[crownver]

Hi all,

I've been hunting for info on proper pump to tank hook up for a home built compressor, but haven't been able to find exactly what I need.

My compressor started with a small free pump and 110 V motor. Graduated to a new free 2 HP industrial motor, which burned out the pump. I bought an Eagle 2 cylinder pump (I think it's 2 stage how do you tell), and it now performs reasonably well. I am going to put in some extra piping for water removal, and I want to make sure I am getting the best possible from this unit.

One issue is that I have the 1/2" pipe from the pump going to the cut-out switch, and a 3/8 pipe from that to the tank. (the tank is a 100 lb. propane tank). It seems to me I should have 1/2 inch line all the way to the tank, but I've been unable to find the correct way to hook this up. Should I go directly from the pump to the tank with a T to the cut-out switch? Also, I don't have a check valve or unloader valve in place. The pump starts fine without the unloader but maybe I should have one on? Where would I put that? There is a small plastic tube coming out of the cut-out switch that I haven't hooked up to anything. What is this for?

If I could find a schematic of the correct way to hook everthing up that would be ideal. Any suggestions? Thanks!

[delawarebill]

a unloader is something u should have... but that little line from the p/s just might be for that.. but anyways.. i'd put the p/s on the tk so it's not seeing pulse from pump. 2 stage pumps are connected to ea other and only one pipe to the tk from the hi pressure cyl. the only way i can tell is if there is a small air filter on the top of ea cyl then u don't have 2 stage, just a double pumper. there are 2 screw on one contact in the sw. the hot wire is cut and goes on them. having a 3/8" pipe u don't have a big pump.. maybe in the 4cfm area.

[crownver]

I guess I have a single stage then. There's a 1/2" line from 1 cylinder to the other, but there is a filter on each cylinder.

The pump has a 1/2" line outlet. At the moment I have that 1/2" line reduced to 3/8" going to the pressure switch, and a 3/8" line from the pressure switch to the tank because that's the size of the inlet/outlet of the pressure switch. If I run a 1/2" line directly to the tank with a tee going to the pressure switch will that work and will it improve the performance?

The pump is an Eagle PV02A rated at 8.3 cfm at 100psi at 1725rpm with a 5.7" pulley on the motor. I have a 6" pulley on it now. The pump has no trouble starting with the motor I have now. I'd like to find out what RPM the pump is running at but the only way I can think of is with a mechanical tach. and I have no idea where to borrow/rent one.

Electrical motor horsepower ratings confuse me. I know the 2 hp motor has a lot more power than most of the compressors you see at the big box stores. It's a 240V single phase from a commercial restaurant roof fan.

[cyclopsblown34]

Your motor nameplate will have the motor speed on it. Get the diameters of both sheaves/pulleys. if they're the same, the pump is turning motor speed. If the motor sheave is bigger than the pump sheave, it's a speed up drive which means the pump is turning faster than the motor, conversely, if the motor sheave is smaller than the pump sheave, the pump is turning slower than the motor. Divide the motor sheave by the pump sheave diameter and multiply the motor speed by the result of dividing those two, that'll give you the pump speed. An unloader would aid in reducing the load on the motor for starting every time.

[crownver]

The motor is 1725 RPM. The pump pulley is 10.625" and I currently have a 7" pulley on the motor, which makes the pump turning at a little over 1100 RPM. The pump is rated at 12.5 cfm at 90 psi at 1230 rpm, and 8.3 cfm at 925 rpm, so at 1100 rpm I would think the pump should be producing 10-11 cfm at 90 psi, but I don't think it is, because it won't keep up to a die grinder. Doesn't the pump drag the motor RPM down?

Also should I change the 3/8" tube going to the tank? Maybe a bigger motor pulley to increase the rpm with an unloader to ease the start up. I don't want to over rev the pump and screw it up.

[oldred]

A lot of things here so lets start with your pump/motor combo and pump speed. First determine if you have a two stage or a single stage, I believe you said you did not know for sure? Easiest way would be to check the cylinder size on that two cylinder pump, a single stage will have both cylinders the same size while the two stage will have a large piston and a much smaller one, of course there are other major differences but that quick check is easy to describe. 12.5 CFM@90 PSI at 1230 RPM is going to be pushing it for a 2 HP motor even in this case where you have what appears to be a real 2 HP, I think you have already noticed the store compressors seem to overate their power, well most of the time they do. The RPM figures you mention could probably be increased just a bit but it is so close to what your motor should be able to carry it probably would not be worth the effort, in any case you have other issues to address before tinkering with the RPM since it sounds about right to start with. Now a couple of things you HAVE to have that apparently you do not have-first you MUST have a back-flow or check valve installed between the pump and the tank, this MUST be there to prevent air from flowing back from the tank to the pump and even if your pump does not leak while at idle (I bet it does whether you have noticed it or not) the overall performance will surely suffer a great deal, this valve is NOT optional! Next, that small plastic line from the pressure switch is also a necessity and you could easily burn out your motor if you omit this circuit even if all seems well. Since the small plastic line is coming from the pressure switch then it seems you have the right switch with an unloader valve installed and all you will need to do is tee this plastic line into the line from the pump to the tank, this MUST be connected between the pump and back-flow/check valve. When properly plumbed in you will get a momentary "hiss" sound from the pressure switch when you reach shut-off pressure and the motor stops, this is pressure being relived between the pump and the tank and should last only a second or so.The purpose of this is to relieve the pressure on the pump when idle so that the motor does not have start the pump spinning while it is under a load. If you do not do these things and do not install the lines/valves then if the pump and motor pulleys are matched right to provide a decent RPM for proper performance then the motor would be under way too much of a load at restart if tank pressure is allowed to stay on it, this over-load condition at start up will damage the motor even if it seems to be starting normally now.



BTW, Yes you do need to change the line from the pump to the tank to 1/2" or larger.


EDIT, I am a bit confused as I just looked at your earlier post and you are saying the pump is a bit over 8 CFM@100 PSI and 1725 RPM? 1725 sounds kind of high for the pump but which specs are correct 8+ CFM@100 and 1725 RPM or 12+ CFM@ 90 PSI and 1230 RPM, 1230 RPM sounds more like a pump rating.

[crownver]

Thanks guys. Oldred, you have answered most of my questions! Both cylinders are the same so that confirms it is a single stage. Sorry about the confusion on the rating. The motor is a 1725 RPM. The pump is rated at 8.3 cfm with a 2 hp motor with a 5.7" pulley and 12.5 cfm with a 3 hp motor and 7.6" pulley.

The only thing I'm not sure of is: can I run a 1/2" pipe directly from the pump outlet to the tank, and tee in fitting to that line to the pressure switch and block off the other fittings? Or do I need to get a new pressure switch with bigger inlet/outlet?

So, if the above works, I would run a 1/2" pipe from the pump to the tank. Off that 1/2" line right after the pump, I would Tee off a 1/4" to the plastic tube on the pressure switch unloader. Next on that 1/2" I would install a check valve. Between the check valve and the tank I would tee off a 3/8" line to the inlet side of the pressure switch and block of the outlet of the pressure switch. Does that sound right?

[delawarebill]

yes u can and should run 1/2" line from pump to tk.. that 3/8 line is a restriction and will reduce cfm's.the flow in any line is calculated by its smallest ID... u can have a 2" pipe flowing through a 1/4" nipple reducer and the cfm's will be figured on that 1/4" line.. if your pump is 1/2 and the tk is 1/2 then use 1/2 for piping.. if the tk is 3/8 then a 3/8 line is all u need.... bill

[crownver]

Thanks Bill,

I'm off to the Mayan Riviera for 2 weeks today so will re-plumb my system when I get back, and let you know how it goes......John