Document Gz67gm6oeKQ6jnGBj9vVgYOoN

774 CHAPTER 39 1949 Guide of their modifications. Only the general purposes and operating char acteristics of the more typical mechanisms are here discussed. Compressor Motor Controls. Two types of controls are used for intermittently starting and stopping compressors. The first of these is a pressure motor control responsive to the evaporator pressure and the second a thermostatic motor control responsive to the temperature of the load surrounding the evaporators. In the first case the compressor operation is indirectly dependent upon the temperature of the load and is controlled by the refrigerant pressure at the point of control location. The second type is dependent upon the temperature of the load being cooled. With the pressure actuated device, the control is frequently located directly on the condensing unit and the low pressure in the suction line or the crankcase of the compressor is used to control motor operation. Such a control usually consists of a low pressure bellows connected through tubing directly to the low pressure control source and an electrical switch operated through linkage by the movement of the bellows. The electrical circuit is closed on rising pressure and opened on falling pressure. The thermostatic type of motor control is usually similar in construction to the pressure control with the exception that a temperature bulb and capillary tube replace the pressure line, and the temperature bulb is located adjacent to the evapora tor itself. In this case motor control is directly responsive to changes in the tempera ture of the load surrounding the evaporator. Frequently a high pressure safety cutout switch is combined with the motor control and operates to cut off the power from the motor in case the high side pressure exceeds a predetermined limit. Solenoid Valves. Solenoid or magnetic stop valves are frequently used in refrig eration systems for control of gas and liquid flow. When applied as liquid stop valves, they are placed in the liquid line between the condenser or receiver and the evaporator, and the line is open to passage of the refrigerant only when the com pressor is in operation. When the compressor is not in operation, leakage of liquid refrigerant in the evaporator is prevented. In some cases such a magnetic stop valve is operated directly by a thermostat located at the point of the load, and the com pressor motor operation is controlled independently by a low pressure switch. Mag netic liquid stop valves are also widely used for the control of the refrigerant flow to individual evaporators in a multiple evaporator system operated by one compressor. In some installations magnetic liquid line and suction line valves are used to isolate completely an evaporator for deirosting purposes. A magnetic valve may be in stalled in a by-pass around one or more cylinders of a multiple cylinder compressor and thereby be used to unload a compressor during starting to reduce load. Addi tional applications are found in control of the circulation of brine in a secondary refrigeration system. . Suction Pressure Valves. Suction pressure control valves, frequently called back pressure regulators or two-temperature valves, are sometimes placed in the suction line to prevent the evaporator pressure and temperature from dropping below a pre determined level. Typical applications of such controls occur in water cooling or milk cooling systems where freezing and other damage would result if the evaporator pressure dropped too low or in multiple systems where several evaporators are sup plied by one condensing unit. Thus, different evaporators may be kept at different temperatures by maintaining a pressure drop between the evaporator and the suction line. Condensing Water Control. The majority of the refrigeration systems, other than fractional horsepower, use water cooled rather than air cooled condensers since the lower condensing temperatures result in more economical operation. Automatic con trol of the water flow to the condenser must be maintained if water wastage is to be eliminated. Such control may be provided through the use of either an electric solenoid water valve or by means of a pressure control valve. With a solenoid valve, the flow is two-position, either off or on, and its operation is simultaneous with start ing and stopping of the compressor motor. With a pressure operated valve, the flow is modulated ana is dependent entirely upon condenser pressure rather than condens ing unit operation. Similar water valves controlled thermostatically by the tempera ture of the water discharged from the condenser are also available. Safety Controls. Many controls are designed not to aid in proper operation'of the system but.to prevent damage in case of improper operation. One such safety control is the high-pressure cut-off frequently combined with the low-pressure motor control as previously described. Another safety control often used is a low voltage cut-off which shuts down the system automatically in case the line voltage drops below a minimum value. High pressure relief valves are used for safety purposes to prevent damage in case excessive condensing pressures are encountered. Oil separators are often installed between the compressor and the condenser to prevent excessive oil Refrigeration 775 Table 5. Maximum Tons Refrigeration for Ammonia Mains - Suction Line Pipe Sue, In. Suction pressure psig ( r temp) 3 i 2 l H H 2 2J -3 3} 4 5 6 8 10 12 5 (-17.2 F) __ 0.6 1.2 2.2 4.4 6.4 12.1 19.1 31.5 46.6 64.0 117. 175. 362. 640. 940. 20 (5.5 F) ' __ l.i 2.2 4.0 8.0 11.8 22.2 35.5 59.0. 87.5 118. 208. 306. 650. 1180. 1850. 45 (30 F) __ 2.0 4.1 7.5 15.0 21.6 42.0 65.0 108. 156. 240. 385. 600. 1200. 2160. Discharge Link Liquid Line Condenser, to receiver' Receiver to system __ 3.1 6.0 11.4 22.4 30.9 62.0 97.5 160. 238. 330. 560. 905. 1810. 3200. 2.5 6.0 14.0 24.0 50.0 77.0 140. 220. 375. 540. 740. 1320. 2030. 4200. 12.0 20.0 75.0 137. 245. 400. 850. 1475. 2400. 3500. pumping from the crankcase into the condenser and the evaporator. Separation of the oil from the refrigerant gas is usually accomplished by slowing down the gas velocity sufficiently to allow the oil to separate out by gravity. REFRIGERATION PIPING The pressure drop which occurs during passage of the refrigerant through connecting piping is similar in effect to that which occurs through suction and discharge valves of the compressor. Thus, the effect of the pressure drop in the suction line between evaporator and compressor requires that a lower pressure be maintained inside the compressor during suction than is maintained in the evaporator. The pressure drop through the connect ing piping between the compressor and the condenser requires that a higher Table 6. Freon-12 Liquid Lines, Tons Capacity per 100 Ft Equivalent Length Line Sue, Inches Peesstjbb Drop per 100 Ft Equivalent Length, Psi 3 5 . 10 20 fOD i OD i IPS iOD i IPS f OD .1 IPS liOD H IPS If OD 1J IPS If OD 2 IPS 2i IPS .3 IPS . 3i IPS 4 IPS 0.88 2.89 4:86 4.86 9.73 10.5 21.4 . 21.4 36.9 36.9 62.0 62.0 124. 230. . 364. . 539. 753. 1.14 3.64 6.81 6.81 12.6 14.1 28.2 28.2' 48.1 48.1 80.2 80.2 161. 297. -469. . 704. 972. 1.80 5.56 10.2 10.2 18.5 21.8 41.3 41.3 70.5 .70.5 114. 114. 231. 426. 676. i005. 1385. . . . Note: -Tonnage values above those underlined give velocities of 300 fpm or less. 2.58 8.50 15.8 15.8 27.0 33.0 60.8 60.8 101. 101. 160. 160. 328. 607. 972. 1430. 1945.