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Minuteman
08-04-2007, 08:36 PM
That's right, when it comes to refrigeration I'm a newbie, and about NH3 I don't even rank as a newbie yet. So if my questions are bizarre now you know why.

I am researching the idea of solar powered air conditioning. If I hadn't happened upon the idea in the RV industry of cooling something by making a flame, I wouldn't be here. But I figured if they can cool a refrigerator with a gas flame one ought to be able to cool a house with a solar collector?

My knowledge of the refrigeration system is limited to what I have picked up online so I am a little bit familiar with the idea of disolving the ammonia in water, heating it to drive it off, condensing it, etc.

In my solar pursuit I have seen that there is a huge amount of inexpensive and plentiful solar heating systems already successfully used for swimming pool heating. Since these heat water, and NH3 refrigeration uses water...

If I was to use swimming pool solar collectors, it presents a limit on the pressure in the system. They are very low pressure (35 psi at 140 degrees F 90 psi at ambient (I think))

So, without anything but theory to back this up, the way to keep the pressure within this range is to increase the circulation rate and keep the temp low. I am assuming this range of temp won't be enough to drive off the ammonia from the water.

But... would it work to inject this now warmed up solution into a vacuum, thus lowering the "boiling point" of it to where the ammonia evaporates out of it?

Thanks in advance - from a newbie

lana
09-04-2007, 10:12 AM
Hi Minuteman,

Welcome to this forum.

I idea you have about ammonia is not going to work. Ammonia (as the refrigerant) and water (as the absorber) is in the Absorption system. Which has a different theory and operation.
The only thing you can do is to heat the water by solar energy then use this water in an absorption system.
BUT BUT BUT BUT .......:eek:

If the required energy can be obtained with this method. In absorption system, usually steam or direct gas fired is used.
Also, absorption chillers come with big capacities which require high heating energy.

Let us hear from other experts before making any decisions.

Regards
LANA

taz24
09-04-2007, 11:06 AM
But... would it work to inject this now warmed up solution into a vacuum, thus lowering the "boiling point" of it to where the ammonia evaporates out of it?

Thanks in advance - from a newbie



How do you intend on creating your vacuum.
The only way at the moment is to use a pump (compressor).
If you are using a compressor to do this then you have a normal compression refrigeration cycle.

unfortunatly your system would not work.

Cheers taz.

Minuteman
09-04-2007, 07:45 PM
Thanks for the replies.

I think what I am having a problem understanding is whether or not the ammonia is mixed in the water as a liquid or as a gas (like CO2 in a soda pop).

I understand enthalpy charts, dew points, wet bulb temp, dry bulb temps, and boiling point temps being affected by pressure.

By itself, ammonia's "boiling point" is very low. It "boils" off below room temp and vaporizes. it doesn't however, boil off from the water mix when it is at room temperature, correct?

Again, my goal is to incorporate the cheap solar building material and design the system around that. My thinking to applying the vacuum was to simply lower the boiling point of the mix to a level low enough to separate out the ammonia.

I just found this in Wikipedia that sounds like what I am thinking - "
Sublimation (chemistry)

From Wikipedia, the free encyclopedia

Sublimation of an element or compound is the change from a solid
directly to a gas with no intermediate liquid stage. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).
At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases the transition from the solid to the gaseous state requires an intermediate liquid state. However, for some elements or substances at some pressures the material may transition directly from solid to the gaseous state. Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.
The opposite of sublimation is deposition. The formation of frost is an example of meteorological deposition."

It had a bunch of links to all these concepts but I couldn't post them because I was new to the forum but you can see the article at en.wikipedia.org/wiki/Sublimation_%28chemistry%29

lana
10-04-2007, 06:49 AM
Hi there,

I suggest that you study the theory and operation of absorption refrigeration system with water/lithium bromide and also ammonia/water.

What you have in mind is little bit confused.
You talk about triple point. Triple point for ammonia is -77.6°C at 0.609 bar(a). This has nothing to do with absorption system operation.
For CO2 yes. Triple point is -56.5°C at 5.17 bar(a). So when high pressure CO2 is reduced to atmospheric pressure it changes to solid state (dry ice).

Cheers:)

Minuteman
10-04-2007, 08:34 AM
Thanks again lana

Yeah, you're right. my last post was way off. I'm not talking about getting it that cold and not getting it to a solid state.

It looks like Raoult's law might be more what I am trying to say. From a writing on it -

"As temperature increases, so does the vapour pressure of a liquid. The vapour pressure/ composition diagram is plotted at constant temperature; a series of these plots at different temperatures (fig 1, right) is needed in order to obtain a boiling point/composition diagram. "

AND

"Boiling-point composition diagrams are usually plotted at Atmospheric Pressure , though for an ideal liquid mixture they’d look more or less the same whatever the chosen pressure. "

When he says " they’d look more or less the same whatever the chosen pressure" I'm understanding that the different values of the chart keep the same relationship to each other(pictorally the graph looks the same regardless of what pressure it is displaying).

So lowering the pressure below atmospheric has the same affect on the vapour pressure as raising the temp, right? That is basically what I wanted to do because the solar collectors won't and can't be gotten too hot.

So it sounds like running the warmed up mix through a vacuum might get it to the vapour pressure and start evaporating the ammonia. I'll have to experiment a little to find out the right amount of vacuum to apply for certain temperatures but that shouldn't be too hard.

So, if that will work, I would now have ammonia gas in the pipe and need to use it for cooling. Since it is still in vacuum if I released it through a venturi the sudden rush through the venturi would have a cooling effect. I'm thinking I'd be able to mix it back into the water right there.

lana
10-04-2007, 12:46 PM
Hi Minuteman,

I suggest that you don't attempt any experiment with ammonia. :eek: :eek: :eek:



So, if that will work, I would now have ammonia gas in the pipe and need to use it for cooling. Since it is still in vacuum if I released it through a venturi the sudden rush through the venturi would have a cooling effect. I'm thinking I'd be able to mix it back into the water right there.


The liquid ammonia is doing the cooling by evaporation not the gas. :eek:

Anyway good Luck to you.