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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 3240
Registered: 4-2003
Posted on Tuesday, 22 December, 2015 - 08:13 pm:   Edit PostDelete PostView Post/Check IP

While the heat soars and we rely on the cooling of our Climate Control HVACs, for the first time I experienced an issue when the thermometer hit 43C. That's 108 degrees to the tiny minority in the Fahrenheit Club.

I had the mandatory bi-yearly refrigeration service done on my Conti R on a typical freezing Canberra July morning this year and it all checked out with new/recycled R134a etc and it remained tip-top until last Friday when I was in Canberra as I often am. The HVAC did itself proud at all times in Sydney and all the way to Canberra. Pottering around, suddenly there was no cool air and it became very unpleasant. The blowers still functioned so I know that the R134a was not depleted.

Once back to 80ks it cooled off and the blowers delivered Antarctic again. It happened again, and once more returned to normal once the environs cooled and the Outside Temperature gauge dropped to 40C.

So.

If the system is charged to the limit with R134a on a freezing morning it may just be overcharged once the weather warms and the traffic slows. Given that the compressor cuts in and out at high pressure, this is the case where the system fails to refrigerate when there is just a little too much gas and the compressor controller thinks that there is no need to cut in.

It would take seconds to release a bit of gas by the Schrader valve, but I shall wait to see how hot it becomes again before venting any gas.

RT.
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David Gore
Moderator
Username: david_gore

Post Number: 1862
Registered: 4-2003
Posted on Wednesday, 23 December, 2015 - 08:43 am:   Edit PostDelete PostView Post/Check IP

Richard,

Interesting experience to say the least and not one I ever experienced during my time in the Pilbara region of W.A. during summer with ambient temperatures above 45deg Celsius - sometimes vapour lock caused by fuel volatilisation when trying to restart the engine after a short shut-down period would be a problem with carburetted vehicles however the modern recirculating fuel injection systems do not appear to have this problem as far as I am aware. I do not recall problems with the air conditioning other than a longer cool-down period if the car remained stationary and not travelling due to the lower air flow through the condenser whilst the vehicle was not moving.

I wonder if your problem was a high under-bonnet temperature due to a combination of high ambient temperature, refrigerant compression heat and low air flow through the engine bay at "pottering around town" speed increasing the temperature of the compressed R134a going into the condenser to the point it could not liquify from the cooling available at low speeds. Would have been interesting if you could have taken the car out on the highway to see if the problem occurred there and then returned to town to see if it came back. Reducing the refrigerant volume may help but I suspect the reduction required might also reduce the capacity of the air conditioning system at more normal ambient temperatures as there would be less refrigerant available to absorb and then dissipate the unwanted cabin heat.
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 3241
Registered: 4-2003
Posted on Wednesday, 23 December, 2015 - 10:04 pm:   Edit PostDelete PostView Post/Check IP

Thanks, David,

It was a temporary curiosity rather tha, a problem.

Given that the refrigeration circuit responds to system pressure, I am only reflecting that too much refrigerant will shut the system down just as will too little.

The compressor cuts in at a low pressure and cuts out at a higher pressure. If the pressure is too low when the R134a is depleted then the compressor never kicks in of course, and in that case the blowers are inhibited.

In the case I cite, the system recognised sufficient pressure when ambient to suggest that the compressor should rest until the pressure dropped. I was caused by overcharging with refrigerant on a winterís morning with insufficient allowance for PV=nrT when hot. The blowers functioned normally as the control system did not recognise an underpressure. I can only assume that marginally excessive R134a in the system caused, in the heat of 40C+ and slow traffic giving maybe 70C+ underbonnet, the refrigeration circuit to pressurise enough to give the aircon a snooze.

I raise this because the common perception is that more gas is good gas. More precisely, the system will not function if there is either too much or too little refrigerant charge.

If your gas guy overcharges the system, it may just cause as much discomfort as when the gas has leaked away.

RT.
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Robert Noel Reddington
Grand Master
Username: bob_uk

Post Number: 811
Registered: 5-2015
Posted on Thursday, 24 December, 2015 - 06:05 am:   Edit PostDelete PostView Post/Check IP

To much gas can cause high side pressure to be too high.
R134a runs at higher pressure than R12. The high pressure switch protects the system from blowing a hose or something. The low pressure switch protects the compressor from being run without gas (which carries the oil) and being damaged.

The correct way to charge is sweep with nitrogen and then sweep with vacuum then charge with the correct weight of gas. The correct weight in pounds or kgs. is available from the suppliers of the gas.

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