暖通空调专业毕业设计外文翻译-暖通(编辑修改稿)

暖通空调专业毕业设计外文翻译-暖通(编辑修改稿)内容摘要：

mes and relatively high pressures. Most reciprocating chillers used in building applications currently employ R22. Modern highspeed reciprocating pressors are generally limited to a pressure ratio of approximately nine. The reciprocating pressor is basically a constantvolume variablehead machine. It handles various discharge pressures with relatively small changes in inletvolume flow rate as shown by the heavy line (labeled 16 cylinders) in Figure . Condenser operation in many chillers is related to ambient conditions, for example, through cooling towers, so that on cooler days the condenser pressure can be reduced. When the air conditioning load is lowered, less refrigerant circulation is required. The resulting load characteristic is represented by the solid line that runs from the upper right to lower left of Figure . The pressor must be capable of matching the pressure and flow requirements imposed by the system. The reciprocating pressor matches the imposed discharge pressure at any level up to its limiting pressure ratio. Varying capacity requirements can be met by providing devices that unload individual or multiple cylinders. This unloading is acplished by blocking the suction or discharge valves that open either manually or automatically. Capacity can also be controlled through the use of variable speed or multispeed motors. When capacity control is implemented on a pressor, other factors at partload conditions need to considered, such as (a) effect on pressor vibration and sound when unloaders are used, (b) the need for good oil return because of lower refrigerant velocities, and (c) proper functioning of expansion devices at the lower capacities. With most reciprocating pressors, oil is pumped into the refrigeration system from the pressor during normal operation. Systems must be designed carefully to return oil to the pressor crankcase to provide for continuous lubrication and also to avoid contaminating heatexchanger surfaces. Reciprocating pressors usually are arranged to start unloaded so that normal torque motors are adequate for starting. When gas engines are used for reciprocating pressor drives, careful matching of the torque requirements of the pressor and engine must be considered. 浙 江 海 洋 学 院 毕 业 设 计 6 FIGURE Illustration of a twinscrew pressor design (courtesy of Carrier Corporation). Screw Compressors Screw pressors, first introduced in 1958 (Thevenot, 1979), are positive displacement pressors. They are available in the capacity ranges that overlap with reciprocating pressors and small centrifugal pressors. Both twinscrew and singlescrew pressors are used in chillers. The twinscrew pressor is also called the helical rotary pressor. Figure shows a cutaway of a twinscrew pressor design. There are two main rotors (screws). One is designated male (4 in the figure) 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。