1 Lec 27: Refrigeration

2 For next time: –Class projects due Wednesday, December 3, Outline: –Refrigeration cycle –Idealized cycle devices –Example Important points: –Distinguish between heat pumps and refrigerators –Know what devices make up the refrigeration cycle.

3 Refrigeration affects many areas of your life The obvious: –Refrigerator/Freezers allow food preservation –Air conditioning makes Texas inhabitable between July 4 and mid-September Refrigerated fishing boats allow preservation of catch Refrigerated trucks are used to ship fruits/meats Refrigeration makes possible medical procedures that call for lowering body temperatures

4 What we call a refrigeration cycle depends on its function The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium

5 Schematic of the cycle

6 Four processes in cycle Isentropic compression (1 to 2) Constant pressure condensation (2 to 3) Isenhalpic expansion (3 to 4) Constant pressure evaporation (4 to 1)

7 Refrigeration cycles are often displayed on pressure-enthalpy diagrams

8 Performance of refrigeration cycles is measured by the Coefficient of Performance For Refrigerators and Air Conditioners:

9 COP for heat pumps:

10 TEAMPLAY A system operates between fixed indoor and outdoor temperatures. It has a fixed w in.would the COP be greater for heat pump operation or cooling operation? Why? T H TLTL System TLTL

11 TEAMPLAY Show that the performance of a heat pump and a refrigerator are related by the expression COP HP = COP R + 1.

12 Simple view of household refrigerator

13 Heat Pump Heats a House in Winter and Cools it in Summer

14 TEAMPLAY Work Problem 8-130

15 TEAMPLAY If the evaporating pressure were reduced to 0.10 MPa in the previous problem, how would the following be affected (up, down, remain the same): Evaporating Temperature? Cooling capacity? Work input to the compressor? Explain why.