Compressor continuous run/head unloaders

Hi all,

I've been doing some research on compressors and have a few general questions regarding continuous run setups, and how to identify them. I think I have it all figured out (ha!), but could use some confirmation. Simple questions I think. I am asking in general about popular industrial compressors (IR, Quincy, Saylor-Beall, etc).

Questions:
If a 2 stage compressor has brass lines connecting the top of the cylinders to each other and then off to something else, does this indicate that the compressor has head unloaders? Is this universally true, or can they be for something else?

If the above is true, is the "something else" above a pilot valve connected to the high pressure tank, which when triggered by high pressure opens the top of the cylinder to the atmosphere (so there is no pressure in the cylinders)?

If you have head unloaders are there also typically centrifugal unloaders? Is the amount of unloading the same? Or asked another way - is a head unloader going to start easier than one without head unloaders?


I am trying to determine the best way to use 50CFM at around a 70% usage cycle by grouping 5 or 10 hp compressors (for a 200# steam hammer). I'd like to keep the current surges low since I have 100amps at the moment. I'm considering the constant use since I think the compressors would cycle a lot, so I'm thinking that if that is the case it might be better to be able to switch modes (during heavy use run continuous, but when I'm not using it as heavily go to auto start/stop). Searching on CL there are usually pictures, but little other information, so I'd like to be able to pick up on the key identifiers without having to email.

I appreciate any help.

Thanks,
Don

I can only speak to Quincy, since it's the only one I've owned. The copper lines you speak of don't necessarily indicate continuous run unloading. My Quincy has hydraulic unloading in which the intakes are held open by tank pressure until sufficient oil pressure is developed, at which time the valves close and pressure builds. If you were to look at a Quincy manual/parts list, you would see that the piping for the continuous-run unloader is similar to the hydraulic unloader with the addition of another valve. Lot's of Quincy experts on here, perhaps someone will be along to give you a more complete (better) answer.

Bob

My Atlas KE2 Vee twin single stage compressor had unloading valves on each head linked as you said with a supply pipe running to a spring loaded pressure valve on the tank. The valve was adjusted to lift at the desired pressure feeding air at tank pressure to the diaphragms that operated the unloading valves. A small orifice in the system leaked a little air to ensure the system behaved itself.

So far as I can see this was a pretty standard set-up for smallish industrial rated compressors of that era. Blocking up the orifice to stop the leak seriously upsets the system. My KE2 was seriously cheap from Mr Scrappy as it had been disposed of for that reason. Set it up for permanent unload by screwing a bolt into the orifice and it won't make air will it?

Clive

Thank you both for the replies. Bob - what pump were you referencing? I looked at the 325 docs and it seems like there are different versions (L, LVS, L??) which have the extra valve you reference.

AFAIK all the 300 series use the same setup for hydraulic unloading. Continuous run unloading valves may differ depending on displacement.

Bob

dschad said:

I am trying to determine the best way to use 50CFM at around a 70% usage cycle by grouping 5 or 10 hp compressors (for a 200# steam hammer). I'd like to keep the current surges low ...

Click to expand...

If 'simple' is part of 'best', your best solution might just be using a separate pressure switch for each compressor, and offsetting the cut-in pressures a tad.

In actual use I don't think there's all that much difference in start up current for a recip compressor with or without unloaders. Once the motor is spinning though the continuous run unloaders make a huge difference.

IMO, the best ones to look for are the Quincy QR25 series. The unloaders aren't all the same and you can't tell by a glance. The continuous run compressors have a far more complex valve that references oil pressure and tank pressure. The best way to tell if it has continuous run unloaders is to look at the control panel- A continuous run setup will have a switch with 2 modes- Auto and continuous.

The only QR series Quincy's I've seen that didn't have continuous run mode were small guys- like 310 and 325. The big 350, 370 and monster 390 pumps all seem to have them from what I've seen.

I have QR350 driven at 400 RPM (super quiet), but the inrush is pretty high and the compressor is from 1960 with umpty million hours on it so I bought a pair of government surplus new 325's with the nice screw on oil filters and a pair of 10HP VFD's. Kind of doing the same thing you are, just milling a lot of steel and need to the air volume occasionally for air blast on my CNC's.

Jim McIntyre said:

If 'simple' is part of 'best', your best solution might just be using a separate pressure switch for each compressor, and offsetting the cut-in pressures a tad.

Click to expand...

So the thinking is that if the compressors are going to be cycling frequently then it might be better to just leave them running during period of heavy usage, to avoid the frequent startups and associated high current draw. I'm keeping my options open.

Garwood said:

I have QR350 driven at 400 RPM (super quiet), but the inrush is pretty high and the compressor is from 1960 with umpty million hours on it so I bought a pair of government surplus new 325's with the nice screw on oil filters and a pair of 10HP VFD's. Kind of doing the same thing you are, just milling a lot of steel and need to the air volume occasionally for air blast on my CNC's.

Click to expand...

Assuming that the 325's are 5hp - why a VFD versus RPC? Do you have them programmed for soft start, etc? I know there is a lot of flexibility with a VFD.

Thanks all,
Don

The real reason for continuous unloaded is to keep from burning the motor out from frequent starts. My 10 HP QR25 has a maximum number of starts per hour specified, either six or ten I forget which. More than that number of starts and the motor builds heat faster than it can dissipate, and Quincy recommends continuous running with unloaded for that situation.
Dennis

dschad said:

So the thinking is that if the compressors are going to be cycling frequently then it might be better to just leave them running during period of heavy usage, to avoid the frequent startups and associated high current draw. I'm keeping my options open.



Assuming that the 325's are 5hp - why a VFD versus RPC? Do you have them programmed for soft start, etc? I know there is a lot of flexibility with a VFD.

Thanks all,
Don

Click to expand...

VFD for ramped acceleration. I run a lot of stuff on a 50HP RPC. some machines will brown out if the they are ramping up their spindle and the compressor starts.