Refrigerator Parallel Pipework

When compressors are installed in parallel and share common suction and discharge lines, they are usually open-type units. Hermetic and semi-hermetic units are not used in case of a motor failure. The high temperatures resulting from the motor failure of one motor compressor can cause a chemical breakdown of the refrigerant and oil, resulting in the contamination of the whole system.

However, two or more hermetic or semi-hermetic units can be used to accommodate a common load. When this is required each compressor is connected to an independent circuit within the evaporator and the condenser, so that they function without the possibility of cross-contamination. A single condenser is normally preferred for ease of control, but a separate condenser may be used for each compressor.

Discharge lines

Discharge lines should be installed so that the horizontal section is pitched downwards to join the common line. This will provide a free draining trap, preventing oil drain back to the idle compressor.

When the condenser is located above the compressors, separate risers should be used. Oil traps should be included at the base of each riser if the net lift to the condenser is more than 2 m. If the riser height is less than 2 m the oil traps may be omitted.

Figure 77 shows a basic pipework arrangement for discharge lines.

Common discharge
Common discharge

Discharge line equalizing

When individual condensers are installed, discharge lines must be equalized before they enter the condensers to allow them to function as one (Figure 78).

The pressure drop within equalizing lines is critical. For example, assume a pressure drop of 0.03 bar (0.5 psig) in the equalizer line. The pressure differential between an idle and an active condenser will cause condensed refrigerant from the active condenser to back through the liquid line to the idle condenser in an effort to equalize.

Discharge and crankcase equalizing in common pipework system
Discharge and crankcase equalizing in common pipework system

Crankcase equalizing

Obviously it is also necessary to prevent unequal oil distribution between the crankcases of the compressors. This is achieved by connecting an equalizer line to tappings provided (Figure 78).

The equalizer line size must be equal to the size of the tapping union. It is also essential that the compressors are mounted on the same level or that the oil levels are at the same height; compressor mounts can be adjusted to make this possible.

Suction line equalizing

Pipework should be routed to a manifold arrangement to equalize pressure at each unit. Common suction lines will drain freely, so this method is suitable for most systems (Figure 79).

When the evaporator is at a lower level than the compressor, the suction line should be pitched downwards. There should be two oil traps approximately 0.5 m (20 in) in height when suction risers are in excess of 3 m (10 ft).

Suction line equalizing
Suction line equalizing


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