Car AC Compressor Conversion

[shadeyinka]

Hello,
I am a final year student from a polytechnic in Nigeria. We are required as part of our group project to develop a cheap Water Freezer that will be powered from an internal combustion engine.

We have decided in our group to use the AC compressor of cars since they are belt driven. The problem now is that
1. What is the average maximum power rating that we can obtain from a single compressor.
2. What will be a suitable refrigerant for this conversion.

Urgently Yours,

Shadeyinka

[hyperion]

At normal ambients, when used on a car these could range from 2 to 3kw. However the duty will be much less when used at the lower temperatures that you plan. Car A/C compressor have not been designed to operate constantly on low pressures. If you get one of a modern car, post 1995 the compressor will have been made to operate on R134a. Whilst this refrigerant is not ideal for freezer applications, you may be able to get it to work. You will need an expansion valve or other metering device to control the refrigerant flow, a suitable drier, a low pressure safety switch and a controlling thermostat. Most car A/C systems use purpose made hoses and connections which are not always compatible with commercial refrigeration products.
Sounds like an interesting project. Good luck and work safely.

[shadeyinka]

Thanks Hyperion,
You have indeed been helpful. At least I know the approximate rating of the
internal combustion engine to use with the Car AC compressor.

2 to 3kW sounds impresive. Is is possible that cascading two car AC compressors
will produce the continous low pressure at the suction side?

I know that some refrigerants are not compartible with some plastic or rubber
valves of the compressor. I wonder if that is a case to worry about suppose i
choose not to use R134a. We hope we can freeze water to a low temperature of at
least -20 oC

[hyperion]

You will need to obtain an evaporator coil or freezing plate. This will need to be in an insulated container or coldroom.
You might be able to find an old ice cream serving cabinet that could be re-used as your basis for the insulated cabinet.
The cooling capacity of the compressor and system is inter-related to the quantity of water that you wish to freeze and also how quickly you require it to reach the target temperature. If you wish to create ice cubes in an ice tray for example to prove that your system works, you will not need all that big a system or a temperature as low as you have stated.
If you require to freeze a larger quantity of water, then the cooling capacity would have to be increased.
You will also need a condenser coil and fan. You might be able to salvage this also from the scrapped vehicle that you retrieve the compresor from.
You may be able to find a complete belt driven compressor/condenser set in a scrap yard which could be easily modified to be driven by your petrol engine. These were very common up until recently over here. This would probably be a more substantial unit that would require less modification.
You might be able to persuade a local refrigeration company to part with an old unit, once they are aware of your project.
To create the sort of temperature that you mention will mean that the R134a suction pressure will be in a vacuum state. On a perfect new system, whilst not ideal this may not be an issue, however there is a great chance of air and moisture being sucked into the system if a leak occurs and has not been repaired. This will adversely affect the performance of the system, eventually causing complete failure.
Using second hand kit of unknown quality could present some ongoing problems, particularly if the pipes have become porous.
If you use a different grade of refrigerant, this might not be compatible with the ex car A/C compressor or condenser coil.

[young gun]

Quick question slightly off topic but still relevant?
Why do most car air-conditioners use Reciever driers, and not the normal inline driers.
My thought is that because dramatic head pressure changes could occur in a short amount of time.
1 minute at the light on a hot day with exhaust fumes blwoing straight into the condenser. One minutes going 100km down a highway with nice coud breeze blowing straight through the coil.
Reciever would assist in sending more liquid to tx valve when such big head pressure changes could occur.
Am i on right track???

[Brian_UK]

^^ Young Gun ^^
^^^ Yes ^^^

Also, they often have a sight glass on them to help with charging.

[shadeyinka]

Hello Hyperion,
Thanks again for your valuable advice. It looks like if we are going to use the car AC compressor we will be stuck with R134a refrigerant. I am afraid, this has torn my team into two camps.

The first camp is looking at the option of cascading two AC compressor pumps to achieve a sufficiently low pressure at the suction side.( Interestingly, you didnt comment about the possible effect of this in your last post).

The second camp is opting to abandon the car AC compressor entirly and go for the (you wont believe this) "TRUCK AIR BRAKE COMPRESSOR"!!!....Their argument is that they are sure they can push up to 120PSI on the high side and with a suitable throttling device they will get a low pressure at the low side of the evaporator. This group also feels that the Truck air brake compressor will be able to handle R407c or R410a

So who is right and which of us is wrong? Thanks in anticipation of your helpful answers.

shadeyinka

[hyperion]

I think that your truck air pump will not with be suitably "leak proof" as it has been designed to work with air and not the much smaller molecular structure of refrigerants.
It would be best to stick with a compressor that is intended for refrigerant use.
The discharge pressures that you mention do not equate with R407C let alone R410A.
Using either of these modern blends for the low temperature that you are attempting will be difficult to achieve. There are limited types of expansion devices available for R410A, other than electronic expansion valves. R407C is better served, but is not normally used for low temperature experimental applications.

[shadeyinka]

Hi Hyperion,
You have been the Most Helpful. Thanks a lot.

We discussed this in my team and i am glad to tell you that all of us agree perfectly with your views. Thanks again for not allowing us to waste our money.

Does any one have any experience with cascading AC compressor pumps?

Theoretically, cascading two compressors should create a larger pressure differential between the High Pressure side and the Low pressure side of the pump. Can we cascade 2 car AC compressors, using R134a to achieve freezer temperatures of the order of -20oC?

[semb]

You could try using R290 (propane) instead of R134a, as long as you are operating somewhere safe (i.e. outside, so a small leak will not cause a problem).

[hyperion]

Advocating a small leak with R290 is not an acceptable practice. Be very carefull if you proceed down this route as you have alrady indicated that you intend to use a petrol engine for the power source. Any propane leak could have an unexpected/explosive result.

[shadeyinka]

Hello,
We have decided to go on usnig the safest method. R134a and the car AC compressor. We are using a 5kW Petrol engine.

To improve efficiency we are planing of using 2 fan+water-cooled Car AC condenser. We are not sure whether to connect them in series or in parallel.

Can we get a very low suction pressure if we use a larger evaporator in our cabinet? We are thinking of using Four Car AC evaporators connected in parallel. We feel that an over-sized evaporator will help us to obtain a low evaporation temperature.

We also intend to use a single car expansion valve(not two or three). This we think will regulate the high pressure side to the capacity of the compressor. We will like to know if we are on track.

Thanks. We have about a week to go into production.