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Refrigeration Lab Report[2037] Flipbook PDF
Refrigeration Lab Report[2037]
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Refrigeration Lab Report Ali Rida Bachir SID 8104461 Abstract: In this experiment, a refrigeration unit (R634) was studied. The unit was allowed to operate normally while different parameters were measured such as temperatures, pressures and flow rates. The results were tabulated and used to construct the thermodynamic cycle on the P-h chart. As well as asses its performance by measuring the isentropic efficiency which was 59.9% and the COPR (coefficient of performance) = 1.949 .
Introduction: Refrigeration is the process of transferring energy from a low energy domain to a high energy domain. According to the second law of thermodynamics, heat cannot be transferred from a cold location to a hotter one unless work is introduced to the process. The devices that apply this process are called refrigerators. Refrigerators are cyclic devices that operate on the vapour – compression cycle (reverse cycle of heat engines) involving four processes; Evaporation, Compression, Condensation and Expansion. For that it requires four components; Compressor (to raise temperature and pressure), Condenser (exchange heat with surrounding), Expansion valve/Throttle (to lower pressure and temperature), Evaporator (absorbs heat from refrigerated area) shown in Figure 1. The purpose of a refrigerator is to maintain the refrigerated space at low temperature by removing heat from it. The working fluid in refrigerators is called the refrigerant it absorbs heat isothermally from a low temperature source in the amount of ‘QL’ (from refrigerated area at evaporator) then rejects heat isothermally to a high temperature sink in the amount of ‘QH’ (to the surrounding at condenser). The objective of this report is to identify and analyse the processes in each component, record data such as pressure and temperature at different locations as well as studying the refrigerant’s phases (thermodynamic states) (Figure 2) to construct the refrigeration cycle on a real pressure enthalpy chart provided by SOLVAY. The chart is for refrigerant Solkane® SES36. Solkane® refrigerants are superior as they are nontoxic, ozone friendly, completely recyclable and easy to handle.
Apparatus used: The apparatus used for this experiment was a refrigeration cycle demonstration unit R634. It consists of a bench mounted vapour compression refrigeration cycle demonstration unit using a hermetic compressor and water cooled flooded glass condenser and evaporator. A floatcontrolled expansion device controls the flow of refrigerant (Solkane SES36). Internal electrical and mechanical safety devices allow for unsupervised operation by students. Instruments that allow the measure of pressures temperature as well as flow rates are fitted in the machine. (© 2011, P.A. Hilton Ltd). F IGURE 1 DEMONSTRATION UNIT R634 (2011, P.A. H ILTON L TD ).
Experimental procedure: 1. Firstly, the cooling water and mains supply to the unit are turned on. 2. Make sure the valves are in normal operation. This ensures that the vapour is drawn by the compressor and the condensed liquid goes to the evaporator. 3. Set the condenser cooling water flow rate to 6 g/s. 4. Set the evaporator water flow rate to 10 g/s. 5. Turn on the main switch. The compressor will start working. If the unit is working normally, we should see two internal lamps light up. F IGURE 2 VALVES POSITIONS FOR NORMAL OPERATION. 6. Set the refrigerant flow rate to 1 g/s. 7. Let the unit to run for 15 to 20 minutes to allow everything to stabilize. (time taken to stabilize may depend on surrounding conditions) 8. Record all parameters in data sheet.
Results: The results obtained from recordings and calculations are tabulated below respectively: T ABLE 1 THESE VALUES ARE VALID AT ROOM TEMPERATURE OF 21O C WHILE P ATM WAS TAKEN AS 100 K N/M 2
QUANTITY MEASURED
RECORDED VALUE
UNIT
Evaporator gauge/absolute pressure Pe
-68/32
kN/m2
Evaporator inlet/outlet water temp.
15.6/13.2
oC
Evaporator refrigerant temp.
3.5
oC
Evaporator water flow rate
10
g/s
Condenser gauge/absolute pressure Pc
53/153
kN/m2
Condenser inlet/outlet water temp.
16/21.9
oC
Condenser refrigerant temp.
33.5
oC
Condenser water flow rate
6
g/s
Compressor discharge temp.
69.6
oC
Compressor power input
170
Watts
Refrigerant mass flow rate
1
g/s
Expansion valve inlet temp.
26.1
oC
T ABLE 2 TEMPERATURES WERE CONVERTED TO SI UNIT KELVIN - (C P = 4.18 K J/KG.K )
FORMULAE/QUANTITY Q’c (Qh) = m’c . CP . (t1-t2)
VALUE
UNIT
0.148
KJ/s
Q’e (QL)= m’e . CP . (t3-t4)
0.1
KJ/s
𝜻 (pressure ratio abs.) = PC/Pe
0.0478
-
COPR = (h1-h4)/(h2-h1)
1.949
-
Isentropic efficiency
0.559(55.9%)
-
Power output =m’ref . (h2-h1)
59
J/s
Pmech (Given)
170
J/s
Discussion: Points 1 ⟶2, the compression process takes place. The refrigerant enters the compressor as a low temperature and pressure vapour saturated (ideally) or super-heated and is discharged as a high temperature and pressure super-heated vapour. The change in internal energy is 𝛥h1,2=Win, compression process is assumed to be adiabatic. Points 2⟶3, condensation process takes place. The high temperature and pressure vapour passes through the condenser where it rejects heat to the surrounding environment and condenses into a saturated liquid (ideally) at constant pressure and temperature. In reality, the refrigerant is allowed to condense beyond the saturated liquid state into the sub-cooled liquid state. That is to ensure complete phase transformation. The change in internal energy 𝛥h2,3 is equal to the heat rejected Qc . Points 3⟶4, the expansion process takes place. The high-pressure liquid goes through the expansion valve that causes a significant drop in the temperature and pressure of the refrigerant. The process is assumed to be adiabatic since its rapid and no work is done. Change in internal energy 𝛥h=0 .
SATURATED
Win
F IGURE 3 MAIN COMPONENTS SHOWN WITH FLOW AND STATE OF REFRIGERANT ( IDEALLY ) Critical point
T1
3
2
T2
Points 4⟶1, when the pressure of the liquid is lowered it starts evaporating. The energy that 4 1 evaporates the refrigerant comes from cooling everything down (e.g. pipes). As it passes through the evaporator (refrigerated area/region), the refrigerant is a mix of liquid and vapour, the energy it absorbs is in the amount of Qe continues to evaporate F IGURE 4 IDEAL PRESSURE VS . ENTHALPY EXAMPLE CHART SHOWING PHASES OF THE FLUID the refrigerant until it is in the saturated vapour state (ideally), in reality, it is allowed to evaporate until a super-heated vapour is obtained. That is to ensure 100% conversion to vapour before it enters the compressor since liquids are incompressible (assumed to be as they require a lot of pressure to accomplish a little compression). Pmech (given) was measured using the electrical supply to the compressor. Pmech > Poutput is an expected inaccuracy as the energy provided to the compressor is not fully used into compressing the refrigerant vapour. Most of the energy is lost due to a lot of factors mainly: -
The quality of the refrigerant vapour. Thermal losses through conduction and convection. Vibrations (sound). Some of the Kinetic energy of the compressor is transformed into heat. Hysteresis loses due to heat exchange between cylinder and refrigerant vapour.
On the other hand, other major errors affected the results of the experimental data including: -
Readings of the P-h chart were inaccurate as the copy provided was a photograph. External influences such as vibrations and room temperature which was assumed. Inaccurate reading of values by the machine Pressure losses due to imperfect seals. Thermal losses throughout the system.
h3,4
h3’4’
h1
h2’
h2
F IGURE 4 SHOWING THE PLOTTED CYCLE ON THE REAL P- H CHART
Conclusion: In conclusion, as for the efficiency of the compressor, 44.1% of the power input is lost due to natural occurrences and poor insulation. Moreover, refrigerant Solkane SES36 is not suitable for an ideal cycle.
References:
Gkanas, E. 2018, thermodynamics [online lecture] module 207MAE. Coventry University available from [september23 June 2018].
Cengel & Boles, Y. M. 1989, thermodynamics: an engineering approach, 5th edition, McGraw-Hill, London.
James M. Watterson. 2018, a simple guide to understanding compressors, 222 East 46th Street, New York, NY 10017.