Question
I'm looking to gain some air for the occasion when I need to run two air sanders, and wondered if an air tank would really be the answer (or if I should save my money for a screw compressor). My compressor is a 5hp, single stage, IR with a 60 gallon tank. Typically, we're only running one sander and it can be a hard day on the compressor if we're sanding all day long. Any thoughts? Budget is a big concern.
Forum Responses
(Cabinetmaking Forum)
From contributor M:
Sounds like you need more CFM. Average CFM for sanders is about 16 and your 5hp compressor is probably putting out at best about 18cfm. Adding an air tank would help but not as much as you might think. Air is expensive energy. Why pay the electric cost of a 5hp motor to run a sander?
The 15hp cost us $2300, and the screw cost $6000. We kept the 15hp as a backup, but have not had to turn it on yet in the 9 months since we got it.
Compressors that first go into unload operation before shutting the motor off are unlikely to suffer the short cycle problem. If this isn't making sense to you, call one of the large compressor dealers, not Home Depot. After installing the sequencing system, spend some time watching it run to be sure all is okay. Where you are likely to see a problem is the point where one compressor isn't quite enough.
I am still not at the point that a screw makes sense, but I expect to buy one in a couple of years. Until then I think having two compressors will give me the flexibility and efficiency I need.
Compressed air is expensive! Additional items on our system: condensate water/oil separator, air to air cooler, coalescing filter, refrigerated air drier, main regulator, 1 1/2" primary piping, 1" secondary loop piping, several secondary loops (all 1" and smaller piping is copper), additional regulators, filters, oilers, all the quick couples, hoses, air tools, spray equipment, pneumatics on the stationary tools, eek! It's surprising that it all works as well as it does. We didn't have the refrigerated drier and coalescing filter until we added the second compressor. That improved just about everything downstream. Main reason we got the drier was to protect the pneumatic systems on the stationary tools. Turns out everything benefits.
From contributor L:
If you buy a second recip compressor, put them both on an alternating switch system. When both are not needed, they will get used alternately and get to cool between starts.
From contributor I:
Great point, contributor L. Can you recommend a simple off the shelf controller that is easy to wire into the magnetic switches?
From contributor L:
The simplest controls only work well on matched compressors (two nearly identical compressors.) You can set one up using parts from Automationdirect. More complex systems that control mixed or more than 2 compressors should be left to the control designers. For controlling two very similar compressors you need to be sure you have adequate storage to prevent short cycling times. The pressure switches should be of the same type and also be matched in their in/out set points. All the simplest controls do is start or load one compressor and run it until either enough air is in the tank to shut it off or until it can't keep up with demand and then starts a second compressor. When demand drops, the first compressor is shut down or unloaded and the 2nd one continues to supply air. If demand rises above what compressor #2 can supply, compressor #1 is started/loaded. When demand has again been exceeded, compressor #2 shuts off or unloads and #1 carries the load. You can see that enough storage is needed to prevent the second compressor from very quickly bringing the pressure up to the shutoff point. That could start a series of short cycles and be very hard on motors.
From contributor I:
Thanks. I see there is a lot more to this than I realized. I am also looking to increase my air supply. I was just going to buy another compressor and set its cut in pressure 10 PSI higher than the first. The setup you are describing keeps the work load evenly distributed over the units.
From contributor L:
We kind of went the same route as contributor A over the years. When we moved to this shop (4000sq ft) I bought a 10hp on a 120 gallon tank. We expanded to 8,000 sq ft and I added a 2nd 10hp same as the 1st one. I tried the thing of just setting the pressure switches slightly different. That made the first compressor run most of the time and the 2nd one only at high demand times. After seeing how hot the one compressor would get running most of the time, I bought a used sequencer. That made things much better. We enlarged the shop twice more (to 25,000'), added more equipment and were running short on air. The worst was the CNC would stop if air dropped below 85#. Took the two 10s out of the compressor room and installed a 25hp screw, installed one of the 10s as a backup and use its tank as extra storage capacity. It can't be run at the same time as the screw as things are now arranged due to the differences in operating pressures. We are starting to run short on air again at peak times.
From contributor I:
Thanks again. Is "sequencer" the technical name for this device?
From contributor L:
I think that is the proper term.
