ammonia-water refrigeration system

[Salah1]

Hi gyus:
if the ammonia evaporation temperature is -33 degrees Celsius at atmospheric pressure, why can not we get freezing temperatures at the evaporator (-15 or -20 degrees Celsius for example??) during ammonia-water refrigeration system??
can we achieve that by running the evaporator at atmospheric pressure or even below ??using a little air fan ??

[SLK001]

Probably because the flow is metered to keep the evaporator temperature above the freezing point of water. If not, it freezes up in no time flat.

[deltap]

You may be confusing a vapor compression system with an absorbtion system. They do not work the same way. Vapor compression uses an electric motor or combustion engine to drive a compressor. Absorbtion uses a heat source like steam or gas flame to drive the system. Small absorbtion systems use ammonia/water with water being the refrigerant. Larger systems use a mixture of lithium bromide and water.

[rdfeil]

As deltap said, we would need to know which refrigeration system you have. I have worked a lot with the Vapor compression type systems and it is no problem at all getting the evaporator temperature below the freezing point of water (32F or 0C). I have worked industrial plants where the final evaporator temperature was below -40F/C.

[reggie_obe]

Nothing odd or unusual about the OP's first post?
While members work professional in a variety of fields, still it isn't Home Refrigeration Engineer.

[Salah1]

Thank you guys for the quick reply,
I plan to run a freezer based on ammonia-water absorption refrigeration system with some solid waste as heat source,
in most articals I have read related to such system the evaporation temperature is usually around zero Celsius or alitte lower (about -5 degrees Celsius).
what might be the reasons for that???
why it is not possible to get much lower freezing temperatures such (-15 or -20 degrees Celsius)).
thank you.

[rdfeil]

Nothing odd or unusual about the OP's first post?
While members work professional in a variety of fields, still it isn't Home Refrigeration Engineer.

reggie_obe , I disagree with your statement. Without the details the question is vague at best. Now with the explanation given above things are 100% clear and people with experience can provide answers. With that said, I am not one of those people as I have no experience with absorption systems. I may be missing your point????

[Erich]

His point, this is a forum for people with interest in and possibly expertise in Machining. If you want expertise in Ammonia absorption refrigerators, there HAS to be a place on the web with a greater concentration of said expertise than this forum.

[Tom S]

His point, this is a forum for people with interest in and possibly expertise in Machining. If you want expertise in Ammonia absorption refrigerators, there HAS to be a place on the web with a greater concentration of said expertise than this forum.

Eh, probably not. This is a niche area of expertise inside of an industry that is already specialized. I bet that greater than 95% of people working in the refrigeration field have never touched an absorption system.

I'm very familiar with small vapor-compression systems, but have almost no knowledge of absorption systems, but I'll highlight some things that might be barriers to an absorption-system freezer.

First off, you need to recognize that for a freezer you need to move more heat than for a fridge. You can offset that through improved insulation, but at the end of the day the colder your box the more heat will infiltrate it. The way to counter that increased heat flow is that you need to move more refrigerant. A pound of refrigerant at a certain condition can only absorb a fixed amount of heat energy. In order to increase the heat transferred via refrigerant you need to increase the mass flow of the refrigerant, how many pounds per hour are being moved through the system. Now, something to note is that at a lower pressure ammonia is able to absorb more heat than at a higher pressure, but you're still going to have to move more refrigerant in the end. With a vapor-compression system this is achieved by increasing the size of the compressor, I'm not sure if you're able to do the same in an absorption system with the absorber/generator.

The compressor in a vapor system serves 3 purposes - it moves refrigerant through the system, it increases the pressure on the high side, and as it increases pressure on the high side it reduces pressure on the low side (in conjunction with the expansion device). The colder the system is designed to run, the greater the pressure differential across the compressor is. We're still condensing using ambient air so the high side remains at the same pressure to match that requirement, but the low side of the system needs to have a lower pressure in order to achieve lower temperatures. The greater the pressure difference the greater the power input - but we're dealing with a mechanical item that has a motor and a piston and a cylinder, so by increasing the size or power it's easy to match that greater pressure requirement. In an absorption system we don't have that same mechanical action but are instead relying on the pressure differential created as the ammonia is driven out of solution via heat input. I think that ammonia systems run on a pretty low pressure differential and I think it will be difficult to increase that pressure differential to the point where you have a legitimate freezer.

The third thing that might give trouble with the system is using water as a solvent. Refrigerating a well-insulated box with minimal airflow you might be able to hold that evaporator right around 30F mark. With hot ammonia coming into the expansion device it's probably just enough to keep any ice from forming at that critical point if there was any moisture making it up to the evaporator. However, with a much colder evaporator for a freezer the risk of that happening increases and it becomes more and more critical that every last drop of moisture is removed from the ammonia to prevent any ice from forming and blocking the expansion device. Moisture is the enemy of vapor systems, not only because it can contaminate the oil, but also because with enough moisture it will cause an ice blockage and prevent the system from operating correctly. An absorption system is designed to work with water, and because of that may be restricted to temperatures where that water cant' cause a serious issue.

These is just my thoughts, and I may easily be proven wrong by someone with more knowledge than me.

Please also be careful and aware of what you are getting into. Ammonia is a dangerous chemical and won't give you warning or a second chance. Service personnel have died working with it when they don't take the proper precautions. It is not something that I think should be experimented with in your garage or workshop. If something goes wrong you can kill yourself and your family without warning and then possibly kill or injure the emergency personnel responding to the situation if they don't know beforehand what they are entering into. I'm not one for fear-mongering, but ammonia is not something to be taken lightly and needs to be handled by professionals.

[J Tiers]

It's been decades since I had to calculate refrigerator related stuff in e-school. BUT.....

Absorption refrigerators exist, and they have legitimate freezers. A friend of mine who is off-grid has two of them. They definitely go as cold as a regular fridge and they do not have an incredibly bulky insulation system. A former el=mployer of mine had a gas air conditioner. It worked fine too, and cooled a large building quite nicely.

A solar-heated A/C system has been used in tropical resorts.

The questions are not "if" it works....

Instead the questions are how well you can make one, and whether that is a good idea to do or not.

[Erich]

The absorption fridge in my RV has a freezer, Makes ice cubes just fine. Keeps ice cream nice and firm. Sometimes the fridge portion gets too cold and the lettuce freezes. So it works.

[Doozer]

I for one welcome intelligent questions and commentary.
Even if not totally machining related.
Booo...If you are trying to be the forum police.
We already have too many people telling others what to do.

-Doozer

[Tom S]

Well Salah1 , looks like you have your answer. There are some commercial samples available, maybe you need to do some more research that way. I was working under the assumption that you had done that research, maybe I need to do a little more looking myself first next time. Anyhow, I still would not recommend playing with an ammonia system for the reasons I stated above.