Parameters and recommendations for optimal system efficiency and operational safety

Optimisation of the control accuracy (CF)

A wide control range of the capacity control ensures stable operation, even with load or power changes. However, this is only possible if the control range of the guide compressor can cover the capacity gaps caused by other compressors when switching on and off. The control accuracy (CF) is calculated from the difference between the power of the guide compressor at maximum and minimum frequency, divided by the power of the following compressor, multiplied by 100%.

The compressor capacity control in the compressor stages should ideally achieve values ≥ 100%. Values < 80% are not good and can cause unstable operating conditions.

See also Technical Information KT-600.

Covering part load/minimum load

Covering of partial load conditions down to minimum load, e.g. by the guide compressor, guarantees a continuous mass flow, stable suction and high pressures, and stable suction gas temperatures. This prevents poor efficiency of the system, possible wet operation, reduced oil return, fluctuating control loops and unfavourable operating conditions for the compressors.

Avoid frequent start-up and shut-offs (cycling rate) of the compressors

High cycling rates lead to increased mechanical stress on the drive gear and thermomechanical stress on the compressor motor. Operating at low compressor frequency or repeatedly switching off risks a lack of oil!

For compressors without frequency inverter:

• Max. six starts per hour and min. 10 minutes between two starts.

For compressors with frequency inverter:

• Operation of the compressors in the starting phase with a frequency of ≥ 40 Hz for ≥ 10 s
  This ensures sufficient oil supply to the drive gear before the control is released.
• Note that frequent starts and shut offs usually occur at night or outside opening hours!
• Cycling rates > 120 per day are critical. Check the control behaviour of the system!
• Cycling rates > 160 per day reduce the service lifetime of the compressor!

If refrigerated display cabinets are retrofitted with glass doors, observe the following points:

• Most systems are ordered and delivered with a large power reserve. Subsequent installation of glass doors reduces the required cooling capacity by a further 40–50% depending on the temperature class and evaporation temperature. The deviation between installed and required cooling capacity thus increases considerably, with major effects on the part load behaviour of the system. Cycling rates, for example, will increase sharply.
• For systems with frequent start and shut-off of the guide compressor, check whether the additional installation of glass doors can be combined with a modification of the system.
  Appropriate measures would be:
• Select a displacement stage smaller than the guide compressor and
• design the downstream compressor with stepped capacity control.
• Also: Observe the notes in the chapter on optimising the control accuracy! (Parameters and recommendations for optimal system efficiency and operational safety)

Control refrigerant mass flow in a stable and carefully manner

Cautious control of all system components, adapted to the conditions of the system, leads to a stable refrigerant mass flow without oscillating control loops.

• Operation with frequency inverter:
  Speed ramps to be aimed for in standard operation: Increasing 1 Hz/s and decreasing 2.5 Hz/s, observe the timer involved (compound control, frequency inverter), set one timer to zero if necessary.
• Avoid unstable operating conditions:
• Such as the "gas loop"/"rinse mode" – do not set any set points for the high pressure control in the critical point area in order to avoid fluctuating amounts of flash gas in the intermediate pressure vessel caused by pressure and temperature conditions between the boiling point line and the dew point at the gas cooler!
• e.g. due to mutually influencing system components (e.g. gas cooler fan and high pressure control valve) on essential process control variables (e.g. the optimum high pressure).

Further recommendations:

• Reduce the start opening of the electronic expansion valves on evaporators with high refrigerating capacity in line with the total refrigerating capacity of the system.
• Design heat recovery systems with pressure boosting with buffer tank on the hot water side. This prevents constant changing of operating modes.

Adhere to the permitted operating conditions

• Maximum discharge gas temperature 160°C, measured inside the cylinder heads, or 140°C at the surface of the discharge gas line (see Operating Instructions KB-130).
• During continuous operation, the minimum discharge gas temperature must not drop below 50°C and the minimum oil temperature must not drop below 30°C.
• The minimum superheating of the suction gas is 10 K – this must not be undercut in systems with cooling by direct evaporation!
• Comply with the maximum operating currents of the compressors!

A standard compound control unit only monitors the variables: High pressure, discharge gas temperature, suction gas superheat, oil level and engine temperature independently and only provides a standard safety cut-out. However, the permitted discharge gas temperature depends on the pressure ratio, the suction gas superheat, the compressor frequency, the operating time and the dynamics of the operation.

A low compressor frequency and a higher suction gas superheat have an influence on the thermal load of the compressor and reduce its application limit. In the figure below, for example, the dashed line ③ indicates the maximum permitted discharge gas temperature (t_max.) for a compressor frequency of 25 Hz with a suction gas superheat of 30 K and thus the limitation of the application limit.

①	tmax, 10 K, 50 Hz	②	tmax, 10 K, 25 Hz
③	tmax, 30 K, 25 Hz	④	toil min, 10 K, 50 Hz

Ensure oil circulation and lubrication

Careful planning of the piping networks ensures the oil return from the system!
Oil migration and coating in the heat exchangers can thus be avoided.

• Observe minimum flow velocities, in rising lines for example, calculate using the equation according to Jacobs.
• Plan vertical rising lines with oil collection and oil breakaway bends.

The high gas solubility of the refrigerant in the oil can lead to strong degassing effects that negatively influence viscosity and tribology.

Continuous operation of the compressors without frequent starts and shut-offs aids good oil return from the system. This also prevents cold oil enriched with refrigerant from being returned abruptly from the evaporators to the compressors, e.g. after defrosting phases or at high refrigerating capacity.

Stable operating conditions also allow the oil level in the oil level controllers to be stabilised and the level to be aligned with the level in the drive gear.