Potter & Somerton - Chapter 10: Refrigeration Cycles

We have previously covered refrigeration and heat pump cycles in detail in ME321 (refer back to Chapter 4) thus this first part should be treated as a review. Be sure to review all four of the Additional problems presented in Chapter4. Before doing these you should review the example solution of Refrigerators and Heat Pumps.

• Additional Problems 4.1 & 4.2 - Problem 4.1 (home refrigerator) and Problem 4.2 (home refrigerator with an internal heat exchanger)
• Additional Problem 4.3 (home heat pump system for space heating)
• Additional Problem 4.4 (home air conditioner and hot water heating system)

In Chapter 10 of Potter and Somerton we will not be covring Sections 10.3 (The Multistage Vapor Refrigeration Cycle), 10.5 (The Absorbtion Refrigeration Cycle) or 10.6 (The Gas Refrigeration Cycle). Instead we will introduce the new hot topic of the refrigeration world i.e. the carbon dioxide (R744) transcritical refrigeration cycle.

We recommend that you attempt as many of the following Supplementary Problems as possible during this week:

• Ideal vapor refrigeration cycle: 10.7, 10.8, 10.10. 10.11. 10.13.
  Note: for all the above problems, ideal cycle refers to isentropic compression from a saturated vapor state, no pressure drops in heat exchangers, and a condenser outlet at saturated liquid state.
  Problem 10.7: replace the phrase "saturation temperature of 40°C" with "a high pressure of 1000kPa".
  Problem 10.8: recall that 1 hp = 0.746 kW.
  Problem 10.11: see page 246 - 1 ton of refrigeration equals 3.52 kW.
• Evaluating Compressor Efficiency (h_C) (refer back to Chapter 6 and review Additional problems 6.1 and 6.2): 10.14, 10.16, 10.17.
• Ideal heat pump: 10.22

Please note that all of these exercises should be accompanied by a P-h (Pressure-enthalpy) diagram. Problems involving non-isentropic compressors should also be accompanied by an h-s (enthalpy-entropy) diagram. All answers to the Supplementary Problems are given at the end of the chapter

The basic refrigerator and heat pump configurations

The basic refrigerator consists of four components as shown below. Heat is removed in the low temperature evaporator and rejected to the ambient in the condenser.

The basic heat pump system is seen to consist of the identical four components, however the heat is now extracted from the cold outside in order to heat the home, as shown below.

The pressure-enthalpy (P-h) diagram

Unlike the situation with steam power plants, it is considered the norm to describe refrigeration (and heat pump) systems in terms of P-h diagrams, and we will use them throughout in place of T-s diagrams presented in Potter & Somerton. In fact, in the Fundamentals of Engineering (FE) exam you are expected to solve all refrigeration and heat pump problems using the P-h diagram provided in the FE Handbook.

The enthalpy-entropy (h-s) diagram

We will use the h-s diagram in order to evaluate a refrigerator or heat pump system with a compressor which has an isentropic efficiency less than unity. Thus only the superheated region is of interest, as shown.

As an example of its usage consider a refrigeration compressor in which R134a refrigerant enters the compressor at 140kPa, saturatedvapor and is compressed to 800kPa. The isentropic process is shown as process (1)-(2s). Assuming a compressor efficiency (h_C) of 85%, the actual process is shown as process (1)-(2).

On to Carbon Dioxide (R744) Refrigeration Cycles

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