# 05.4 - Refrigeration

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### OVC Intro

Refrigeration Cycle Introduction (LearnChemE.com, 3min) explains each step in an ordinary vapor compression (OVC) refrigeration cycle and the energy balance for the step. You might also enjoy the more classical introduction (USAF, 11min) representing your tax dollars at work. The musical introduction is quite impressive and several common misconceptions are addressed near the end of the video.
Comprehension Questions: Assume zero subcooling and superheating in the condenser and evaporator.
1. An OVC operates with 43 C in the condenser and -33 C in the evaporator. Why is the condenser temperature higher than than the evaporator temperature? Shouldn't it be the other way around? Explain.
2. An OVC operates with 43 C in the condenser and -33 C in the evaporator. The operating fluid is R134a. Estimate the pressures in the condenser and evaporator using the table in Appendix E-12.
3. An OVC operates with 43 C in the condenser and -33 C in the evaporator. The operating fluid is R134a. Estimate the pressures in the condenser and evaporator using the chart in Appendix E-12.
4. An OVC operates with 43 C in the condenser and -33 C in the evaporator. The operating fluid is R134a. Estimate the pressures in the condenser and evaporator using Eqn 2.47.
5. An OVC operates with 43 C in the condenser and -33 C in the evaporator. Assume the compressor of the OVC cycle is adiabatic and reversible. What two variables (P,V,T,U,H,S) determine the state at the outlet of the compressor?

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### Reading A PH Chart

How To Read the Pressure-Enthalpy (PH) Diagram for Propane (uakron.edu, 9min) A chemical process may need refrigeration to operate the condenser of a distillation column at cryogenic conditions. The process in this video operates between -100F and 80F in its refrigeration coils.
Comprehension Questions: Assume zero subcooling and superheating. (ie. The "approach temperature" is zero.)
1. Download the table of saturation properties for propane from the student supplements section of chethermo.net. Estimate the pressures and enthalpies exiting the condenser and evaporator. How do these compare to the values reported in the video?
2. Suppose the condenser outlet operated at 100 F. Estimate the condenser pressure (MPa).
3. Suppose the condenser outlet operated at 100 F. Estimate the condenser outlet enthalpy(J/g)
4. Suppose the condenser outlet operated at 100 F. Estimate the condenser inlet temperature(F)
5. Suppose the condenser outlet operated at 100 F. Estimate the condenser inlet enthalpy (J/g)

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### COP of OVC

COP of OVC Cycle (uakron.edu, 7min) operating at -100F in the evaporator with a compressor that takes the saturated vapor from the evaporator to 10 bars and 180F. This presentation continues the example of compressor efficiency, using similar outlet conditions to those developed previously. You may wish to refer back to that example if you haven't watched it lately. Also, you may want to refresh your chart reading skills before you watch this first video because it is based on chart reading for the compressor work calculation. As an alternative to chart reading, (uakron, 10min) you might be interested in using the ideal gas equation of state to estimate the compressor work and the propane tables to look up the saturation properties. The significance of PHi/PLo is somewhat more obvious when using the ideal gas equation. This last presentation is slightly different from the compressor efficiency problem in Section 4.10 because the compressor inlet saturated vapor (exiting the evaporator) is determined from the propane table instead of the SCVP eq. Comparing the chart calculation to the calculation based on tables and the ideal gas assumption shows that the estimates are not so different. You should have the skills to perform the calculation either way.

Comprehension Questions:
1. Suppose you wanted to run a refrigeration process between the same condenser and evaporator temperatures using HFC134a as the working fluid. How would that affect the COP? How would the pressures in the process change? Would there be some practical concerns about the pressure ranges involved with either propane or HFC134a?
2. Suppose you replaced the throttle with a turbine. How would that affect the shape of the cycle on a P-H diagram?
3. What was the numerical value of the compression ratio, PHi/PLo, applied in the presentation using the ideal gas assumption?

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