This invention relates to a method and control for defrosting the outdoor coil of a heat pump in a manner which optimizes efficiency and conserves energy.
When a heat pump operates in its heating mode, frost builds up on the pump's outdoor coil. As the frost thickness increases, heat transfer from the outdoor air decreases and the efficiency of the heat pump drops significantly, a substantial amount of energy therefore being wasted. Hence, it is necessary to periodically defrost the outdoor coil. This is usually accomplished by reversing the refrigerant flow in the heat pump which will heat the outdoor coil and melt the frost.
It is recognized that there is an optimum point of frost accumulation at which the heat pump should be switched to its defrost mode of operation. If defrost is commanded too soon or too late, energy will be wasted and efficiency will suffer. Unfortunately, it has been very difficult to achieve such optimum operation in the past. Moreover, these previous defrost systems are unreliable in operation and/or are not adaptable to all types of outdoor coils.
Substantially less expensive defrost control systems have also been developed, but these systems are not capable of adjusting to the prevailing weather conditions. In one such system, the differential between the outdoor ambient (dry bulb) temperature and the refrigerant temperature in the outdoor coil is measured. The outdoor coil temperature decreases as frost builds up, and this increases the temperature split or difference between the outdoor ambient temperature and the coil temperature. When the temperature split increases to a predetermined value, the outdoor coil is defrosted. These prior temperature differential type defrost controls, however, fail to take the weather conditions into account. The temperature split between the outdoor ambient air (dry bulb) temperature and the refrigerant temperature in the outdoor coil for clean coil operation is a function of the outdoor wet bulb temperature and not the dry bulb temperature. For example, when the outdoor ambient air has a 35.degree. F. dry bulb temperature, a 34.degree. F. wet bulb temperature, and a relative humidity of about 90%, the refrigerant temperature in the outdoor coil of a typical three ton heat pump may be about 23.degree. F. when the outdoor coil is frost-free, the clean coil temperature split (namely, the outdoor ambient temperature minus the outdoor coil temperature) thereby being 35.degree.-23.degree. or 12.degree.. (All temperatures mentioned herein will be F or Fahrenheit.) For the same outdoor dry bulb temperature, an outdoor wet bulb temperature of 28.degree. and an outdoor relative humidity of about 40% may then provide an outdoor coil temperature of about 17.degree., resulting in a clean coil temperature split of 35.degree.-17.degree. or 18.degree.. Neither humidity condition is uncommon in most areas. Thus, if the defrost control were set, when the ambient air has a 34.degree. wet bulb temperature, to initiate defrost at a temperature differential of, for example, 5.degree. above its expected clean coil condition, defrost would occur when the temperature differential became 12.degree.+5.degree. or 17.degree. and dry weather conditions would result in the system continually defrosting itself without time for frost buildup on the outdoor coil.
Even if the temperature split, at which defrost should occur, is properly determined when the outdoor coil is frost-free, long before frost builds up and that temperature split is reached the weather conditions (namely, the outdoor temperature and/or relative humidity) may change significantly, and that previously determined temperature split may no longer be appropriate or valid. If there is a decrease in outdoor temperature between defrost modes, excessive frost would build up on the outdoor coil and defrost should now be initiated at a smaller temperature split, not the one previously determined. On the other hand, as the outdoor temperature rises the same system may go into needless defrost because the control would assume that frost is building up on the coil, when it may not.
This phenomenon may be appreciated and more fully understood by observing FIG. 1 which provides a graph of the performance of the typical three ton heat pump mentioned previously. The graph plots the wet bulb temperature of the outdoor air versus the outdoor ambient or dry bulb temperature at different outdoor relative humidities. The graph shows the liquid line temperature, which is essentially the same as the outdoor coil temperature or the coil surface temperature, under clean coil conditions at various wet bulb temperatures. The clean coil temperature splits (the outdoor dry bulb temperature minus the liquid line temperature) for different weather conditions, namely at different points on the graph, may easily be determined by subtraction of one temperature from the other at the point that represents the weather conditions. The graph clearly illustrates that the liquid line temperature is strictly a function of the wet bulb temperature, and thus the moisture in the outdoor air.
It will be assumed that on a given day at about 7 a.m. the weather conditions in a particular area are as depicted by point 11 in FIG. 1, namely about 12.degree. outdoor ambient temperature, 10.5.degree. wet bulb temperature and about 77% relative humidity, the liquid line temperature for clean coil conditions thus being about 4.5.degree. to provide a clean coil temperature split of 12.degree.-4.5.degree. or 7.5.degree.. Point 12 indicates the assumed weather conditions on the same day at 10 a.m.--29.degree. outdoor dry bulb temperature, 23.degree. wet bulb temperature, about 40% relative humidity and a liquid line temperature of about 13.5.degree., the clean coil temperature split thereby being 29.degree.-13.5.degree. or 15.5.degree.. This corresponds to an 8.degree. increase (15.5-7.5) in the temperature split for a clean outdoor coil. If the control system were programmed, in accordance with the data at 7 a.m., to initiate defrost after there is a 4.degree. temperature increase in the clean coil temperature split, a needless defrost cycle would occur with no frost build up on the outdoor coil. Points 13 and 14 in FIG. 1 depict the assumed weather conditions at 4 p.m. and 11 p.m., respectively, on the same given day. The graph indicates that the clean coil temperature split would change downward from about 18.degree. to 11.5.degree., or about 6.5.degree., between 4 p.m. and 11 p.m. Thus, a 4.degree. programmed differential would require that the initial 18.degree. clean coil split at 4 p.m. would have to increase to 22.degree. before defrost would occur, whereas the optimum defrost split (the difference between the outdoor temperature and the coil temperature when the defrost mode should be initiated) for the weather conditions at 11 p.m. would be 11.5.degree. plus 4.degree., or 15.5.degree.. Hence, the split would increase 6.5.degree. (from 15.5.degree. to 22.degree.) above the optimum defrost condition before defrost would be initiated and excessive frost would accumulate. The conditions assumed in explaining the FIG. 1 graph are not uncommon, since the outdoor temperature and relative humidity may experience wide variations over a 24-hour period.
The defrost control system of the present invention is a substantial improvement over those previously developed. The system is not only relatively inexpensive but the initiation of outdoor coil defrost is timed to occur at the optimum point regardless of changing weather conditions so that defrost only and always occurs when it is necessary, thereby increasing the efficiency of the heat pump, conserving energy and improving system reliability. Any time there is a significant change in the weather conditions, the control system of the present invention will effectively recalculate when a defrost cycle should be initiated.