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Handbook of air conditioning and refrigeration / Shan K

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12.6 CHAPTER TWELVE<br />

System Components<br />

Most <strong>air</strong>-source heat pumps consist <strong>of</strong> single or multiple compressors, indoor coils through which<br />

<strong>air</strong> is conditioned, outdoor coils where heat is extracted from or rejected to the outdoor <strong>air</strong>, capillary<br />

tubes, reversing valves that change the heating operation to a cooling operation <strong>and</strong> vice versa, an<br />

accumulator to store liquid refrigerant, <strong>and</strong> other accessories. A typical ro<strong>of</strong>top packaged heat<br />

pump that uses HCFC-22 as refrigerant is shown in Fig. 12.3. In this heat pump, dual circuits,<br />

consisting <strong>of</strong> two compressors, two indoor coils, two outdoor coils, two throttling devices, <strong>and</strong> two<br />

reversing valves, are <strong>of</strong>ten used for better capacity control <strong>and</strong> better defrosting control for larger<br />

heat pumps.<br />

Compressor. Reciprocating <strong>and</strong> scroll compressors are widely used in heat pumps. In Bucher<br />

et al. (1990) the median service life <strong>of</strong> the compressor in a heat pump is 14.5 years, <strong>and</strong> the median<br />

service life <strong>of</strong> a heat pump system is 19 years, depending on the conditions <strong>of</strong> operation <strong>and</strong><br />

FIGURE 12.3 A typical ro<strong>of</strong>top heat pump: (a) schematic diagram;<br />

(b) reversing valve, cooling mode; (c) reversing valve, heating mode.

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