【正文】 nges 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. High isentropic and volumetric efficiencies can be achieved with screw pressors because there are no suction 11 or discharge valves and small clearance volumes. Screw pressors for building applications generally use either R134a or R22. 12 中文 譯文 空調系統(tǒng) 過去 50 年以來，空調得到了快速的發(fā)展，從曾經的奢侈品發(fā)展到可應用于大多數(shù)住宅和商業(yè)建筑的比較標準的系統(tǒng)。C) (39 to 45176。F). An azeotropic mixture behaves like a single ponent refrigerant in that the saturation temperature does not change appreciably as it evaporates or condenses at constant pressure. R410A has a small enough temperature glide (less than 176。F) and a condensation (dew) point of –37176。C (150176。 2 Vapor Compression Cycle Even though there is a large range in sizes and variety of air conditioning systems used in buildings, most systems utilize the vapor pression cycle to produce the desired cooling and dehumidification. This cycle is also used for refrigerating and freezing foods and for automotive air conditioning. The first patent on a mechanically driven refrigeration system was issued to Jacob Perkins in 1834 in London, and the first viable mercial system was produced in 1857 by James Harrison and . vapor pression, there are two less mon methods used to produce cooling in buildings: the absorption cycle and evaporative cooling. These are described later in the chapter. With the vapor pression cycle, a working fluid, which is called the refrigerant, evaporates and condenses at suitable pressures for practical equipment designs. The four basic ponents in every vapor pression refrigeration system are the pressor, condenser, expansion device, and evaporator. The pressor raises the pressure of the refrigerant vapor so that the refrigerant saturation temperature is slightly above the temperature of the cooling medium used in the condenser. The type of pressor used depends on the application of the system. Large electric chillers typically use a centrifugal pressor while small residential equipment uses a reciprocating or scroll pressor. The condenser is a heat exchanger used to reject heat from the refrigerant to a cooling medium. The refrigerant enters the condenser and usually leaves as a subcooled liquid. Typical cooling mediums used in condensers are air and water. Most residentialsized equipment uses air as the cooling medium in the condenser, while many larger chillers use water. After leaving the condenser, the liquid refrigerant expands to a lower pressure in the expansion valve. The expansion valve can be a passive device, such as a capillary tube or short tube orifice, or an active device, such as a thermal expansion valve or electronic expansion valve. The purpose of the valve is toregulate the flow of refrigerant to the evaporator so that the refrigerant is supe