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Old 06-25-17, 11:47 AM   #11
oil pan 4
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If you can figure out how to zombie a variable speed front loader motor, post that up it has lots of applications.

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Old 06-25-17, 04:42 PM   #12
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I started a thread on that subject in the billiards room a short while ago. I have been fixing broken ones for awhile and selling them for big return on investment. So far, I haven't gotten one that wasn't economical to repair, but I've still been looking into how the thing is doing what it does. Once I have figured out how to get it to do whatever I want, I'll put it up in that thread.
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Old 06-29-17, 05:43 AM   #13
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As far as I know, many refrigerant compressors are not able to start up against a backpressure. Normally after stopping the compressor, after a while the pressures in condensor and evaporator will be equalised. So when the compressor has to start up again, the pressure in the dischargeline is equal to the pressure in the suctionline.
However when you connect multiple compressors in parallel to one evaporator and one condensor, the first compressor will be able to start up, but the second (and 3rd, 4th, etc) have to start up against a pressurised dischargeline.
This is needing much more torque and many units have not been built for that.
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Old 06-29-17, 07:33 AM   #14
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An unloader valve can be installed to overcome that obstacle.

This nifty device is also called a capacity control valve. It is a three-port solenoid valve. It has many uses, but in this situation, it connects the discharge and suction pipes of the compressor group to each other. It has a check valve at its outlet (or merely closes the discharge port when unloading occurs), so that the high-pressure refrigerant built up in the rest of the system cannot rush backwards into the suction side.

When another compressor in the group is called into action, the unloader valve is energized, and the pressure across the compressor group equalizes at suction pressure. The extra compressor is started while the running compressor group is unloaded as well, so there is no excessive electrical load generated. All of the running compressors draw minimum current, and the summoned compressor starts easily. Once the extra compressor is online, the valve is de-energized and the refrigeration cycle starts doing its job again. In most cases, this operation happens so fast that the pressure built up in the condenser doesn't have time to bleed off.
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Old 07-30-17, 02:35 PM   #15
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Having 4 compressors in DIY setup is a recipe for disaster. Too many things to consider, to much plumbing.... solenoid valves, controls, etc.

How about having 4 small compressors 4 small condensers and one 4x times evaporator coil splited to have 4 independent circuits. It is like having 4 separate units working as needed all together or one by one.

4 small systems is cheap to build with off shelf parts and they will work predictably and reliably.

Going with "4 stage" system you will fight capacity change which is determined by compressor size, but whole system must be balanced in order to work reliable. One of the biggest issues is TXV or whatever other magic metering device you are going to use. It must cover whole capacity range. Piping can be a trouble because of oil return etc.

Multi stage systems even in commercial world with huge budgets are always a challenge. I am not saying DIY is impossible, it is, but at what price and time?

Last edited by Vlad; 07-30-17 at 09:36 PM..
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Old 09-19-17, 09:53 AM   #16
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I built one of these a few years back:
http://ecorenovator.org/forum/geothe...heat-pump.html

In the end I abandoned it as it appeared it didn't work but it turned out I actually had a faulty SSR controlling one of the compressors but I didn't realise until I had dismantled everything and had the same SSR in a different machine :-(
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Old 09-20-17, 07:12 PM   #17
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A 2 stage commercial condenser unit is very common , one of our guys was wiring one up earlier today .

This was straight A/C , not a HP .

Two sets of refrigeration lines . C , R , G , W1 , W2 , Y1 , Y2 for controls .

We have an inverter drive 12,000 BTU ductless mini split HP in our bedroom . We love it . MS's are very quiet . Also , a traditional MS in the living room .

I think inverter drive is slowly working its way into the residential & commercial traditional split systems .

Wiring a VFD would not be that difficult , more a matter of the controls .

Best wishes, :-)
Wyr
God bless
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Old 09-25-17, 11:19 PM   #18
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Quote:
Originally Posted by jeff5may View Post
An unloader valve can be installed to overcome that obstacle.

If three phase compressors are used with individual VFDs, there should be no problem starting up against the pressure differential.

The trouble is that most of us are using single phase motors with capacitor start which has very slow start and low rotor torque. My 4 ton unit is single phase.

These days, small 1HP VFDs are cheap and common and they will make 3 phase from standard 240V single phase. Controlling a bank of several of them with an inexpensive surplus PLC is a relatively simple setup.

Multiple compressors would be an interesting project for sure.
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Old 09-26-17, 01:33 AM   #19
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If you have a variable speed setup the you shouldn't need more than one compressor unless you are trying to go big.
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Old 09-26-17, 01:59 AM   #20
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Quote:
Originally Posted by oil pan 4 View Post
If you have a variable speed setup the you shouldn't need more than one compressor unless you are trying to go big.
In terms of practical efficiency gains vs cost, I agree.

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