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BASIC HEAT PUMP THEORY By: Lloyd A. Mullen By: Lloyd G. Williams Service Department, York Division, Borg-Warner Corporation THREE FUNCTION CONTROL REQUIRED Controls are the most interesting components in a heat pump system. More controls are needed for a heat pump than for a cooling unit....Why? Because it must perform three functions instead of one. it must cool, it must heat, and it must defrost outdoor coil when ice accumulates on it. Why does ice collect on the coil? Contrary to indicating poor design, this is just a natural result of extracting heat from relatively cold air by use of a colder coil. CONDENSER COIL ICING This condition can be qualified by consulting a psychrometric chart. Assume outside air is 45°F D.B. and relative humidity 70%. Hypothetical unit, of three-ton capacity, has a suction pressure of 50 lbs. A T.P chart will show suction temperature of about 25°F. As relatively warm air flows through the colder coil, D.B. temperature drops, but moisture content remains the same. When air reaches 35°F D.B. it will have a relative humidity of 100%. This is dewpoint and water is formed on coil. Air temperature continues to drop to 32°F, at which point water will freeze and form ice. Ice collects in the coil, and, prevents air from passing through, air that does get through will get colder and more ice will form. Eventually, the coil becomes blocked with ice and must be defrosted. DEFROSTING METHODS It is obvious that in order to defrost the coil, heat must be added to melt the ice. Of two methods available, one, incorporating electric heaters imbedded in the coil is rarely used, although it was tried on older units. The more common method is to reverse the cycle making the outdoor coil the condenser, and using discharge temperature to clear the coil of ice. To increase temperature of condenser, we can raise head pressure by stopping outdoor fan. DEFROST CONTROLS Timer Control There are numerous methods of signaling the control circuit when unit needs a defrost cycle. A timer, temperature control or pressure control can be used. The use of a timer is a simple method requiring no sensing bulbs or external devices, but this control cannot sense need for defrosting in advance of the critical point, causing excessive defrost periods or in rare cases, delaying a defrost cycle until coil is completely frozen. Figure 26F06C illustrates another type of timer defrost initiating control... The capillary tube and sensing bulb control defrost cycle termination and will be covered later in the text. Service Application Manual SAM Chapter 620-54 Section 18 11PDF Image | BASIC HEAT PUMP THEORY INTRO
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