Pump down system

In the case of a pump down system, the thermostat or the superior controller switches the solenoid valve upstream of the expansion valve. The compressor continues to pump down refrigerant until it is shut off by a low pressure switch. This reduces the amount of liquid in the evaporator. When the solenoid valve opens, the evaporator is fed. The suction pressure rises and the low pressure switch switches on the compressor.

With pump down systems, it is also possible to safely operate systems in which there can be strong liquid displacement in the evaporator, suction gas line or compressor during longer shut-off periods. The risk of liquid displacement also exists in systems whose compressors or suction-side pipe sections and suction accumulators can become colder than the evaporator(s). The pump down system is designed to protect compressors from liquid slugging and frequent operation with liquid in the refrigerant being drawn in.

A distinction is made between:

• Automatic pump down system (also called "pump-down"): Extraction of refrigerant, carried out repeatedly if necessary.
• Single pump down system (also called "pump-out"): One-time extraction of refrigerant.
  An additional auxiliary contactor ensures that no further pumping operations are possible after suction and compressor shutdown. Only upon the request of the room thermostat or after completion of a defrost cycle can it be switched on again.

The following recommendation applies to flooded evaporators:

• Mount the solenoid valve directly at the outlet of the suction gas line from the evaporator,
• with combined start-up regulator function,
• closed when the system is at a standstill.

Raised standstill pressure can be avoided, if necessary, by means of a draining device to the high pressure side. Mind the collector volume.

For water as a coolant, minimum pressure limits must be minded which refer to a saturation temperature of the refrigerant of >0 °C in order to avoid damage to the heat exchanger.

For the low pressure setting, the minimum pressure must be taken into account, which refers to the lowest evaporation temperature of the application limits.

Schematic wiring diagrams

The following schematic wiring diagrams show control examples for automatic and single pump down systems. The simplified wiring diagrams only show the relevant details – the rest of the control structure corresponds to the schematic wiring diagrams, e.g. in the Technical Information AT-300.

If there is a load demand during standstill, the liquid injection to the evaporator concerned is opened first, e.g. via solenoid valve M20. The suction pressure rises to a preset value at which the compressor is switched on via a low pressure switch (B55). When the load decreases, the process is reversed: The solenoid valve closes, thereby "pumping down" the evaporator to a pressure that is also preset. Only then is the compressor shut off.

Components F01 to F03, B10, B11, B32, B41, B42, B53, B60 to B62 as well as the time relay K02T must be arranged in the safety chain upstream of the control devices of the pump down system. This ensures that the solenoid valve (M20) cannot open during fault shutdowns and during the pause time. Separate activation of the solenoid valve can lead to liquid flooding of the evaporator in the cases mentioned above.

With the automatic pump down system, the auxiliary relay K14 (optional) enables combined control. The compressor is always switched on directly, the pump down system is then active mainly during standstill. This circuit variant protects against liquid flooding of the evaporator even if the values of the low pressure switch of the pump down system (B55) cannot be set sufficiently low. However, an additional low pressure switch (B11) is required to protect the system against insufficient suction pressure.

With automatic gear shift: Limit the cycling rate!
If the pressure rises again from the high to the low pressure side due to leakage when the compressor is at a standstill and the solenoid valve is closed, the compressor will pump down again with an automatic pump down system. This can lead to undesirably frequent switching. Therefore, the low pressure switch (B55) and the time relay for the minimum shut-off period (K02T) must be set so that each compressor can start max. six times per hour. If necessary, this function is assured by the compressor protection device (see Technical Information CT-120).

The switch-on value of the low pressure switch (B55) must be lower than the saturation pressure on the suction side which can occur during standstill. The saturation pressure on the suction side usually corresponds to the temperature of the evaporator package. If the pressure setting is too high, refrigerant can condense in the cold evaporator before the compressor switches on.