cycle; figure 2 shows the ideal cycle on a pressure-enthalpy diagram.

Heat out

3 2

Condenser

Throttle Work in

Valve

Evaporator Compressor

4 1

Heat in

Figure 1: Vapour Compression Cycle components

Figure 2: Ideal cycle on pressure – enthalpy diagram.

In ideal vapour compression cycle the refrigerant vapour is compressed isentropically to a higher temperature and pressure (1→2). The compressed vapour is then condensed isobarically which results in heat rejection to the surroundings (2→3). The next step is the adiabatic throttling of the refrigerant to the low temperature and pressure (3→4). The final step is where the refrigerant is evaporated isobarically at low temperature and pressure, which results in the absorption of heat from its surroundings (4→1). In an actual cycle states 1 and 3 should not lie on the saturation line as there is subcooling (helpful to obtain state 3 in the diagram) and superheating (necessary to avoid droplets (two phase region!) in the compressor).

The usual measure of performance of a refrigerator or heat pump is the Coefficient of Performance COP which for a refrigerator COPR is defined as:

(1a)

For a heat pump COPH:

(1b)

where E, C, R, H stand for Evaporator, Compressor, Refrigeration, and Heat pump respectively.

Experimental Apparatus

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The usual measure of performance of a refrigerator or heat pump is the Coefficient of Performance COP which for a refrigerator COPR is defined as:

(1a)

For a heat pump COPH:

(1b)

where E, C, R, H stand for Evaporator, Compressor, Refrigeration, and Heat pump respectively.

Experimental Apparatus

The new experimental apparatus is based on an electrically driven compressor. Unlike in domestic units which are controlled by switching the compressor on and off at certain temperatures, the power consumption of the compressor in this experiment is fully adjustable. Furthermore, in domestic units the heat exchanges in the evaporator and condenser occur by forced convection with air. For the evaporator in this apparatus the rate of heat transfer in the heat exchanger can be varied by adjusting the flows of glycol and water in the primary and secondary circuit respectively. Glycol is used in the primary circuit so that the refrigerant can be evaporated at temperatures below 0 °C (down to almost –20 °C), where water would freeze. In the condenser, a single water circuit can be used, because the temperature is always well above the freezing point of water.

The rates of heat transfer at the plate heat exchangers can be determined quantitatively by measuring the inlet and outlet temperatures and the flow-rates. The flow-rates of glycol and water can be measured with rotameters, however the rotameter for the glycol flow has to be calibrated with an additional measuring cylinder during the experiment. The flow-rate of the refrigerant can be measured with a turbine flow meter.

The refrigerant is R-134a (1,1,1,2-tetrafluoroethane) for which an enthalpy-pressure diagram is provided. Pressure gauges measure the pressure at positions between each of the four components (corresponding to the states in the cycle, see figures 1 and 2). The compressor draws its power through a kilowatt-hour meter and the power consumption can be altered via an external frequency changing controlling unit. Note: the frequency changer draws its power through the kilowatt-hour meter so correction is required.

Thermocouples measure the temperature at 12 different points, at the inlet and outlet of all three heat exchangers [the evaporator (glycol/refrigerant), the condenser (water/refrigerant), and the primary/secondary circuit (water/glycol)]. All the temperatures are read from the same digital display by changing the thermocouple channel. The table below shows which temperature corresponds to which thermocouple channel.

Where R, W, G mean refrigerant, water and glycol respectively;

E, C, I/II for the heat exchangers: evaporator, condenser and first/second circuit respectively;

in and out stands, of course, for the flows in to or out of the respective heat exchanger.

Procedure

Turn on the glycol pump by switching the 230 V switch on (this will also switch on the refrigerant flow meter).
Set the float in the glycol rotameter to any flow in the range 20 to 22.5 cm, and then turn on the valve of the calibration tank to measure the actual glycol flowrate using a measuring cylinder and a stopwatch.
Measure the ambient pressure using the barometer on the second floor of the semi-scale lab.
Turn on the water associated with the glycol circuit and the condenser and set their rotameters to within ranges 16 to 19 cm and 18 to 20.5 cm respectively.
Start the compressor after having set its frequency to 50 Hz (make sure, that the suction side won't be over evacuated; if it's going rapidly below zero kPa (gauge) open the throttling valve a little bit)
Set the throttle valve to read any flow between 0.6 and 1.5 L/min on the metering handle.
Check that the system has come to steady state by taking selected (see prelab exercise) temperature and pressure measurements every minute until no further change is observed (should be 5 to 15 min).
At steady state, measure power consumption with the kilowatt-hour meter and a stop watch.
Take readings of the four pressures, 12 temperatures, and the refrigerant flow-rate and check that the one glycol and two water flow-rates and all the pressures have remained constant.
Change the setting of the throttle valve to read a different flow also between 0.6 and 1.5 L/min on the metering handle.
Take another set of measurements.
Repeat the same 2 sets of measurements at compressor speed of 40 Hz.
For shutdown, stop the compressor, wait 5 minutes then switch off the glycol pump and close the water control valves. Then switch off the main power supply and the Fluke 52 thermocouple device (to save batteries).

WARNING:

Make sure that the water in the I/II heat exchanger does not freeze (not less than 2 °C at TW,I/II,out). This is only an issue if your glycol temperature after the evaporator drops below zero.

Also take care that superheating in the evaporator is greater than 6 °C as droplets of refrigerant in the compressor might contaminate the oil thus destroying the device. Try to maintain subcooling after the condenser (otherwise you might run into trouble getting state 3 on the chart).

Calculations and Discussion

For each speed, plot the cycle on the p-h (pressure-enthalpy) chart for R134a (remember to add the ambient pressure to your gauge readings).

For each throttle valve setting (at the same compressor speed), calculate and plot the COP for each cycle as follows:

The theoretical COPt for refrigeration and for heat pump: (COPR)t and (COPH)t from the charts.

The actual COPa for refrigeration and for heat pump: (COPR)a and (COPH)a by performing an energy balance over the heat exchangers and the compressor.

Carnot COPc for refrigeration and for heat pump: (COPR)c and (COPH)c , a Carnot refrigeration or heat pump cycle would be, if feasible, the most efficient cycle because all the steps in the process are reversible. It can be shown that the COP of Carnot refrigerator is only dependent on the temperatures at which heat is absorbed Tcold and rejected Thot specifically:

(2a) (2b)

For each compressor speed (at constant throttle valve setting), calculate and plot the coefficient of performances (COP)t , (COP)a , (COPR)c for refrigeration and for heat pump.

Take the evaporator inlet refrigerant temperature as Tcold and use the condensation temperature as Thot (you will need to read this temperature from the pressure-enthalpy chart). Plot all actual (COPR)a and (COPH)a against (Thot-Tcold). Discuss whether the temperature dependence of the actual COP is consistent with what you understand about COP for Carnot refrigerators and heat pumps.

Consider each process of the cycle separately, discuss how ideal each process is.

The operating conditions of the refrigerator are controlled by adjusting the throttle valve (and hence the refrigerant flow-rate). Discuss why all the temperatures, pressures and power consumption of the compressor are dependent on this variable and discuss the difference between adjusting the compressor speed and adjusting the throttle valve.

Reference

Smith, J. M., Van Ness, H. C., Abbott, M. M. “Introduction to Chemical Engineering Thermodynamics”, McGraw-Hill Book Company, 5th edition 1996, pp 295-314.

Mills, A.F. “Basic Heat and Mass Transfer”, Irwin, 1995.

Pre-Lab Exercise

Draw a line diagram of the apparatus (situated in the middle of the low ceiling area of the Semi-Scale Lab in the Denham Building). Identify the position of each of the four states in the refrigeration cycle (i.e. each state lies between two of the four mechanical components in the cycle, identify which lies where). Identify the pressure and temperature measurement points. Between the evaporator and the compressor lies an additional so-called accumulator, which accumulates liquid refrigerant (in the case of no superheating), to avoid damage to the compressor.

You can't check all temperatures at the same time; hence you have to decide which temperature(s) to measure as an indicator that the system has come to steady state. Explain.

Given the following information:

Ambient pressure: 100.93 kPa

Evaporator water flow-rate: 1.49 L/min

Evaporator water inlet temperature: 20.6 °C

Evaporator water outlet temperature: 16 °C

Condenser water flow-rate: 0.88 L/min

Condenser water inlet temperature: 20.7 °C

Condenser water outlet temperature: 30.9 °C

Compressor power consumption: 0.411 kW

Compressor inlet (state 1) temperature : 20.3 °C

Compressor inlet pressure: 30 kPa (gauge)

Compressor outlet (state 2) temperature: 101.9 °C

Compressor outlet pressure: 940 kPa (gauge)

Condenser outlet (state 3) temperature : 30.7 °C

Condenser outlet pressure: 930 kPa (gauge)

Evaporator inlet (state 4) temperature: -9.5 °C

Evaporator inlet pressure: 100 kPa (gauge)

Plot the refrigeration cycle on the pressure-enthalpy chart provided and calculate the theoretical refrigerator COP (section 9.2 and example 9.1 of the reference may be helpful). Calculate the actual refrigerator COP as defined in the introduction. You may assume that the density and heat capacity of water and glycol are constant in the temperature range involved (Mills 1995).