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Working on a two-stage compressor with my new fridgy. Turns out its 3 or 4 times stronger than my old one, going by the strength of the air stream coming out of the output. So I rigged up the old compressor's output to the new one's input and now all I have to do i epoxy a fitting on the end of the second outlet. I'm expecting it to fill my SCUBA tank to 300 psi in less than 5 minutes.
What do you guys think about pressurizing the first stage to about 15-30 psi with my shop compressor? Would that make the pressure too high by the time it gets to the second fridge compressor?
Awesome. I have an old HF20 tank laying around here which I started to mess around with for a portable air source, possibly for MFIC actuation. Good stuff!
I'd say that's an okay idea. I don't think that'd be a problem -- are you planning on using a pressure switch at all?
No, just gonna watch he gauge and switch it off myself. I'd imagine pressure switches get pretty expensive up in the 300-400 psi range.
So my dilemma now is whether my stage-1 compressor is even going to deliver enough air to stage-2 for it even to be worth it. But if I pressurize the inlet of stage-1 it might be feeding stage-2 too much air... Only way to tell is install a pressure gauge between the two stages and monitor the air usage. Then I guess I could fine-tune it.
That's what I'd do honestly. I really don't know what effects certain air delivery rates would have. There's one way to find out.
I want to be able to use this without my shop compressor so it would be good if my stage-1 could retain some pressure in the stage-2 input.
Knowing the normal range of pressure a fridge compressor deals with is the first step. Below is a pressure gauge from a service tech's manifold.
On it are several scales. Note the temperature scale. Think fridge. What temp do they run?
From temp, you find Pressure.. Say the freezer in the fridge runs 0F you can see the pressure for the refrigerant at that temp. This would be just fine for a pressure to feed into a compressor. Now the hot side.. The warm side typically will run about 20F hotter than the room to give off heat. so a warm room at 80F would have a condensor pressure relating to 100F on the gauge. This is a normal high side pressure. Compressors will take some varations such as starting with the fridge at room temp instead of cold inside etc.
On this gauge 0F is about 50 PSI for this refrigerant. 100F is about 300 PSI, so feeding the compressor 50 PSI in to get 300 out is normal operation for a fridge compressor.
This is a low side gauge why the 100F is in the danger zone for inlet pressure.
Here is a high side gauge showing higher temp with higher pressure.
This shows temps for 3 older refrigerants. R12 is obsolete.
If you work with absolute pressure, volume scales with inlet pressure. 1ATM provides vol 1, 2 ATM provides vol X2.
So ~15 psi doubles volume, 30 triples it and 45 is about 4X. Note higher inlet pressure increases compressor load, not reduce it as some think. This is why compressors burn out on AC units on the hottest days of the year.
The unloader valve is much like a tire valve that is hit by the pressure switch.
Last edited by Technician1002 on Thu Aug 08, 2013 11:22 am, edited 2 times in total.
Well that makes more sense. I forgot about the temperature variances. My wife's A/C Compressor went out on her car years backl and I remember checking the high/low pressures. Now I know why; thanks Technician.
That's perfect. I usually feed mine 30-40 psi to get 300 psi or 350 max, so I shouldn't be straining my compressor at all. I just need to figure out how much stage-1 is feeding it now which I will probably do today.
@Solar- I've looked for high pressure pop-off valves and they all cost several hundred dollars which is way out of my budget. The only thing I could do it replace the burst disk on my two aluminum cylinders that I will be filling with material that is soft enough to blow around 500 psi.
In addition to a relief device, a pressure switch can be used to shut it off when the desired pressure is reached.
Just remember that the pressure will need to be bled off before attempting to re=start the compressor. Many won't start with high pressure on the outlet. Most fridges will bleed the high pressure into the low pressure side while the compressor is cycled off so it can restart. This is why fridges don't like a power interruption as they may try to start too soon while the high side is still under high pressure.
Normal air compressors us an unloading valve that releases pressure from the pipe between the compressor and tank so they can restart with some pressure in the tank.
The unloader valve is on the pressure switch and works with the tank check valve to release the pressure between the compressor and tank when the compressor shuts off. This is the short hiss of air head when an air compressor shuts off.
Two air lines connect to a typical pressure switch, one measures the pressure and the other is the unloader valve which releases head pressure between the compressor and tank check valve.
Thanks for the info on the unloader valve. Now that you say that, I understand why my compressor hisses when I turn it off. Cool
The picture you edited into your last post is certainly some food for thought. Gives me a good idea for a homemade pressure relief valve and pressure switch. Didn't see that picture until just now!
I always bleed my outlet before restarting. Lately I've just been using a braided SS line to connect to the half inch outlet on my scuba tank, so when I take the hose off it exhausts itself. Previously, I used a quick connect coupler and just pushed the moving part inside to bleed the outlet.
However I will probably incorporate a check valve into my compressor so that even when I connect it to a tank with high pressure it will start with no problems.
Today I found out that my new fridge compressor takes in air faster than my littler stage-1 compressor can feed it. So there goes the two-stage compressor idea. Looks like I'll be using my compressors separately with pressurized inputs. Using them in parallel was my original plan but upon realizing that the other compressor was so much stronger I figured it wouldn't be worth the extra effort of plumbing them together with check valves and all the other parts I'd need.
It previously took me 7 minutes to fill my scuba tank to 250 psi. Today we will find out how long it takes with my new fridgy. Hoping that its less than 4 minutes!
With a ~25 psi input, it took just over 4 minutes to get about 250 psi. It's hard to tell the pressure though because my gauge goes from 0-3000 and between 0 and 500 there are no graduations. Considering I normally feed my other compressor's inlet 40 psi, I think it's safe to say my new compressor is about twice as strong/fills twice as fast as my old one.
Well I didn't get this stuff for free but I may as well have.
Anybody care to guess how old this oxygen cylinder is?
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