The use of a CO2 gas cooler for an Automobile Air Conditioning system 2

[supercriticalc02]

Hello all,

I'm trying to design a gas cooler for an automobile air conditioning system using CO2. I understand the supercritical cooling process of CO2 and its potential to be used as a refrigerant however I was wondering if anyone could help me with regards to the heat transfer and pressure drop characteristics of a mirco-channel gas cooler within an automobile air conditioning system using CO2. Hope that makes sense!

Thanks in advance
David

 

[MintJulep]

The entire automotive industry has been attempting to develop a CO2 based air conditioning system for ten years or so, and thus far has failed to come up with a commercially viable solution.

But perhaps you have access to better resources, so good luck to you.

 

[vpl]

school project?

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[supercriticalc02]

Its a project im doing, but im really stuck, the aims are

a) :to understand the heat transfer and pressure drop characteristics of the micro channel gas cooler within the automobile air conditioning system using co2

b)To recommend suitable design methods for heat transfer and pressure drops in CO2 gas cooler

c)To show a case study for gas cooler with potential designs and provide a recommendation for future design and research

Ive done the reasearch into the cooling of CO2 and understand it but im stuck on where to go, a point in the right direction would greatly help!

 

[MintJulep]

Get a bunch of heat exchangers.

Put them in an operating system.

Instrument the heck out of things.

Get data. Review data. Draw conclusions.

Design new exchanger based on your conclusions.

Put new exchanger into system.

Confirm that it is better.

 

[supercriticalc02]

Im so lost its unreal.

 

[MintJulep]

Wander here

http://www.r744.com/

if you're lost.

 

[supercriticalc02]

Thanks MintJulep! That website is really good!

 

[Montemayor]

Supercritical:

There is nothing “supercritical” about a typical CO2 mechanical refrigeration process used for creating a refrigeration temperature. I designed, fabricated, installed, and operated CO2 systems for many years – most of them had a refrigeration requirement for storing, handling, and transporting the liquefied CO2 at low saturation temperatures (normally 250 psig & -8 oF). However, note that I use the word “typical” for refrigerant condensers working with the refrigerant condensed at ambient (70 – 90 oF) temperatures.

You ask for help in the heat transfer and pressure drop characteristics of a mirco-channel gas cooler within an automobile air conditioning system using CO2. Therefore, since you haven’t described your refrigeration cycle (conditions at the compressor suction, the condenser, and the evaporator) I have to assume all these things as I conceive them for an A/C system:

Condenser outlet = 1,200 psig and 115 oF (supercritical state);
Evaporator outlet = 490 psig and 32 oF (saturated state);
Compressor suction = 487 psig and 37 oF (superheated vapor).

The above conditions require a compression ratio in the compressor of 2.42, which is OK and a good design for a reciprocating compressor. However, note that the absolute values of the suction and discharge pressures are relatively high and demand a very heavy and robust compressor cylinder, piston, and valves. The tubing using will be heavy gauge.

I have assumed an air-cooled condenser and therefore, can only condense at approximately 115 oF. The necessary Liquid CO2 receiver will require a pressure vessel with an MAWP of approximately 1,500 psig – a relatively heavy steel vessel.

The heat transfer in the air-cooled condenser and the evaporator will not be a difficult or challenging task. Both exchangers will, of course, have the CO2 in the tube side (because of the relatively high pressures) and the tubing employed will have to be of a heavy gauge. The same robust quality will also apply to the expansion device or valve.

The expansion device will carry out an isenthalpic expansion that can be easily followed on any Mollier Diagram or a CO2 database. CO2 in the pressure range I have described will behave as any other refrigerant – as long as you stay away from the Triple Point (-70 oF & 60.5 psig).

Of course, you must be aware that the thermodynamic efficiency of the CO2 refrigeration cycle is rather low when compared with other conventional refrigerants – such as Ammonia. This means that you are going to consume more energy than a conventional refrigerant cycle. That inefficiency and the higher capital cost of the high pressure equipment (compressor, condenser, receiver, evaporator, and tubing) are the main trade offs when using the CO2 refrigerant.