I think first we have to look at the places RV's go - and what may or may not be available there.

They are much the same as fully kitted out BOATS! You don't always have a power station nearby, so you can't just plug in a conventional refrigerator. Even IF YOU CAN - there is only so much power (amperes) that you can expect to go through an extension cord...

(Let's face it, that's what an RV power cable is)

So you want something that does the job with very little power, or even better "PORTABLE POWER"


RV refrigerators are designed based on a very strange example of physics.

The expansion of liquids into gasses causes an apparent change in temperature - they become "COLD".

When gasses become liquids - the temperature of them increases because the molecules are more closely packed together.

In the different states they are in heat can be radiated off, so that when they change state the difference is magnified, or they are cold in relation to what is around them so that they absorb heat energy from what they are surrounded with.

These are two key principles that are the basics of refrigeration and air conditioning....

BUT!

As regards RV refrigerator systems the methods get even wierder yet:

It is a standing fact that materials have different properties in different atmospheres. Ammonia for one, acts completely different in an atmosphere of natural air than it does when surrounded by HYDROGEN GAS (yep - the same stuff that ruined the Hindenburg Zeppelin). RV refrigerators contain both of these things, as well as a small amount of dye to tell if they are leaking. For future reference it is bright yellow, so if you see that your cooling coils have ruptured...

This process is so much like alien science that it will take a while to complete, so expect this POST (I intend to edit it continuously so that all of the information is RIGHT AT THE TOP) to take a while to be finished.

In an RV refrigerator a heat source is applied at the bottom of a riser tube that carries the evaporated ammonia to the top of a system of criss-crossing channels in which it condenses and travels back to the bottom - where it collects again so it can be heated and rise to the top once more.

The tubing that the channels are made of are in contact with the inner surfaces of the refrigerator and transmit the "COOLING EFFECT" to the inside of the refrigerator compartment.

TAKE NOTE HERE that heat is energy - cold is the "ABSENSE OF HEAT ENERGY", so what we think of as cooling is really the absorbtion of heat energy from something that has less heat energy in it. We absorb heat from where we don't want it, and hopefully radiate it out in a place where we don't care about it...

Air conditioning and refrigeration are essentially "WASTEFUL", we are throwing energy away. HEAT energy, or BTU's (British Thermal Units) are transported by various means. We pick up as much as we can from a material that has been rendered able to absorb more heat, and then concentrate the heat some way so that excess heat can radiate off in a place that is cooler. We then change the material in such a way that it become comparably cooler because of molecular change.


"Oh dear - I wonder if I have just lost all of you..."

 

Tracking your description, and had been wondering how this thing works. Thanks for filling us in.

 

Good info. It is indeed strange how you can get cooling from a fridge that has a burner.

 

neat! I love science.

 

very interesting, did you post this on RV.NET ?

 

I look forward to the completion as well.

I understand how they work. What I don't understand is why all our refridgerators don't work the same way. The RV system seems so efficient, particularly on gas, why don't we have natural gas fridges in our homes?

 

yup one little flame the size of a pilot light works pretty good all summer & very economical too.

 

I think the main reason this type of system is not used in homes is that it would require a lot of modification to the home to have one, versus just plugging in a standard electric fridge. You need a cavity behind the fridge to allow air movement and removal of the collected heat.
Many commercial systems use a similar setup with ammonia absorption, but they typically have a heat exchanger on the roof of the building that handles this, with a circulated cooling fluid going to the fridge/freezer.

 

you have to consider where the heat is moved to once it leaves the refrigerator's interior.

in an RV and most commercial applications, the heat is moved directly outside. If any of the coils should rupture, the gases will be vented directly outside. When using ammonia for a refrigerant, that is a good thing (story below).

Indoor refrigerators move heat from the inside to the room around them. Should a coil rupture, you will soon have a room full of whatever refrigerant you were using... hopefully it wasn't toxic.

Story:
I worked with a root & vine crop harvesting outfit for a little while. One of the vine crops was cucumbers (the pickling type). To keep cucumbers crisp after harvest they have to be chilled in a hurry before being transported. The way this is done is by pouring thousands of gallons of 40°F water over the cucumbers. To chill the water, there are a few ammonia refrigerators (not enough in my opinion, as they couldn't keep up) with absolutely no safeguards in place. One of which was a previously portable unit...(no longer portable, as it had long since been blocked in by the infrastructure of the operatiron) This particular unit required an outside supply of water to cool the high pressure coils... but would run just fine without it.

On one particularly long harvest day, we were using both systems to keep the water as cold as we could, and somewhere along the way, the cooling water supply was shut off to the secondary unit. Amazingly enough the unit lasted the entire day, and waited until the last truck had left before it started bursting coils. The bursting coils meant one thing... Escaping ammonia gas. Ammonia gas is invisible and burns the eyes, and the nose. It was a good thing the refrigerators were outside, or else the gas would not have been able to escape to the atmosphere as it did.

 

There where ammonia gas fired absorption refrigerators (low pressure) in houses long before there where electric ones mostly in rural areas before electricity made it out in the country. We had one in our cabin in the 60s because the only electric we had was from a generator.

Commercial systems that use ammonia don't use propane or natural gas but use a electric refrigeration compressor to compress the gas and condensing units condense it back into a gas under high pressure. Ammonia is a good cheap refrigerant but very very dangerous to work with under high pressure and is not very widely used. In my business I would not work on a ammonia system life is to short.

Denny

 

The limitations on location make a lot of sense. A home system similar to an RV system would ideally be on an outside wall and possibly even serviceable from the outside. In the event of a leak you would want a flow path toward the outdoors. That would be possible on new construction, but very limiting for interior design.

It sure would be economical though, and the fridge uses a lot of juice.

 

One of the big food supply warehouses here had an ammonia leak a few years ago. I remember because our summer camp didn't get a shipment, and the cooks had to scramble first to get us through the week, then find another supplier for the rest of the summer. The warehouse was shut down for at least a week or two before they got it all aired out and cleaned up. Everything refrigerated/frozen was lost. They were using loaders to fill the dumpsters, that is after the air had been cleared off all the ammonia, so everything was good and well rotted.

I'm sure if it was more efficient and safe, we'd be using it in our homes, but as mentioned, the logistics for doing it safely would be hard to design around.

 

Remember up until the last few years when the gas company's had natural gas A/C units. They where ammonia absorption units that cooled water mixed with antifreeze and ran it through a A coil in the furnace. I replaced a few of them that sprung a leak between the ammonia and water side and it was not fun. If you want to know how dangerous a commercial ammonia system is, you have to wear protective eye and breathing apparatus before you can enter the compressor room. it wasn't for me there was plenty of good money to be made elsewhere.

Denny

 

Looking forward to more information on this subject. I firmly believe there is no such thing as "Too much information" about anything.

If I don't know something about the subject, I haven't received enough information yet. Ask my wife, she'll be happy to tell how much I don't know.

 

Denny