AC work load

[Eagle]

This is not a K5 question, but I am having a debate with my father in law. The debate is over a cars air conditionings work load. He is trying to tell me that by turning down the fan while keeping the temperature gauge at the coldest setting will reduce the work load of the air conditioner. I believe that by turning the fan down only reduces the blower; the actual compressor is still working just as had. So what is it?

 

[Blazer_Boy]

Neither to be honest. Prior to 1977 A/C was controlled by set pressure points. When the pressure got to the correct point, the compressor backed off. Not good for R-134 that works with higher pressure. In 1977 they changed to a switch that worked off of tempature to keep things from icing up. R-134 never really gets cold enough to make to that happen so the compressor runs alot more than it would with 12. Moving the lever to cold only changes where the fan blows over. The compressor doesn't care where the switch is on the controls. Blowing less air over the coil may give it a chance to cool down enough to trigger the switch to turn off the compressor. Its an all or none system.

 

[K5 NUTT]

A/C load is determined by a number of factors...most important is sun load...a K5 has more than most cars due to the windows and the size of the interior...humidity plays a factor also...it is the job of the a/c system to remove humidity and heat from the air..thus conditioning the air. R134A vs. R12....it is true that R134a runs slightly higher pressures than that of R12....however it really does not effect things greatly. In 73 gm trucks started useing the CCOT system that differs greatly from the 72 and below STV POA valve setups...73 to 75 orifice tubes were larger and used sintered metal as a filter screen...in 76 thru 91 it was made of plastic....73 thru 76 used a high psi cutout switch in the discharge line close to the orifice tube....along with a thermostatic switch to keep the evap from flooding and sending liquid refrigerant tot he compressor and to keep the evap from iceing over...and thats why the heater core is so close to the evap...77 up used only a low psi switch to regulate the system...which is still used on modern day peterbilt tractors...allthough they add 2 more psi switches and they run r134a. Using recirc mode and or Full cold...reduces load on the a/c system as you are only circulating the air in the interior...on the other hand you are sucking in outside air and cooling it....another load factor on the a/c system that most people forget is condensor drop...the temp drop across the inlet and outlet of the condensor should be 40 degree's...if it is not the front end is usually packed up..ie:...dirt..sand..bugs...clean it out once a year with a pressure washer...air works also but it can't break down and disolve dirt...if the condensor can't dissapate the heat (ie: hot interior air)...the high side operating pressure rises..usually above 300 psi...your duct temp will suffer and you risk blowing a hose or the pressure relief valve in the rear of the compressor.

Azblazer
ASE Master Tech

 

[95 Silverado]

you could think of it in these terms, once the interior is cooled down and you just want to maintain temperature, slowing the blower speed will reduce the air flow volume over the evaporator core, most systems sense evaporator temp, either directly or by low side pressure, they switch off the compressor at a given point to allow the evaporator to warm up, so it doesn't freeze up, the vent air temp will remain cold, once the evaporator warms up the clutch kicks in again to cool it back down, by slowing the blower down, the warming process would take longer and thus the compressor would be shut off for a longer period of time before kicking back on, and it would cool down faster allowing it to cycle off again sooner, so over a given amount of time the compressor would be off for a larger portion of the time.