Maybe it's a stupid question but Google isn't producing satisfactory answers.

I see many people here building cold rooms and kegerators and keezers and they all look marvelous. Of course, now I want to build one of my own. I keep wondering about the design of refrigerators. This is probably going to be confusing and long winded so please bear with me.

My understanding of refrigeration:

Refrigeration works on the principle of "absorbing" heat with a gas and then transferring it out of the desired room where it is compressed back to a liquid form so that it will release it's heat in a dispersion coil. These are endo and exothermic reaction properties inherent to certain gasses and some are better at this than others.

So with that in mind, I have these questions:

1.) Hot air rises correct? Then why on earth are the evaporator coils of refrigeration units usually on the BACK interior walls of refrigerators? Wouldn't logic dictate that, in order to make more efficient use of your endothermic reaction, your evaporator coils be on the uppermost portion of your refrigerated unit with fans just slightly above these coils to gently encourage the warmer air towards the coils and their output then deflected out and away from the coils along the ceiling?

2.) Why is a window style air conditioner ideal for cold rooms instead of building it much like a giant walk in cooler based on a (by my possibly sloppy logic) more efficient refrigeration model?

I ask these questions because I'm looking into building a walk in cooler built to my specifications with the most efficient cooling properties possible. I know that I don't know everything there is to know about refrigeration and I have lots to learn. But I guess my google-fu is weak. I've been searching for answers on this topic for approx. 48 hours now and have yielded some ... less than satisfactory results. Any inputs would be appreciated and I can sketch out what I'm envisioning if anyone is having a hard time seeing what I'm talking about.

Here's to hoping I don't get flamed.

*edit* If this is in the wrong forum, please move me. I just figured that since I'm wanting to build this myself, it should be in the DIY forum.

Ok so a few things....

The coils are on the back for a couple of reasons...

More readily accessable
More square footage to cover
N it was the way fridges been done for years sonny...

heheh

Window air con is NOT the most ideal
---Its the easiest in so much as a maintenance and build perspective
---Also easily swapped out and replaced
--- Easily pulled and made into a regular room again...

Its a cost value game... The added cost of a real cold chamber would be much much higher then a well drawn out fermentation chamber as per spec anywhere on her...

Yup, this should open a huge debate(argument)... fun fun

In a similar situation... After seeing all these guys build rooms it got me doing research aswell! I have just as many questions as you and will be watching this for information.

But, from what I've seen a large window unit is a few hundred dollars and the few walk-in referigerant systems I looked at were a few thousand dollars. So cost is a big part of it.

The evaporator in most refrigerators that I have seen are located in the freezer compartment. Typically, the evaporator is a sort wrap around plate mounted to the top, back and bottom of the freezer compartment, but there are a number of different configurations used. The refrigerator compartment is cooled by moving cold air from the freezer compartment with a small fan. When a fan is used, it really would not make much difference where the evaporator was physically located. Keep in mind that the manufacturers must balance a lot of cost benefit stuff when building and marketing these units. Utmost efficiency often gets cut off by higher manufacturing costs. I'm sure they could build units that would be twice as efficient as most, but the cost would higher than the market would bear. This pretty much applies to nearly everything we buy from cars to vacuum cleaners and such.

Hot air doesn't rise, heat moves from high temp to low temps, it just so happens to be that colder temps are located higher in the atmosphere giving the illusion that hot air rises.

The coils are placed where they can get the most surface area contact with the inside of the fridge to aid in the removal of heat from the cold box that is your refrigerated area.

The coils get hot because they are removing heat from the air inside the fridge. The R-134a coolant inside the coils goes through several stages to cool your intended product.

could go on for several pages but I will stop here. PM me if anyone wants a more in-depth explanation

Eric_Leifermann said:

Hot air doesn't rise, heat moves from high temp to low temps, it just so happens to be that colder temps are located higher in the atmosphere giving the illusion that hot air rises.

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Hot air is less dense than cold air, and will rise (e.g. hot air balloons).

Eric_Leifermann said:

Hot air doesn't rise, heat moves from high temp to low temps, it just so happens to be that colder temps are located higher in the atmosphere giving the illusion that hot air rises.

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We know energy is exchanged from high energy systems to low energy systems, but hot air does rise. The temperature effects the density, effecting the buoyancy. Like RDWHAHB pointed out, that's what makes hot air balloons possible. Ever sit in a sauna on the bottom bench, then the top bench? Notice a temperature difference?

Are you a Tech student? What's your major?

TB

Hot air doesn't rise

Yes it does and it's a fact.

heat moves from high temp to low temps

Yes, so long as the heat energy is out of equilibrium it will do so according to the second law of thermodynamics IIRC.

it just so happens to be that colder temps are located higher in the atmosphere giving the illusion that hot air rises

This statement doesn't even make sense. If there were an illusion of some kind, the upper atmosphere would be warmer, not colder. It's colder at higher altitudes because the atmospheric pressure is less. It has nothing to do with hot air rising or not rising.


The coils are placed where they can get the most surface area contact with the inside of the fridge to aid in the removal of heat from the cold box that is your refrigerated area.

Yes, sometimes they are configured that way, but not always, and more often than not, the evaporator is in the freezer compartment, not in the refrigerated area. There are exceptions to this, but generally speaking, that's what I have observed.

The coils get hot because they are removing heat from the air inside the fridge

I would guess that you are referring to the condenser coils. These are typically located on the back of the fridge, but some models have them embedded in the outside walls. The compression of the refrigerant causes it to take on heat and this heat is dissipated by the condenser.

I could be wrong on the above, but I don't think so.

Catt22 said:

heat moves from high temp to low temps

Yes, so long as the heat energy is out of equilibrium it will do so according to the second law of thermodynamics IIRC.

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If the two energy systems were in equilibrium, there wouldn't be high and low temps (assuming constant pressure). But yes, you are correct.

TB

The quick version of the cooling cycle in the refrigerator,
The compressor squeezes the cold gas from the evaporator into the condenser where it gives up heat to condense into a high pressure liquid.
High pressure liquid is fed through a restriction where the pressure drops and the liquid needs to absorb heat to boil in the evaporator.
The cold gas returns to compressor to repeat cycle.
The evaporator temperature is a function of the pressure of the boiling liquid, the lower the pressure, the colder it gets, and it traditionally has been located in the top of the cold compartment where the cold air can drop down and cool the rest of the box.
Using window air conditioners for cold rooms will work, but moisture is the biggest enemy of continued operation, moisture flows from the surrounding warm areas and ends up on the evaporator coils where it usually freezes. The competitive nature of consumer products means that the least amount of material you can get by with in a product and not have it blow up will make it profitable. The evaporator coils in the window shakers are designed for 80 degree incoming air and the leaving air temp is usually 50 degrees, refrigerant is usually near freezing trying to move the heat from an undersized cooling coil. When you put the cooling coil into a cold box the coil temperature and refrigerant temperatures go well below freezing and will ice up if ran very long.
The commercial units are designed to run the evaporators at above freezing coil temperatures to avoid ice up when operating with long duty cycles.

Most comercial refrigeration systems have a set hot gas period that allows hot gas to flow through the coils which in turn melts the frost/ice accumulation. With a window A/C unit in a chill box setup at home you should do a manual hot gas (shut it off and open it up to let the frost/ice melt). That will be the best preventative maintenance you can do on the A/C unit.

There's a lot of information floating in these threads that dabbles into what I'm talking about but never really gets it all out there. Kind of the same way google has been treating me.

I'll try to explain what I'm wanting to build in text and then, failing that, I can post a diagram.

I want to take the compressor, condenser coils and evaporator from a deep freeze type refrigeration unit. Remove the evaporator coils and install my own. I want to line the inside ceiling of my proposed "walk in kegerator" with 1/4" ID copper tubing (approx. 50 feet worth, more if needed). Between the ceiling and the copper "coil", I want to install low velocity fans to coax the air upwards across the coils and then deflected out and away from the rest of the ceiling. Maybe I'm over engineering this. But it's something I want to at least make a go of and see if it works.

I have sufficient skills as a plumber and electrician to make it all work in a fabrication sense, but I'm wondering about the efficiency of this design. The condenser coils would obviously be outside the unit as well as the compressor. Maybe this will be one of those "build it and see" type projects.

That is a huge undertaking seeing how you are going to need all sorts of refrigeration gear and an EPA certification. Brazing, vacuum pump, recovery equipment, nitrogen, etc.

Okay, here goes
What size enclosure are you looking at?, the surface area, insulation "R" value, thickness, and temperature difference will get you to the amount of heat to remove from cooler.
Next, what is the heat removal capacity of the cooling system components you want to use. You can look up the compressor information from the tag on the compressor.
Last, it would be better to look for a refrigerator with finned coil exchanger that can be used without breaking refrigerant piping as these units are not easy to rework the coolant lines and recharge successfully.

Ok, lets start this with a few disclaimers and legalities.

That is a huge undertaking seeing how you are going to need all sorts of refrigeration gear and an EPA certification. Brazing, vacuum pump, recovery equipment, nitrogen, etc.

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I'm not going to be doing any dumping and removing of refrigerant. As a plumber, I know several HVAC guys that ARE licensed to remove and purge refrigerant systems. (so please don't lecture me about that.) And also please don't assume that I am completely inept and unqualified as a brazer/solder applicator in the lines of copper plumbing. My friends in the HVAC industry can and will vacuum test my system once it's installed when I think I'm done with it. And as for putting refrigerant back into it, the equipment needed AND the refrigerant can be obtained at WAL-MART!

Now then, to the constructive side of the replies in order to stay on topic.

The size of this "walk in cooler" is proposed to be about 6'x10'x8' outside dimensions. I'm figuring that I'll have 1"x6" stud walls to install r-30 insulation. (I am not positive of the thickness of this insulation at the moment.*edit* r-30 is too think to be reasonably applied here. 9.5") I also do not have a compressor/donor system selected yet.

This is early in the planning stages yet I find it amusing that some people like to immediately assume the absolute worst things possible and start preaching about it.

*edit* Sorry, got distracted by an empty beer. Anyway, with using 1x6 stud construction, this should give me interior dimensions of roughly 5'x9'x7'. Thus giving me a cubic volume of 315 cu. ft to cool. This, of course, does not include vapor barrier (minimalist in space consumption) and interior wall coverings. I figure on using some type of easy to clean paneling. Like the white stuff you see in cheap a$$ public bathrooms. I suppose I could ask my HVAC buddies about this but they're currently in the states whereas I am not. I know this would be easier to calculate and figure out in person but that is currently not possible. So I came here.

Fans create heat, so design with as few as you can get away with.

True about the fans. I was thinking about using some kind of 12 vdc cpu fan that required little to no current to run. That or a squirrel cage fan (the kind used in your car heater or other larger CFM applications.)

This would require that the space above the refrigerant coils be mostly enclosed in order to make efficient use of the fan/fans that are in use. And with a 7' ceiling, that doesn't leave much room for an air duct.

iggie said:

Ok, lets start this with a few disclaimers and legalities.
I'm not going to be doing any dumping and removing of refrigerant. As a plumber, I know several HVAC guys that ARE licensed to remove and purge refrigerant systems. (so please don't lecture me about that.) And also please don't assume that I am completely inept and unqualified as a brazer/solder applicator in the lines of copper plumbing. My friends in the HVAC industry can and will vacuum test my system once it's installed when I think I'm done with it. And as for putting refrigerant back into it, the equipment needed AND the refrigerant can be obtained at WAL-MART! .

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Not trying to lecture anyone. You asked questions and stated your intentions and I was just stating things that came to mind that might complicate it. Having HVAC buddies help will be a great thing and your brazing skills might be great.

Here are some more things to think about as well. How are you going to calculate how much more refrigerant the unit will need? Is it a hermetic compressor? If so, unless you can isolate the compressor and keep it under a refrigerant blanket, then moisture will get in and ruin the oil which will in turn harm the piping system.

Just throwing some knowledge out there since others might want to try this in the future as well.

snackson said:

Is it a hermetic compressor? If so, unless you can isolate the compressor and keep it under a refrigerant blanket, then moisture will get in and ruin the oil which will in turn harm the piping system.

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Now here's something that I'm not clear on. I was under the impression that all refrigeration compressors were hermetically sealed within their system. The good ol' Push n pull type. I was not aware that there were other types. Basically, I'm just looking for whatever would be used in a deep freeze setup. Like I said, I'm going to salvage the materials for the compressor, evaporator and condenser coils. Would it be possible that commercially available deep freeze compressors are not hermetically sealed in this regard? If so, how do they last so long with the inevitable impairment you bring up? As for calculating how much refrigerant I will need, I'm unsure. When I bring the HVAC guys in I could ask them, but if you know of a way to make this calculation, feel free to enlighten me. I'm sure it will make things much smoother if all I have to do is vacuum check then fill instead of start guessing. Overcharging is just as inefficient as undercharging the system.

The system itself is hermetic. Most residential deep freezers come with completely hermetic systems. If you do the cutting out of the old evaporator coils and braze the new ones in within a day or two you should be fine. From their your buddies can pressure test with nitrogen, vacuum test and you can re-charge. I don't know how to calculate out how much more refrigerant you need. One way to do is to recharge from the vacuum with the old refrigerant (charging gas first until pressure is above 0 psi, then liquid). After that you can buy the cans that you referred to from Wal-Mart and slowly add a few ounces at a time, let the system settle out and run for awhile and add more as needed. Your HVAC buddies will be able to help with this. I'm sure there is a way to calculate but I would need to dive into some books to figure it out.

I'm fairly certain that I will not be re-charging the system with the originally recovered and then topping off with Wal-Mart refrigerant. Mixing refrigerants is extremely ill advised. (I've been lectured about explosive results.) I'll probably wind up trying to get a measure of what they recover from the system and then figure out what the volume of the original evaporator coils would have been. Then, calculate what the volume of my NEW evaporator coil is and then start doing some math. Since I don't even have it in place yet, that will have to wait.

The big thing at this point is determining the maximum efficiency of the system and exploiting any strengths it has while correcting any weaknesses anyone familiar with cold room design and refrigeration can identify at this juncture.