Size Of Equalizer Lines And Thermosyphon Vapor Lines

Under normal circumstances equalizer lines between the receiver and the inlet or outlet of the condenser can be small, because it is not expected that they need carry high flow rates. They only need to convey a volume flow rate of vapor equal to the rate of change of liquid volume. There is one situation where the equalizer line should be generously sized and that is during a shutdown of a plant located in a cold ambient temperature but with the receiver located in a warm machine room. There will be a constant vaporization of the liquid in the receiver causing vapor to flow through the equalizer line to the discharge gas header where it will condense in the condenser. The pressure drop resulting from the vapor flow in the equalizer line must be compensated by a liquid column that will develop and back liquid into the condenser. For high pressure drops of, for example, 25 kPa (3.7 psi), an ammonia liquid column of 4.6 m (15 ft) may be needed. This elevation could be enough to push all of the liquid of the system into the condenser resulting in difficulty in restarting the plant.

As was pointed out in the previous section, the vapor line between the thermosyphon receiver and the discharge gas header (Figure 7.33) is more than a low-flow equalizer line, but carries a heavy flow of vapor and will be of larger
size than an equalizer line. Table 5.3 in the chapter on screw compressors listed recommended pipe sizes for this line, which are reproduced here in Table 7.4 along with recommended equalizer line sizes.

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