Systems with flooded evaporator

Risk of large concentration shifts in the flooded evaporator, and as a result also in the circulating refrigerant mass flow.

Risk of large concentration shifts in the flooded evaporator, and as a result also in the circulating refrigerant mass flow.

Only use refrigerant blends without a significant temperature glide (azeotropic refrigerants) in systems with flooded evaporator!

The use of flooded evaporators requires separate oil return from the evaporator or low-pressure separator and is only possible with refrigerants without significant temperature glide. The oil-refrigerant mixture should be taken at several tapping points, from the oil-rich phase of the liquid level.

The refrigerant must first be evaporated with the aid of a heat exchanger, e.g. in counterflow to the warm refrigerant liquid. The oil is then fed back into the suction gas line. If the liquid level fluctuates greatly, it may be useful to tap at the lowest point or downstream of the circulation pumps. However, it must then be checked individually whether the oil and refrigerant are sufficiently miscible under the operating conditions in the evaporator or separator – with some combinations, there are pronounced miscibility gaps, see

Depending on the oil circulation rate, phase separation can occur, with the oil depositing on the liquid level – therefore the previously described arrangement of the tapping points is usually mandatory.

To achieve the minimum oil circulation, oil separators must always be individually designed for systems with flooded evaporators (upon request). Depending on the system design and operating conditions, a secondary oil separator may be necessary.

CS. and CSV. with flooded evaporator

CS. and CSV. compressors are also designed with flooded evaporators, particularly in liquid chillers. As shown in the figure below, oil or oil/refrigerant mixture is then drained from the evaporator and returned to the suction side of the compressor using an ejector (jet pump).

Oil return from the flooded evaporator to CS. and CSV. compressors, driven by a hot gas bypass with ejector. The illustration is highly simplified - in fact, three parallel bypasses with one ejector each are often necessary, for example.

The ejectors are regulated by a solenoid valve with time control. How long the intervals should be and how often they are necessary must be determined individually for each system. During standstill, an additional solenoid valve between the evaporator and ejector should shut off the bypass, as otherwise refrigerant may enter the suction gas line.

Note for systems with R717 (ammonia)

In R717 systems, the oil is always taken from the lowest point of the evaporator or suction accumulator. The oil has a higher density than liquid R717 and therefore collects under the liquid refrigerant and is returned from there directly to the suction side of the compressor(s). In the usual design of R717 systems, primary and secondary oil separators connected in series are necessary (design with the BITZER SOFTWARE).

For details on R717 systems: