Author Topic: Compressor conversion  (Read 7057 times)

berwick

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Compressor conversion
« on: March 19, 2015, 07:18:24 pm »
Probably a stupid question but here goes. I have a dual holding plate spillover system that is run by a 115 V/AC Tecumseh AE2415A compressor (1525 BTU, 1/3 HP) All the other stuff (dryer, gauges etc) are mounted on the compressor chassis. For various reasons I want to switch to a 12 volt system. Without tearing the fridge apart for the holding plates (not sure even how it was done) I would like to change the compressor to a 12 volt one but still use all the old stuff.
Can this be done?
If so any suggestions?

Thanks
Richard

Richard

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Re: Compressor conversion
« Reply #1 on: March 20, 2015, 08:24:11 am »
Yes a conversion is possible but many questions need to be answered first :

To determine 12 volt compressor size needed how well did the 110 volt AC compressor perform? and how many hours per day does it run in the warmest climate that you plan on cruising in? Small 12 volt compressors are limited to 800 Btu at +10 degrees F. Larger 12 volt belt or shaft driven compressors are always water cooled requiring more space and are expensive.

Evaporator tubing diameter inside holding plates is important if a small 12 volt compressor is used. It is also necessary to know size of refrigerant control device, capillary tube or thermo expansion valve and if there is a refrigerant receiver tank?

Is present system’s condenser Air or water cooled?

Pictures of all components in this system would help provide answers to other question.



berwick

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Re: Compressor conversion
« Reply #2 on: March 22, 2015, 01:01:07 pm »
Hi Richard, here in the Bahamas I run it for 40 minutes twice a day. This gets a thin layer of frost on the plates. I am only using this as a fridge (one side colder than the other due to the spillover). I have a separate stand alone Engel that I use as a freezer. If I decided not to use the Engel then I think that an hour of run time twice a day would work as a fridge/freezer combo. Our cruising would be here for the next while. I have a bunch of pictures but I can't even put one on as it is over 192 KB, any ideas?
thanks

Richard

berwick

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Re: Compressor conversion
« Reply #3 on: March 25, 2015, 12:02:23 pm »
Richard, can I send some pictures to your Email vice this forum then I won't have this 192K limit?

Richard

Richard

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Re: Compressor conversion
« Reply #4 on: March 25, 2015, 01:50:40 pm »
Yes. Email Pictures to ricnard@kollmann-marine.com

I am unable to find specs on that compressor, 1525 Btu is a rating at what evaporator (holding plate) temperature compaired to smaller 12 volt compressor. 

Richard

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Re: Compressor conversion
« Reply #5 on: March 29, 2015, 05:18:13 pm »
The Pictures you emailed me provided a better understanding of your present system. This is a homemade unit using standard over the counter refrigerant components. Holding plate non standard type evaporators were not pictured. This system is a good job and may be linked to my DIY book although I never used external equalized Thermo expansion valves in my books.

From picture 4240 It is difficult to determine age of condensing/compressor unit if prior to 1996 it would have been designed for refrigerant Freon R12, after 1996 it would be a 134a refrigerant system. Because someone marked 134a on fan shroud and no information on web available to confirm to which refrigerant, one must assume it has been running two years OK with 134a refrigerant in it. This same picture shows the low pressure switch set to stop compressor at a low suction pressure of 10 psi and restart compressor after a ten psi pressure rise. These pressure settings are only reference marks so actual settings will need to be determined later in a performance run that I will recommend.
The low pressure switch may be used to protect compressor from running at too low a press or in a vacuum or it may be intended to replace thermostat control over compressor. Can you confirm if there is also a thermostat?

Picture 4243 shows refrigerant line sizes to be ½ in OD for suction side of system and 3/8 OD for high pressure side. Without holding plate pictures I assume their inside evaporator coils are ½ inch OD and this would mean a Danfoss 12 BD compressor would not provide adequate velocity to return compressor lubricating oil for very long. Line size then would rule out small 12 volt compressor as a replacement for present 110 volt compressor.

Picture 4241 has an oversized start capacity to provide a boosted compressor start when power source is week and is not a standard Tecumseh condensing units but again shows this is a well though out system design.

Picture 4245 shows the compressor start relay and a restart timer that prevents compressor from restarting for a period of time up to 8 minutes when power is interrupter for any reason including thermostat or low suction pressure activation.

Picture 4250 is a mystery as there are two TXVs. One valve controls flow of refrigerant through one or two holding plates. The second TXV has no liquid high pressure line connected to it. Is there a plate that does not get cold? Are there four lines connected to each holding plate? It may be that this system was intended to be a Hybrid with a second 12 volt system installed to freeze plates in addition to 110 volt system.

Picture 4244 is of both permanently installed High and Low pressure gauges. These gauges allow for a refrigeration performance review at any time during the life of a system and generally found only on systems with operating engineers or a service man is onboard. Holding plates are continuously in state of change which means the refrigerant pressures and box temperatures are never constant for long when compressor is running unlike standard evaporator boxes.

I would like you to run this test.
Record every 15 minutes the following for two hours beginning to record before first compressor startup. It is not important to start with warm plates.
Box temperature
Blue gauge low pressure
Red gauge High pressure
If compressor stops record time
When Compressor restarts record time
Report where frost is present and where there is no frost.
There are two refrigerant flow sight glasses in this system I want you to report what you see every 15 min in the very small glass on top of the black receiver tank in pictures 4243 and 4242. This glass will show in any order of time from clear to a white color to large bubbles to bubbles and clear spaces.

 
 


berwick

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Re: Compressor conversion
« Reply #6 on: March 29, 2015, 09:31:08 pm »
Hi Richard, wow thank you for all the info! A few years ago (Aug 11) the system developed a leak which was fixed and the tech said it would be better for me to switch to R134A, thus the change. The two cold plates have two lines as it was an engine driven system as well as a 110 VAC compressor. The original owner could never get the engine driven part to work properly and he had tried everything, so I just took out the compressor. There is a switch to put it in manual mode? which I use now but I can also put it in auto mode and there is a RANCO control that may be the thermostat that you mentioned? I will send you a picture of it. I can run the tests with the control on auto but I don't have any sort of thermometer with me and we are still in the Bahamas. Do you want me to do the rest of the tests?

thanks
Richard

berwick

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Re: Compressor conversion
« Reply #7 on: April 05, 2015, 09:52:31 pm »
Hi Richard and others, finally got a thermometer and this excel program is what the results are,
Richard

Richard

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Re: Compressor conversion
« Reply #8 on: April 07, 2015, 09:18:03 am »
 Your two hour refrigeration performance test does indicate a High side pressure problem as recorded:

Box temperature
The area of box where temperature was recorded would not represent common refrigerator temperatures. At 120 minutes of compressor running time temperature is not very low although if this were first days run with holding plates box temperature would be slow to react to change. If temperature recorded was plate temperature instead of box temp system performance would need to be questioned.

Low suction refrigerant pressure (Blue Gauge)
The low pressure side of a system containing thermo expansion valve and holding plates will get maximum flow of refrigerant as valve will be wide open and progressively close as plate temperature changes. After 15 minutes 13 psi low pressure is wide open valve orifice for your 1/3 HP compressor. I am surprised to see 8 psi after 120 minutes but is possible if  holding plates are very large. Normally good holding plate refrigeration performance with 134a refrigerant the low pressures will be somewhere below 8 psi generally 6 to 4 psi.   

High liquid pressure (Red Gauge)
When refrigerant pressure remains above 150 psi on these small low temperature refrigeration systems it is almost impossible to maintain a steady flow of liquid refrigerant at expansion valve. High pressures can be expected when valve orifice is wide open but as evaporator coil inside plate cools valve slowly closes, at this point low and high pressure should drop. A desirable high pressure of 135 psi would be a great improvement over 170 psi at 120 minutes running time.

Small Sight Glass
I prefer to use the small sight glass because it is much easer for someone to understand  refrigerant volume on small systems. Bubbles at all times as you report would lead most people to believe system is low on refrigerant but on this unit adding refrigerant would only increase high pressure and shorten compressor life do to excessive compressor heat.

Conclusion and recommendation.

The main cause for this system’s poor performance is the condenser is not efficient enough to dispose of the process heat. Either the heat in area where condensing unit is located is not exhausted adequately elsewhere or warmed air passing through condenser coil or fan is insufficient. Without seeing condenser air flow temperature in and out I can not make a recommendation.  Typical solutions for this problem is, install an additional fan to dispose of heat elsewhere or a another fan to push more air through condenser or a combination of both.

berwick

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Re: Compressor conversion
« Reply #9 on: April 07, 2015, 08:07:17 pm »
The condenser and compressor are in the lazarette near the generator and engine. I worked in there during the entire running test changing oils etc so maybe I didn't give it fair test. Richard, do you want me to run for two hours again? The thermometer was above and about 3" away from the top cold plate, where should I place it if I run it again?
Also do you do "house calls"?

Richard

berwick

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Re: Compressor conversion
« Reply #10 on: April 08, 2015, 11:50:03 am »
Hi Richard and others, did a second test with the lazarette open to keep it cooler. The thermometer was placed at the same level as the lower plate thus the lower temperatures. The box continued to drop to 38 F after the compressor was off when I checked half an hour afterward.
The new spread sheet is attached
Thoughts?

thanks
Richard

Richard

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Re: Compressor conversion
« Reply #11 on: April 11, 2015, 10:08:06 am »
In order to solve the poor performance problem you have High pressure (RED gauge) must be reduced to 125 psi if possible. I should have asked before if the condenser coil is free of restrictions from dust and hair. Try temporally adding any kind of fan in front of condenser to force more air through it. Looking at present fan its shroud is not very efficient. Second area to be concerned with is exhausting heat from that compartment naturally or mechanically.

After high pressure is lowered Low pressure will drop producing lower temperatures and hopefully clearing up sight glass when plates are frozen

CaptnSteve

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Re: Compressor conversion
« Reply #12 on: May 06, 2015, 06:55:04 am »
I'm trying to find out how to post a problem and request for advice;  I can't see a button or means to start a new topic.   I am a new user.  Can anyone help?

Richard

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Re: Compressor conversion
« Reply #13 on: May 06, 2015, 12:29:49 pm »
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