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Starting up and Stopping Refrigeration Compressor and Plant

Updated on October 3, 2015


  • The heating rod in the crankcase must be energized at least 12 hours before starting up the compressor in order to boil any refrigerant out of the compressor oil. At the same time, the suction stop valve must be open.
  • Check oil level in crankcase. The oil level must always be visible in the oil sight glass.
  • Check correct setting of safety automatics on compressor.
  • Open discharge stop valve at compressor.
  • Set capacity regulator at minimum capacity.
  • In order to avoid excessive pressure reduction in the compressor on startup, the suction stop valve must be opened a few turns, as there is otherwise a risk of oil foaming in the crankcase.
  • Open all other stop valves except for the main valve in the liquid line and possible by-pass valves serving other purposes.
  • Start condenser cooling, brine pumps, fans at air coolers as well as any compressor cooling device. (All of this is usually done automatically)

Refrigeration Compressor
Refrigeration Compressor


If oil separator is used and the oil separator at standstill is colder than the
condenser, the valve in the oil return pipe must not be opened until the oil
separator has warmed up.

  • Start compressor motor and check suction and oil pressures.
  • Carefully continue opening suction stop valve to its full open position.
  • Open main valve in liquid line.
  • If the oil in the crankcase foams, or knocking sounds are heard from the compressor because droplets of liquid are being fed in with the suction gas, immediately throttle suction stop valve.
  • The compressor is now operating. Increase capacity stepwise, allowing the compressor to adjust to new conditions before switching to next stage.
  • Check carefully whether oil is foaming and whether oil pressure is correct.

  • Check whether oil return from oil separator is working, the pipe should normally be warm. (If oil separator mounted)
  • Do not leave the plant for the first 15 minutes after start-up and never before it has stabilized.

Stopping and Starting-Up Compressor during a Short Period of Standstill

Before stopping the compressor, its capacity must be reduced to the lowest capacity stage for a few minutes, before it stops. (Capacity regulating is not standard.)

During short periods of standstill, it is not necessary to shut off the suction stop valve and the discharge stop valve. The heating rod must be energized. Compressor start-up must always take place at the lowest capacity stage, after which capacity is increased at suitable intervals, in order to avoid that a sudden excessive pressure reduction in the evaporation system causes slugging in the compressor and oil foaming in the crankcase.

Stopping the Plant for Brief Periods (Until 2-3 Days)

  • Shut-off liquid supply to evaporators for a few minutes before stopping the plant.
  • Stop compressor and shut-off suction and discharge stop valves. Close valve in oil return.
  • Stop of Condenser Cooling, Pumps, Fans and any Compressor Cooling.
  • Cut-off power supply to both master and control currents.

Stopping the Plant for Lengthy Periods (More than 2-3 Days)

  • Shut-off main valve after receiver and pump down evaporators. If necessary, adjust low-pressure cut-out on unit to a lower pressure during evacuation.
  • Allow temperature in evaporators to rise, then repeat evacuation.
  • When suction pressure has been reduced to slightly over atmospheric pressure, stop compressor. Shut-off suction and discharge stop valves and close-off stop valve in oil return.
  • Shut-off condenser cooling. If there is a risk of freezing, draw-off coolant.
  • Cut-off power supply to master and control currents. Inspect receiver, condenser and pressure vessels as well as piping connections and apparatus for leakage.

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