【正文】 igure) and the other female (6 in the figure). The pression process is acplished by reducing the volume of the refrigerant with the rotary motion of screws. At the low pressure side of the pressor, a void is created when the rotors begin to unmesh. Low pressure gas is drawn into the void between the rotors. As the rotors continue to turn, the gas is progressively pressed as it moves toward the discharge port. Once reaching a predetermined volume ratio, the discharge port is uncovered and the gas is discharged into the high pressure side of the system. At a rotation speed of 3600 rpm, a screw pressor has over 14,000 discharges per minute (ASHRAE, 1996). Fixed suction and discharge ports are used with screw pressors instead of valves, as used in reciprocating pressors. These set the builtin volume ratio — the ratio of the volume of fluid space in the meshing rotors at the beginning of the pression process to the volume in the rotors as the discharge port is first exposed. Associated with the builtin volume ratio is a pressure ratio that depends on the properties of the refrigerant being pressed. Screw pressors have the capability to operate at pressure ratios of above 20:1 (ASHRAE, 1996). Peak efficiency is obtained if the discharge pressure imposed by the system matches 浙 江 海 洋 學 院 畢 業(yè) 設 計 7 the pressure developed by the rotors when the discharge port is exposed. If the interlobe pressure in the screws is greater or less than discharge pressure, energy losses occur but no harm is done to the pressor. Capacity modulation is acplished by slide valves that provide a variable suction bypass or delayed suction port closing, reducing the volume of refrigerant pressed. Continuously variable capacity control is most mon, but stepped capacity control is offered in some manufacturers’ machines. Variable discharge porting is available on some machines to allow control of the builtin volume ratio during operation. Oil is used in screw pressors to seal the extensive clearance spaces between the rotors, to cool the machines, to provide lubrication, and to serve as hydraulic fluid for the capacity controls. An oil separator is required for the pressor discharge flow to remove the oil from the highpressure refrigerant so that performance of system heat exchangers will not be penalized and the oil can be returned for reinjection in the pressor. Screw pressors can be direct driven at twopole motor speeds (50 or 60 Hz). Their rotary motion makes these machines smooth running and quiet. Reliability is high when the machines are applied properly. Screw pressors are pact so they can be changed out readily for replacement or maintenance. The efficiency of the best screw pressors matches or exceeds that of the best reciprocating pressors at full load. High isentropic and volumetric efficiencies can be achieved with screw pressors because there are no suction or discharge valves and small clearance volumes. Screw pressors for building applications generally use either R134a or R22. 譯 文 冷水機組 1995 年，在美國，冷水機組應用在至少 4％的商用建筑中。C) (39 to 45176。F). The chilled water is then distributed to watertoair heat exchangers spread throughout the facility. In these heat exchangers, air is cooled and dehumidified by the cold water. During the process, the chilled water increases in temperature and must be returned to the chiller(s). The chillers shown in Figure are watercooled chillers.