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Re: Shop realities

Posted: Sun Nov 04, 2018 4:23 pm
by BadDog
I went from a 60 gallon "5 (chinese) horsepower" oil bath cast iron "Industrial" (so the label said) Husky. It was quite adequate for almost everything I wanted to do. But the blast cabinet would bring it to its knees. It would run constantly, and never catch up until I gave up and let it. This Curtis is only 20 gallons bigger on capacity, but it's in a completely different league. It works to keep up with the blast cabinet, but even die grinders, orbital sanders, and buffers only make it earn it's keep. I had to modify (clearance and slot some hole in 3/16-1/4 steel) some parts on my latest desert truck with carbide burrs yesterday. 3 die grinders mounted with different burrs hot swapped as needed. I didn't count, but seems like it cycled maybe 5 or 6 times tops, certainly not running constantly, couldn't say how much run time, but it wasn't a little. The same thing on my old Husky would have had it running constantly to melt down if I didn't give it breaks. And even better than that, you can carry on a fairly normal conversation while it's running 20 feet away.

Anyway, 60 to 80 gal isn't the primary differentiator. It's the pump rating combined with motor rating. As someone explained to me when I was thinking of adding a secondary tank to help my old Husky, adding capacity is a very small part of the equation, and can actually hurt by forcing an overloaded pump/motor to keep running longer to top up the capacity in order to take a cool down break. Curtis used to have a lot of useful detail on their website regarding how motor power combined with their various pumps produced different capacity, but now it's been converted "sales driven", and none of that is available that I could find. I couldn't even find the page that used to differentiate the pumps.

Re: Shop realities

Posted: Sun Nov 04, 2018 4:30 pm
by BadDog

Re: Shop realities

Posted: Sun Nov 04, 2018 8:51 pm
by SteveHGraham
I guess Curtis doesn't make the one I bought any more.

Re: Shop realities

Posted: Mon Nov 05, 2018 12:50 am
by choprboy
SteveHGraham wrote:
Sun Nov 04, 2018 3:42 pm
I don't really know how CFM are measured. I never really thought about it. I guess it must be the pump output, not the flow at the hose end, since any pressurized container will release gas at the same rate through the same size nozzle.
Compressor pump output is nominally measured at standard atmospheric pressure (sometimes, depending on the manufacturer, plus a fair amount of wishful thinking)... not at the pump pressure rating. Performance drops as pressure in the tank climbs from empty to full, particularly so at higher pressures. Good manufacturers will tell you the rated capacity at various output pressures as well.

For my shop compressor, I replaced my 15 year old portable Dewalt (noisy, slow, but has always worked) with a franken-compressor I built from old and surplus parts for a few hundred dollars. An old 60gal air tank (cleaned and tested), a Harbor Freight "5HP 145PSI twin cylinder pump", and a true 5HP 220V motor, mag starter, and pressure switch from SurplusCenter. Add a bit custom piping and sheetmetal. It is way quieter than the old one and works surprisingly well. I can run my sandblaster or die grinder continuously while the pump cycles on and off.

You can calculate the actual average CFM of your compressor pretty easily by knowing the volume of the tank and timing how long it takes to go from empty to a full known pressure. It is a rough measurement (does not take into account thermal expansion, non-linear compression ratio, etc.). You can do the same measuring time between certain pressure intervals to calculate your compressor output at different tank pressures.

So if you have a 60gal tank:
60 gal = 8.02 cuft

If we fill this tank from 0psi to 125psi, we increase the pressure:
125 psi = 8.51 atm

So volume in the tank at nominal atmospheric pressure is:
8.02 * 8.51 = 68.25 cuft

If you compressor takes 4min 32sec from 0psi to 125psi, then your average CFM (under load):
68.25 /4:32 = 15.1 CFM

If you take time splits between pressure readings you can calculate output at different tank pressures:
0 - 25 psi @ 50 sec = 16.4 CFM
25 - 50 psi @ 49 sec = 16.7 CFM
50 - 75 psi @ 51 sec = 16.1 CFM
75 - 100 pis @ 56 sec = 14.6 CFM
100 - 125 psi @ 66 sec = 12.4 CFM

The above are my actual calculated numbers for the HF pump, which is pretty good considering it is rated 15.2CFM@90psi, interpolation measured at 15.1CFM, and I am only running it a 90% of the rated shaft speed.

Re: Shop realities

Posted: Mon Nov 05, 2018 6:51 pm
by SteveHGraham
Common sense suggests that 17.5 CFM means you can run the pump without a tank at typical compressor pressure (maybe 100 psi) and get 17.5 CFM without losing steam. I am not a compressor engineer, however.

Your post reminds me why I hate imperial units.

Posted: Tue Nov 06, 2018 2:33 pm
by TRX
Harold_V wrote:
Sat Nov 03, 2018 3:44 am
My lines are reasonably tight, so it doesn't cycle often.
My lines are still airtight 28 years after installation. The chucks always leaked a little air, and still do. Enough that I plugged off several of them that I seldom use. I probably ought to put an elbow, pigtail, and ball valve on each air drop now that ball valves don't cost a fortune.

Posted: Tue Nov 06, 2018 2:39 pm
by TRX
John Hasler wrote:
Sat Nov 03, 2018 11:56 am
pinhole leak in the bottom of the tank
Check your local propane place. They may have a collection of expired propane tanks that are no longer lawful to refill. Unscrew the valves, wash them out with soap and water (the odorant builds up inside the tank), then manifold as many of them together as needed for the volume you want. Extra points if you manifold them inverted so any condensation falls out and down to a convenient drain valve in the manifold.

Posted: Tue Nov 06, 2018 2:46 pm
by TRX
liveaboard wrote:
Sat Nov 03, 2018 2:39 am
Long runs of sewer line are tricky with those macerator toilet pumps; I've installed many of them. The check valves always leak, so the pump cycles from time to time. if the power is off, bad things can happen.
Despite being a fairly established product - I remember ads in Popular Mechanics etc. for them back to the early 1960s - the consumer reviews on those things are *terrible.* That's one of the few times Amazon's review system has actually been useful for me. There being no products on the market that even met casual definitions of "reliable", I concentrated on what the failure modes were. Mostly the computerized control boards (for a toilet?!) and float sensors. Pump and motor failures didn't seem to be a big problem.

After consideration and downloading the manuals for several products, I figured if the electronic doodaddery died, wiring a simple switch and listening for the pump sucking wind would work OK if the electronics failed.

Re: Shop realities

Posted: Tue Nov 06, 2018 3:30 pm
by John Hasler
TRX wrote:
Tue Nov 06, 2018 2:39 pm
John Hasler wrote:
Sat Nov 03, 2018 11:56 am
pinhole leak in the bottom of the tank
Check your local propane place. They may have a collection of expired propane tanks that are no longer lawful to refill. Unscrew the valves, wash them out with soap and water (the odorant builds up inside the tank), then manifold as many of them together as needed for the volume you want. Extra points if you manifold them inverted so any condensation falls out and down to a convenient drain valve in the manifold.
Good idea (though they might not be willing to sell intact expired tanks). For now, though, I'll stick with the one 60 gal tank. This project has already chewed up too much time. I'm not even going to fabricate a belt guard at this time since the belt will be next to the wall. Making tools to make tools to make tools...

Re:

Posted: Tue Nov 06, 2018 4:35 pm
by Harold_V
TRX wrote:
Tue Nov 06, 2018 2:33 pm
Harold_V wrote:
Sat Nov 03, 2018 3:44 am
My lines are reasonably tight, so it doesn't cycle often.
The chucks always leaked a little air, and still do.
I use Milton almost exclusively. However, I rebuilt each of them (don't have a count, but it's more than 20), replacing the original poppet with a new one made of stainless, and I also made new rubber seats. None of mine leak. The upgrade I performed worked exactly as I hoped.

H

Re:

Posted: Tue Nov 06, 2018 5:09 pm
by liveaboard
TRX wrote:
Tue Nov 06, 2018 2:46 pm
There being no products on the market that even met casual definitions of "reliable", I concentrated on what the failure modes were. Mostly the computerized control boards (for a toilet?!) and float sensors. Pump and motor failures didn't seem to be a big problem.
I've built them, bought them...
The little cheapo ones that fit behind the toilet and have just a little lift are pretty bad; I always kept a spare unit to swap out.

A few years ago, city regulations required me to fit expensive ones with specific requirements for power and pressure.
I fitted 2 German KSB units, and an Italian Wilo one at another location.
They were VERY expensive.
They have all been working ok; motors are fine, electronics are fine.
All the check valves leak a little. All the pumps cycle at least once an hour to push it back.
I figured someone must have solved the problem by now, so I went to a guy I know who installs and services hundreds of them in Amsterdam.
He told me no one has solved the problem. He's tried them all, they all leak back.
It's the toilet paper. It does weird things you don't expect.

Re: Shop realities

Posted: Tue Nov 06, 2018 6:59 pm
by wlw-19958
Hi There,

I don't see that anyone mentioned anything
about single stage vs two stage air compressors.

Good Luck!
-Blue Chips-
Webb