Performance analysis of Air Conditioning

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Abstract:

This experiment is mainly intended over analyzing the COP obtained for the case of refrigeration kit. We have computed refrigeration effect produced by the kit and even the compression work required for running the kit. They were computed for three different conditions which included varying input to the pre-heater. This three varying input to the pre-heater included 0 kW, 0.5 kW and 1.5 kW. It can be defined as the ratio of the refrigeration effect to the work input at the compressor. Their refrigeration effect and heat flowing was computed which helped in analysis of its performance and even for computing the COP of the complete refrigeration kit. Even we computed the mass flow rate of the air through the evaporator which is helping in terms of heat transfer to the closed medium from the refrigerating medium. Comparison was done for the heat transfer through the air which is flowing through the evaporator and net refrigeration effect produced.

Introduction

Air conditioning is very important in today’s era where maintaining temperature of a closed room and also the humidity of the surrounding environment is essential for comfort of the individuals. Air conditioning doesn’t only removes the heat from the closed room but even maintains humidity for comfort feeling. Refrigeration effect is produced by means of thermodynamic cycles which includes vapour-compression, absorption and reversed Brayton Cycle. These cycles by use of refrigerant fluid extracts heat from the source and transfers that heat to the surrounding environment which can be termed as heat sink.

Refrigeration is a term which simply means cooling of the system below the temperature of its surroundings. This process is accomplished by means of different components which includes evaporator, compressor, condenser, expansion valve and working fluid. All these components together constitutes the refrigeration system. Heat is extracted from source via evaporator where latent heat of evaporation of the working fluid is absorbed for cooling the source. This heat absorbed is further rejected to the heat sink via complete cycle through condenser. Below figure shows the schematic diagram of vapour compression refrigeration cycle and arrangement of its different components.

Figure 1 Vapor Compression Refrigeration Cycle[2]

Let us now look at various process which undergoes inside this arrangement for the process of cooling.

1. Compressor: In compressor required work input is supplied in terms of Mechanical Energy. This mechanical energy is utilized for compressing the refrigerant vapour. This refrigerant is compressed from low pressure and low temperature to high pressure and high temperature. This is mainly because latent heat of vaporization is quite lesser at higher pressure. This

reduces the amount of cooling required for condensing the vapour in the condenser. During this compression process mixture which is consisting of liquid and vapour has to be compressed in compressor isentropically. This kind of compression is known as wet compression which is very difficult especially with the reciprocating compressors. Even though some kind of compressors can tolerate this presence of liquid in vapour, but since reciprocating compressors are most widely used in refrigeration, mostly dry compression is preferred over wet compression.[1]

2. Condenser: Vapour which is superheated by means of its compression in compressor is cooled in condenser at constant pressure. Temperature of the vapour gradually reaches the saturation temperature while causing the vapour to condense into a liquid by means of losing its latent heat of vaporization. This liquid refrigerant is then collected in a receiver which is placed at the end of the condenser. By means of losing its latent heat of vaporization it gets converted from the vapour phase to corresponding liquid phase.

3. Expansion: This is a process of expansion of the condensed liquid which has released its latent heat of evaporation during condensation. High pressure liquid from the receiver is then further passed via expansion or throttling valve which causes pressure and temperature to drop. It causes some of the liquid for flashing into vapour which results in two phase liquid-vapour mixture. Even the enthalpy across the expansion valve is constant as there is no work done by means of expansion valve. In ideal condition this process is adiabatic.

4. Evaporation: Two phase liquid which is coming out of the throttling device finally enters into the evaporator. It absorbs the latent heat of evaporation in the evaporator and boils by means of gaining heat from space which is to be cooled. It ultimately causes the refrigeration effect. For the case of avoiding liquid drops of refrigerant from entering the compressor this mixture is heated in such a manner that it exits in form of saturated or superheated vapour.

Figure shows the schematic representation showing heat sink and heat source which are correspondingly attached to condenser and evaporator. In this particular case turbine is used in place of expansion valve. Turbine ultimately works as an expansion device which extracts energy of the expanded steam received and converts the same into Mechanical Energy. There’s a mechanism shown where Turbine is connected with Compressor which transfers some of the extracted energy to Compressor for its working thus saving some of the heat loss. In this case heat source is the place from where heat is extracted or where refrigeration effect is produced and heat sink is a place where heat is rejected which forms the outer surroundings.[1]

Figure 2: Schematic diagram of refrigeration cycle[1]

Beside figure is showing the T-S diagram for the vapour-compression refrigeration cycle where all the processes undertaken are represented. Here 1-2 process is compression process which is governed by means of constant entropy and increase in temperature and pressure of vapour. Process 2-3 is for the case of condensation where vapour will extract its latent heat of vaporization and will turn into liquid. In process 3-4 it is shown for the case of expansion valve where pressure and temperature reduces with slight increase in entropy. At last process 4-1 is governed by evaporation of liquid

and vapour mixture where it absorbs heat from the region which is to be cooled and increases the entropy. In this case temperature remains constant as there is only phase change and heat energy absorbed overcomes the latent heat of vaporization. Same is the case shown for 2-3 where we can see straight line even for a heat transfer which governs the phase change energy transfer.

Objective of this experiment is distributed in three cases. First case among them is for calculating the net refrigeration effect which is produced and also the compressor power consumption. It will thus be calculating the net Co-efficient of Performance of the Refrigeration Cycle (COP). This will be done by use of [Eq. 1]. Second case among them includes calculating the mass flow rate. This is calculated by means of utilizing [Eq. 2]. After all this in case three we will be carrying out the energy balance across the evaporator. It includes comparing the amount of heat which is removed from air to net refrigeration effect. It is termed as the heat gained by the refrigerant. We will compare them and find out whether they match and if not why does they don’t match.

COP = Refrigeration EffectCompressorWork [Eq.

1]

m air = ρair∗Cd∗A∗√ 2∗g∗h∗ρwρair

( kgs ) [Eq.

2]

where,

ρair = Density of Air ( kgm3 ) Cd = Co-efficient of Discharge of the orifice = 0.5 (given)

A = Area of the Orifice (Diameter of Orifice = 8 in) (m2)

1 Source: Lab Manual

g = Gravitational Constant (ms2 ) h = Pressure Head (m)

ρw = Density of water ( kgm3 ) COP=

mr (h1 '−h4)mr (h2−h1)

=(h1'−h4)h2−h1

= enthalpy differenceacross evaporatorenthalpydifference acrosscompressor

[Eq. 3]

Setup

Figure shows the test rig which is going to be used for the experiment for analyzing the refrigerating effect. Various components connected with the test rig includes blower which is providing the air flow for the system. It is rated at 1300 CFM (36.84 m3/min). Air flow can be varied by varying the speed of the blower. It can even be controlled by using the

blower speed control knob. Next component includes the Stabilizing Duct. It consists of orifice at the exit and a pitot tube at the entry. Manometer is connected across the orifice and pitot tube for measuring the differential pressure across them. It is in order to calculate the air flow. Another component which is to be used includes Re-heater and Pre-heater. They consist of two resistance element heaters which are rated at 0.5 KW and 1 KW. They are used for altering the temperature of air leaving and entering the evaporator coil.

Next component includes evaporator in which refrigerant enters as a low pressure liquid vapour mixture. This warm air which is circulating in the duct further passes through the evaporator. Here two phase refrigerant mixture absorbs the sensible and/or latent heat of vaporization and further starts to boil. In this setup forced air evaporator is used in which a direct expansion coil is made of the copper tubing. It is supported in metallic tube sheets which are having aluminium fins. This fins increases the heat transfer efficiency. Condenser is also an important part of the experimental setup. Here heat is absorbed by the refrigerant during the process of evaporation and is further released to the condensing medium. Magnitude of heat which is released from the condenser is always greater

than the heat absorbed during the evaporating process. Additional heat is due to the compressor work.

Reciprocating type compressor is used for increasing the pressure of the refrigerant to a value such that the saturation temperature is higher than the temperature of the cooling medium in the condenser. It even reduces the pressure over the evaporator side while maintaining the evaporator temperature lower than the temperature which is required by means of application. Thermal expansion valve even controls the amount of refrigerant flow into the evaporator while causing pressure drop in the working fluid used. Rate of flow is controlled by means of needle type plunger which varies the opening of the orifice. Needle is controlled by means of three forces which are namely due to evaporator pressure, force due to superheated spring and pressure exerted by the charge in the thermal bulb.

Operating Procedure

In this basic performance testing of refrigeration system, initially we will be switching on the blower before switching over to the compressor. It can thus appreciably load the compressor. Corresponding wick will then be wet of the wet bulb thermocouples by means of use of distilled water via opening which is provided. After that required flow rate is set of air into the system by means of varying the speed of the blower. It is done by means of using the speed control knob. Inclined manometer reading will further be noted at the exit for calculating the mass flow rate of air in the system. System is then allowed to run until steady state is reached of the system. This experiment is conducted by means of a damper in its closed position and we will further be noting down the thermocouple readings at the inlet of the system which is at the evaporator and at the exit of the system for the case of air and across the compressor, across the expansion valve and across the evaporator for refrigerant.

We will further note down the inlet and exit pressures of the refrigerant across the compressor and local pressure which is provided by the instructor. Even the refrigerant flow rate from the rotameter and use of calibration chart which is attached for converting the scale reading to flow rate are noted down. Power consumed by blower and compressor will even be noted down. Experiment will further be repeated while following the steps being mentioned above with only 0.5 kW preheater switched on. Again the experiment will be repeated while following the above steps with both of the 0.5 kW and 1kW preheater switched on.

Results and Discussion

Table 1 P vs H Enthalpies

1’ 1 2 3 4425 kJ/kg 475 kJ/kg 490 kJ/kg 245 kJ/kg 245 kJ/kg1’ 1 2 3 4431 kJ/kg 460 kJ/kg 485 kJ/kg 250 kJ/kg 250 kJ/kg1’ 1 2 3 4445 kJ/kg 450 kJ/kg 478 kJ/kg 256 kJ/kg 256 kJ/kg

Table 2 Psychometric Diagram Enthalpies

0.0 kW Inlet 0.0 kW Outlet 0.5 kW Inlet 0.5 kW Outlet 1.5 kW Inlet 1.5 kW Outlet

26 kJ/kg 23 kJ/kg 43 kJ/kg 36 kJ/kg 51 kJ/kg 56 kJ/kg

After conducting various calculations COP of the refrigeration rig was found to be around 2. It included refrigeration effect of around 85 kJ/kg and compression work of 41 kJ/kg for the case of 0 kW preheater. Mass flow rate of the air was obtained to be around 0.1297 kg/sec. Volumetric flow rate of 0.000129 m3/sec is obtained. We even calculated the heat removed from air which is far lesser than the refrigeration effect. This is mainly because refrigeration effect is the total heat removed or heat removal effect produced by the refrigeration kit, whereas heat removed from air is just part of the total refrigeration effect. Heat removed from air is the actual refrigeration obtained in the closed surrounding where air is directly in contact. One can even say this is the actual refrigeration produced. Even volume of the air being quite large for specific quantity of water which also reduces the heat removed from a specific weight of the air which is having higher entropy or randomness.

Table 3 Table of Results

Pre-Heater Power (kW) COP

Refrigeration Effect (kJ/s) Δ Q (kJ/s)

0 2.066659956 85.00 0.930.5 1.343043486 55.00 0.361.5 1.968755794 85.00 0.98

As this is a real system, it will not be having ideal situation for the case of refrigeration effect. It is being equal to the heat loss which is by means of passing air through the evaporator. Difference in the refrigeration effect and even the heat lost by means of air is possibly due to the errors obtained in the temperature measurement system. System is not being fully insulted and that of the air conditioning is causing fluctuations in the room temperature.

Summary

In this experiment we have analysed the refrigeration rig where its refrigeration effect was evaluated and compared with the compressor work required to be supplied. COP was found by means of comparison of the refrigeration effect across the evaporator to the work input which is obtained from the compressor. This gave us the COP of that refrigeration kit. Enthalpies across the compressor and evaporator were used in this experimental for finding out the COP for 3 of the different experimental situations. This situations were having varying levels of power supplied to the pre-heater. We even evaluated the refrigeration effect and work input by means of evaluating the pressure vs enthalpy chart and enthalpy difference across each of the devices. We even focused over comparing the energy balance across the evaporator where the refrigeration is taking place. Heat which is absorbed by the refrigerant in the evaporator was compared with the heat which is lost by the air which flows over the evaporator. Even we calculated the mass flow rate of air occurring through the device. This mass flow rate is signifying the air flow rate through the evaporator which is helping in terms of refrigeration provided in the specified closed dimension space. We even compared the amount of heat rejected from the air flowing through the evaporator and corresponding refrigeration effect produced. Heat rejected from the air flowing through the evaporator is far lesser than the refrigeration effect which we discussed in the result section of the report.

References:

[1] Vapour Compression Refrigeration Systems, Lecture 10, Page 3-18, IIT Kharagpur, http://nptel.ac.in/courses/112105129/pdf/RAC%20Lecture%2010.pdf

[2] Lab Manual

[3] Moran, Incropera, and Munson. "Chapter 8: Vapor Power and Refrigeration Systems." MAE240/MEE340 - Thermofluids I and Thermofluids II.