E2.9 Refrigeration Cycle Experiment

SCHOOL OF MECHANICAL AND AEROSPACE ENGINEERING

2006/2007

E2.9 REFRIGERATION CYCLE EXPERIMENT

THERMODYNAMICS LABORATORY

(THERMAL AND FLUIDS ENGINEERING DIVISION)

N3-B2a-01

Name of students:

Kng Qiao Mei

Neo Huai Jin Bernard

Tang Jingwei

Yeo Yul Chen Janan

Lab Sub-Group:

AE18

Name of Supervisor:

A/P Jeffery Low

References and Acknowledgements

1. Thermodynamics – An Engineering Approach, 5th Edition, Yunus A Cengel & Michael A Boles, McGraw-Hill 2006

2. AE2006: Thermodynamics: Refrigeration and Heat Pump System Notes, Associate Professor Ho Hiang Kwee, Nanyang Technological University

3. ME372 – Thermodynamics Notes, University of Nevada - Reno, Mechanical Engineering

4. ME354 – Heat Transfer Notes, University of Waterloo, Canada, Microelectronics Heat Transfer Laboratory

We would like to thank the above sources for their valuable information and A/p Jeffery Low for his kind guidance.

Further Discussions

Describe the major differences between an industrial refrigerator and an commercial refrigerator.

There are mainly 2 versions of industrial refrigerators in the market.

1) Cascade Vapor Refrigeration Cycles

Cascade Vapor Refrigeration Cycles are used industrial applications to boost the performance of vapor compression cycles.

The use of two or more refrigeration cycles in series is called Cascade Cycles.

Cascade cycles allow for the use of two separate types of refrigerants to suit the varying operating conditions of each individual cycle, and are seen in application where relatively low temperatures are desired.

Fig 3.1 - Schematic Diagram of Cascade Cycle

F ig 3.2 – T-s Diagram of a Cascade Cycle

From Fig 3.2, we can see that compressor work decreases and the amount of heat absorbed from the refrigerated space TL increases as a result of cascading. Therefore, industrial refrigerators that make use of the Cascade cycle enjoy a higher coefficient of performance (COP) compared to normal commercial refrigerators.

If a certain temperature is desired, the number of cascading stages can be increased but the increase in processes also mean a higher cost.

If the refrigerants used in the cascade refrigeration cycle are the same, the heat exchanger between the stages can be replaced by a mixing chamber (known as a flash chamber) since it has improved heat characteristics, then the Cascade Refrigeration Cycle will be know as a Multistage Compression Refrigeration System

2) Absorption Refrigeration Systems

Another form of refrigeration that becomes economically attractive when there is a source of inexpensive heat energy at a temperature of 100 to 200oC is absorption refrigeration, where the refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small.

Since in liquid form, work input into the system is minimal because liquid requires significantly less energy to pump compared to gases. It is also environmentally friendly, as it makes use of generally non-CFC or HCFC refrigerants.

Though it has a relatively lower COP compared to other industrial refrigerators, it can be improved by multi-staging.

Fig 3.3 – Schematic Diagram of an Absorption Refrigeration Cycle

Improvements to the vapor compression refrigeration cycle:

The performance of vapor compression cycle can be improved by the following methods:
-- Cascade Vapor Refrigeration Cycle
-- Multipurpose Compression with intercooling
-- Multistage Compression Refrigeration cycle
-- Gas liquefaction

Cascade Vapor Refrigeration Cycle
Cascade cycles are two or more refrigeration cycle arranged in series. In figure 1 below, the cascade cycle is shown in which two vapor-compression refrigeration cycles, labeled A and B are arranged in series with a counterflow heat exchanger linking them. In the intermedium heat exchanger, the energy rejected during condensation of the refrigerant in the lower temperature cycle A is used to evaporate the refrigerant in the higher-temperature cycle B.

Figure 1: Cascading Cycle

The desired refrigeration effect occurs in the low-temperature evaporator and heat rejection from the overall cycle occurs in the higher temperature condenser. In the cascading cycles, the coefficient of performance is the ratio of the refrigeration effect to the total work input.

A significant feature of the cascade system is that the refrigerants in the two or more stages can be selected to have reasonable evaporator and condenser pressures in the two or more temperature ranges. In double cascade system, a refrigerant would be selected for cycle A which has a saturation pressure-temperature relationship that allows refrigeration at a relatively low temperature without extensively low evaporator pressures. In turn, the refrigerant for cycle B would have saturation characteristics that permit condensation at the required temperature without excessively high condenser pressure.

Multistage Compression with intercooling
Net work output can be increase by decreasing the compressor work consumed. This can be achieved by multistage compression with intercooling the gas between stages. Intercooling is achieved in gas power systems by heat transfer to the lower-temperature surroundings. In refrigeration systems, the refrigerant temperature is below that of the surroundings for much of the cycle. Thus, other means must be employed to accomplish intercooling so as to achieve the attendant savings in the required compressor work input.

Figure 2: Multistage Compression with intercooling

Multipurpose Compression Refrigeration cycle
Refrigeration at several temperatures can be achieved in a refrigerator with a single compressor by throttling the refrigerant in stages. One type of multistage compression cycle is the house-hold refrigerator. In the refrigerator, the flow of cool air to the multipurpose storage chamber is selectively controlled in accordance with a variety of operational modes. In order to accomplish such a cool air flow control, a refrigerating compartment fan and a damper are provided at positions above a refrigerating compartment evaporator.

Figure 3: Multipurpose Compression Refrigeration cycle

Gas liquefaction
Refrigeration by liquefaction of gases is the process of refrigerating a gas to a temperature below its critical temperature so that liquid can be formed at some suitable pressure, also below the critical pressure. Gas liquefaction is a special case of gas refrigeration. Firstly, The gas is compressed to a higher pressure in an ambient-temperature compressor. This high-pressure gas is passed through a countercurrent heat exchanger to a throttling valve or expansion engine. Upon expanding to the lower pressure, cooling may take place, and some liquid may be formed. The cool, low-pressure gas returns to the compressor inlet to repeat the cycle. The purpose of the countercurrent heat exchanger is to warm the low-pressure gas prior to recompression, and simultaneously to cool the high-pressure gas to the lowest temperature possible prior to expansion. Both refrigerators and liquefiers operate on this same basic principle.

Figure 4: Gas liquefaction

Results/Discussions

a) Following the instruction given in the Appendix, construct the cycles representing the system at the two operating conditions on the attached p-h diagram.

Figure 2: T-S graph

From the enthalpies of the state points in the refrigeration cycles:

b(i) Calculate the work done by the compressor and compare it with the electrical input into the compressor

From figure 3, it was observed that the power supplied to the compressor is always greater than the work done by compressor. The refrigerant was compressed from state 1 to 2 in an irreversible process as heat was lost to the surrounding, thus explaining for the loss of energy in the system to surrounding.

Figure 3: Excerpt from table 2 – energy balance at compressor

b(ii) Calculate the heat rejected by the refrigerant at the condenser and compare it with the heat gained by the condenser coolant.

Figure 4: Excerpt from table 2 – energy balance at condenser

From figure 4, we can see that the heat rejected by refrigerant at condenser is always greater than the heat gained by condenser cooling water. This can be explained as the heat transferring process was non-ideal, i.e. some heat was lost to the surrounding during the course of the experiment.

In both cases, explain the discrepancies:

Temperature fluctuations during the experiment result in inaccurate data readings and thus result in erroneous calculations. This can be explained by the fact that the thermistor is a highly sensitive device. The website below gives a detailed explanation of the thermistor’s sensitivity:
www.omega.com/temperature/Z/pdf/z036-040.pdf

c) Determine the coefficients of performance COPc, COPh, COPc,o and COPh,o for the two sets of readings. Discuss the differences in the coefficients of performance.

The calculated results are tabulated below.

Figure 5: Excerpt from table 2 – Coefficients of performance

Firstly, it was observed that COPc is always smaller than COPh. COPc is essentially the measurement of how much heat is extracted in by the refrigerant whereas COPh is the measurement of how much heat is extracted out of the refrigerant.
Secondly, COPc,o and COPh,o are smaller than COPc and COPh. The loss in heat dissipation in converting electrical energy supplied by the compressor motor to the shaft of the compressor explains for the differences

Lastly, the COPs for result 1 is greater than that of result 2. By partially blocking the evaporator outlet, condenser water flow rate was reduced. This means that lesser heat was transferred from the cooler reservoir. Hence, the thermal efficiency of the refrigerator is reduced.

Experiment

(1) Start up of the equipment.
(2) Adjust the condenser cooling water to 35g/s and allow the unit for stabilize (around 5 minutes)
(3) When stability is established, take the required readings and record them in the attached Data sheet (as presented in our Results).
(4) Reduce the condenser water flow rate so that p2 increases by approximately 200kPa. Adjust t4 (the evaporation temperature) until it returns to its initial value by partially blocking the air flowing across the evaporator if necessary.
(5) When stability is established, record the second set of readings.

Equipment

In this experiment, the apparatus we are using, as shown below in figure 3.1, comprises of four major components:

A hermetically sealed, reciprocating compressor
A condensor
A thermostatic expansion valve
An evaporator

The refrigerant used in the refrigeration unit is R-134a and the condensor coolant used is water. Temperature and pressure at relevant state points in the cycle can be easily read off from a digital temperature reader and the 2 pressure gauges. In addition, refrigerant and condensor coolant flowrates can be obtained directly from flowmeters located on the unit itself. Finally, electrical power input to the motor can be obtained by timing how long the disc of the watt-hour meter take to complete one revolution.

Fig. 3.1 Schematic layout of experiment apparatus

Theory

One important application of the thermodynamic cycles is refrigeration. The transfer of heat from a higher temperature to a lower one is a natural phenomenon, but the unnatural reverse process happens in refrigeration cycles. The devices that produce refrigeration are called refrigerators and the working fluids are called refrigerants.

Fig. 2.1 Schematic diagram of refrigerator

A refrigerator is shown schematically in Figure 2.1. Here, QL is the magnitude of the heat removed from the refrigerated space at temperature TL, QH is the magnitude of the heat rejected to the warm space at temperature TH, and Win is the net work input to the refrigerator.

Refrigeration can be classified as:
1. Vapor cycle, and
2. Gas cycle

Vapor cycle refrigeration can further be classified as:
1. Vapor compression refrigeration
2. Gas absorption refrigeration

Fig. 2.2 Ideal vapor compression refrigeration cycle

An ideal vapour compression refrigeration cycle is defined as a refrigeration system that removes thermal energy from a low temperature region and transfers the heat to a high temperature region. The diagram in Figure 2.2 shows the schematic and the T-s graph for an ideal vapor compression refrigeration cycle.

For an ideal vapor compression refrigeration cycle, it consists of 4 main processes:

The refrigerant enters the compressor at state 1 as saturated vapour and is compressed isentropically to the condenser pressure. The temperature increases during this process to well above the temperature of the surrounding medium. The refrigerant then enters the condenser as superheated vapour at state 2 and leaves as saturated liquid at state 3 as a result of heat rejection. The temperature at this state is still above the surrounding temperature.
The saturated liquid at state 3 is throttled to the evaporator pressure by passing it through an expansion valve. The refrigerant enters the evaporator at state 4 as a low quality saturated mixture and is completely evaporated by absorbing heat from the refrigerated space. The refrigerator leaves the evaporator as saturated vapour and re-enters the compressor, completing the cycle.

However, in real life situations, the ideal cycle rarely occurs due to friction in the pipe and irreversibilities that occur in pipes and devices in the refrigeration cycle. The deviation from the ideal cycle is as shown below in Fig. 2.3.

Figure 2.3 Schematic and T-s diagram for the actual vapor compression refrigeration cycle

Objective

Comparison of practical and ideal refrigeration cycle on a pressure-enthalpy (p-h) diagram and establishing the energy balance on the condensor and compressor.

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