Some basic objectives in designing and installing the pump and its inlet piping are:
• provide generous NPSH
• install two pumps, one of which is standby
• provide a region for oil to separate from ammonia and be drained
• design with a minimum of fittings to avoid excessive pressure drop
• choose angle valves in preference to straight-through valves
Section 8.9 emphasized the need for adequate NPSH as the dominant method of guarding against cavitation. There are numerous successful piping configurations that provide positive suction head, and two are shown in Figures 8.22a and 8.22b. The difference in elevation between the liquid level in the lowpressure receiver and the pump should generously exceed the minimum NPSH. The arrangement in Figure 8.22a incorporates a large diameter liquid leg from which the liquid to the individual pumps is drawn. The configuration in Figure 8.22b shows separate lines running from the low- pressure receiver to each of the pumps.
The package should be equipped with two pumps, each capable of providing the required liquid flow rate by itself. In other words, one of these pumps is standby. If only one pump is available and that pump fails the entire recirculation system would be out of service. The discharge line of each of the pumps should be equipped with a check valve to prevent backflow through the idle pump.
The low-pressure receiver is the expected location for the accumulation of oil, and oil drain provision should be made. One of the advantages of the liquid recirculation system over individual flooded or direct-expansion coils is that the oil is brought back to one location—the low-pressure receiver. Figures 8.22a and 8.22b show different locations of the oil drain. In both cases the drain is from a region of stagnant motion where oil, being heavier than liquid ammonia, will separate.
Contributing to the relentless struggle to prevent cavitation is the effort to reduce the pressure drop in the inlet pipe and fittings as much as possible. High friction losses in these pipes only detract from the NPSH that has been provided. Methods for maintaining low friction include (1) keeping the horizontal distance short, (2) using angle valves in preference to straight-through valves wherever possible, and (3) avoiding high velocities because of pipes that are too small. Table 11.1 in the chapter on refrigerant control valves shows that the pressure drop in an open angle valve is much less than in an open straight through valve. The arrangement in Fig. 8.22a uses straight-through valves, so there must be adequate liquid head to overcome this pressure drop. The individual suction lines in Fig. 8.22b are adaptable to angle valves. In some recirculation packages, the orientation shown in Figure 8.22c is chosen to accommodate angle valves. A strong preference for angle valves in the discharge line does not prevail because the pressure of the liquid is high enough that vaporization is not a problem.
It may seem that the precaution against cavitation would be best served by choosing liquid suction lines as large as practical to achieve very low liquid velocities. This would normally be the case, except in instances where the pressure in the low-pressure receiver fluctuates. If the pressure falls rapidly, bubbles of vapor form almost immediately to absorb heat, which brings the liquid temperature back in conformity to the saturation temperature. If these bubbles form in the suction pipe, they have no opportunity of venting to the vapor space of the receiver. The objective becomes one, then, of passing the liquid out of the suction pipe in a reasonable amount of time. Lorentzen recommended velocities between 0.8 and 1.0 m/s (160 to 180 fpm) as a compromise to combat cavitation due to pressure fluctuations and without encountering high pressure losses.
When drawing from the liquid leg as in Figures 8.22a and 8.22c, there is little likelihood of vapor bubbles entering the suction tube, but in the arrangement of Figure 8.22b a vortex may develop above the inlet which draws vapor into the pipe. To break up any such vortex, an eliminator as shown in Figure 8.23 can be placed at the inlet.