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BASIC HEAT PUMP THEORY By: Lloyd A. Mullen By: Lloyd G. Williams Service Department, York Division, Borg-Warner Corporation Figure 26F07 Air Pressure Sensing Control When ice builds up on outdoor coil, it allows less air to pass through coil. This resistance can be measured by using a control such as shown in Figure 26F07A. This is a diaphragm type control with pressure taps on each side. One tap is connected to outside of unit. The other is mounted inside unit to sense air flow through outdoor coil. When this differential, which is increased by ice build-up, reaches a pre-set point, the control causes unit to go into a defrost cycle. Figure 26F07A In contrast to a straight timing device, the last two types of defrost initiation devices illustrated do exercise some control over ice accumulation and thus avoid unnecessary defrosting. DEFROST CYCLE TERMINATION Having initiated a defrost cycle, a method must be provided to terminate defrosting when ice has melted off coil. Again, there are several ways of accomplishing this. One could be a timer with switching arrangement so that after a period of running on defrost cycle, it would put the unit back on heat cycle. Another method could be a head pressure control which would sense a rise in head pressure as ice melted off and coil warmed up. To speed up this process, the outdoor fan is stopped during defrost period. Temperature of liquid out of outdoor coil could also be measured. This would give the same reaction as pressure difference. As temperature rose after ice melted a thermostat bulb on liquid line would sense this change and terminate defrost cycle. A few examples of defrost controls are given in the following: Figure 26F06C illustrated a timer initiating defrost control with temperature sensing bulb and capillary tube, with bulb attached to liquid line from coil, this control terminates defrost by measuring temperature of liquid leaving outdoor coil. Another type of temperature defrost termination control is one in which the diaphram type sensor is clamped to liquid line outlet from condenser. Service Application Manual SAM Chapter 620-54 Section 18 13PDF Image | BASIC HEAT PUMP THEORY INTRO
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