AIR CONDITIONER An air conditioner uses a refrigerant system and an air moving system to cool air. A typical “Special Purpose Air Conditioner” operates as follows: Heat is transferred from the enclosure components by circulating air around and through them, the air is then cooled, dehumidified and returned to the enclosure without the admission of air from the outdoors. The heat is removed from this air within the air conditioner and discharged by means of a vapor compression refrigeration cycle. This takes place in a hermetically sealed system, utilizing either an air-cooled or water-cooled condenser coil. A schematic of a typical air conditioner is illustrated below.
Air conditioners that are used to cool enclosed equipment differ radically from room air conditioners. In the area of temperature control, for example, most electronic systems are adversely affected by large line transients typical of air conditioner compressor cycling. Electronics also exhibit sensitivity to electromagnetic interference caused by thermostat contacts. The control system of an air conditioning package must be designed accordingly. In addition, the field experience of many compressor manufacturers has indicated that the frequent start/stop cycling, typical of standard air conditioner operation, shortens compressor reliability. These factors have led to the development of techniques for close control of internal temperature over a wide range of ambient conditions, without turning the refrigeration compressor on and off and without employing electrically-controlled solenoid valves. Recent developments in temperature requirements for enclosed components have led to the addition of adjustable Low Temperature Control thermostats in all KOOLTRONIC Air Conditioners to prevent over-cooling.
HEAT EXCHANGERS (AIR-TO-AIR) Advanced air-to-air heat exchanger designs for cooling enclosures include two types of heat transfer methods. One design consists of a finned-tube coil which contains liquid refrigerant. The warm air exhausted from the equipment cabinet to the heat exchanger is directed past the coil, causing the refrigerant to boil and absorb heat. The resultant refrigerant vapor rises to the upper portion of the tubes, where the heat is removed by the cooler ambient air and the refrigerant condenses back to liquid, completing the cooling cycle in a continuous process. The most recent developments in enclosure heat exchanger design employ high-efficiency heat transfer elements fabricated of embossed convoluted metal foil or thin-film polymer material, constructed into two totally separate air paths. The air leaving the hot enclosure is directed through one side of the exchanger, where the heat passes through the element walls into the ambient-side air stream and is dissipated.
Figure 1 illustrates heat transfer in air-to-air heat exchanger applications.
HEAT EXCHANGERS (WATER-TO-AIR) If ambient air cannot be utilized directly as a cooling medium, another cost-effective method of cooling is a water-to-air system (Figure 2). Water is used to remove heat from air circulated within the electrical enclosure. Cooling water is circulated through a finned-tube coil, which is installed in a compartment isolated from the enclosure to protect the contents from possible leakage of water. As the heat-laden air circulates through the coil, the heat is absorbed by the water and carried away, in a continuous process. Water-to-air systems are easy to install and usually require minimum maintenance. The water used must be reasonably clean and cold enough to ensure proper operation of the cooling system under the most severe anticipated conditions. In cases where sufficiently cold water is available, below ambient-temperature cooling can be achieved.