Document bB087v2Xw2OZ1m3Edv3gyJ8G3

920 CHAPTER 36 1957 Guide the discharge valve in the compressor .head (uniflow arrangement). The valves themselves are usually classified as either poppet, ring-plate or flexing. Imbrication and Cooling. Lubrication of modern compressors is accomplished by either splash lubrication or forced lubrication. The latter is used on large com pressors, while simple splash lubrication is used in the smaller units. Large compressors are usually water cooled with the water jacket either cooling the cylinder walls, or both the cylinder walls and the conmrcssor head. Small com pressors are either water cooled or air cooled with extended finned surfaces cast on the exterior of the cylinder: In a few cases small compressors may be found in which there is no attempt to add any purposive cooling other than through non-finned surfaces to the lower temperature air. . Water cooling is more effective than air cooling, but even under the best condi tions cylinder cooling removes only a portion of the superheat in the refrigerant gas. This removal of heat from the cylinder results in some decrease in the work of compression, as well as reduction in condenser load. Control of Reciprocating Compressors Capacity Control of reciprocating compressors to match the imposed load is accomplished by a controller responding to load variations, and operating one of the several means for changing the compressor capacity. This controller may respond to the condition being controlled, such as space temperature in the case of a direct expansion system, or water tem perature in the case of a water chiller; or it may respond to suction pressure, which varies with the imposed load. The common methods of varying the compressor capacity are: 1. Starting and stopping the compressor. This is more common on small units where frequent cycling of the compressor offers no serious complications. On large installations two or more smaller compressors may be employed and these may be started and stopped in squence, thus providing more capacity steps. Sequence con trol of multiple compressors is accomplished by the use of a program or step con troller, and in most cases this is arranged to return to the offposition when the system is shut down or in the event of power failure, and to provide a suitable time delay between steps on start-up to prevent the full electrical load from being thrown across the line at one time. Means for manually or automatically varying the sequence to equalize wear on the compressors are sometimes employed. 2. Cylinder Unloaders. These usually are available on all but the smaller com pressors and are used on both single and multiple compressor installations. They consist of a device built into the cylinder head to hold the suction valve open for unloading the cylinder. Oil pressure or head pressure is applied to these devices to load the cylinders, hence the compressor always starts at minimum capacity. De pending upon the make of compressor, the application of pressure to the unloaders is accomplished by: a. Solenoid valves usually operated by the program controller in sequence with starting and stopping of the compressor or compressors. The program controller starts the compressor at minimum capacity, then loads the cylinders in sequence as the demand increases. If more than one compressor is used, it starts and loads the second compressor in sequence after the first is fully loaded, etc. b. A self-contained stepping valve built into the compressor and acutated by a bellows measuring suction pressure. This valve is supplied with oil pressure from the compressor oil pump and has an oil outlet connection to each cylinder un loader. This valve supplies oil pressure to, or releases it from, the cylinder unloaders progressively as the suction pressure increases or decreases. The suction pressure bellows is opposed by a spring which has an external manual adjustment for selection of the desired operating range. The other side of the bellows may be open to atmosphere so that the suction pressure is controlled with reference to atmospheric pressure, or it may be sealed and provided with an air connection which permits variation of the operating range as a function of air pressure from a pneumatic controller. For example, if the spring is so adjusted that the basic operating range is 35 to 40 psi suction pressure when the pressure from the pneu matic controller is zero, it will be increased to 45 to 50 psi when the control Pres' sure is 10 psi. Thus these self-contained cylinder unloaders not only can control the suction pressure within a fixed range, but this range can be moved up or down automatically to meet the varying demands on the system. Pneumatic controllers Refrigeration 921 used for this purpose often are supplied with air pressure up to 25 psi to permit a wide range of suction pressure variation. Most of these self-contained cylinder unloaders also can be arranged for mechanically varying the suction pressure by means of an electric motor operator acting against the manual adjustment spring to permit the use of proportional electric or electronic controls. 3.Speed Control.. Multi-speed motors are sometimes used, actuated by a multi stage thermostat or a proportional thermostat and a program or step controller. . Safety Controls are provided on reciprocating compressors to stop the motor whenever certain conditions are outside of safe limits. These con ditions include: 1. Low Suction Pressure. This is a pressure controller responsive to the suction pressure. It stops the compressor when the suction pressure drops below a safe limit. It is sometimes also used as a capacity control, as described in preceding sec tion, paragraph numbered 1, when it is desired to start and stop the compressor in accordance with the load demand as determined by changes in suction pressure. When other means are provided for starting and stopping the compressor in accord ance with load, the low pressure control functions solely as a safety device. 2. High Head Pressure. This is a pressure controller responsive to the high side pressure. It stops the compressor when the high-side pressure exceeds a safe limit. This is strictly a safety device and frequently is of the manual reset type so that the compressor cannot again start until the reset button is manually operated. In many cases the high and low pressure switches are combined into a single unit. 3. Low Oil Pressure. This is a pressure controller which stops the compressor in the event of low oil pressure. It also is frequently of the manual reset type. 4. Low Line Voltage. This is a low-voltage cutoff, which stops the compressor when the line voltage drops below a minimum value. 5. High Motor Current. This may be built into the motor or into the motor starter. It stops the motor in the event excessive current persists for more than the brief interval required in starting. 6. High Pressure Relief. This is a device for preventing damage if excessive head pressures are encountered, and provides protection from damage beyond that pro vided by the high-pressure cut-out. It may be a relief plug which blows out in the event of excessive pressure. This has the disadvantage of losing the entire re frigerant charge. A relief valve, which functions like a pop-safety valve, is preferred since it wastes only enough refrigerant to relieve the danger. 7. Low Water Temperature. If the compressor is used for chilling water, a lowlimit thermostat usually is installed in the chiller to stop the compressor in the event E !Ie?z'nS *emPeratiire is approached. This is in addition to a controlling thermostat which also may respond to water temperature. Centrifugal Compressors Centrifugal compressors are used with very low pressure refrigerants; sometimes both evaporator and condenser work below atmospheric pres sure. Trichlorotrifluoroethane (C2CI3F3) and monofluorotrichloromethane (CCljF) are the refrigerants commonly used in centrifugal machines. Compression of the refrigerant is accomplished by means of centrifugal > force; therefore, this type of compressor is inherently suitable for large volumes of refrigerant at low pressure differentials. Two or more stages ure usually required and high speeds are necessary to obtain good efficiency. The evaporator is usually constructed as an integral part of the centrif ugal type condensing unit, to chill water which is then circulated to the uir conditioning system. This is done because it would not be economical f pipe these large volumes of refrigerant any distance. Centrifugal compressors, like reciprocating compressors, can be divided into two general types, open and enclosed. In general, the open type com pressor is geared to the driving mechanism, and operates at higher speed ban the driving motor or turbine. A modem, completely enclosed, direct- riven centrifugal compressor is illustrated in Fig. 12. Centrifugal refrigeration compressors are particularly well. suited to