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Old 10-03-2019, 09:12 AM   #21
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Engel just uses the Danfoss compressor just like Norcold, Dometic and some others. China has started to make their own small DC compressors.


The swing compressor is pretty interesting and in theory more efficient. They humm a little. For our freezer box we use a stirling cycle linear cooler from Global cooling. Mostly used in the medical world where money is less of an objection. It humms a little also.


My 500 watt high frequency pure sine wave did not reliable start my old fridge with a tube condensor on the back.. .I liked the fridge because of the separate condensor but I think the motors from the 10 years ago are less efficient then some of the more current motors.


Anyhow, heat pumps are fascinating to play around with.


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Old 10-03-2019, 09:21 AM   #22
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Engel just uses the Danfoss compressor just like Norcold, Dometic and some others.
No, completely false

Engel and (older?) ARB & Norcold use Sawafuji "swing reciprocating" compressor, different type from Danfoss.

Dometic (Waeco) make their own, more similar to Danfoss style.
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Old 10-03-2019, 09:50 AM   #23
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You are absolutely right, completely false.
Thanks Johan
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Old 10-03-2019, 11:30 AM   #24
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My Norcold has the Sawafuji compressor and it made quite a racket initially. But it turned out that the noise was from resonance in the piping. A little bending and a few pieces of foam fixed that. Now, it is very quiet.


Here is the fate of the freebie fridge:

Rear panel removed with the condenser pipe visible.
20191003_100606 (2).jpg
The copper pipe was taped to the sheet metal shell, which acts as the heat exchanger. A design like this needs to have clearance around the sides and back. I should also be clear that trying to keep a space cold while surrounding it with hot metal on the outside of the foam insulation is not the most efficient way.

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20191003_104716.jpg

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20191003_115637 (2).jpg
The black thing on the bottom of pic is the compressor. White U shaped plate on top is the evaporator that gets cold. The copper pipe in between is the condenser that sheds the heat removed from the fridge plus the energy supplied to the compressor to the ambient air. The little white box on the cables is the thermostatic switch.

Date plate
20191003_100630 (2).jpg

Compressor tag
20191003_100615 (2).jpg

This refrigeration unit is ideal for my needs because of the long condenser pipe. I can get the main condenser section with heat sinks in the useless space between the wall and the dinette back rest and have the freezer box adjacent to the bench without having to break the loop.

Just need to increase the surface of the evaporator to keep some pizzas frozen. I assume that the two plates this evaporator is made from are soldered outside of the embossed channels, could be drilled there, and bolted to an aluminum plate. If that assumption turns out to be false, they were kind enough to specify the amount of R134a for a recharge after installing a larger evap.

I also did some back of the envelope calculations and it looks like the condenser would benefit from a large thermal mass because of the duty cycle. For about 10% of the time the heat from the refrigerant would get dumped quickly into the sink (fluid to solid conductive heat transfer) and then you have 90% of the time to dissipate that into the air via convection through a low conductivity interface while the compressor is off.

The unit will cool nicely the way it sits in the photo, evidenced by the frost on the evaporator, but the temperature drop across the condenser is rather pathetic without coupling a larger exchange surface or a larger thermal mass to the condenser pipe. I could also hear that the expansion orifice was a little liquid starved. Partially submersing the condenser pipe in a bucket of water really got the unit cranking.
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Old 10-04-2019, 01:11 AM   #25
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This thread sure gets my mind thinking about alternate arrangements for the frige.
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Old 10-04-2019, 06:32 AM   #26
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Very nice job taking that apart...surprised with the low pressure of 86 psi.. I thought mostly a couple of psi on the low side. Nice idea to get thermal mass on the condenser. Wonder what that does for the cop.

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Old 10-04-2019, 08:40 AM   #27
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Originally Posted by joeblack5 View Post
Very nice job taking that apart...surprised with the low pressure of 86 psi.. I thought mostly a couple of psi on the low side. Nice idea to get thermal mass on the condenser. Wonder what that does for the cop.

Johan
You raised an interesting point with the low side pressure. I had not paid attention to the values on the tag after taking the pictures.

I thought at first that the low side pressure on my tag would be an error but looking online I found tags of other fridges with similar values.

What I do not understand is how they get the evaporator plate to freeze over at these low side pressure levels. According to the charts, 85 PSIG equates to 79F for R134a.

Maybe what they call low side pressure is really equilibrium pressure with the system off. I have never seen more than one service port (e.g. crimped and brazed pipe) on a fridge. But why even mention that pressure as it is just a function of temperature and not an indication of charge volume or system performance?
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Old 10-04-2019, 09:29 AM   #28
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Yes , I do not know, but I recharged a r134a fridge recently to that spec and.. it does not work...After googling around .. and as you said.. that pressure makes no sense.




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Old 10-04-2019, 12:09 PM   #29
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The high side on R-134 is 290-300 psi, it cools off from the pressure drop passing 300 psi through a small orifice. Also, most systems are charged by volume/weight. So if you need to recharge it determine the volume/weight of a similar compressor to know how much to replace. The line size is negligible on that size unit so you don't need to calculate that into the total volume.

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You raised an interesting point with the low side pressure. I had not paid attention to the values on the tag after taking the pictures.

I thought at first that the low side pressure on my tag would be an error but looking online I found tags of other fridges with similar values.

What I do not understand is how they get the evaporator plate to freeze over at these low side pressure levels. According to the charts, 85 PSIG equates to 79F for R134a.

Maybe what they call low side pressure is really equilibrium pressure with the system off. I have never seen more than one service port (e.g. crimped and brazed pipe) on a fridge. But why even mention that pressure as it is just a function of temperature and not an indication of charge volume or system performance?
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Old 10-04-2019, 01:56 PM   #30
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The high side on R-134 is 290-300 psi, it cools off from the pressure drop passing 300 psi through a small orifice. Also, most systems are charged by volume/weight. So if you need to recharge it determine the volume/weight of a similar compressor to know how much to replace. The line size is negligible on that size unit so you don't need to calculate that into the total volume.
There was no concern about the high side pressure indicated on this label:

20191003_100630 (2).jpg

But notice the low side pressure and look up the corresponding vapor temperature in the tables or P-h diagram for R134a. ~79 F evaporator outlet temperature does not make sense for a refrigerator on planet Earth.

I speculated that this pressure is NOT the suction pressure and a friend who owns a commercial refrigeration business confirmed that an hour ago. He stated that this is the leak test pressure for a newly assembled system, but his technicians test with Nitrogen above high side pressure to be on the safe side before they charge the system with (many pounds of) refrigerant.

Something must have gotten lost in the translation from Chinese to English and we should ignore what they call "low side" pressure on the label.

I agree that filling by weight is always the better strategy. Then fine tune by measuring pressures AND temperatures, taking into account the desired levels of superheat and/or subcooling. This is especially important in an orifice system where the superheat is not governed by a TXV.
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Old 10-08-2019, 11:54 PM   #31
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Sound ideas tend to gain popularity.


We should ask ourselves whether the operating voltage or the country of purchase are direct reasons for the efficiency difference or whether there is some other causality that follows the laws of physics.

The Kenmore office fridge has a condenser hidden beneath the side walls. You can feel them getting pretty warm when the fridge runs. What separates the space you are trying to cool from this hot area is less than one inch of injected foam. Cheap to make but certainly not efficient to operate.

My marine fridge with 12VDC swing compressor has 2" of insulation and the condenser coil is separated from the fridge by another 2" of air. After studying several compessor spec sheets, l guesstimate that the refrigeration components account for maybe 10% of the lower electricity consumption and that the major efficiency gain comes from better insulation of the cold space. Even that could be improved in the door area.

The next experiment with the free office fridge will be to extract the refrigeration components and putting the evaporator (the plate getting cold) in a very well insulated chest, custom built for my food storage needs and the available space in my vehicle.

You can buy refrigeration kits for building marine 'ice boxes' but these kits are as expensive as the good 12VDC fridges/freezers due to the low production volume. Let's see how less costly, high-volume components perform in a carefully designed context.
I have a similar plan. I got a deal on a 12/24 volt ice box kit that will refrigerate up to a 5 cubic foot space or freeze a 1.8 cubic foot space with good insulation. I'm looking at using a well insulated "Yeti like" cooler around 110 quart in size for the fridge.

A friend gave me a 12/24 volt 65 liter fridge that worked but the interior was in rough shape. I pulled the evaporator and compressor off of it and plan on installing it onto a 45 quart cooler to use as a beer/beverage fridge or freezer. This would avoid opening the big fridge and letting heat in every time someone needed a new beverage.

Ted
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Old 10-13-2019, 09:31 PM   #32
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To add some more nice hacking opportunities....while I was wandering thru our basement I found my earlier experiments with water coolers that I thought to reuse/convert into pet bed coolers..
The water coolers are very easy to take apart without compromising the freon circuit.
Probably good as human cooler as well. I am wearing a military vest with many small water tubes. It was used in helicopters and tanks and others....?? Here in the picture.. I am using an ice / water box with air driven pump..
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Old 10-14-2019, 12:26 AM   #33
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Originally Posted by joeblack5 View Post
To add some more nice hacking opportunities....while I was wandering thru our basement I found my earlier experiments with water coolers that I thought to reuse/convert into pet bed coolers..
The water coolers are very easy to take apart without compromising the freon circuit.
Probably good as human cooler as well. I am wearing a military vest with many small water tubes. It was used in helicopters and tanks and others....?? Here in the picture.. I am using an ice / water box with air driven pump..
Thumbs up. Iíve wondered about making a water cooled and water heated bed or clothing. The water cooler idea may be the ticket.
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Old 10-14-2019, 12:45 PM   #34
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You guys are making my brain hurt reading all these calculations. I'm going to use a chest freezer with either a Johnson Controls or an inkbird temperature controller. They cost roughly $50 and will turn the compressor off when the temperature hits a preset temperature like 38 degrees.

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Old 10-14-2019, 01:14 PM   #35
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Same here, and if it looks like I might want a refrigerated vest, I will just go somewhere cooler. Most parts of Canada are quite nice in the summer.
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Old 10-14-2019, 03:59 PM   #36
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Same here, and if it looks like I might want a refrigerated vest, I will just go somewhere cooler. Most parts of Canada are quite nice in the summer.
Yeah, it might even hit like 49/50 degrees up there.
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Old 10-14-2019, 06:09 PM   #37
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Just hook it up to the coolant loop of the bus....and an umbilical cord and you will stay nice and toasty
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Old 10-14-2019, 07:02 PM   #38
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Has anyone done a chest freezer to refrigerator conversion using the temperature control on skoolie.net?
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Old 10-14-2019, 07:28 PM   #39
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Yeah I used that system for a while but that freezer was a pain in the ass. We upgraded to a 12v rv refrigerator about a year ago. It worked ok as as far as keeping appropriately cold but it was hard to get stuff out and keep it organized.
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Old 10-14-2019, 09:13 PM   #40
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Interesting topic. I'd like to see more.
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