SCREW COMPRESSOR CAPACITY CONTROL 1

[bassonet]

We have a screw compressor (Aerzen Delta Screw VLM150)that delivers too much air to process with no built in way of controlling the capacity. It delivers air at 0.75 Bar delivery, intake 10000 m3/h free air at 1300m altitude, 300 kw motor at 2950 rpm.

What is the quickest way of reducing delivery capacity given a remote location with on site plant pipe fabrication facilities?

 

[zdas04]

Do you have the ability to slow the machine down? A VFD in that application is the most effective way to control capacity.

I'm not familiar with that brand of compressor, but does it have internal capacity control (on air compressors, if they're available, they are usually called "turn valves" or a "poppet valves")? If so, your compressor control panel can usually drive an unloader scheme.

Finally, if those options are not available to you, you can put a recycle valve to dump excess air.

David

 

[Compositepro]

The screw compressors I've seen, regulate capacity with 'unloading" valves that close the suction to the compressors. With the suction closed the compressor does no work. If the compressor stayed unloaded for more than 15 minutes the motor would be turned off.

On expensive equipment like this it would be wise to consult the manufacturer of your compressor.

 

[zdas04]

The "unloading valves" do not throttle the suction to the compressor (that would increase compression ratios and create some really interesting heat and hp issues), nor do they slam the suction shut which would create horrendous windage losses. What they do is dump some of the flow back to suction from a little ways into the screws (i.e., you add a couple of pounds of compression to be able to overcome the suction pressure). Some machines can dump as much as 40% of rated capacity.

I've seen compressors running partially unloaded for decades, and have rearely seen a flooded screw in oil & gas service that wasn't partially unloaded.

David

 

[bcs5274]

A bypass or back pressure valve downstream a possibility? Ask the oem if there is a different VI or gear set for this particular unit.

 

[Compositepro]

Zdas you are incorrect in this case. As I said, a compressor with a blocked suction does no work, it is simply spinning like a vacuum pump that has a compression ratio of 1000 or more. Dumping compressed air is the most inefficient way to control load.

 

[Compositepro]

I should add that the design details of the compressor determine how it should be unloaded.

A roots blower, for example has no valves so the lobes are always working against outlet air pressure. To unload a blower you would put a check valve in the outlet and then dump air to atmosphere before the check valve.

A reciprocating compressor can be unloaded by locking inlet or outlet valves open or closed. There are potential problem each approach that need to be addressed. For example, under vacuum the cylinder could slowly fill with oil.It would not be good to try to compress that oil.

 

[bassonet]

Thank you all, as we are in a remore location we do not have the immediate option of procuring unload valves etc, also the compressor is not equipped with a slide valve to return compressed air internally. However, rough and ready options seem to be either simply dumping air (inefficient yes) or one that you guys put me onto to stick an orifice plate after the filter in the suction, cheap an nasty but should result in the required reduced capacity w/o strain or unnecessary loss of energy untill a control valve may be delivered.