The ability to determine the pressure drop in a refrigerant line may be crucial, but there remains the decision of how much pressure drop (or drop in saturation temperature) to specify. While the optimization process discussed in Sec. 9.5 would be ideal, designers usually resort to some conventions that at least give reasonable pipe sizes. The various pipe sections are addressed individually:
• Suction to compressor. The total drop in saturation temperature is usually chosen to be 0.5 to 2°C (0.9 to 3.6°F). The exceptions are vertical risers both for halocarbon direct expansion and for ammonia liquid overfeed coils. For halocarbon direct-expansion systems the velocity of the refrigerant vapor must be high enough to convey oil back to the compressor. For ammonia liquid-overfeed coils the vapor velocity in the riser must be high enough to blow the liquid out so that it doesn’t fill the riser.
• Discharge from compressor to condenser. The total drop in saturation temperature is usually chosen from 1.0 to 3.0°C (1.8 to 5.4°F). A given drop in saturation temperature in the discharge pipe is slightly less penalizing in
compressor power than the same drop in temperature on the suction side.
• High-pressure liquid. A pressure drop in this section may exact no penalty whatsoever on system performance, because the pressure drop that does not occur in the pipe will take place in the expansion device or level control valve. The expansion device provides the final reduction of pressure to the intermediate pressure (in two-stage compression) or to the low pressure (in single-stage compression). The concern about pressure drop in this line arises more in assuring that the pressure does not drop to the saturation pressure corresponding to the existing refrigerant temperature. Were the pressure to drop to that point, the liquid would flash into vapor, aggravate the pressure gradient, and possibly restrict the flow through the expansion device. Refrigerant velocities chosen for liquid lines range from 1 to 2.5 m/s (3 to 8 ft/s).
• Liquid/vapor return from evaporators to low-pressure receiver. The line from the evaporators back to the low-pressure receiver in liquid recirculation systems carries a mixture of liquid and vapor. Calculations of pressure drops in the flow of liquid/vapor mixtures, while possible, are complex. To avoid cumbersome calculatons, yet still make allowances for the presence of liquid, some designers choose the line size, first by determining the appropriate size if the pipe were carrying only vapor, then step up to the next pipe size to allow for the combined flow of liquid.
• Hot-gas defrost lines. To make an intelligent choice of pipe size, the required flow rate of hot gas as a function of the evaporator size should be known. A rough estimate of the hot-gas flow rate is that it is twice the refrigerant flow rate used during refrigeration service. With this assumption, the recommended sizes of ammonia hot-gas branch lines proposed by Hansen9 use as a basis a velocity of 15 m/s (3000 fpm) with 21°C (70°F) hot gas. This velocity would be appropriate for hot-gas branch lines serving a single evaporator of a cluster of evaporators defrosted at the same time. The hot-gas mains may be sized for carrying half the total of all connected evaporators on the assumption that no more than half the evaporators would be defrosted at one time.
Recent efforts to operate plants with as low a condensing temperature as possible impacts the desired size of hot-gas lines. The ultimate criterion is the saturation temperature at which the defrost gas can condense in the evaporator being defrosted, so the drop in saturation temperature in the hot-gas line reappears as the most appropriate basis for selecting the pipe size. As the condensing temperature of the plant drops, the defrost gas becomes less dense, and when the condensing temperature of the plant drops from 35°C (95°F) to 15°C (59°F), for example, the drop in saturation temperature for several common refrigerants doubles.