Test on Vapor Compression

TEST ON VAPOR COMPRESSION REFRIGERATION

INSTRUCTED BY: Mr. H. M. K. Anjana

GROUP MEMBERS:

NAME

Bandara K.D.

Abeysinghe W.D.D.J.

COURSE

B.Sc. Engineering

Ashroff M.A.

INDEX NO

090046 C

Atapattu K.R.

MODULE

ME 4382

Bandara K.D.

DATE OF PER

21.06.2013

Chamika W.N.R.J.

DATE OF SUB

09.09.2013

Dayanath B.A.A.V.

INTRODUCTION The vapor compression cycle basically consists of an evaporator, condenser, throttling valve, and a compressor. Figure below illustrates the components and flow arrangement for it.

Fig 1 : Schematic diagram of a Vapour Compression Refrigeration Cycle

In the shown cycle (Fig 1), refrigerant in the form of a cool mixture of liquid and vapor centers the evaporator at low pressure (4). Then using relatively warm air or water, the liquid refrigerant will boil. The resulted vapor (1) is pumped by the compressor, which raises refrigerant vapor pressure and temperature. This hot refrigerant vapor at high pressure (2) enters the condenser, where heat is released to a lower temperature ambient air, and the refrigerant vapor turns into a liquid, then passes to the expansion device (3), where the pressure of the refrigerant is reduced to the pressure of the evaporator, cooling the staying liquid refrigerant to the wanted evaporator temperature. Then mixture of the cool liquid and vapor refrigerant (4) passes to the evaporator and repeats the cycle. Pressure reducing valves are used to reduce the refrigerant pressure when it flows from the condenser to the evaporator. There are six main types of refrigerant controls 1. Automatic expansion valve 2. Capillary tube 3. Low pressure side float valve 4. Thermostatic expansion valve 5. Thermal electric expansion valve 6. High pressure side float valve Performances and applications of above pressure control valves are slightly different to each other.

OBJECTIVE 1. A detail study of a simple refrigeration cycle with the help of commercial refrigeration training board. 2. Study the performances of expansion valves involving in a simple refrigeration cycle. THEORY Refrigeration effect is calculated by measuring the air flow in the evaporator duct and temperature. Heat pumped out to the surrounding is calculated by measuring the air flow in the condenser duct and its temperature. COP of refrigeration cycle

= Refrigeration effect / Compressor work done

COP of heat pump cycle

= Heat rejected by the condenser / Work done

COP heat pump

= COP refrigeration + 1

W isentropic is calculated by assuming isentropic compression. Actual amount of power consumed by the compressor is calculated by means of electrical Wattmeter, W. Then, isentropic efficiency of the compressor is calculated as follows; Isentropic efficiency

= W isentropic / Wattmeter reading

PROCEDURE 1. Suitable electrical connections and the valves were arranged. 2. The refrigerant was allowed to flow, when the capillary tube was in action 3. The refrigerant flow was maintained by adjusting the valve manually which was placed on the high pressure liquid line. 4. Sufficient time was provided to the system to reach the steady state condition and the readings were taken. 5. Similarly, readings were taken for the different types of valves.

CALCULATION Comparison coefficient of performance of refrigeration cycle and the heat pump cycle. Capillary tube Electrical energy supplied

= 136.96 W

Evaporator heat absorption

= mair x Cair x ΔT

= 57.18 W COP refrigeration cycle

= Refrigeration effect/Work done = 57.18/136.96 = 0.4175

Condenser heat rejection rate = mair x Cair x ΔT = 62.39 W COP heat pump

= Condenser heat rejection/ Work done = 62.39/136.96 = 0.4555

Isentropic compressor work = 0.244 x (307-283) = 5.5856 W Isentropic efficiency

= Wisentropic/Wactual = 4.28 %

Thermostatic expansion valve Electrical energy supplied

= 136.96 W

Evaporator heat absorption

= mair x Cair x ΔT = 38.99 W

COP refrigeration cycle

= Refrigeration effect/Work done = 38.99/136.96 = 0.2847

Condenser heat rejection rate = mair x Cair x ΔT = 85.07 W COP heat pump

= Condenser heat rejection/ Work done = 85.07/136.96 = 0.6211

Isentropic compressor work = 0.657 x (312-280) = 21.024 W Isentropic efficiency

= Wisentropic/Wactual = 15.35 %

DISCUSSION

Coefficient of performance (COP) of the refrigeration cycle

Thermostatic Expansion Valve

Capillary Tube

0.2847

0.4175

1.2847

1.4175

0.6211

0.4555

Coefficient of performance (COP) of the heat pump cycle = COP (Refrigeration cycle) + 1 Obtained COP of heat pump

Above derived values are not satisfying the given equation. That is because, There are a number of differences in the calculations involved in an ideal refrigeration cycle and the actual (experimental) refrigeration cycle. In the ideal refrigeration cycle it is assumed that the pressure drops are negligible, the compression is isentropic, and the evaporator and condenser are adiabatic. However the measured values will include the pressure drops due to friction, the compressor is not isentropic and the evaporator and condenser are not adiabatic. Since the compressor is not isentropic there will be more work required to get from state 1 to state 2. The entropy difference across the compressor at state points one and two reflects the entropy generated due to the irreversibility’s. Thus the actual enthalpy and temperature at state two will always be greater than the enthalpy and temperature at state 2 for the ideal process (isentropic). This results in a greater amount of work to be applied to the working fluid in comparison to the isentropic case. Since COP is defined as the benefit over the cost, increasing the work will decrease the COP. Hence the actual COP will be less than the COP obtained for the ideal system. There are other reasons for this miss match. Gas leaving temperature in compressor is 65˚C for thermostatic expansion valve and 60˚C for capillary tube which are greater than the room temperature (i.e. 30˚C). So there is a heat loss from the pipes going from compressor to condenser. On the other way there is some heat absorption from atmosphere by the pipes from evaporator to compressor.

Expansion Valves: Thermostatic expansion valve (TEV)

Fig 2 Schematic diagram of a TEV

Thermostatic expansion valve is the most versatile expansion valve and is the most commonly used in refrigeration systems. A thermostatic expansion valve maintains a constant degree of superheat at the exit of evaporator; hence it is most effective for dry evaporators in preventing the slugging of the compressors since it does not allow the liquid refrigerant to enter the compressor. The schematic diagram of the valve is given in Fig 3. This expansion valve has bellow or diaphragm, bulb, spring, adjusting screw, needle and seat. The bulb is attached to the evaporator exit. It is important to have a good thermal contact between bulb and the evaporator exit to have same temperature in both. If evaporator liquid gets super-heated due to overloading (i.e. rising of ambient temperature) then liquid and vapour solution in the bulb will increase its pressure (i.e. Pb). This will push the needle downwards allowing more refrigerant to flow. If the evaporator is facing under-load condition then pressure inside the bulb will reduce. This will raise the bellow and reduce the refrigerant flow. This will protect compressor by not letting liquid refrigerant flooding into the piston. The advantages of TEV compared to other types of expansion devices are: 1. It provides excellent control of refrigeration capacity as the supply of refrigerant to the evaporator matches the demand 2. It ensures that the evaporator operates efficiently by preventing starving under high load conditions 3. It protects the compressor from slugging by ensuring a minimum degree of superheat under all conditions of load, if properly selected

Capillary tube This is a throttling device which uses friction to reduce the pressure. Small diameter tubing is called capillary tubing. For throttling purposes, capillary tubes have diameters varying from 0.5mm to 2.28mm. The pressure reduction in a capillary tube occurs due to the following two factors: 1). The refrigerant has to overcome the frictional resistance offered by tube walls. This leads to some pressure drop. 2). The liquid refrigerant flashes (evaporates) into mixture of liquid and vapour as its pressure reduces. The density of vapour is less than that of the liquid. Hence, the average density of refrigerant decreases as it flows in the tube. The mass flow rate and tube diameter (hence area) being constant, the velocity of refrigerant increases since

. The increase in velocity or

acceleration of the refrigerant also requires pressure drop. This is mostly used in domestic air conditioners, refrigerators and freezers. Advantages of capillary tubes 1. It is inexpensive. 2. It does not have any moving parts hence it does not require maintenance 3. Capillary tube provides an open connection between condenser and the evaporator hence during off-cycle, pressure equalization occurs between condenser and evaporator. This reduces the starting to torque requirement of the motor since the motor starts with same pressure on the two sides of the compressor. Hence, a motor with low starting torque (squirrel cage Induction motor) can be used. 4. Ideal for hermetic compressor based systems, which are critically charged and factory assembled

Automatic expansion valve An Automatic Expansion Valve (AEV) also known as a constant pressure expansion valve acts in such a manner so as to maintain a constant pressure and thereby a constant temperature in the evaporator. The schematic diagram of the valve is shown in Fig 4.

Fig 3 Schematic diagram of an AEV

The automatic expansion valves are used wherever constant temperature is required, for example, milk chilling units and water coolers where freezing is disastrous. In air-conditioning systems it is used when humidity control is by DX coil temperature. Automatic expansion valves are simple in design and are economical. These are also used in home freezers and small commercial refrigeration systems where hermetic compressors are used. Normally the usage is limited to systems of less than 10 TR capacities with critical charge. Critical charge has to be used since the system using AEV is prone to flooding. Hence, no receivers are used in these systems. In some valves a diaphragm is used in place of bellows. Electronic Type Expansion Valve The schematic diagram of an electric expansion valve is shown in Fig. 5. As shown in the figure, an electronic expansion valve consists of an orifice and a needle in front it. The needle moves up and down in response to magnitude of current in the heating element. A small resistance allows more current to flow through the heater of the expansion valve, as a result the valve opens wider. A small negative coefficient thermistor is used if superheat control is desired. The thermistor is placed in series with the heater of the expansion valve. The heater current depends upon the thermistor resistance that depends upon the refrigerant condition. Exposure of the thermistor to superheated vapour permits thermistor to selfheat thereby lowering its resistance and increasing the heater current. This opens the valve wider and increases the mass flow rate of refrigerant. This process continues until the vapour becomes saturated and some liquid refrigerant droplets appear. The liquid refrigerant will cool the thermistor and increase its resistance. Hence in presence of liquid droplets the thermistor offers a large resistance, which allows a small current

to flow through the heater making the valve opening narrower. The control of this valve is independent of refrigerant and refrigerant pressure; hence it works in reverse flow direction also. It is convenient to use it in year-round-air-conditioning systems, which serve as heat pumps in winter with reverse flow. In another version of it the heater is replaced by stepper motor, which opens and closes the valve with a great precision giving a proportional control in response to temperature sensed by an element.

Fig 4 Electronic type expansion valve

Use of Ammonia instead of R12 and what prevents using it There are a variety of different chemicals that have been used as industrial refrigerants over the years, but ammonia is one of the few that is environmentally compatible. Made of nitrogen and hydrogen, ammonia occurs naturally .As such, ammonia poses no danger to the environment and doesn't harm the ozone layer, unlike many CFCs and other chemicals that have been used as refrigerants in the past. On the other hand ammonia is flammable. This is why it is not recommended for use in domestic applications. R12 is Fluorocarbon which is neither explosive nor inflammable. Ammonia is generally known to be an extremely efficient refrigerant. According to Goodway.com, ammonia is 3 to10 present more efficient than other CFCs that have been used as chemical refrigerants in the past. This efficiency means there is less energy required to keep an ammonia-based refrigeration system at its proper levels over time. There will be less electricity used, and the ammonia-based system will be cheaper to run overall.