14 replies to this topic

[Gouthami]

Hi, I m a 3rd yr chemical engineering student from National Institute of Technology, India. I came across gas refrigerators and have been wondering really hard why it is not a commercial success... If ammonia is poisonous, cant some other low boiling liquids be used?

Can a low boiling liquid like acetone be employed with a fan over it in a box kept in thermal contact with some substance which is to be cooled.

I meant, if the fan is above acetone, low pressure is created... so acetone starts vaporising and above the fan there will be high pressure ... So, if we can keep a narrow tube(with lots of constrictions in it) jus above the fan, will that condense the vapour acetone... thus the system acting as a refrigerator...

Any suggestions for practical feasibility of this ???

[Art Montemayor]

Gouthami:

I don't know about India, but in the USA refrigeration without electricity has been a commercial AND industrial success for more than half a century.

I was the eldest of 9 children and my mother kept us fed by using a Servel refrigerator in 1950 to preserve food and also to keep it frozen. The Sevel used the Planten-Munters process of refrigeration. This process is well-known and is discussed in almost every Thermodynamics book I've read and have. You should become very aware of this process because of its importance and your need to know its existance and capabilities. Our Servel refrigerator not only preserved food, it also produced ice cubes. And it did all this without making a single sound - it had no mechanical devices or movement! We never knew when the Servel was making ice or not, since we couldn't hear it working. It used only natural gas to operate - although we had to manually fill the ice maker with water in order to keep making ice.

The Absorption Refrigeration System - of which the above Platens-Munters process is an example - is an important industrial process. There are many refrigerants that can be used in this process - ammonia being just one of them.

In fact, WATER is one of the best refrigerants that you can use - with a limitation of approximately 35 oF on the low side. Your course(s) in Thermodynamics should clearly point this important point out. I have designed and employed water as a refrigerant by merely using a jet evacuator and a couple of vessels. The motive force for producing chilled water at 40 oF is simply low pressure steam - nothing else! Of course, if you want to distribute the chilled water you have to furnish a pump - but this too can be driven by a steam turbine. So, in essence, you can chill water for process or other uses by relying on low pressure steam - which is often a "waste product" at many plants. Many companies have exploited the use of their low pressue steam to produce chilled water - or even produce lower tempeatures by relying on the Platens-Munter process.

You are probably just getting to Thermodynamics in your ChE course work at university, so all this sounds strange to you. However, what you are discovering is some of the most basic and most interesting subjects in Thermodynamics.

Isn't Thermo COOL?

[Qalander (Chem)]

Dear/Respected Art.
Thanks and Best Regards for this highly useful info for us indeed!
Qalander

[Gouthami]

Dear Art,
Thank you so much for that insight.
Is the hydrogen used in such models, blown across ammonia, creating low pressure for evaporation.... Can a fan also do the same thing ??
I was thinking why not have the entire thing in one set up completely instead of such an elaborate arrangement ..... that is we can evaporate, condense in a single vertical stand..

We can have the low boiling liquid in a container.. with a fan over it.... fixed to a vertical tube with lots of constrictions for condensing... I ve shown it pictorially in the attachment...


What do you think of it ? Is tis possible... Also, I read in the papers recently that a few families in USA are now trashing their refrigerators for contributing to cut down the pollution...
Will it not be great if refrigerator companies start researching more on this kind of technology for cooling....?? With Obama very concerned about climate crisis.. he may jus encourage buying of such refrigerators... It would be a great economy boost for the current crisis.

The exhaust from a vehicle... the smoke can be used as force for driving the fan in a cooler, it will provide to be a portable cooler

I feel with a little initiative.. all of us from different parts of the world can work on this to do somethin tht can make a big change !!!



Gouthami

[DRS]

Vapor Absorptions Systems are very old in India (for commercial purposes) and I have been operating them since I started my career (say 13 years). Ammonia+Water and Water+LiBr systems are in vogue here. The possibility of whether all refrigerated systems can be converted from VCS to VAS depends upon two critical factors. One is bulkiness and second is COP. VAS can be used as an alternative to VCS but it doesn't make business sense in many of the cases Most of the areas where we have waste heat, VAS is preferred and is followed.

I have an opinion that you overlooked Mr Montemayor's last comment. You can have answers for your questions if you understand the second law of thermodynamics and the theory of refrigeration properly.

You have some very basic questions that an engineer should answer themselves.

For example, 1) Using a very low BP liquid and making a compact system - As we all know that refrigeration systems should be closed loop, so how are you going to condense the low BP liquid efficiently? Suggestion - Refer to refrigeration cycle principles and Boyle's law

2) We can have the boiling liquid in a container with a fan over it...... - When you are restricting the flow then there is pressure drop and this is against what you should do to condense the gas back to the fluid state. Suggestion - Refer to Boyle's law

Mr Montemayor has mentioned the essence of VAS including the disadvantages which are generally ignored. The last time I worked out on an Air Conditioning Project of 1500 TR capacity, vapor compression systems seemed to be advantageous.

There are many technical issues like capacity control, crystallization, corrosion issues and low COP etc. Read the earlier post carefully and do google search on vapor absorption machines and you will be clear. I would suggest reading Thermodynamics by PK Nag. If possible, Thermodynamics by Zemansky is a delight.

[Gouthami]

It is true that if i have the fan over a closed container, there will be restricted flow leading to pressure drop... But if I allow some gap between the fan and vessel wont the vapors try to move up creating higher pressure on the upper side...

Just like in a normal fan where there is high pressure on the side opposite to that creating low pressure .... Like in a helicopter... high pressure will develop on the bottom side ?

[DRS]

The primary and rudimentary technique of cooling any substance is by sublimation or evaporation of another substance and this is a destructive method. Economically, it is better to recover the refrigerant and reuse it interms of a closed cycle refrigeration. I presume you are discussing about a closed cycle.

All closed cycles operate with final heat rejection to atmosphere (which is called as sink), either by means of cooling towers are air coolers. No closed cycle can continuously perform unless the final heat rejection is done to a system whose temperature remains constant, irrespective of the amount of heat it absorbs.

Thus, the minimum condensing temperature of a refrigerant is restricted to the temperature of the atmosphere (either wet bulb or dry bulb depending upon the type of condenser). Low BP gases have low condensing temperatures and only method to increase the condensing temperature is to increase the pressure.

Fans are low pressure ratio devices and may not be able to increase the refrigerant pressure to such an extent that condensation is possible at ambient temperatures. Plot the cycle on a P-H or T-S chart of the refrigerant under discussion and see whether it is possible.

[Gouthami]

yes i meant a closed cycle...

And yes, i felt the draw back myself. The high pressure may not be enough..but with some design calculations maybe we can construct something suitable.

Actually i thought we ll have a fan outside the closed system ... now this can help power the fan inside(above the refrigerant) ... also it can push the air around outside the closed system.. so it ll be a good heat sink...

We can power this fan outside by employing some waste gas...

This is just an idea.. I hav no theoretical proof that it will work.. I m in pre-final now n I felt this could be a great final yr project for me... I have received a lot of help on this aspect from all of you... thanks a lot.. With some more "real" working on this i ll get back

[Gouthami]

Hi,
In my coll we recently conducted the international techno management fest "Pragyan". I organised one of the events, called delta-T where the participating team has to build a model to cool water without using electricity.
The 3rd place went to 2 teams - one, used a brute force method, they employed just liquid nitrogen.. but their design was good; Heat removed was very high. Another team actually brought down a vapor absorption refrigeration system ...but as they lost the vacuum in their system while travelling .. so heat removed was very less.
The 2nd place went to a team that employed thermosyphoning principle...
& the 1st place went to team that had a very simple model employing just the idea of a cooling tower.(evaporative cooling)Small plastic buckets constituted the cooling towern with water entrainment , & they just had to blow in air.

Lots of other teams employed endothermic reactions to drop the temperature. Some employed multiple principles.

Over all it was a good show. I will keep updating this space everytime i come across a new method.

[Gouthami]

I wish to know only this..

In utilising thermosiphoning principle, for condensing i can employ just something similar as in refrigerators rite ??

[shadow0000]

Hi, I m a 3rd yr chemical engineering student from National Institute of Technology, India. I came across gas refrigerators and have been wondering really hard why it is not a commercial success... If ammonia is poisonous, cant some other low boiling liquids be used?

Can a low boiling liquid like acetone be employed with a fan over it in a box kept in thermal contact with some substance which is to be cooled.

I meant, if the fan is above acetone, low pressure is created... so acetone starts vaporising and above the fan there will be high pressure ... So, if we can keep a narrow tube(with lots of constrictions in it) jus above the fan, will that condense the vapour acetone... thus the system acting as a refrigerator...

Any suggestions for practical feasibility of this ???

The advantage it has over other flame powered refrigerators is that you don't need propane or kerosene to run it and it will cost far less. All you need is a cook fire.

Engineering being the art of compromise, I can see a disadvantage in that you will have to build a fire to heat this device up every 24 hours to keep food cool. There is no free lunch. It will consume cooking fuel and because it is labor intensive, it will add to the time constraints and physical energy drain on a given villager who already expends a lot of time and energy working in fields, collecting water and fuel.





_________________
Ice Machine Parts

[Gouthami]

energy can also be provided by some wind catcher right ....All i need to do is some mechanical energy, building a fire neednt be the only option

[Neelakantan]

dear ms gauthami,
vanakkam

your surmise that an easily evaporating fluid (you would have let acetone deliberately on your back of the palm and would have seen the cooling effect; who doesnot!) can be used for refrigeration is well known and accpeted; and you are aware of mann-panai (earthen pots) where water evaporates through pores and keeps the bulk reasonably cold; (the same principle is used for cooling towers!)

now for a succesfful refrigeration, you have to go beyond ideas and define numbers. being a young chemical engineering student, i am sure you are comfortable with calculations! (and regional engineering colleges admit top of the pool!)

let us consider an insulated container *you define the volume, dimensions of your choice". let us also consider that a small stream of water flows through it in a perfectly conducting tube. say a litre a minute. now to cool the water say from 25 deg c to 0 deg C!, we need the acetone cooler than 0 deg c (thermondynamics law: heat flows from higher temp!)

now see if u can have ur acetone at say -10 deg C; and if practicable, see how much heat is required fby the sensible heat of water alone, and how much vapours of acetone are generated. now look at your fan, which will essentially drive the cold vapours from atm pressure to a slightly atmospheric perssure and wht is the condensing temperature!? /pressure? let us assume you are able to draw out the vapours at atmospheric condition and fan pushes it to say condense at 0 deg C. From the property tables work out the pressure. now calculate the load of the fan!

this will give an idea of the system, power requirement etc. the other part is then to see the size of the tube required for cooling 1 lit/min of water, and the heat transfer rate, area etc

nice project na!
best of luck
neelakantan

[papaya]

hi, i don't know much about thermodynamic but these got something to do with heat transfer and temperature change from high pressure to low pressure?

But surely all these electricityless methods still require some sort of energy driver to run it?

[Neelakantan]

now see if u can have ur acetone at say -10 deg C; and if practicable, see how much heat is required by the sensible heat of water alone, and how much vapours of acetone are generated. now look at your fan, which will essentially drive the cold vapours from atm pressure to a slightly atmospheric perssure and wht is the condensing temperature!? /driver to run it?

hi
there is a small fallacy to be captured on the above statement, remembering that acetone is a pure component with a unique pvt relation;

one more point to note is the arrangment of the closed circuit for the condensed acetone to return to the evaporation area (container)

cheers
neelakantan