A characteristic of industrial refrigeration plants is the use of parallel condensers and compressors. This fact is especially pertinent to ammonia systems, and not always true for halocarbon systems where the oil transfer between parallel units sometimes becomes a problem. Plants of even moderate size are usually designed for parallel condensers to offer flexibility in meeting a wide range of load variation. When condensers are piped in parallel, the following rules should be observed:
– trap the liquid drain lines
– provide a generous length of vertical drain line
– install an equalizer line between the receiver and the entrance of the condensers
The purpose of trapping the liquid drain lines is to aid in drainage of liquid from all condensers. The potential drainage problem with multiple parallel condensers is illustrated in Fig. 7.31 where the condenser on the right has, at least at this moment, a low flow rate of refrigerant. Some reasons for the low flow rate include: a different design of condenser than the other or the fans are completely or partially shut down.
The pressure drop through both condensers must be the same because there are two common points in the piping—at the inlet and the outlet of the condensers. The only way the active condenser on the left can operate with the low pressure drop of the condenser on the right is if liquid backs up into the condenser tubes. Assume that the drain lines are large, so the liquid head in the tubes on the left condenser supplements the available pressure difference.
To avoid the problem of liquid backup into one of the condensers, liquid lines from both condensers should be trapped, using arrangements such as those shown in Fig. 7.32. Once again, the pressure drop through the two condensers from their common inlet to their common outlet is identical, but because the liquid lines are trapped, the drain line is full, and the difference in liquid level in the vertical drain line compensates for the difference in pressure drop. Thus, the liquid head in the left condenser builds up in the drain line and not in the condenser tubes where it would adversely affect the condenser performance. The bottom inlet receiver in Figure 7.33 inherently provides liquid traps for each condenser.
Implied in the correct piping of Figures 7.32 and 7.33 is an adequate length of the vertical leg. An estimate of the needed length of liquid column can be derived from the knowledge that the maximum pressure drop in an operating ammonia condenser is usually about 3.4 kPa (1/2 psi), which is the maximum pressure difference to be compensated for between an idle and an operating condenser. A liquid column of 0.6 m (2 ft) would compensate for this pressure difference, but most designers attempt to place the condenser high enough to permit a 1.2-m (4-ft) column of ammonia. Liquid R-22 has twice the density of liquid ammonia which would suggest that only half the length of liquid column would be needed for R-22, but the pressure drop in an R-22 condenser is about four times that of an ammonia condenser. The reason for the higher pressure drop with R-22 is because of its low latent heat, requiring perhaps six times the flow of R-22 compared to ammonia for a given heat-rejection capacity of the condenser. The recommended length of liquid column in an R-22 installation is 2.4 m (8 ft).
An equalizer line is required since the vapor from the top of the receiver cannot vent back to the condensers through the drain line because of the liquid traps as it could in the single condenser in Figure 7.28. Also the pressure at the outlet of the condenser tubes may be different, so the equalizer line must be connected to the top of the condensers where the pressure is the same for both condensers.