1. Field of the Invention
This invention relates to a wrapped fin heat exchanger wherein the heat exchanger is divided into a plurality of specific circuits. More particularly, the present invention relates to the arrangement of loops forming a circuit for a wrapped fin heat exchanger including both an inner set of loops and an outer set of loops. The loops are arranged to promote defrost when refrigerant is circulated through the heat exchanger during a defrost cycle.
2. Prior Art
In many air conditioning and refrigeration applications a heat exchanger is used under conditions wherein water is deposited on the heat exchange surfaces. For example, the outdoor heat exchanger of a heat pump operating in the heating mode serves as an evaporator absorbing heat energy from ambient air being circulated thereover. As the ambient air temperature is decreased its ability to hold water vapor is additionally decreased and excess water vapor will be condensed and deposited on the heat exchange surface as water. If this surface is below freezing, ice will accumulate and the heat transfer efficiency between air and the heat exchanger surfaces will be diminished. In addition, if it is raining or snowing, this moisture may be drawn into the heat exchanger by its air handling apparatus or forced onto the heat exchanger surfaces by the wind.
In a cold room or other similar applications where an evaporator is operating below the freezing temperature of water to cool the air being supplied to the room a similar problem may occur. The reduction in temperature of the air being circulated over the heat exchanger below its dew point acts to condense out moisture which may freeze on the evaporator surfaces impeding heat transfer.
Most heat pump systems include means for eliminating frost from the coil surface. One of the most common means of defrost is to reverse the heat pump placing the heat pump system in the cooling mode of operation wherein heat energy is discharged to the outdoor coil then serving as a condenser. Heat energy is supplied by the hot gas from the compressor being circulated to the outdoor heat exchanger wherein it serves to raise the temperature of the heat exchanger and to melt the frost accumulated thereon.
It has been found in various heat exchangers that frost tends to accumulate towards the bottom of the heat exchanger. The accumulation at the bottom is especially acute since water vapor condensed on the surface of the heat exchanger tends to drip towards the bottom where it collects and is more likely to become frozen. The condensate from the air as it is cooled collects on all the circuits and thereafter tends to drip downwardly to the lower areas of the coil. As the frost accumulates it builds up on the lower areas of the coil not only effecting heat transfer between refrigerant flowing through the heat exchanger and air flowing thereover but actually may impede air flow between the heat transfer surfaces. Under some frost conditions it has been found that frost accumulates primarily on the outer row as well as on the bottom portion of the heat exchanger.
In order to effectively direct hot gaseous refrigerant to the location where the frost has accumulated the present invention provides for a circuiting arrangement in a wrapped fin type heat exchanger such that hot gaseous refrigerant is supplied directly to the lowermost portion of the coil and thereafter to the exterior surface of the coil to effect defrost. The refrigerant circuit is arranged such that the hot gaseous refrigerant is circulated first to the highest frost accumulating areas and thereafter to the lesser frost accumulating areas.