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two fridge compressors + tanks for 1000 euro

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two fridge compressors + tanks for 1000 euro

Unread postAuthor: POLAND_SPUD » Thu May 14, 2009 2:13 pm

used compressor with two fridge pumps and goodies for ~ 1000 euro

new one costs 1590 euro

@BTB and psycix maybe we should start manufacturing them ?? :wink:

notice that there is no air cooling (at least I can't find it in the pics) and that the shape of the 'fridge' cases is designed to help in heat dissipation

EDIT
ohh I've found their web page -> clisky notice that in PDFs there are nice graphs showing how flow changes as the pressure rises

I wonder if their customers know that how they got ripped off....
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Unread postAuthor: Hotwired » Thu May 14, 2009 2:42 pm

Tsk tsk, being ripped off is subjective.

From the sellers point of view, they're buying in components, assembling the system and billing for their time and expenses.

If you sell at the cost of materials then what you're doing is basically making things for free and not earning anything for the time you spend.

Do you work for free?
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Unread postAuthor: Biopyro » Thu May 14, 2009 2:52 pm

Hot damn! That looks nice and neat. 1500 euro's is pretty steep, but I suppose it depends how long it took to make.

To me the BL-520 seems like a rip off (I know it isn't) but that's because I can make something which does the same job (loosely) for less.

Looks like a profitable enterprise though, especially if you can get hold of the pumps after they have been legally emptied of CFCs.

As hotwired said, it's subjective though.

Those datasheets seem to suggest the pressure is only good to 120psi!? I get the impression that price tag is because of their 30dB volume.
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Unread postAuthor: POLAND_SPUD » Thu May 14, 2009 2:58 pm

@hotwired
Do you work for free?
I work as a teacher in middle school... so yeah... almost for free :?

notice that they use off the shelf parts - so apart from the frame (which is cheap to make) it's just assembled... have a look at other models as well - some of them are as simple as my first fridge compressor was..


besides the point of this thread is not 'ohh look I could make it 10 times cheaper' but 'if you have any doubts that fridge compressors are no good look at this'

Those datasheets seem to suggest the pressure is only good to 120psi!?
most people don't need more than that becasue most tools run at circa 10 bar... besides most of the parts used are rated by their manufacturers for 10 bar... notice that there are pop-offs so they are most likely set to 12 bar or so...
obviously if there are almost no tools that need more than 10 bar there is no point in spending more cash on parts rated to that pressure
I get the impression that price tag is because of their 30dB volume.

economy is governed by supply and demand.. if you produce something that's difficult to get (like silent compressors) then you can overprice it

quite a lot of things are expensive as hell but manufacturing them is cheap there are lots of examples - since we're building spudguns let's use potatoes for this comparison -> if a farmer sells X kg of potatoes for 10 euros they will cost 20 euro when bought at the supermarket

since these days most people live in big cities they can't get potatoes directly from farmers... so I think that most of them don't even know how cheap they could buy them if they bought them directly from the farmers... sure transport and stuff costs but sometimes you can rip-off people simply becasue you know you can...

lol let's be honest biopyro - if there there were people ready to pay 1000 euro for a basic combustion (they don't know they can build it cheaper and that they can build them themselves) would you tell them that you want only 100 euro for each launcher ??
:wink:
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Unread postAuthor: Technician1002 » Thu May 14, 2009 3:40 pm

Since we are talking about compressors, I have a tidbit for those who want massive flow at high pressure, well sorta massive flow anyway..

Due to the displacement of the fridge compressor being very small to work with higher pressure and atmospheric pressure is below the normal pressure of the compressor, they are not very efficient and pump quite slow. If you feed a fridge compressor at it's designed inlet pressure, it will output the designed volume at the designed outlet pressure.

If we work from absolute pressures, we can see how to cascade compressors. To make the math easy, I am calling 1 atm 1 unit volume of air in a given volume. A fridge compressor will then take in 1 unit by itself with no boost. Assuming a fridge has an icebox running at about +10 F, the freon (R143a) boils at about 10 PSI or slightly over 1 atm. The freon condenses at above room temperature at a higher pressure often in the 100 to 120 degree range for design outlet pressure of up to 150 PSI.

If we ditch the freezer box and worked with a fridge designed to never make ice you get into air conditioning compressors. Assuming they boil freon on the evaporator (the cold side) at 25 degrees F and the tubing remains above the frost point, we would want an inlet pressure in the range of near 30PSI or +2 units. Note, if the compressor takes in twice as much on each stroke, it delivers twice as much. :D Half the pumping time or twice the flow for a BBMG. On the down side, the compressor is now working twice as hard to deliver the output. Keep this in mind. Raising the inlet pressure increases the load.

An advantage of an AC compressor is they are not designed to condense the freon at room temperature :D They are designed to dump the heat outside and condense it at outside temperatures. Woo hoo. High pressure baby.

This high pressure, high volume heat pump is why most air conditioners draw way more than a refrigerator. They are bigger!

Back to cascading, several fridge compressors can be used in parallel to feed a single AC compressor's input. I am not sure how many would be needed, but if the fridge compressors were the same displacement as a AC compressor, then 2 compressors in parallel will provide 2 units of air per stroke, but at 2 atm pressure the space occupied would be 1 volume and the A/C compressor would take that volume in each stroke. This would only be a boost of 15 PSI or 1 bar. To boost to 30 PSI or 2 bar, 3 compressors are needed, 3 bar or 4 atm would need 4, etc.

I am assuming an AC compressor being a bigger beast would have a larger displacement, so I would assume that more fridge compressors could be paralleled to feed one A/C compressor. If you do this, then serious considerations need to be given to the electrical requirements.
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Unread postAuthor: POLAND_SPUD » Thu May 14, 2009 4:17 pm

On the down side, the compressor is now working twice as hard to deliver the output. Keep this in mind. Raising the inlet pressure increases the load.


well I don't think it increases the load... I am not an engineer so I had a hard time explaining people that there is a possibility to 'cascade compressors'

I used a simple example to do this - if you used your 300 psi shock pump on a planet with 1/10 the pressure on earth then you won't get past 300 psi as easily as you can on earth - if you could - then it would mean that you get something out off nothing

that's just an example but it helps to realize this as most people who hear about my idea immediately claim that 'if you provide pressurised air to the air inlet then it would be harder to pump to the same pressure not easier' (sorry tech I couldn't resist :D )

so in other words IMO it works like this:

if the compressor can use atmospheric air (so air at 1atm.) and pump it up to 20 bar then I assume that if was feed with 0.1 atm then it could pump it up to twenty times that pressure - that is - to 2 bar using the same amount of energy as it used before...

the idea is that you can do it in the other way round that is that the compressor can pump X * 20 pressure so if you feed it with 10 bar you could get 10 * 20 = 200 bar or something like that

generally I am not a big fan of AC compressors (as I can't get them :wink:) so I think that it would be easier to use normal compressors to provide pressurised air - if you think about it even a setup with just one fridge compressor could pump more air at higher pressures than most users could safely use (as it's rather diffiult to get cheap parts rated specifically for 600 psi or so)
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Unread postAuthor: Technician1002 » Thu May 14, 2009 4:30 pm

POLAND_SPUD wrote:
On the down side, the compressor is now working twice as hard to deliver the output. Keep this in mind. Raising the inlet pressure increases the load.


well I don't think it increases the load...

I used a simple example to do this - if you used your 300 psi shock pump on a planet with 1/10 the pressure on earth then you won't get past 300 psi as easily as you can on earth - if you could - then it would mean that you get something out off nothing

that's just an example but it helps to realize this as most people who hear about my idea immediately claim that 'if you provide pressurised air to the air inlet then it would be harder to pump to the same pressure not easier' (sorry tech I couldn't resist :D )

so in other words IMO it works like this:

if the compressor can use atmospheric air (so air at 1atm.) and pump it up to 20 bar then I assume that if was feed with 0.1 atm then it could pump it up to twenty times that pressure - that is - to 2 bar using the same amount of energy as it used before...

the idea is that you can do it in the other way round that is that the compressor can pump X * 20 pressure so if you feed it with 10 bar you could get 10 * 20 = 200 bar or something like that

generally I am not a big fan of AC compressors (as I can't get them :wink:) so I think that it would be easier to use normal compressors to provide pressurised air - if you think about it even a setup with just one fridge compressor could pump more air at higher pressures than most users could safely use (as it's rather diffiult to get cheap parts rated specifically for 600 psi or so)


The energy boost and pressure boost work for fluids. Compressed gasses is another animal.

Take your hypothetical shock pump. Make it a vane type so there is no dead space at the output. Put a big crank and flywheel on it like a compressor. Feed it 1 atm and let it rip to 20 bar. Lots of work required. Now feed it 0.1 bar. (assume sealed container if it helps) and look at the force needed on the crank at 1/2 stroke. 1 bar input would be at 2 bar. The 0.1 bar would be at 0.2 bar.

Do the same for the 9/10ths stroke. That's 9 bar and 0.9 bar. See any force difference? If the piston can go all the way to zero volume, both can deliver at 20 bar. (remember the vane pump).

Now do the same for an inlet of 2 bar. 1/2 stroke is 4 bar. 9/10 stroke is 18 bar.

The above that I wrote is how to feed 2-3 bar to an A/C compressor to fully load it to capacity.

I hope this helps.
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Unread postAuthor: POLAND_SPUD » Thu May 14, 2009 5:45 pm

what you don't take into account is the fact that in any pump atmospheric pressure is not only feed to the air inlet but it also acts on the other side/surface of the piston

since fridge compressors are housed inside sealed units you can increase the rpessure acting on the other side of the piston by providing pressurised air to the air inlet...

let's forget about no deadspace pumps and assume that air gets compressed 10 times.... assuming that there is no deadspace only makes things harder to understand and fridge compressors do have deadspace

at 9/10th of the stroke there would be the same compression ratio but for 1 atm input it would be 9 atm and for 0.1atm it would be 0.9 atm

now add forces acting on the other side of the piston
in the first situation it would be 1 atm vs 9 atm - so the force (pressure * surface) differential is 8
in the second situation it would be 0.1 atm vs 0.9 atm - so this time it's only 0.8

so IMO it won't be easier to pump to higher pressure from lower pressure as it seems illogical... I doubt I can provide a good equitation for load since as I said I am not an engineer
it would be easier if some one could clarify this as it really seems illogical for me that one can achieve the same pressure easier (with less force) if he starts from lower pressure
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Unread postAuthor: Technician1002 » Thu May 14, 2009 6:23 pm

POLAND_SPUD wrote:what you don't take into account is the fact that in any pump atmospheric pressure is not only feed to the air inlet but it also acts on the other side/surface of the piston

since fridge compressors are housed inside sealed units you can increase the rpessure acting on the other side of the piston by providing pressurised air to the air inlet...

let's forget about no deadspace pumps and assume that air gets compressed 10 times.... assuming that there is no deadspace only makes things harder to understand and fridge compressors do have deadspace

at 9/10th of the stroke there would be the same compression ratio but for 1 atm input it would be 9 atm and for 0.1atm it would be 0.9 atm

now add forces acting on the other side of the piston
in the first situation it would be 1 atm vs 9 atm - so the force (pressure * surface) differential is 8
in the second situation it would be 0.1 atm vs 0.9 atm - so this time it's only 0.8

so IMO it won't be easier to pump to higher pressure from lower pressure as it seems illogical... I doubt I can provide a good equitation for load since as I said I am not an engineer but I think it would be something like this:
F=(Y/X - X) * surface area of the piston
where X is inlet pressure and Y output pressure

it would be easier if some one could clarify this as it really seems illogical for me that one can achieve the same pressure easier (with less force) if he starts from lower pressure


Good catch. However in the last part;

it would be easier if some one could clarify this as it really seems illogical for me that one can achieve the same pressure easier (with less force) if he starts from lower pressure


The high force is only present in the very last 1/20th of the stroke when the pressure is high enough to exit the check valve. All the rest of the 95% of the stroke, the force is less with lower inlet pressure.

Try it.

1 atm 0 bar. discharge 20 bar. assume no dead space (scroll or vane compressor) 50% stroke 1 bar in pressure 1 bar on crank. End of stroke has a short high pressure spike of 19 bar differential

2 atm 1 bar 50% stroke 4 atm or 3 bar, crank pressure is differential of 2 atm outside and 4 atm inside or 2 bar. End of stroke spike is longer but not as high with a 18 bar differential

4 atm 3 bar 50% stroke is 8 atm inside and 4 atm outside or 4 atm differential. End of stroke spike is even longer and not has severe with only 16 bar differential.

See the trend in power to reach 20 bar where the check valve opens to the storage tank? You are correct in noting the end of stroke force is less, but this is a short portion of the compression cycle. In fluids, the entire stroke is at the discharge pressure so the commonly assumed higher inlet is easier applies. There is no compression work in pumping fluids. (assuming zero compressibility of liquids for this example)

As you approach the tank pressure on the inlet, then yes, the head pressure provides less end of stroke resistance and the end of stroke (venting to tank) time is longer. Where the power requirement actually falls instead of climbing I do not know.

so IMO it won't be easier to pump to higher pressure from lower pressure as it seems illogical...


The end of stroke where the 20 bar exits the check valve, the force is greater at lower inlet pressure. The power used for the entire stroke is much less. Don't confuse pressure for power.
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Unread postAuthor: POLAND_SPUD » Thu May 14, 2009 7:00 pm

my equation is obviously wrong but still I can't believe that you can get 20 bar easier if you start at lower pressure

apparently it's not that you can get 10 times the pressure at the air inlet with the use of the same amount of work... well it would be pretty strange if you could get 200 bar with just a fridge compressor

but I think it seems logical that it's easier if you start at higher pressure but I don't know how much easier

Just think about it... why there are multiple stage pumps if you could get the same output pressure with just one stage?
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Unread postAuthor: Technician1002 » Fri May 15, 2009 12:15 am

POLAND_SPUD wrote:my equation is obviously wrong but still I can't believe that you can get 20 bar easier if you start at lower pressure

apparently it's not that you can get 10 times the pressure at the air inlet with the use of the same amount of work... well it would be pretty strange if you could get 200 bar with just a fridge compressor

but I think it seems logical that it's easier if you start at higher pressure but I don't know how much easier

Just think about it... why there are multiple stage pumps if you could get the same output pressure with just one stage?


Multi stage pumps are so you don't break connecting rods at the end of the stroke pushing a big piston into a really tiny space.

All piston compressors have some dead space that the piston won't fit into between the piston and check valve. Any air here will re-expand filling the space back on the return stroke. If the space is 1/20th the cylinder volume, then the piston will only compress air to 20 X the inlet. If the outlet is at this pressure, then no air flows out the check valve. The air in the dead space re-expands back to 1 bar on the return stroke and no air is taken in the intake valve. In short, there is a limit to how much a compressor can increase the pressure in each stage.

Most compressors have a dead space of about 15 to 20% of the cylinder volume limiting the pressure output to about 15-20 times the inlet where they simply fail to pump anymore.
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