【正文】 cement pressor. On the intake stroke of the piston, a fixed amount of gas is pulled into the cylinder. On the pression stroke, the gas is pressed until the discharge valve opens. The quantity of gas pressed on each stroke is equal to the displacement of the cylinder. Compressors used in chillers have multiple cylinders, depending on the capacity of the pressor. Reciprocating pressors use refrigerants with low specific volumes 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).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 9 runs from the upper right to lower left. 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. 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. A cutaway of a twinscrew pressor design. There are two main rotors (screws). One is designated male and the other female . 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. 10 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 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. Hi