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Unread postAuthor: roughboy » Tue Apr 28, 2009 7:59 pm

How much is the cheapest, brand new, AC compressor cost and also how much is the cheapest, brand new, fridge cost?

i've googled it and all it shows me is auto compressors and a few AC compressors but without the price.
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Re: compressor

Unread postAuthor: Moonbogg » Tue Apr 28, 2009 8:18 pm

roughboy wrote:How much is the cheapest, brand new, AC compressor cost and also how much is the cheapest, brand new, fridge cost?

i've googled it and all it shows me is auto compressors and a few AC compressors but without the price.


Go to
www.dealextreme.com
see what you find there.
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Unread postAuthor: daberno123 » Tue Apr 28, 2009 8:55 pm

http://www.spudfiles.com/forums/air-conditioner-compressors-on-ebay-t17665.html

There's a link there where I got my A/C compressor for $20, free shipping. Brand ne. :D
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Unread postAuthor: Biopyro » Wed Apr 29, 2009 10:04 am

Why would you want a brand new fridge if you're just going to remove the compressor?
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A/C VS Refrigerator compressor comparisons.

Unread postAuthor: Technician1002 » Wed Apr 29, 2009 12:35 pm

daberno123 wrote:http://www.spudfiles.com/forums/air-conditioner-compressors-on-ebay-t17665.html

There's a link there where I got my A/C compressor for $20, free shipping. Brand ne. :D


Instead of bumping the other thread, I have a comment on a post in that thread on Air conditioning vs Refrigerator compressors since you mentioned an A/C compressor. I do have some refrigeration background.

The post in question;
Does anyone think this is worth it or not? I remember reading in some thread on here that these can pump up to 300 psi when a fridge pump can (as per mfg specs) only pump up to 150 psi. But in fact, as we all know, they can pump a bit more.

So does anyone think an air conditioner compressor can pump higher or does it just have more flow?


The output pressure capability is NOT the big difference although it is a factor. The type of refrigerant is not the big difference, but it plays an important role also.

A little background. A skinny bicycle pump pumps less volume but may reach higher pressure with the same effort. A fat bicycle pump can pump a higher volume at lower pressure with the same effort. Effort and the pump displacement is the differences between an A/C pump and refrigerator pump.

Toss out the differences in refrigerants used at the moment and follow along. Pretend for a moment that they both use the same refrigerant. It will make this easier.

Using the bicycle example above, the expected result is matched well to the expected engine, your arms. A compressor is no different.

If the two applications use the same refrigerant and both are dumping heat at the same temperature, the output pressure is the same in both systems. The pumps are NOT the same. The difference is the amount of gas compressed on each stroke and the pressure increase. Let's compare.

Refrigerator with an ice box. Low temperature. Fairly high pressure differential. Maybe needs a small displacement like a shock pump.. Wrong.. Has a big displacement because it gets little gas to compress on each stroke.

A/C compressor, hot side the same. Cold side much warmer, don't want to make ice and frost it up and block the air. Gas feeds into the pump at much higher pressure so the displacement must be less to not stall the motor.

For the same horsepower a Refrigerator/Freezer pump has a higher displacement pump!

Now the BTU ability, cooling a small insulated box doesn't take much power. Umm Small compressor. A/C cooling a huge box full of heaters, warm people, sunshine in windows, more leaky door seals, bathroom fans, etc. In short A/C pumps tend to be huge in comparison to a fridge.

To keep pipe sizes manageable and pressure in reason, higher temp (A/C) uses lower pressure high volume refrigerant. A freezer may have trouble pulling the low pressure needed to boil enough freon at low temperature, so a higher pressure refrigerant is used so it will boil and provide enough pressure for the compressor to grab it.

With the other refrigerant, the pressures on the high side in a fridge tend to be higher than in an A/C unit. Due to the low capacity of a fridge (Small motor) they tend to have a fairly low displacement pump.

A/C compressors on the other hand working at about the same pressures due to the other refrigerant, need to shove a high volume and have a big motor and large displacement.

How to choose? Fridge compressors are low power and push small volumes to high pressure. It is unlikely you will stall one.

A/C compressors have a big motor and large compressor. The inlet pressure at atmospheric pressure is less then they typically run in use. This means they are Lightly loaded up to normal output pressures. Running the pressure up may begin to strain them, but you are unlikely to stall one.

Look at the graph below. Notice the curve for R22. It has high pressure for low temperatures. It is a good match for freezers. R134A has relatively low pressure at higher temperature. It's a good match for A/C.

For fast fills, go with the big boys. It's OK. Note R12 is obsolete to protect the ozone. Refrigerants near the same curve are good substitutes.
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Common Refrigerant temperature pressure graph.
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Unread postAuthor: Biopyro » Wed Apr 29, 2009 12:48 pm

Was definately worth bumping the other thread with that, so if someone searches aircon compressor they'll see that post in the right thread.

I take it you used to work in refrigeration?
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Unread postAuthor: POLAND_SPUD » Wed Apr 29, 2009 12:57 pm

well that's an interesting explenation... but is it really like this ??

I thought it's more like AC compressors are exactly like fridge compressors but they have higher flow while being able to provide the same or slighly higher pressures

well anyway judging from your post it seems that I was right when I suggested a setup with several fridge compressors to multiply flow --> http://www.spudfiles.com/forums/viewtop ... tml#242222
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Unread postAuthor: Technician1002 » Wed Apr 29, 2009 1:06 pm

Biopyro wrote:
I take it you used to work in refrigeration?


I worked in a highly classified data center. We did most AC stuff in house to prevent shutdown to allow outside contractors in. Believe it or not, my A/C stuff is directly related to my electronics. I worked on R22 systems up to 100 Ton capacity using chilled water.

The jokes there were fun. My job was so secret even I didn't know what I did. :D

Do you have use for a 5 ton compressor? I have a new spare about 30 years old. Never used. Needs 3 phase 208 volt to run it. It's about the right size for a Golf Ball Machine Gun. :D It's high temp (A/C) R22.

POLAND_SPUD wrote:I thought it's more like AC compressors are exactly like fridge compressors but they have higher flow while being able to provide the same or slighly higher pressures


Actually, with the inlet pressure way below normal, the pump is lightly loaded at normal discharge pressure. For A/C work in the summer, the condenser temp is typically at 120 - 140 degrees F. use the graph for 134a. Normal is about 175 PSI. Due to the inlet being 1 atm instead of 1 atm + the pressure at about freezing, the compressor is expecting the volume of being pressure fed at about 4 bar or 5 Atm. It is instead getting the volume of 0 bar or 1 atm or only about 1/4 the normal input volume. With the light load on the motor, go ahead and run the pressure up. You won't hurt it unless you blow out something from the pressure. The big motor and larger capacity will provide faster fill.
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Last edited by Technician1002 on Wed Apr 29, 2009 1:20 pm, edited 1 time in total.
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Unread postAuthor: Biopyro » Wed Apr 29, 2009 1:16 pm

Oooooh please, I have a 208V 3 phase socket in my bathroom. £100 seller pays shipping to UK? :P

Summary:
Air conditioning = higher flow, lower pressure.
Fridge = higher pressure, lower flow?

Have you ever used a compressor like this hooked up to a standard connection?
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Unread postAuthor: Technician1002 » Wed Apr 29, 2009 1:24 pm

Biopyro wrote:Oooooh please, I have a 208V 3 phase socket in my bathroom. £100 seller pays shipping to UK? :P

Summary:
Air conditioning = higher flow, lower pressure.
Fridge = higher pressure, lower flow?

Have you ever used a compressor like this hooked up to a standard connection?


Excellent summary. One addition, AC more power!

Standard connection, like regular house power? If so, it won't start. 3 phase has no start circuit. A rotary phase converter will be needed to provide 3 phase. A start cap won't do it.
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Unread postAuthor: POLAND_SPUD » Wed Apr 29, 2009 1:40 pm

@tech's reply

well I know someone who tested one and it had problems with reaching 150 psi AFAIK remember (I don't know if it ok with them to share this becasue the discussion was via PMs) and then it broke down

It seems that what you say expleins this to some extent


Due to the inlet being 1 atm instead of 1 atm + the pressure at about freezing, the compressor is expecting the volume of being pressure fed at about 4 bar or 5 Atm
now do exactly the opposite with a fridge compressor - that is provide pressurised air to the air inlet and voila you can increase it's pressure output - I am not sure if you are familiar with discussion on this idea... can you offer any useful input about this ?

the way I see it it's that it can be safe as long as:
1. the pressure at the air inlet was within rating of the outer body of the compressor and
2. the pressure generated by the compressor was below pressure rating of tubing at the air outlet (any internat part that can withstand for example 500 psi can withstand 500 psi + pressure of air provided to the compressor/presssure inside the case)
is my logic flawed in any way??
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Unread postAuthor: dewey-1 » Wed Apr 29, 2009 1:47 pm

Technician1002 wrote:I worked in a highly classified data center.
My job was so secret even I didn't know what I did. :D


That seems to be the same syndrome our elected government officials
have sometimes! :D

But in all due respect, I also felt that way before retirement. :)
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Unread postAuthor: Technician1002 » Wed Apr 29, 2009 2:03 pm

POLAND_SPUD wrote:@tech's reply

well I know someone who tested one and it had problems with reaching 150 psi AFAIK remember (I don't know if it ok with them to share this becasue the discussion was via PMs) and then it broke down

It seems that what you say expleins this to some extent


Due to the inlet being 1 atm instead of 1 atm + the pressure at about freezing, the compressor is expecting the volume of being pressure fed at about 4 bar or 5 Atm
now do exactly the opposite with a fridge compressor - that is provide pressurised air to the air inlet and voila you can increase it's pressure output - I am not sure if you are familiar with discussion on this idea... can you offer any useful input about this ?

the way I see it it's that it can be safe as long as:
1. the pressure at the air inlet was within rating of the outer body of the compressor and
2. the pressure generated by the compressor was below pressure rating of tubing at the air outlet (any internat part that can withstand for example 500 psi can withstand 500 psi + pressure of air provided to the compressor/presssure inside the case)
is my logic flawed in any way??


Logic is flawed. Let me explain. It has to do with the motor and the amount of work load placed on it. You can feed the input up to the normal range of volume it expects if you keep the output at the proper pressure. But just like using a tire hand pump at higher pressure, the energy needed goes way up if you are trying to deliver a high output pressure at the same time. It is often incorrectly assumed that the difference between the inlet and outlet is the only power requirement. This is WRONG

If you do the energy in compressed gas at different pressures you can see it. Take air at 0.1 ATM and feed it into a compressor and output at 150 PSI. Most of the stroke will be easy and the volume of air delivered is low simply because there is little in there on the intake stroke. Now feed in a 60 PSI boosted inlet and press that to 150 PSI. Sounds easier.. NO. The huge volume needs lots of force the entire stroke. Same applies to refrigeration compressors. High inlet pressure is more work. Boosting the inlet pressure to boost the outlet pressure will overload the compressor.


Big systems use suction throttle valves to adjust the load. Reference in the link. It's used to throttle the system to prevent ice in AC units.
http://www.tpub.com/content/constructio ... 50_303.htm
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Unread postAuthor: POLAND_SPUD » Wed Apr 29, 2009 2:26 pm

well either my logic is completelly flawed or you don't know that the other side of the piston is exposed to the pressure inside fridge compressors/provided at air inlet

Image

so due to pressure differential the load stays the same

(for simpliciy sake lets assume that both sides have exactly tha same area)

for 10 atm output at 1atm input load is the difference between output and input

so IMO load is the same for 20 atm output at 10 atm input



actually there was a guy who did that and he claimed that he had achieved 1700 psi before a gasket on his compressor ruptured
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Unread postAuthor: Technician1002 » Wed Apr 29, 2009 2:47 pm

POLAND_SPUD wrote:well either my logic is completelly flawed or you don't know that the other side of the piston is exposed to the pressure inside fridge compressors/provided at air inlet

Image

so due to pressure differential the load stays the same

(for simpliciy sake lets assume that both sides have exactly tha same area)

for 10 atm output at 1atm input load is the difference between output and input

so IMO load is the same for 20 atm output at 10 atm input



actually there was a guy who did that and he claimed that he had achieved 1700 psi before a gasket on his compressor ruptured


Take a volume of air at 0.1 Atm and compress it to 150 PSI. Inside the sealed case is fine. I understand that. Now do the same and compress 60 PSI inlet to 150 PSI, inside the same sealed case. It logically looks like it should be easy to see that it would be much easier as you are only boosting the pressure by 90 PSI instead of 150 PSI.

The answer is simple obvious and wrong.

I have to carry some stuff up 150 stairs. I start with a single book. Easy job. Lets make it easy and carry 240 books up 90 stairs instead. I hope this helps. show the increased load. If you try it, hook up a watt meter to the compressor. On A/C stuff we watch the load change drastically as the inlet pressure climbs when a building is hot inside. Other than both a higher discharge pressure, the higher inlet pressure is what causes so much A/C electrical overload in hot weather.

The differential is the same, but the compressor requirement is not. It's same work to compress from 150 to 175 as it is to compress from 50 to 75 lbs but you need a smaller displacement compressor to do this at the higher pressure and same horsepower. This is why multi stage compressors are smaller each stage by the same power requirement for each stage.
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