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578 CHAPTER 38 1960 Guide 'Table 7.... Approximate Suction-line Capacity Factors for Equal Pressure Drop of CCItFj (Refrigerant 12) . Sqturolod Section Teapero- 50 40 30 20 to tin, F. 0 -10 -20 Factor.. 1.09 1.00 0.92 0.86 0.80 0.74 0.66 0.56 difluoromethane, and 0.64 psi per foot for monofluorotrichioromethane. Where there is a possibility of vaporisation of some of the liquid before reaching the expansion valves, means for subcooling should be provided. Since a reduction of suction pressure at the compressor results in an appreciable reduction in capacity and more powet input per ton of refrigeration, great care should be given to the proper dang of suction lines between the 'evaporator and the compressor. Although comparatively high velocities, 500 to 5000 fpm, may be used, the optimum value will depend upon the refrigerant and the operatingpressure range. Since return of the oil to the compressor must be considered in the case of the fluorinated hydro carbons and methyl chloride, for these refrigerants the minimum velocity should be 500 fpm for horizontal runs and 1000 fpm for vertical runs. For the former, the usual ;design velocities range between 1000 and 2000 fpm. Too ' high velocities create noise problems and excessive pressure drops. The total pressure drop in the suction line should 'be between one and two psi, if the velocity can be kept within the specified limits. Compressor discharge or hot gas lines may be designed with velocities from 1000 to 5000 fpm, except for dense 'gases such as carbon dioxide, where noise considerations will reduce the upper limit. A pressure drop of 2 to 4 psi is recommended for the discharge lines. Extensive tables are available in the literature for the determination of pressure drops through refrigerant lines with various refrigerants. The capacities listed in Tables 5, 6, and 7 are published in AR1 Equipment Standards 520 (1946), of the Air-Condi tioning and Refrigeration Institute, and are used by piermission. Table 5 shows the tonnage capacity normally allowed for CCUF* liquid lines per foot equivalent length of pipe, and Table 6, the maximum tonnage for suction and discharge CC1F, lines. Table 7 presents suction-line capacity factors for equal pressure drop. ACCESSORIES Dehydrators, oil separators, strainers, vibration elimi nators, sight glasses, and various types of valves are ac cessories frequently needed for the proper installation and operation of refrigeration systems. Refrigerant-line de hydrators or dryers usually consist of copper containers fitted with tubing connections at either end, and contain a desiccant such as silica gel, activated alumina, or calcium chloride. The liquid refrigerant is circulated through the dryer during operation of the system, and the moisture content of the. refrigerant charge is thus kept to a minimum. Oil separators are installed between the compressor and condenser to prevent excessive oil removal from the com pressor crankcase and its passage into the condenser and evaporator. The oil is separated from the gaseous refrig erant by gravity during its passage through a chamber of sufficient size to reduce the velocity. A float-operated valve maintains a maximum oil level in the separator, and ad ditional oil is forced by pressure difference through a line back to the crankcase. Screen strainers are frequently installed in the liquid line piping before solenoid valves and expansion valves, as well as before regulating valves in water lines leading to water cooled condensers. Sight glasses that permit visual inspec- Fig. 16 .... Performance Characteristics of Compression Refrigeration Machines at Constant Speed' Fig. 17 .... Performance Characteristics of Compression Refrigeration Machines at Constant Speed Refrigeration 579 Capacity Tent Table 8.. .. Basis of Equipment Selection Majority Used Some Used 'fewUnd Oto 10 Unit systems in conditioned space. Unit central systems using duct Built-up central systems. distribution. 10 to 25 Unit central systems using duct distribution. Unit systems in conditioned space. Built-up central systems using re ciprocating compressors, adsorp tion, and absorption systems. 25 to 100 Built-up central systems using re ciprocating compressors. Unit central systems using duct distribution. Central systems using adsorption systems or centrifuged refrigera tion. 100 to 200 . Built-up central systems using re ciprocating compressors. . Built-up central systems using ab sorption and centrifugal com pressors. Built-up central systems using steam jet. 200 and Over Built-up central systems using cen- Built-up central systems using Built-Up central systems using re . trifuga) compressors. steam jet or absorption systems. ciprocating compressors. tion of the condition of the refrigerant are sometimes in stalled on factory-assembled commercial unit systems. It is particularly advisable to place such a fitting before the expansion valve, if the evaporator is located above the condenser. Flexible vibration eliminators, usually consisting of a bellows design covered with woven copper wire, are some times installed in copper lines where units"such as com pressors are installed on flexible mountings, or where vi bration is otherwise a problem. Packed or packless shut-off valves are necessary where it may be required to isolate portions of a system. EQUIPMENT CHARACTERISTICS AND SaECTlON The various types of compression systems have quite' different characteristics of capacity and power with varying evaporator and condenser temperatures, as may be noted from curves in Figs. 16 and 17. From Fig. 16 it may be observed that power require ments for the centrifugal compressor, increase much more rapidly than for the reciprocating compressor, with in crease in evaporator temperature. Similarly, the capacities of the steam ejector and centrifugal compressors increase more rapidly than those of the reciprocating compressor, with increase in evaporator temperature. Thus, both the steam-jet and centrifugal machines tend to be more selfregulating than the reciprocating. It is also evident from Fig. 16 that the steam-jet equipment is best suited for operation at high evaporator temperatures. The effectof condenser temperature upon the power and capacity of the different types of compressors is shown in Fig. 17. It may be noted that the power required by the reciprocating compressor increases rapidly with increase in. condenser temperature, while the power curve for the centrifugal compressor is relatively flat. It is also evident that the capacity of the steam-jet compressor is independent of condenser temperature until a certain point is reached, Table 9 .... Typical Operating Conditions for Two Types of Load type of Cndoror* Ratio lead Bhi Sensible per Hr. to Total Heal Air Entering' Cod Toaip. Rri. F H%` ' Operating Balance Point Evapo rator Tamp. ConPro*Pri Sens ible Hoot % X Restaurant Sensible Latent Total 103.000 45,000 148.000 0.695 82 45 34.4 123 69.9 Office............ Sensible 121,000 Latent 27.000 Total 148.000 0.820 82 45 42.2 100 82.1