Oil management

Oil and compressed gas temperature

• Generally use oil heater, especially during standstill phases.
• Oil temperature: 30°C (20°C = absolute minimum value!).
• Recommended suction gas superheat 20 K – if necessary, provide heat exchanger to minimise refrigerant concentration in the oil.
• Lower suction gas superheat is possible, provided minimum oil and discharge gas temperatures are maintained. When cooling by direct evaporation, avoid values < 10 K!
• Minimum discharge gas temperature = condensing temperature t(_c) + 40 K.
• The discharge gas temperature during operation must be determined as a function of the peak pressures. During continuous operation, the temperature of the compressed gas should not fall below 50°C!
  Depending on the high and low pressure, very high discharge gas temperatures can occur even when operating with saturated suction gas!
• Maximum discharge gas temperature 140°C, measured at the discharge gas line at a distance of 10 cm from the compressor discharge gas connection.
• The influence of different load conditions (Parameters and recommendations for optimal system efficiency and operational safety) and the use of different system concepts (e.g. flash gas bypass) on the operating conditions of the compressors must be taken into account and included in the calculations. If necessary, consultation with BITZER is recommended.

Oil separator and oil return

• Use oil separators with coalescence filter cartridges.
• Only use oil separators without a float valve. The (electronic/optical) oil level monitoring in the oil separator opens a solenoid valve in the line to the oil reservoir if required.
• Only use electronic oil level controllers that are suitable for the high maximum opening pressure differences (MOPD= Maximum Operating Pressure Difference).
  The permitted pressure figures for mechanical controllers are not sufficient for R744 systems, especially booster systems.
• The amount of oil in the oil reservoir should be at least as large as the amount of oil in all compressors. However, the oil reservoir itself may only be filled to maximum halfway with oil.
• It is not recommended to use a controlled intermittent oil return that feeds oil into the reservoir (low pressure oil reservoir) or the compressors (high pressure oil reservoir) regardless of the amount of oil in the oil separator.
• To ensure oil return to the compressors, the pressure in the oil reservoir must be higher than the highest suction pressure in the system.
  Degassing takes place in the corresponding suction gas line. In systems with parallel compression, degassing always takes place at the intermediate pressure level and not, for example, at the suction pressure level of the compressors in the medium temperature compressor stage.
• Differential pressure valves should be used in the degassing line. The required pressure difference depends on the boundary conditions. Pressure differences between 2.5 and 4.5 bar are usually used.

In addition, observe or coordinate the following parameters and variables:

• Nozzle cross-section of the oil level controllers used in the respective compressor stage.
• Opening time of the oil level controller when refilling oil.
• Amount of oil held in the oil reservoir.
• Nominal maximum pressure difference in the oil reservoir.
• Length and cross section of the oil lines from the oil reservoir to the oil level controllers.