Hitachi Screw Compressor Unloading Setting

[Inverter man]

Can any one help?

On an infinity controlled Hitachi 6005SC-Z screw compressor, what is the maximum recommended prolonged discharge pressure without causing detremental operation of the compressor or its components.

The compressors in question serve an air cooled chiller (Climate/ Multiclima) there are unload HP switches on each circuit set circa 20 to 21 bar however these have previously been adjusted by others so the settings are not uniform and the origin setting info has been lost. These HP switches command the main controller to unload all compressors for a time period (circa 20 minutes). This basically unloads them to minimum capacity which can be an issue during warm periods.

I have details of the HP unload switch setting for stepped 6002SC-H compressor which unloads the compressor to 75% which is 'Cut In @ 23.5 Bar/ Cut out @ 19.5 Bar' this would be less of a probem as the compressors do not unload fully and are allowed to load back up when the pressure falls to 19.5 Bar.

For Reference:
HP cut outs are set for 24.5 Bar
These compressors are old babies and are close to bearing renewal (22000 hrs), however this will not take place due to cost and and are being kept alive until the chillers are replaced in the near future.

I have also considered that higher operating pressures reduce the life of the compressor bearing.

Basically can anyone offer advice on the setting of the unloader HP switches?

[install monkey]

hitatchi website is a bit vague
Specification of Screw Compressor 5 Series

Model
4005SC-H(Z)
5005SC-H(Z)
6005SC-H / 6005SC-Z


Overall
dimensions
mm(inch)
Height
505(19-7/8)
530(20-7/8)


Width
413(16-1/4)
458(18)


Length
1,093(43)
1,179(46-3/8)
1,209(47-5/8)


Net Weight
kg(lbs)
360(794)
420(926)
440(970)


Piping
connections
(mm)
Suction O.D.
60.5


Discharge O.D.
42.7
48.6


Oil Charge
liter
6.0


Frequency
Hz
50
60
50
60
50
60


Cooling Capacity
(R22)
kW (×1000Btu/h)
132.2
(451.1)
159.6
(544.6)
163.8
(558.9)
196.9
(671.9)
201.7
(688.2)
242.6
(827.8)


Motor input
kW
32.5
39.0
40.0
48.0
49.5
59.5


Displacement
m3/h
137.4
165.6
169.5
204.2
208.7
251.5




Condensing Temperature
40.6http://www.refrigeration-engineer.com/forums/image/screw_spec/c.gif(105http://www.refrigeration-engineer.com/forums/image/screw_spec/f.gif)


Leaving Liquid Temperature
40.6http://www.refrigeration-engineer.com/forums/image/screw_spec/c.gif(105http://www.refrigeration-engineer.com/forums/image/screw_spec/f.gif)


Evaporating temperature
4.4http://www.refrigeration-engineer.com/forums/image/screw_spec/c.gif(40http://www.refrigeration-engineer.com/forums/image/screw_spec/f.gif)


Suction Gas Temperature to Compressor
12.8http://www.refrigeration-engineer.com/forums/image/screw_spec/c.gif(55http://www.refrigeration-engineer.com/forums/image/screw_spec/f.gif)




The compressors are driven by two pole motors directly mounted on the compressors shaft, and with a rotation of 2,880 rpm for 50Hz and 3,470 rpm for 60Hz, at the rated voltage.
Refrigerant R22, R407C and R134a are applicable to these compressors. Oil is charged in these compressors at the time of shipment.
Models of 4005SC-H, 5005SC-H and 6005SC-H are designated for STEP capacity control.
The capacity shall be 100％, 75％, 50％ or 25％ (except for 4005SC-H).
The capacity shall be 100％, 75％, 50％ or 33％ (only for 4005SC-H).
4Models of 4005SC-Z, 5005SC-Z and 6005SC-Z are designated for INFINITE capacity control.
The capacity ranges between 100％ and 25％ (except for 4005SC-Z).
The capacity ranges between 100％ and 33％ (only for 4005SC-Z).
Piping sizes show the external diameter of piping in mm, which is the size of the flange that compressors are equipped with.
The standard color is light green (Munsell Code 10G5/2). Other colors are available upon request.
The cooling capacity and the motor input are based on the following conditions:
The overall dimensions and the net weight are subject to change depending on order specifications.
These Compressors should be installed indoors or in a location equivalent to an indoor environment..

[install monkey]

comp service manual
http://www.mediafire.com/view/?wujwjdsvs5az2dt:cool:

[Inverter man]

Ta Monkey Man, I appreciate your efforts, alas I already have the same info, did some trawling before asking.

On reflection I have a chart on site that details overhaul periods against discharge operating pressure, from memory, above 19 - 20 bar the period is reduced markedly. I Know I'll have to keep any excessive operating pressure/ conditions to a minimum.

[Inverter man]

Have found out more:-

the chiller manufacturer factory setting for unloading is 22 Bar cut in (cut out @ 19.5 Bar).

When the pressure reaches 22 Bar this enables a pulse timer which cyles the unloading solenoid for that circuit, in the mean time the other circuits continue to load if required. When the said circuit pressure falls to 19.5 Bar the pulse timer is disengaged and an additional timer is brought into play whch prevents all circuits from loading for circa 10 minutes. After this period, the compressors are allowed to load as required.