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Fridge Compressors


rtbarton

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Rog and Luke,

Far from me to criticise, Im game for anything which involves experimentation and general weirdness ( ;) ).

But in a practical world..

You have both played with the principle and proven the theory, but is it not a waste of time? I know Rog says he tops up at night time using economy 7, but it takes 1.5hrs to get from 80psi to 150psi? Surely it must be cheaper to use a 3hp motor that wold do that in a matter of mins?

Not digging or anything chaps, its projects like this which bring new ideas to the world, and Im so jealous of Lukes ability to wire that control system! The useage of "free" hardware deserves applause. Well done.

I understand exactly where you're coming from.

In my particular position I have a workshop in a residential area which severly limits the hours I can work, I do have a 3 HP piston compressor in an outside 'dog kennel' but noise still escapes. As I build my system up it supplements the existing unit, but hopefully it will replace it eventually.

Obviously I can increase the output by using multiple fridge compressors, I reckon on needing 30 :blink:

Also I have this 3 hp compressor pumping heat into outer space and come in after work and switch a 3 kw heater on, placing the fridge compressors around the workshop/office should save money and cut down on electricity used - CO2 emissions blah blah.

Further, if one motor fails on the 3 hp compressor I lose 100% of my air, if one fridge motor out of 30 fails I lose 0.3%

I reckon it would cost me around £2000 to get this much air using silent compressors.

I intend to prove that genius really is next to madness :D

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more than likely overheating and tripping out the thermal overload due to no cold suction vapour. This is usually a simple 'klixon' type bi-metal arrangement, arranged alongside the relay.

That's what stops the motor, but why does it only start tripping after being disconnected from the rest of the fridge for a couple of days?

It trips due to overcurrent, this induced by the motor running sluggishly which is caused by the oil being contaminated? At this point the compressor still has its original oil in. The effect is much greater with the thicker oil and lower temperature.

In a fridge that has been switched off for a while there would be no cold suction vapour anyway, but would the fridge still have difficulty starting?

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Rog and Luke,

Far from me to criticise, Im game for anything which involves experimentation and general weirdness ( ;) ).

But in a practical world..

You have both played with the principle and proven the theory, but is it not a waste of time? I know Rog says he tops up at night time using economy 7, but it takes 1.5hrs to get from 80psi to 150psi? Surely it must be cheaper to use a 3hp motor that wold do that in a matter of mins?

Not digging or anything chaps, its projects like this which bring new ideas to the world, and Im so jealous of Lukes ability to wire that control system! The useage of "free" hardware deserves applause. Well done.

Waste of time? for me, no. I agree that the application is marginal, but it holds merit for my use. My garage is the ground floor of the (3 floor) house, the end of a row of three, slap bang in the middle of Sheffield. I often work with the garage door closed, and the noise of the compressor is deafening. Using anything like an air ratchet has the compressor on and off every 5 minutes or so, which is annoying. The freezer pump runs almost silently and with the 300L tank, there's plenty of air as and when i need it.

As an aside, the development of the system is very much a learning process, valuable to me for both my degree and, i hope, my career. The electronics are a product of research and investigation, trial and error, and help from others (including a few from here - Roger, Fridge, Si).

Luke

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I understand exactly where you're coming from.

In my particular position I have a workshop in a residential area which severly limits the hours I can work, I do have a 3 HP piston compressor in an outside 'dog kennel' but noise still escapes. As I build my system up it supplements the existing unit, but hopefully it will replace it eventually.

You've got this the wrong way round Roger, you should have stuck your existing 3HP compressor in the fridge to sound proof it and binned the fridge compressors! ;););););)

Steve :)

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Waste of time? for me, no. I agree that the application is marginal, but it holds merit for my use. My garage is the ground floor of the (3 floor) house, the end of a row of three, slap bang in the middle of Sheffield. I often work with the garage door closed, and the noise of the compressor is deafening. Using anything like an air ratchet has the compressor on and off every 5 minutes or so, which is annoying. The freezer pump runs almost silently and with the 300L tank, there's plenty of air as and when i need it.

As an aside, the development of the system is very much a learning process, valuable to me for both my degree and, i hope, my career. The electronics are a product of research and investigation, trial and error, and help from others (including a few from here - Roger, Fridge, Si).

Luke

As mentioned Luke, not criticising, indeed Im applauding, perhaps its not for the masses, but the benefis of silent air are good, and heating the workshop - bonus - in winter anyway :)

You keep experimenting mate, Im going to be calling on your electronics ability in a short while, might need a mad proffesor brain for it!

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You've got this the wrong way round Roger, you should have stuck your existing 3HP compressor in the fridge to sound proof it and binned the fridge compressors! ;););););)

Steve :)

Bo**o*ks - there's always one :ph34r:

Actually a large chest freezer would keep the compressor cool and my air dry - two big problems I have. the freezer would have to be capable of handling the heat though, they reckon 1 Kw of electrical power can pump 3 kw of heat, so for a 3 hp (near enough 3kw) we're looking at shifting 12 kw of heat (3 from the motor and 9 from compressing the air), so the freezer would have to have a 3 kw motor (I think) :)

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Well done chaps!

I used a Danfoss Air Con scroll compressor for a while. 3Hp 3 phase, very quiet and produced a large amount of air! It came from a scrappy. At the start of the summer he usually gets a load of single & 3 phase compressors.

It only died because the filter on the inlet had fallen off & it was sucking in grinding dust from behind my bench!

Si

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I like experimental tech! Keep it up.

Going down a similar but alternative track, wouldn't a rotary aircon compressor like a Sanden from a range rover be fairly quiet if driven by a belt from a 240v electric motor?

Regards,

Diff

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I like experimental tech! Keep it up.

Going down a similar but alternative track, wouldn't a rotary aircon compressor like a Sanden from a range rover be fairly quiet if driven by a belt from a 240v electric motor?

Regards,

Diff

Don't see why not, more work involved in mounting the motor and pump, I'll try to post a photo of my triple compressor unit, held together with cable ties (can't get baler twine round these parts.)

Rangie aircon compressors probably cost though, even from a scrappy.

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As mentioned Luke, not criticising, indeed Im applauding, perhaps its not for the masses, but the benefis of silent air are good, and heating the workshop - bonus - in winter anyway :)

You keep experimenting mate, Im going to be calling on your electronics ability in a short while, might need a mad proffesor brain for it!

Rog,

It's all constructive :) , I was just trying to justify what, as you said, appears to be a bit pointless :huh:, It's certainly not for everyone, but this post may sow a few seeds for someone.

Any extra heat i can get into the garage, the better! it's getting cold down there!

Mad professor brain? I think i may well just be mad :D . Chuck me a PM or email any time.

Cheers

Luke

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That would be pretty magic if it were true. We would have to re-write the laws of thermodynamics.

Sorry, no more energy is coming out than went in.

No - it can PUMP that amount - ie move it from one place to another, the hot side of the freezer will radiate heat at the rate of 3 kW at the same time the cold side absorbs heat at the same rate. The energy required to move the heat from cold to hot is approx 1/3rd the heat moved.

This is the principle heat pumps work on and a fridge is just a heat pump.

Energy is not lost or created, it is just moved. The total output will be 4 kw ie 3 kw from the cold side of the machine plus 1 kw from the electricity used.

Lets start a new thread "What happens whe you leave a fridge open in a closed room" :P:ph34r::blink:

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No - it can PUMP that amount - ie move it from one place to another, the hot side of the freezer will radiate heat at the rate of 3 kW at the same time the cold side absorbs heat at the same rate. The energy required to move the heat from cold to hot is approx 1/3rd the heat moved.

This is the principle heat pumps work on and a fridge is just a heat pump.

Energy is not lost or created, it is just moved. The total output will be 4 kw ie 3 kw from the cold side of the machine plus 1 kw from the electricity used.

Lets start a new thread "What happens whe you leave a fridge open in a closed room" :P:ph34r::blink:

I think what red90 is trying to say, as the compressor has a 3HP motor, it will consume around 2-3kW, therefore the input of 3kW it cannot generate more than 3kW of heat (hence the thermodynamics comment)

A quick google shows many motors are around 70-90% efficient, so as 3HP is around 2.25kW, assuming 75% efficiency you get losses of 750w (3kW input to motor), most of this energy is lost as heat so heat gain of 750w.

To move 750w with a fridge or heat pump type device I would estimate that 190-250w of power would be required.

Thats my theory anyway... :D

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That's what stops the motor, but why does it only start tripping after being disconnected from the rest of the fridge for a couple of days?

It trips due to overcurrent, this induced by the motor running sluggishly which is caused by the oil being contaminated? At this point the compressor still has its original oil in. The effect is much greater with the thicker oil and lower temperature.

In a fridge that has been switched off for a while there would be no cold suction vapour anyway, but would the fridge still have difficulty starting?

Is it trying to start against a 'head' ??

these little motors have bugger all starting torque and require pretty much zero pressure differential between suction and discharge at start-up otherwise they stall, another nail in the coffin for air compressor use.

In a refrigerator/freezer, a capllary is used as the expansion device which effectively unloads the compressor on the off cycle, enabling start up.

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I like experimental tech! Keep it up.

Going down a similar but alternative track, wouldn't a rotary aircon compressor like a Sanden from a range rover be fairly quiet if driven by a belt from a 240v electric motor?

Regards,

Diff

this is how some of the heavy duty refrigerant recovery units work. Javac use a Sanden compressor driven off a 240V motor, with an oil separator in the discharge line feeding the oil back to the compressor.

And FWIW, Sanden's are a positive displacement compressor (current ones are mostly 7 cylinder) activated by a swashplate. The model # is the giveaway. eg. SD7H15 = 7 cylinder, approx 150cc.

The only downside is that the oil they use OE is PAG (polyalkylglycol) which is horrifically hygroscopic and not really compatible with anything else. I think that the old OE mineral oil was either an ISO 46 or 68 oil, around 7-8 cSt @ 100*C.

Endless Air use a Sanden with (supposedly) grease in the sump. Not sure what they've done to make that work.

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Is it trying to start against a 'head' ??

these little motors have bugger all starting torque and require pretty much zero pressure differential between suction and discharge at start-up otherwise they stall, another nail in the coffin for air compressor use.

In a refrigerator/freezer, a capllary is used as the expansion device which effectively unloads the compressor on the off cycle, enabling start up.

No - free air.

I've got fridge motors that will start with 120psi on the input when the oil is warm, the bigger ones are better at it though. In my final design I shall use a vent valve to ensure that they start under no load conditions.

My 3 hp piston compressor struggles to start under load if the vent valve blocks (which it did, it was full of Loctite when I got it, good old Clarke :rolleyes: )

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I've got fridge motors that will start with 120psi on the input when the oil is warm, the bigger ones are better at it though. In my final design I shall use a vent valve to ensure that they start under no load conditions.

Further to this, the one that started under load decided it didn't want to today!

I think for practical purposes you have to assume they're not going to, except that the Machine Mart Sssshhhhh compressors which appear to use fridge motors don't use the dump valve in the pressure switch so they must start under pressure.

Having said that, they are bigger motors and they cut out at about 80 psi.

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A lot of pressure switches on compressors dump the pump side pressure when they get up to pressure and switch off. You can hear a hiss as they stop.

Even though this only leaves the volume of the connecting pipe to pressurise when the motor starts, it's enough to unload it.

There are a few photos here of my workshop air-con/heat pump which was mostly built out of Rangey parts but used a 3hp mains motor to drive the AC Compressor (Sanden RR type) Here

Si

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A lot of pressure switches on compressors dump the pump side pressure when they get up to pressure and switch off. You can hear a hiss as they stop.

Yup, the trouble with them, as Roger found, is the fridge/freezer compressors don't output enough volume to close the dump valve on startup. running a few pumps into one valve would generate enough volume, i'd think, but then you lose individual control.

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Yup, the trouble with them, as Roger found, is the fridge/freezer compressors don't output enough volume to close the dump valve on startup. running a few pumps into one valve would generate enough volume, i'd think, but then you lose individual control.

Exactly that.

When the switch closes to start the compressor the dump valve stays open under spring pressure and doesn't close until the pressure reaches about 35 psi.

It takes the output of 5 of my fridge comprssors to build this pressure against the air bleeding out. Also if my receiver is empty this would also have to reach 35 psi before the valve shuts. Grown up compressors use a non-return valve (NRV) which doesn't open until the pressure in the pipe from the pump has closed the dump valve, my NRV doesn't do this.

The situation could arise with multiple fridge compressors that once 5 of them had got going pressure could start building which could make it difficult for the others to start.

The solution has got to be a solenoid operated dump valve which is closed once all compressors that are going to start have done, there would have to be some input to the system that detects that a compressor is actually running, possibly an opto device on the drive shaft?

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The solution has got to be a solenoid operated dump valve which is closed once all compressors that are going to start have done, there would have to be some input to the system that detects that a compressor is actually running, possibly an opto device on the drive shaft?

I've got a working solenoid dump valve - 12v water/heating solenoid! cheap as chips, though not good for much else.

A shaft position/speed encoder is easy to implement.

alternatively, you could deduce start up procedure by measuring the change of pressure in the line to the NRV.

try to start a pump

close the solenoid

see if the pressure is increasing

open the solenoid

try to start the next pump

close the solenoid

see if the pressure is increasing, quicker than before (ideal factor of 2)

iterate for more pumps

Would be easy to do in software, saves extra hardware and complication that includes.

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