Lec 27 Refrigeration 1 For

Lec 27: Refrigeration 1

• For next time: – Class projects due Wednesday, December 3, 2003. • Outline: – Refrigeration cycle – Idealized cycle devices – Example • Important points: – Distinguish between heat pumps and refrigerators – Know what devices make up the refrigeration cycle. 2

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 3 temperatures

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 4

Schematic of the cycle 5

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) 6

Refrigeration cycles are often displayed on pressure-enthalpy diagrams 7

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

COP for heat pumps: 9

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

TEAMPLAY • Show that the performance of a heat pump and a refrigerator are related by the expression COPHP = COPR + 1. 11

Simple view of household refrigerator 12

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

TEAMPLAY Work Problem 8 -130 14

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. 15