Compressor Support

Screw Compressors

Please read this before connecting a Screw Compressor.

Screw Compressors e.g. Hitachi 6002SC-H; Fu Sheng SR4H.

Screw compressors are generally driven by Star delta or Part wind start motors, for details of electrical connections the notes provided for these elsewhere in this guide apply. The critical factors to consider with Screw compressors are those areas associated with initial Direction of Rotation, ensuring a good Head Pressure is established and System Cleanliness.

Screw compressors only work when operated in a particular direction. However, with no external shaft, establishing Direction of Rotation can only be done by close monitoring of the gauges.

Because most Screw compressors utilise a discharge non return valve, if they do rotate the wrong way the reversed pumping effect is minimal. The most reliable method to establish correct DOR is to monitor the suction gauge, which should fall immediately the compressor starts. IF IT REMAINS LEVEL OR SHOWS ANY INCREASE STOP THE MACHINE IMMEDIATELY, then reverse direction (see below) and repeat the attempt.

Reversing Direction of Rotation

Part wind starters Both part wind must be kept synchronised, so swap matched pairs as follows....
  In terminal box 1 with 2 & 7 with 8 or 2 with 3 & 8 with 9 or 3 with 1 & 9 with 7
Or At starter Swap any two incoming phases before they split off to the individual partwind contractors.
Star delta starters Phase fields must remain synchronised, or major motor running problems will ensure. Ideally reverse two incoming phases before they split off to the main & delta contactors. 
In terminal box follows  Pay strict attention to leads to be swapped as U-X V-Y W-Z U V W or X Y Z

 Firstly check field continuity as follows, then verify that the Main contractor is feeding. Swap U wth V & Z with X or V with W & X with Y or W with U & Y with Z.

Installed Phase Rotation Device (if fitted)

Many Screw compressors incorporate phase rotation sensors to prevent reversed running. However, if the compressor has a rewound stator there is a 50% probability the compressor will still rotate the wrong way. This is because the motor connection at the stator itself could have been reversed during the rewind process, thus whereas the phase rotation meter shall detect correct rotation of the supply, the motor may still reverse.

IF PHASE REVERSAL IS NECESSARY THEN THE PHASE ROTATION SENSOR SHALL ALSO REQUIRE TWO OF ITS THREE LEADS SWAPPING.

Establishing Head Pressure

Screw compressors rely solely upon the discharge head pressure to provide differential pressure required for lubrication. The oil used in most Screw compressors is very viscous (typically ISO 220), this is needed to provide an effective seal where the rotor tips marry with the root of the opposing rotor. However, this thick oil requires an effective differential to ensure the proper oil flow. The rolling elements bearings in most semi hermetic screws can run for several minutes on the oil film provided at assembly, but any longer and they will suffer.

If possible try to allow the machine to run for several minutes at first start up and allow the head pressure to rise above 175 psi / 12 bar. In cold ambient conditions consider overriding condenser fan controls to establish the head pressure as soon as feasible. Once this is established the compressor will generally run quieter, and the new bearings will really appreciate the plentiful supply of oil to allow them to run in smoothly.

System Cleanliness

Screw Compressors are very sensitive to contamination, particularly dirt or moisture. Even trace quantities of moisture can lead to excessive Electrolytic Copper Plating. The moisture reacts with the refrigerant o form a mildly acidic refrigerant, this in turn forms an electrolyte, the electrolysis deposits a layer of copper on the close running components, which can lead to binding and eventual seizure.

Always ensure the oil in the compressor is fresh, and if topping up or oil change is required, the oil must only come from a freshly unsealed container, preferably metal not plastic. Always thoroughly evacuate the compressor whenever the interior has been disturbed, even for the simplest task.

Final Checks before starting

  1. Have you checked the connections?
  2. Are you ready to check the Suction Gauge?
  3. Has the machine been thoroughly evacuated?
  4. Is the refrigerant you are adding clean, pure and dry? Save your recovered gas for non screw plant!
  5. Have you overridden fan controls to establish good head pressure as soon as possible after start?

Once Running

  1. Immediately check Suction pressure falls. Listen for tinkling noises - liquid slugging.
  2. Monitor motor amps are balanced. Is vibration level acceptable?
  3. How does compressor sound? Once the head pressure establishes the oil flow should make the compressor run quieter. Throughout the Set to Work it is essential to eliminate what may have caused the previous compressor failure.

If you have not identified the root cause of previous failure the new compressor will be endangered.

In our experience most repeat failures will occur within the first 48 hours running. Close attention to all running conditions at initial commissioning is therefore essential.