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Unread postAuthor: POLAND_SPUD » Tue Sep 02, 2008 6:19 am

well if that was the case then it can work.. but for me a close circut stops being one when you let something from or into it....

anyway.. as you said it won't make much sense to do what you have described... it won't be as efficient as a normal setup and there are no practical reasons to use it

so I think it's a good moment to stop discussing partly closed circuts and stuff


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



points 1,2 and 3 are ok... but point 4 is more less the same as the partly closed circut idea... it will cost more than points 1,2 and 3 but it won't be any better than them....

AFAIK there are pop off vavles that have threaded NPT exhaust ports so it won't be that difficult to do... but as you and I agreed earlier this idea is not practical...


all in all it will be much cheaper to get an old co2 FE or scuba tank and use it as an air resorvoir and either manually turn the thing off when needed or add a pop off valve... instead of spending extra $ on low pressure tanks and stuff you can buy more high pressure vessels
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Unread postAuthor: jimmy101 » Tue Sep 02, 2008 12:38 pm

POLAND_SPUD wrote:anyway.. as you said it won't make much sense to do what you have described... it won't be as efficient as a normal setup and there are no practical reasons to use it

so I think it's a good moment to stop discussing partly closed circuts and stuff

The advantage of the "partially closed loop" is that the pump doesn't have to restart. That avoids starting the pump with the outlet pressure already above the inlet pressure. Since frig compressors are not designed to be started under those conditions their ability to do so may be limited. In many frigs the motor is small enough that the startup surge current is too great for the motor to start without assistance. The assistance is from either a capacitor or by a second winding in the motor that is only used at startup.
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Unread postAuthor: POLAND_SPUD » Tue Sep 02, 2008 3:22 pm

@jimmy... yeah but the reason why I said that it is not practical is becasue you can achieve the same result with just a pop off valve... and the compressor don't have to restart it can run continously...

if you can do something in a cheap and easy way there is no need to overcomplicate things if it does not have any advantages over using a pop off valve
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Unread postAuthor: jimmy101 » Tue Sep 02, 2008 3:39 pm

POLAND_SPUD wrote:@jimmy... yeah but the reason why I said that it is not practical is becasue you can achieve the same result with just a pop off valve... and the compressor don't have to restart it can run continously...

if you can do something in a cheap and easy way there is no need to overcomplicate things if it does not have any advantages over using a pop off valve

There are many ways to do things. Some are simple and cheap. Some are not. Some complex ones end up being simple and cheap if the parts are already available.

If you don't like the closed loop idea then ignore it.
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Unread postAuthor: POLAND_SPUD » Tue Sep 02, 2008 5:34 pm

hey I really appreciate the fact that you want to build something new...it's ok to do so... I am not criticizing you... but I am trying to find out all the possible advantages and disadvantages of your idea...

so far I think that your idea:

1. is not any better than a simple setup with a tank and pop off valve rated to desired pressure (there are pop offs rated to higher pressure the only problem is to find them + you can mod a standard pop off valve to open at higher pressres)

2. for the same output presssure a partly closed circut setup will put more load on a compressor

3. partly closed circut idea requires an expansion valve... you can use either a needle valve or a pop off valve - it's easier to find a cheap needle valve rated to fridge compressor pressures than a pop off valve to do the same so that's the main advantage of the idea

4. for safety reasons it would be good to have a pop off valve anyway...

5. you say that the partly closed loop will help to lower the pressure.. but if you want to let atmospheric air into the system you can't really predict the output pressure... at that point you have no guaranty that the pressure won't be the same as the pressure produced by a compressor without a closed loop setup....

6. you have to open/close inlet valve manually... since you have to do that it's not any better than a manually turned on/off setup



again.. I am not criticizing you... but it is not practical... if there is a guy who wants to build a gun with a soda bottle for chamber and pump it up with an unregged HPA tank... people will tell him not to do so...

it's not about showing who is right...
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Unread postAuthor: FULLMETALJACKET » Sat Sep 13, 2008 5:02 am

wow all that made my brain hurt.

to poland_spud:
switch them on one at a time, dang why didn't i think of that. 2 pop offs just in case great idea, will do that. also, using rubber hose as a makeshift pop off pretty slick. i was thinking of using a 1/2npt union and soda can aluminum as a burst disk 4 safety. that will need a muffler.

a while ago surpluscenter.com had programmable pressure switches. i believe they were for hydrolic applications, with a wide psi range. i think 100 to 2000 psi. the wicked part was variable on/off at the touch of a button. figure something like that rigged to a high amp relay would be perfect for multiple fridge pumps. the only downside is they were like 50-100 bucks and i didn't have the scratch.

did some digging and i am certain this is it. they looked exactly like the one pictured at this site. http://www.flow-network.com/digital_pre ... h_pdd.html They are now made by kobold. koboldusa.com 'PDD - Digital Pressure Switch'. searched 'PDD pressure switch' through google shopping and they list @ $330.00. Dang now wish i got one way back then. fastenal.com lists a couple made by smc @ $169.50. Guess i am gonna have to sell/recycle some stuff, in order to get my grubby hands on one of those.

i have a 2 medical oxygen cylinders @~ 1/2 and 1 gallon size. I want to use the larger one for my pressure vessel. just don't want to waste the O2 that's in them, i think their full. might use a 20lb propane tank at a lower pressure. I'll try to get something together soon.
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Unread postAuthor: POLAND_SPUD » Sun Sep 14, 2008 7:39 am

the idea to use a burst disk is great... at least you could replace them easily and select the number of layers to regulate the pressure at which they burst...


pressure switches are pricey - of course you need them to build a proper setup that turns off and on when needed but their cost (at least ~150 $) and the cost of all the wiring, switches & relays needed makes the whole thing really expensive...

IMO opinion it would be better to first make the simplest possible setup running continously with pop-offs... and spend the money you would normally have to spend on a pressure switch and all that stuff on as big and safe pressure vessel/s you can get....

my reasoning here is as follows - to make a safe setup you need pop-offs anyway... and instead of spending money on all the stuff mentioned above and ending up a compressor that turns on/off but has a small reservoir it's better to first buy as large air vessel/s as possible and have it running continously... at least you will have a working setup and you can always add a pressure switch and all that stuff later
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