de-rating an Air Compressor..

[celica-fiend]

Hi folks.. Ive been lurking around here awhile admiring everyones toolboxes and organization etc.. and now have stumbled into something I need to ask about..

I share a shop with a few buddies and we've a little upright, oil-less 3700rpm air comp that doesnt keep up with the plasma well, and realllly doesnt keep up with a die grinder..

So when this 'act now!' type deal showed up last night I went for it..

Ended up with a Swan HWU-310 air compressor, 2 stage, 3 cylinder, 120gal, 150psi, 36cfm .. apparently the pump has an unloader as well (whatever that means )

It apparently used to have a 10hp motor on it.. but I didnt get it with one at all.. Ive got a new pressure switch, rebuilt cylinders/reed valves, mufflers, but no motor. (It was $100 and he's moving today, had no space to store it anymore so take it now or go away type thing..)

Now I haven't 3 phase in the shop and from what I recall, we've a 100A service.. Im not about to try and find/install a 10hp single phase motor on it.. and, if worse comes worse, I'll shouldnt have many problems finding a 3 phase to run it and another to build a phase convertor..

.. but what I really wanted to ask is, if we wont be stressing this thing with our fairly light requirements anyways.. is it possible to de-rate the unit in either working PSI or flow to say allow a 5hp single phase or maybe 7.5hp single phase to run it?

From google-ing about, this appears to be my compressor head here:

http://www.forbesaust.com.au/pdf files/HW310 facts.pdf
*fixed link to reflect correct compressor head*

thanks for any advice you can offer..

Brett

 

[Jim Johnstone]

Is your a 2 stage 3 cylinder? Because the link you showed is single stage 3 cylinder.

I would email or call the manufacturer. The Devilbiss pump manuals I've seen have a list of all the hp ratings the pump is able to run on, and the corresponding RPM for each size of motor. Perhaps Swan has a similar table that will tell you what RPM to spin it at for a 7.5 or 5 hp motor. Also, I would derate the speed, but keep the pressure the same.

 

[celica-fiend]

hah, good catch, now I forget where I saw it said 2 stage.. back to the google.

Brett

 

[celica-fiend]

Now I can't find the info I was looking for.. and while I snapped pics of the data plates while I was out at the shop, I didnt get an overall shot of the compressor. Two of the cylinders have mufflers however, and a third feeds the tank.. so Im going to say 2 stage, while I continue to dig.

There we go:

http://www.forbesaust.com.au/pdf files/HW310 facts.pdf

Brett

 

[oldtractors]

Repully it so it runs twice as slow as normal, and it should be half the CFM and require half the horsepower.

 

[Mike007]

FYI, the unloader is a "de-rater".

 

[larry_g]

If I did the math correctly it looks that it would take 6.12 hp to run that compressor at 150psi at the minimum speed of 630 rpm at the compressor. That said what you need to understand is that 630 in the minimum speed of the compressor head, and 50 psi is the minimum operating pressure. From that information you run the numbers, use the numbers that are in the shaft input list. Also note that the input shaft numbers are at 'normal speed', 1000rpm. Figure out the speed and pressure you need to have to have the air supply you need to operate the shop. Its all proportional numbers. If you up the speed then you have to up the motor power or lower the pressure. When you go lower in the motor power you have to slow down the pump or lower the working pressure or both. You have the variables of HP, speed, and pressure. HP = speed X pressure. or from the chart KW=kPa X 1000(rpm)


Once you set up the system and have a motor then you have to make sure that you don't exceed the FLA of the motor while operating.

lg
no neat sig line

 

[kartracer55]

Repully it so it runs twice as slow as normal, and it should be half the CFM and require half the horsepower.

I'm not so sure it works like that.

Also, these pumps will also have a minimum speed rating on them, and this is derived from lubrication needs. On this machine, nominal speed is 870, minimum is 630 rpm.

 

[celica-fiend]

thanks everyone for your input.. I wasnt aware of minimum speed requirements for the compressor head..

All this info in hand, I'll now go try and hunt down a motor..

thanks again,

Brett