SpudFiles

Alot of starting problems with any kind of pumps is that there is poor to no presure relieve on the piston/s and valves. Thats why common aircompressors give off a hiss after they stop running. There is a relieve valve built into the compressor pump letting the presure off/out.

For those of you who might not have known that......

@jrrdw yeah but there are no pressure relief valves in firdge compessors so one would have to add a solenoid valve hooked up with pressure switch and a timer to vent the air before and while the compressors are starting

AFAIK some pressure switches have inbuild unloaders but I've never seen a pressure switch rated to more than 180 psi


@fullmetajacket
There is a simple way to do it without having to add pressure switches and unloaders... what is more, there wouldn't be any problems with excesive current when they are starting because you could turn them on one by one...

To accomplish this you can have them running continously - and have a pressure relief valve to limit the pressure.....

of course for safety reasons it would be good to have at least two pop off valves... just in case one breaks down

i really want a fridge compressor, and i found a site that sells them new but they are like £80+ and i can't find a source anywhere else

I wonder if you could limit the peak pressure of an this kind of compressor with a needle valve on the inlet? No air flow at inlet = outlet pressure stops rising.

Isn't that how the pressure is regulated in a frig?

@ jimmy - I don't think it's the best idea... it'll just limit the flow and put much greater load on the electric motor and all the parts

POLAND_SPUD wrote:@ jimmy - I don't think it's the best idea... it'll just limit the flow and put much greater load on the electric motor and all the parts

Then how do these compressor limit pressure when they are installed in a frig? Many high pressure compressors operate just fine at their bank-off pressure. (Same with high vacuum pumps.) Limiting, or stopping, the inlet flow just uses the inlet / outlet pressure differential to limit the output pressure.

I assume these are piston based pumps. The inlet will get pumped down to near zero PSIA, the piston will require very little force to compress the gas charge so the load on the motor should go to just about zero. You do loose whatever cooling the inlet air provides to the cylinder.

I've never fiddled with one of these compressors but based on other peoples post it sounds like they reach a pretty impressive outlet pressure. Is the frig designed to handle that outlet pressure? All that metal tubing on the back is strong enough for the max pressure of the pump? Or is there some type of pressure limit? A pop-off would work as a safety to keep the thing from exploding but it would disable the frig if it ever triped. Is there a pressure switch? Or do these compressors just use the inlet to outlet pressure differential control the maximum system pressure?

the amount of refrigerant is limited becasue it is a closed circut... therefore the pressure can't increase indefinately...

AFAIK most fridges run at relatively low pressures (as low as 30 psi) I am not complettely sure if I am right but the fact that fridges have no problems with starting up supports this..


limiting inlet air limits the flow as well so I don't see any point in doing so

POLAND_SPUD wrote:the amount of refrigerant is limited becasue it is a closed circut... therefore the pressure can't increase indefinately...

AFAIK most fridges run at relatively low pressures (as low as 30 psi) I am not complettely sure if I am right but the fact that fridges have no problems with starting up supports this..


limiting inlet air limits the flow as well so I don't see any point in doing so

A closed system can indeed increase pressure indefinately, all you need is sufficient pressure drop through the system. Hydraulic systems are closed circuits and can get to crazy high pressures. Besides, fridges and AC units do have a flow restrictor in the loop, it's the expansion valve. I wonder, could you replace the expansion valve with a regulator and feed the low side of the reg back to the pumps inlet? You would probably need a second regulator to control the inlet of more air if you are actually taking compressed air out of the system.

Good point about how easy the the frig compressors startup, there must be relatively little flow restriction around the loop and through the expansion valve.

Limiting the inlet flow will limit the outlet pressure. If you installed a valve on the inlet and a gauge on the outlet you could just close the inlet when the desired outlet pressure is reached. The pump could be left running and the outlet pressure wouldn't rise. Now if you could figure someway to control the inlet valve automatically based on the outlet pressure ...

Generic AC system with expansion valve:
http://en.wikipedia.org/wiki/Vapor-comp ... rigeration
AC system with temperature dependent expansion valve:
http://www.hvacmechanic.com/txv.htm

@ jimmy - fluids can't be compressed easily - that's why you can get 'crazy pressures' in hydraulic systems.. but it's different with gasses...

you see if we assume that the volume on one side of the expansion valve is the same as on the other then the max pressure you can get on one side of the valve is

2 * (amount of refrigerant in the system/the volume {max capacity} of the system)



thx for the links - I've seen that wikipedia article, because I am trying to do my reaserch when discussing, but the other one is great... as you can see in one of the pictures the pressure is as high as 85 psi so it's not something to worry about - especially given the fact that those thin copper pipes can take thousands of PSI IIRC

Limiting the inlet flow will limit the outlet pressure

well I am not sure about that - I am sure that it will limit flow... but since a needle valve is not a regulator it'll be just a restriction to the flow

I assume that the max pressure would be reached anyway but it would take more time... (just imagine there are two huge air tanks with a needle valve in between.... after some time the pressure would equalise... so I assume that there is hte same logic here - since pressure s not regulated then only the flow would be lowered...) and you have to keep in mind that limiting the flow is not something you normally do on compressors - ussually you want max flow/output possible

of course if you close the air inlet the pressure won't rise.. but all in all it would be easier to have the compressor running continously and limit the max pressure with a pop off vavle/valves... your way of doing this is not any more complicated than doing it in the proper way from the very beginig - that is to have a pressure switch to turn the thing on/off when needed and an unloader to help with starting up

ALIHISGREAT wrote:i really want a fridge compressor, and i found a site that sells them new but they are like £80+ and i can't find a source anywhere else

Then just go down to a scrap yard, rip one out and make your own.

POLAND_SPUD wrote:@ jimmy - fluids can't be compressed easily - that's why you can get 'crazy pressures' in hydraulic systems.. but it's different with gasses...

No, that is not true. The maximum pressure you can obtain with a compressor is basically independent of what the working fluid is. Maximum pressure is limited only by the maximum force on the piston and piston area.

It's the work stored in the working fluid that is vastly different between a gas and a liquid, not the max pressure.

Of course, with a gas system you have the complication of the possibility of the gas condensing, but still, the max pressure is based on the piston and power source, not the nature of the fluid. A gas compressor also has some heating issues but those can be fixed with proper heat sinking.

A compressed air system can be built to match the max pressure of any liquid based hydraulic system. Indeed, to a first approximation, taking a hydraulic system and operating it with air as the fluid will give a system with the same basic max pressure limit. The reverse is also true, put oil in an air compressor and it'll still basically work, it'll just reach maximum pressure in a fraction of a stroke insted of requiring the thousands of strokes it would have taken with air.

@jimmy
sorry but you are missing my point....

I'll try to exlain it better... the amount of refrigerant is limited... if you have only 1" X 10" of air at 1 atm then you can compress it to 1" by 5" at 2 atm... 1" by 2.5" at 4 atm etc.
since the volume of one side of the expansion valve is constant (I am now talking about the side where pressure builds up) then it can't rise indefienatelly...

it does not matter how powerful is the pump.... if you want to compress refrigerant you need some refrigerant.... if the volume capacity on both sides of the expansion valve is X then you can pump output side to twice the starting pressure... and you'll end up with ideal vacuume on the other side...

no refrigerant to pump = no pressure increase

if the pressure increased indefinatelly it would mean that the compressor is producing refrigerant... since it's not possible and the amount of refrigerant and volume capacity of the whole system is constant you can't increase pressure indefinatelly

I have no idea how to explain it in a simpler way but for me it's obvious...

Poland:

Yes, I understand, the maximum pressure in a closed system is basically the ratio of the total system volume over the volume beteeen the pump cylinder and the expansion valve. If that second volume is 1/10 the total volume then the max pressure is 10X the starting pressure (ignoring heat affects).

Now add a needle valve into the low pressure side of the loop. Close the needle and start the system. The pressure rises to whatever the volume ratios is. The system is self regulating in terms of the max pressure. Now crack the needle valve. The pressure starts to rise again since you are adding working fluid to the system. The pressure will continue to rise until you reach the force limit of the piston and the motor stalls (or something blows up because you've exceeded the pressure rating). If you close the needle before max pressure the system's pressure will stop rising.

This should work as long as the low pressure side is less than ambient pressure. The pressure of the low pressure side can be controlled by the size of the expansion orifice or by replacing the expansion orifice with a regulator.

You could also put the needle valve in the expansion orifice. The high velocity flow through the orifice would act as an aspirator and suck air in through the needle valve. The inlet air would move in a direction opossite of what you would predict based on the pressure in the orifice and ambient pressure.

Edit: The pump has another pressure limit, besides the total and high pressure volumes. The pump can't exceed the pressure differential defined by the cylinder's compression ratio (again, ignoring heating affects). If the compression ratio is 10:1 then the max pressure increase across the pump is 10X regardless of what the other volumes are.

uhmm... I am sure that what I am saying is logical...

assuming it's a closed circut (a fridge) when you close the needle valve you'll end up with a complete vacuume on inlet side.. so there is nothing to be pumped.... therefore there won't be any pressure increase above some point...

when you open the needle valve then whatever will be pumped to the high pressure side comes from (surprise surprise!!) high pressures side... so as you pump refrigerant to the high pressure side you remove the same amount of it from there... so there is no pressure increase

if we assume that it's not a closed circut (so a firdge pump is being used to pump stuff) then you can't limit output pressure with the use of a flow restricting valve... only a regulator would work...

but it won't make much sense to do so... the pump would stop when it reaches it's max output pressure (pressure differential between pressure at the inlet {which is the same as the pressure acting on 'the other' side of the piston} and the pressure of air on the 'pumping' side of the piston) and even if that will be lowered due to the fact that the inlet air has lower pressure it won't be good for the pump


now what we are discussing here has one one practical use... it's been discussed here sometime ago... in theory you can increase the max output pressure by supplying the inlet of the compressor with air at pressure higher than atmospheric... (of course assuming that the pressure acting on the other {'not pumping'} side of the piston depends on the pressure at the inlet)

EDIT
Am I right or am I missing something obvious ?? any comments?

Ah, I think I see where we are miscommunicating.

What I'm talking about is a closed loop frig system that still has it's expansion valve (venturi, needle valve, whatever) installed. Then add an extra needle valve to the low pressure side.

You close the added external needle valve.
You start the pump.
The high pressure side rises and the low pressure side drops.
The actual final pressures depend on the expansion valve (venturi, needle valve, whatever) and,
the maximum force and piston area of the pump and,
the system volume, mass of gas, etc.

You now open the needle valve (that wasn't part of the orig frig setup) that was added to the low pressure part of the loop. If the low pressure side of the loop is below ambient pressure then you'll suck air into the system.

Since you've increased the mass in the system you can get the max pressure of the hide side higher than it was before.

Alternatively, if the low pressure side of the system is always above ambient pressure then you can't suck air into the system. So the added needle valve won't work. It seems silly to add another pump to pump air into the frig pump so ...

If the expansion valve is a venturi (narrowing pipe diameter) you could drill a port into the venturi. When gas is moving through the venturi at high velocity the pressure in the venturi drops. This is how an aspirator works. An aspirator appears to pump from low pressure to high pressure (it actually doesn't but it looks like it does). So, the aspirator can suck air into the system even if the system pressure is greater than atmospheric pressure. This gives you a way to increase the mass of gas in the closed loop.

As you said though, this probably isn't all that useful. We really want the low side open to the atmosphere, the high side connected to a resevoir. But I believe we got started on this trying to figure out other ways of keeping the frig compressor or pipes, or resevoir, from overpressurizing. Obvious solutions include;
1. monitor the pressure and shut off the pump manually
2. install a pressure switch just like a regular shop compressor uses, when the pressure exceeds a limit the pump is shutoff
3. install a pop-off and just let the system pump and vent

or
4. Figure out a way to recycle from the high pressure to the low pressure side when the pressure limit is reached. Instead of the low pressure side sucking in fresh air the regulator switches over to pass gas from the high pressure side back to the low pressure side. The system is now self-limiting and is just cirulating the gas around the loop. If we take compressed air out of the storage tank (the high pressure side of the loop) then the regulator has to switch the air source to the pump from the high pressure side of the loop to atmospheric.

BTW, frig (and other cooling systems) compressors are working against 0 PSIG at startup. When a frig compressor (still installed in the frig) shutoffs there is a pressure difference between the high and low pressure side. But the pressure equilizes pretty quickly as the gases bleed through the expansion valve. Within a few minutes the high and low pressure sides of the loop are at the same pressure. In some frig's you can even hear this happening. The frig will sort of hiss for a while after the compressor shuts off. When the compressor restarts there is zero pressure differential across the pump. Even though there is no pressure differential most frigs have a big ass capacitor wired to the motor to give the motor a bit extra kick at startup.