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742 CHAPTER 70 1962 Guide And Data Boot of the evaporator should be mada in the suction linn to pre vent liquid refrigerant from draining into the compressor during shutdown. Diagram B Fig. 5 represents a wing!* evaporator below the compressor. An inverted loop is unnecessary since the evap orator will trap all liquid refrigerant. Diagram C of Fig. 5 shows multiple evaporators on dif ferent floor levels with the compressor below. F^h individual suction line should be looped to the top of the evaporator be fore being connected into the suction main, to prevent liquid from draining into the compressor during shutdown. Diagram D of Fig. 5 has multiple evaporators stacked on the same floor level, and operated from one liquid solenoid valve, with the compressor below. Here it is possible to use one loop to serve the purpose. Where coil hanks on the tm> floor level have separate liquid solenoid valves feeding each coil, then a separate suction riser is required from each coil, sim ilar to the arrangement in C and E of Fig. 5 for best oil return performance. Where separate suction risers are not pnrihl^ pipe according to Diagram F of Fig. 5. Diagram G shows multiple evaporators located on the same level mid the compressor located below. The individual suc tion lines out of each evaporator drop down into a mmmnn suction header which then rises in a single loop to the top of the coils before going down to a compressor below. When automatic compressor pump-down control is nd, then evaporators located above Che compressor can be free draining to the compressor without the benefit of protective loops as described above, since the evaporators will auto matically be kept free of liquid by the compressor pump-down operation. Hie small traps shown in the suction lines, immediately after the coil suction outiet, are recommended.to prevent erratic operation of the thermal expansion valve. Hie ex pansion valve bulb is located in the suction lina between the coil and this trap. The trap serves as as drain area pre vents liquid accumulating under the expansion valve bulb during compressor off cycles. liquid lying in the linp. under the bulb would cause erratic operation of the expansion valve when the compressor starts up again These traps, after the thermal expansion valve bulbs, are required only where straight runs or risers are encountered in the suction linp leav ing the coil outlet. They are not required when the suction linp from the coil outlet drops to the compressor or suction header immmediately after the expansion valve bulb location. fig. 7 .... Minimum Gas Velocity for Oil Entrainment Up Vertical Hot Gas Risers (Refrigerant 12)1 Suction Piping to Prevent Oil Trapping in Idle Evaporators Suction lines should be designed so that oil from an active evaporator does not drain into an idle one. Diagram B of Fig. 5 shows multiple evaporators on dif ferent floor levels and the compressor above. Each suctioo line is brought upward and looped into the top of the com mon suction line. This prevents oil from draining down into either coil that may be inactive. Also, note the small loop in the suction line leaving each evaporator. The purpose of this loop is to provide free drainage away from the thermostatic expansion valve bulb. Diagram F shows multiple evaporators stacked with the compressor above. Oil is prevented from draining into the lowest evaporator because the common suction line drops below the outlet of the lowest evaporator before entering the suction riser. In Diagram G, multiple evaporators are shown on the same level and the compressor is located below. The suction line from each evaporator drops down into the common suction line, so that oil cannot drain into an idle evaporator. An alternate arrangement is shown in Diagram H for cases where the compressor is above. Fig. 6 illustrates typical piping for evaporators above and below a common suction lioe. All horizontal runs should be definitely pitched toward the compressor to insure oil return. fig. 6 .... Typical Piping from Evaporators Located Above and Below Common Suction tine DISCHARGE (HOT GAS) UNES The design of hot gas lines with self-contained condensing units is not a problem, inasmuch as they are equipped with 53SW Refrigerant Piping Systems for Refrigerants 12, 22 and 500 743 fig. 8 .... Minimum Velocity for Oil Entrainment Up Vertical Hot Gas Risen (Refrigerant 22)* this line all aswpmhlwri at the factory. In the case of remotely located condensers, however, the hot gas line must be properly designed as part of the system. Hot gas lines should be designed: 1. For correct size to provide practicable pressure drop (Chapter 69). 2. To avoid trapping oil at partial load-operation. 3. To prevent refrigerant from condensing and draining back to the head of the compressor. 4. With connections to multiple compressors (from a com mon line) carefully selected. 5. To avoid developing excessive noise or vibration as a result of hot gas pulsations, compressor vibration or both. Oil Entrainment Up Risers at Practical Loads Even though a low pressure drop is desired in hot gas lines, they should not be oversized to the extent that gas velocities ere reduced to a point where the refrigerant will not be able to carry along any entrained oiL In the usual application this will not be a problem in hot gas lines. In the case of multiple compressors with capacity control, it is wise to make sure any hot gas risers will carry oil along at all possible loadings. Minimum Gas Velocities for Oil Entrainment In Risers Minimum ga* velocities for oil entrainment in hot gas line risers are shown in Figs. 7 and 8. On multiple compressor installations, t.hw actual hot gas volume in cfm, being trans mitted through the hot gas line at the lowest possible system loading, should be calculated and a riser size selected to give at leegt the minimum velocity indicated in the curves for successful oil entrainment, and preferably about 23 percent greater. Sometimes in installations of multiple compressors having capacity control, a vertical hot gas line sized to entrain oil at minimum load has an excessive pressure drop at maximum Ipftd, When this problem exists, either a double riser, or a fringlw riser and an oil separator can be used. Double Hot Gas Risers A double gas riser can be used in the same manner as it is used in a suction line. Fig. 9 shows the double riser principle applied to a hot gas line. The principle of operation and sizing technique is de scribed in the section Double Suction Risers. Single Riser and Oil Separator As an alternate arrangement, an oil separator can be lo cated in the discharge line just ahead of the riser (Fig. 10). This permits siring the riser for a low pressure drop without regard for oil entrainment velocities. Location of the oil separator at this point results in catching practically all of the oil and returning it to the compressors. Any oil draining back down the riser, under light load or on shutdown, would accumulate in the oil separator. Precautions required with this type of arrangement are discussed later in the section Oil Separators. In all 'Aftoq, horizontal lint* should be pitched downward in the direction of gas flow to facilitate travel of oil through the system and back to the compressor. Piping to Prevent Liquid and Oil Draining Back to Compressor Head Whenever the condenser is located above the compressor the hot gas line should loop to the floor near the compressor lfl__C-t COND. PROM .COUR FROM COUP. DISCHARGES fig. 9 .... Double Hot Gas Riser rl K K. . HOT GAS DISCHARGES PROM 3 COMPRESSORS OIL DRAIN TO OIL EQUALIZING LINE CAtmCM'SHOULO NOT BE USEO WHERE UQUIO CAN BOIL 0FT IN CONOENSCR AND CONDENSE IN HOT GAS RISER DURING SHUT DOWN. fig. 10.... Hot Gas Riser with Oil Separator