Questions

[Dtime35]

Hello I have been reading up on refrigerants and was wondering is there no way to make a system that uses them in a cold state always, so you can allow air to by pushed over the coils to be cooled? Do you have to have the compressor and valve? Basically can you take an enclosed area and make the refrigrant in it a cool form and keep it that way in the enclosed area?

[Peter_1]

No, you can't Dtime.
And why? well,....nature laws or thermodynamics with a more scientifice term
Two adds via Google
http://www.freestudy.co.uk/thermodynamics/t2109.pdf
http://www.freestudy.co.uk/thermodynamics/t5201.pdf

have perhaps a look at http://www.freestudy.co.uk/thermodynamics2.htm for all their files

[Ahmed_LNG]

hi dtime..
what happens in any chiller is heat transfer the air is hot , the refrigerant is cold .
air takes the cold temperature from the refrigerant and the refrigerant it self takes the heat from air ... so by time refrigerant will be just hot vapor

[Brian_UK]

Sorry, perpetual motion has not been invented yet.

[star882]

As long as the area/device being cooled can be warmer than the place the heat is discharged into, it can work without a compressor. I've seen that being used in laptops and certain high performance desktop/server computers. The idea is to boil a refrigerant (usually R134a or an alcohol) with the heat from the device being cooled, and then condense it with the heatsink. The refrigerant returns to the evaporator by gravity. Done right, it will work like a material with a very high thermal conductivity.
Now, if you need the temperature to be lower, then you need a compressor.

[Peter_1]

Sounds like the principle of a heatpipe.

[US Iceman]

A thermosyphon loop could also be used for this, but again, the lowest achievable temperature is fixed by the heat sink temperature that the heat is transferred to. This seems to be the same thing star882 is talking about.

You can do the same thing for heat recovery in air systems during the winter time. The lower dry bulb temperature provides a much lower heat sink temperature to recondense the refrigerant. However, most of these systems are dependent on gravity.

So, the installation is critical to work properly.

[abdulazman]

Impossible?????
An impressive imaginative idea but lets be realistic.

[US Iceman]

abdulazman,

What are your concerns with this? I have done this several times and it works very well.

This is no different than the use of a thermosyphon for oil cooling on large screw compressors.

Regards,
US Iceman

[Lazarus]

Well designed pipework and a massive condenser.... whats the problem... Didnt somebody do this with, i think it was council offices in England. Had to force run the compressors once a week to keep the bearings lubricated.

[Lazarus]

It was a long time ago.... the only reason i remember is that we had the problems with the bearings in the compressors because the things were never run. A pricey install but if the end user has the foresight and the right duty requirements, a good system.

[US Iceman]

Enhanced heat transfer surface use depends on the film coefficients for each side of the surface. When using refrigerant to cool or heat air, the air film coefficient is much lower than the refrigerant side.

Therefore there is little to be gained by using enhanced heat transfer surface on the refrigerant side.

For water to refrigerant, the water film coefficient can be quite high, so enhancement on the water side would be beneficial. In this case, the refrigerant side enhancement can help.

The lower film coefficient can be said to be the controlling limit, so if the enhancement is on the wrong side very little is gained at a potentially significant cost.

The High-flux tubing is very interesting. One of the drawbacks to it is that you need some minimum temperature difference to start the nucleation and capillary effect of the sintered tubing. In some cases (from reports I have read) this type of heat transfer surface can take some time to reach it's full potential for heat transfer.

If I remember correctly, the tubes must first heated to initiate the capillary action of the surface, then the entire surface must reach it's full potential for all nucleation sites on the tubing.

[TXiceman]

Thermosiphon cooling is old school and works very well. But you still have to have a heat sink to dump the heat in any case.
Ken

[phil68]

A thermo-syphon is used on the thermo-electric bottle cabinets I've been asked to carry out warranty work on recently. It's attatch to the hot side & looks very similar to a domestic fridge condenser. Don't know what the medium is inside the tubing tough.

[US Iceman]

We looked at the High-Flux tubing from UOP for flooded ammonia chillers. It was an absolutely fascinating material, but too expensive for our use.

It was actually cheaper to manufacture a larger prime surface exchanger, than the smaller one using the UOP enhanced tubes at the time.

All of the nucleation/capillary stuff came from the research papers I reviewed when we were looking into this. It seems to me that in one case the researchers mentioned it could take up to one hour before the tubing performed at full potential.

The increase was not due to the surface area of the tubing so much as it was the capillary action and the many nucleation sites on the tube due to the sintered finish. Similar to smooth tubing versus a tube with a rough finish.

Another interesting item for enhanced heat transfer is made by Cal Gaven there in the UK. They are tube inserts called HiTran. It looks like a spring made from paper clips. Also very fascinating...

We are off the original topic quite a bit, but worth mentioning.