Pressure Cycle in Auto AC

[david.gnaniah]

yesterday I tested a Auto AC which we are developing for a new vehicle. We are using R134a refrigerant. When the system was tested in windtunnel for different operating conditions, it was observed that the discharge pressure was showing a cycle of 3 bar pressure difference every 3 minutes. for example 18 bar for 3 min and 21 bar for the next 3 minutes and so on.

There was no compressor tripping during this phase. I am not able to get the reason for this kind of phenomenon. I have not observed this kind of phenomenon before in any vehicle nor have my colleagues.

Surprisingly this phenomenon was not observed during vehicle idling condition and also as the vehicle speed increases the cycle time came down.

Can anyone throw some light on this.

[NoNickName]

Vehicle thermostat or damper switching on and off?
What was the setpoint?

[david.gnaniah]

Set point for thermister was 2°C and discharge pressure was 32 bar.

But the none of these was observed as compressor did not trip. The discharge pressure was going in a sinusoidal wave form.

Can TXV settings be responsible for this. we were using 1.2 TR 3°C superheat exp valve.

[NoNickName]

Yes, if the TXV does not settle, it will hunt the superheating.

[david.gnaniah]

yes thats quite true. but will the super heat setting affect the discharge pressure. That too in a cycle of 3 min peak and 3 min crest.

I can accept that if the pressure is fluctuating continously. but here it is stabilising for 3 min and goes down stabilizes for 3 min. looks bizzare to me.

Any way thx for the reply. if anyone can throw more light it will be good.

[frank]

Set point for thermister was 2°C and discharge pressure was 32 bar.

A discharge pressure of 479psig on R134A - com'n - you're pulling my leg.

[Brian_UK]

Is the pressure variation due to the condenser fan cycling or the engine cooling fan cycling ?

As the head pressure goes up the heat rejected is absorbed by the engine cooling radiator.

[david.gnaniah]

The Condenser fan was given 12V external supply, so there is no chance for condenser fan cycling. The engine cooling fan was of viscous coupling type so the speed is more or less constant through out.

Yes the as the head pressure goes up the radiator temp should also go up. but how does the pressure comes down again.

For the case of 32 bar. it is the discharge pressure setting at which compressor trips off. This condition is observed only when the vehicle is running at very low speed or idling.

[Andy W]

For the case of 32 bar. it is the discharge pressure setting at which compressor trips off. This condition is observed only when the vehicle is running at very low speed or idling.

Remind me not to stand next to one of your systems on a warm day, I have witnessed that pressure on an high temperature dehumidifier, scary springs to mind.

[NoNickName]

A discharge pressure of 479psig on R134A - com'n - you're pulling my leg.

Let us know the car where this AC is installed. We will never buy it. And also, you'll probably never be able to sell it, since MAC directive is in place.

[david.gnaniah]

Remind me not to stand next to one of your systems on a warm day, I have witnessed that pressure on an high temperature dehumidifier, scary springs to mind.

Its not the case of our system. In case of auto airconditioing this is the set point for compressor trip off. Which normally does not happen. U can observe this behaviour only in high ambient conditions (above 40°C) that too when the vehicle is in idling condition for more than 30 min.

Let us know the car where this AC is installed. We will never buy it. And also, you'll probably never be able to sell it, since MAC directive is in place.

The normal operating discharge pressure for auto AC is from 15 to 21 bar depending upon the vehicle speed. As the compressor is driven directly by the engine, the compressor RPM varies with the vehicle speed.

32 bar is the set point for safety purpose. This pressure is normally observed when the vehicle is tested at extreme controlled climatic conditions and vehicle idling condition. So u wont find this kind of pressure in ur car r any car.

If u test ur car at extreme condition then it will also have very high discharge pressure.

[NoNickName]

The point is that the EU phased out F-gas for mobile air conditioners.

[david.gnaniah]

But in Asia we have much more time than u guys to phase out R134a in mobile airconditioning.

[Brian_UK]

The Condenser fan was given 12V external supply, so there is no chance for condenser fan cycling. The engine cooling fan was of viscous coupling type so the speed is more or less constant through out.

Yes the as the head pressure goes up the radiator temp should also go up.but how does the pressure comes down again.
<snip>.

A Viscous coupling is just that, it is designed to vary the fan speed according to the temperature sensed by the coupling itself. Have you checked the speed of the fan ?

If the fan speed is increasing with temperature then that is what will bring the pressure down.

[david.gnaniah]

A Viscous coupling is just that, it is designed to vary the fan speed according to the temperature sensed by the coupling itself. Have you checked the speed of the fan ?

[/COLOR].

No I did not check the speed of the fan. Will check it and let u know. thx.

[david.gnaniah]

Thannks Brian. I checked the viscous coupling fan. that was the culprit. thanks again.