Screw Compressor Slider Valves

[planck121]

I do not have much working experiencing on oil lubricated screw compressors. Can somebody please explain on the exact function of the slider valves on such screw compressors in general. What does it mean when the operating manual says slider valve 0% open (does this mean there is gas coming into the suction or the other way around).....I am not sure how these slider valves in, lets say a two stage, screw compressor functions.....

Thanks for the input.

 

[zdas04]

Slide valves are usually used in compressors designed for process service (air conditioning usually compressing something like ammonia or propane). For air compressors you usually see "turn valves" or "poppet valves". They all do the same thing.

The unloader valve will dump gas back toward suction early in the travel of the gas through the rotors. At that point nearly zero work has been done on the gas and the cost of dumping it is pretty low. At 0% open, no gas is being dumped. At 100% open, maximum gas is being dumped. Depending on the compressor manufacturer, 100% equates to some value between 40% and 70% of full capacity. Each manufacturer sets that number based on their own design considerations.

I'm not aware of any screw compressors set up as two-stage units. Generally that implies intercooling and I've never seen a skid with an intercooler loop. When you need to do more compression ratios than the screw can handle most people set it up so that the screw discharges into another technology (I've seen screws go into 1 and 2-stage recips for years, but I'm starting to see a few go into centrifugal machines).

The unloader valves tend to be more efficient at capacity control than a suction controller or a recycle, but not as efficient as changing the compressor speed.

David

 

[planck121]

Thanks David,
What is the purpose of dumping the gas back to the suction. I mean is it to serve flow control through the compressor. I was reading our operating manual and it says to keep the silder 0% open during start-up and once the pressure builds up to 2150 psi then to open the slider to 25%. I am not sure what practical purpose does this serve in compressor start-up.

Cheers

 

[petrochemical87]

Hi,
The slide valve basically changes the length of the screw available for compression, thus moving a slide valve from 100 percent open to closed makes more of the screw available for compression and increases the inlet volume.Yes you can say it is like flow control. This is an energy efficient method for capacity control as only a small amount of work is done on the gas volume dumped to suction when the valve is open as zdas04 points out.
Most screws I have seen are started with the slider at 100 percent open.