Dehumidification in warm weather has typically been provided by conventional air conditioning systems and heat pumps operating in a cooling mode. Those systems are controlled primarily by a thermostat sensing room or space temperature. For cooling operation, the air conditioner or the heat pump is generally cycled on and off by the thermostat. Space humidity is controlled indirectly by cooling the air below its saturation dew point to condense moisture out of the cooled air. When cooling was not called for, the space humidity load was not addressed by such systems.
For special applications where humidity control was important, a separate humidistat would be provided to force the air conditioner or heat pump to operate and cool even though the thermostat was not calling for cooling. When this happened, the space would be overcooled. Where a heat pump with supplemental heating was provided, the supplemental heating could be forced on during cooling to reheat the air. However, because of the inherent inefficiencies of overcooling and then reheating, it has generally been too costly both in terms of dollars and natural resources to provide such systems for use in residential or like commercial applications.
Regenerative type periodic flow devices are conventionally employed for the transfer of heat and moisture from one fluid stream to another, and thereby from one area or zone in space to another. Typically, a sorptive mass is used to collect heat or moisture from one fluid stream which flows over or through the sorptive mass. The flowing fluid is rendered either cooler (in the case of heat sorption) or less humid (in the case of adsorption of moisture). The sorptive mass is then taken "off-stream" and regenerated by exposure to a second fluid stream which is capable of accepting the heat or moisture desorbed with favorable energetics.
In many instances, the sorptive material is contained within a vessel or distributed within a bed structure. It is desirable that such material be provided with maximum surface area, and that the fluid flow through the sorptive material matrix in a smooth (non-turbulent) and regular state. Once the sorptive material has been saturated (i.e., has reached its maximum designed capacity for sorption), the vessel or bed is then removed from the fluid flow path and exposed to a second fluid flow to regenerate the sorptive capacity of the material by, for instance, cooling the sorptive material or desorbing material taken up during "on-stream" operation. After such regeneration, the sorptive material is once more placed back "on-stream" and the operation continues.
From such single cycle systems evolved multiple vessel systems which permitted semi-continuous (or semi-batch) operation by synchronously alternating two or more sorptive vessels between on-stream and off-stream operation. The choice of numbers of vessels and cycle structures depends on many factors, but most importantly the ratio between consumption rate of the sorptive capacity of the vessel, and regeneration rates for that same vessel.
In some applications, semi-continuous systems have evolved into continuous flow systems where the sorptive media itself is moved between two or more flowing fluid streams. The most common construction employed for such systems is a porous disk, often referred to as a wheel or rotor. In its simplest form, such a wheel is divided into two flow zones, and fluid is passed over the sorptive surface of the wheel (typically flowing through the thickness of the disc parallel to the rotational axis of the cylinder) as the wheel is rotated to carry the sorptive material from one zone, into the other, and back again to complete a revolution. In a heat exchanger wheel, for instance, one zone of warm fluid and one zone of cooler fluid are present. Heat is adsorbed by the material of the wheel in the warm flow zone, and is carried away from the wheel as the sorptive material passes through the cool flow zone. U.S. Pat. No. 4,594,860 discloses such a continuous flow system and is hereby incorporated by reference.
Generally, a moisture transfer wheel assembly is provided which is separated into two sections to provide an intake path and an exhaust path through the moisture transfer wheel. A heat exchanger wheel assembly, also partitioned to provide intake and exhaust paths, is located substantially adjacent to the moisture transfer wheel, separated only by a heat regeneration coil. An auxiliary heating coil may be placed in the system for use in cold months when it is desirable to heat the interior of the area to be conditioned, rather than to cool it. The heat regeneration coil and heating coil include fluid pipes which are interconnected with standard heating units, such as a solar heating unit. The system terminates in a pair of evaporator pads.
Heat regeneration coils and heating coils are generally of conventional structure. That is, in conventional coil arrangements, the tubing is mechanically connected to fin sheets. The fin sheets are used to extend the surface area of the tubes to increase the coil's heat-transfer effectiveness. That is, a typical condenser or cooling coil is made up of finned tube sheets with good transfer conduction material, typically aluminum or copper, with holes punched in a pattern array through which the tubes are inserted. Through various means, the tubes are expanded to make good contact with the finned tube sheets. In a typical application, air passes over the tubes and fins and is either cooled or heated by the fluid flowing inside the tubes.
Since heat is conducted not only to the area that passes over the fin sheets, but from the warmest area on the finned surface to the coolest area on the finned surface, because of the high conduction of fin sheets, the fin sheets tend to average out the temperature across the coil, even though the fluid in the tube is at different temperatures between front and back. Thus, the upper and lower temperature limits to which the air can be heated or cooled is therefore limited, which directly affects the total heat transferred by the coil and the energy efficiency of the system. Thus, a need has arisen for a condensing or cooling coil which can minimize the effect of averaging out the temperature across the coil without losing the benefit of the use of fin sheets.
In one known system, generally referred to as a "recirculation" system, process air is drawn from within an enclosed and conditioned space, processed by dehumidification and/or cooling, and then returned to the enclosed space. As is well known, moisture is removed from the process air by the moisture transfer wheel. When moisture is removed from the process air, the temperature of the process air increases. The process air is subsequently cooled upon passing through heat exchanger wheel, which lowers the temperature of the warm, dry air.
In some of the known systems, evaporator pads are used to add moisture to the process air, thus reducing the temperature further and supplying cool air to the conditioned area. In other known systems, an evaporator coil of a conventional air conditioning system is used to cool the dehumidified process air. However, the amount of cooling of the dehumidified process air could not be accurately controlled, because the amount of cooling was based on the evaporator size and the compressor load, often resulting in over-cooling or under-cooling of the dehumidified air.
The foregoing problem of not being able to control the return temperature of the dehumidified process air has not been previously addressed, and generally, the process air is over-cooled before it is returned to the enclosed space, resulting in a less efficient system.
The present invention solves the foregoing problem by providing several controllable stages of dehumidification and cooling through the use of two process evaporators and a recovery evaporator such that the return temperature of the dehumidified process air can be accurately and efficiently controlled.