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BASIC HEAT PUMP THEORY By: Lloyd A. Mullen By: Lloyd G. Williams Service Department, York Division, Borg-Warner Corporation Energizing the main "RC" valve causes discharge gas to flow to indoor coil. Resulting high pressure on the bottom end of 'slave' "RC" valve causes slide to move up. Liquid will now flow through 'slave' valve, to expansion valve. It will then travel through 'slave' valve upwards to outdoor coil, and back to main "RC" valve and compressor. Thus far, the basic heat pump circuit, consisting of compressor, "RC" valve and associated piping and various types of expansion devices and coil circuit arrangements has been the major topic of this presentation. At this point air circuits should be covered briefly, as knowledge of this phase of heat pump operation is equally important. AIR CIRCUITS In a cooling only unit there are two fixed air circuits–indoor air and outdoor air. In a heat pump, however, indoor or outdoor air can be either condenser or evaporator air at any given time. Therefore, the unit and the installation must be designed to handle varying conditions. CONDENSER AIR On the cooling cycle when outdoor coil is the condenser, conditions are those we are familiar with. Any obstruction of air flow will cause high head pressure. Duct-work or external resistance cannot be applied to propeller fans. Air quantity is usually fixed and cannot be varied without radically changing the unit. If air is too cold, problems of low head pressure arise. This can head to low suction pressure and evaporator freeze-ups. When the indoor coil becomes the condenser, as it does on the heating cycle, several new conditions arise....Air quantity is variable, either by use of adjustable pulleys on belt driven blowers, or by restriction of duct-work applied. Here duct design should take into consideration the fact that nominal air quantity is needed. In defining restrictions of the indoor coil, the old nemisis dirty filters should not be overlooked. Since this coil is the condenser, restriction caused by such filters will result in high head pressures. Too much air, and this is seldom a factor, can result in lowered head pressure. EVAPORATOR AIR Normally, the indoor coil is the evaporator. Everyone should be familiar with this application. When the outdoor coil becomes evaporator on heat cycle, conditions arise which must be recognized. First, suction pressure will be lower than normally seen on cooling units. Second, a lower coil temperature will exist. Since the unit is removing heat from outdoor air, air off temperature will be lower than ambient temperature. Operating in this manner with coil temperature below 32°F the coil will frost even at ambient temperatures above freezing. Service Application Manual SAM Chapter 620-54 Section 18 10PDF Image | BASIC HEAT PUMP THEORY INTRO
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