Dozman

Unfortunately opinions are opinions and facts are facts.

The function of the a/c system is to operate above 32 degrees constantly till comfortable conditions are achieved in the cabin, which is the fastest way to cool the cabin. The function of the a/c system per design is not to cycle on a saftey, in this case a feeeze stat. For instance a safety fails, compressor damage can happen.

It may take power to run a compressor, but when the system shuts down due to a saftey failure. Then after the freeze stat lets the system operate, the temp in the cabin is rising, as well as humidity which has more heat content (latent heat). So simply letting the system bounce off of a saftey and shut down on freeze stat is not a design criteria of correct operation.

No, the function of the system is designed to operate above 32 degrees. Thus the evap has no chance to freeze. If you try and run the evap below 32 degrees, you remove less heat from the air since ice will start to form on the evap. If the freeze stat keeps cycling, then over a period of time the average of BTU's removed via the system is less, than operating per design conditions for the same amount of time in the same ambient conditions, which is to operate the evaporator saturation pressure/temp above 32 degrees.

If air should be as low as possible as you may think, why not design a system that is similiar to a reach in freezor at a store that operates at 0 to -10 degrees? Because you need to defrost either by electric reheat and or hot gas by pass to defrost the evap. But that is done in off cycle. In turn done when cooling is at a minimum. But as we drive our cars we want cold air all the time, not cold and then warm then cold etc...

How much power are you saving by running the system not by design, cycling on a saftey versus running a system per design? Actually over a 3 hour drive or any drive, I bet running under charge does not cool and keep the cabin as cool constantly as a system charged per design.

Instead of opinions, please post facts or data.

If you operate a system undercharged in that type of heat sooner or later the evap will freeze up, or cycle before it completely ices over, then it will cycle back on sooner or later. Till it comes back online the cabin temp will go up. Now, with a system operating with the correct charge will operate continously without cyckling off, and as it cool the cabin, the air temp will lower. The system operating per design will operate better than one under charged. This is not my opion but facts.

If it was 50 degrees outside there is no reason to use the a/c. If one did use it, the temp slider will blend the air in the evap so there will not be a charge of less than 32 degrees.

You can design a system that won't operate below 32 degrees. Thats one reason there is a an air blender.

I can not find in the manual sequence of operation were it states the correct operation is for the system to cycle on the freeze stat.

Freeze stats are not exact, they need to be calibrated and or fail. Thats why they are a safety. A/c systems are not designed to cycle of a safety device, they cycle of control devics.

Why is there a freeze stat? to keep the evap from freezing up, alas a saftey. If it fails the system can be damaged by the evap freezing. A couple people here complain their systems blow ice cold than warm. Warm, warm in the summer (because of cycling due to freeze stat, or the evap icing up blocking air flow). So if their a/c system blew say 28 degrees, if they corrected the pressure the system operates to say the system puts out say...34 degrees not many people can actually feel the differnce and the system would run continously, and they would not detect warm air blowing. Fixng the problem.

Now, your wrong about house units, there are TXV's, you can also run 2 evaps by using one condensor unit via using epr's and CPR's. There are also low pressure high pressure saftey switches as well for house use.

Your last sentence is correct for any system, be a low, medium or high refrigeration system. Matter of fact the smaller the system, the charge of refrigerant becomes more critical. Since our cars system refrigerant charge is less than most house charges, are charge is more critical than a house system. Unles of course ones house is so large you need to operate a centrifugal chiller. Again this is facts.

I know for fact what I have stated is facts not theory nor hear say nor opinions.

If one charges an a/c system to design specs as in a car, and the system operates correctly, no one should see the evap ever freeze up, nor see the evaporator saturation pressure/temp 32 degrees or below.


Now, I will cut and paste a few things even from "Wally p"

30 psi =35degrees
As Wally P agrees the sytem does not cycle on and off by the freeze stat.

In case of a problem the compressor will cycle on a SAFETY.

I don't understand the logic of a system in a car operating a evap saturation pressure/temp 32 degrees or below.

I will agree to disagree with you on this subject. I was factory trained by York International, worked in a union as a HVAC service technician with too many classes seminars under my belt to name.

The fix for the complaints above is not to just calibrate or change the freeze stat, it's also to determine the system is charged correctly to spec, and ensure the system operates correctly. This way no warm air will be felt, and no excessive water pool will drain from the iced up evaporator.

bd0nalds0n

You got my vote. I'll have the charge tested. Thanks for being thorough.

SharkSkin

Just to throw a monkey wrench into the discussion, 78/79 cars(and most early cars of any make) do not have any sort of warm/cold air mixing. Evap temp is controlled by charge and/or freeze switch.

Also, AC is needed frequently at below 50° ambient -- any time you use the defrost setting. Without the AC it can take a long time to clear the windows if you climb into the car with a wet coat when it's very cold.

Dozman

The reason the a/c is used during defrost is to dehimidify...

The system was not engineered to cycle on a freeze stat. But if one cares to beleive that, thats not a problem. All my cars a/c work properly and put out min 34 +/- degrees to 38 +/- degrees, with no worries if the compressor will be damaged due to a few things.

Just to throw a monkey wrench into things...Oil is entrained in the refrigerant, so lubrication is due to oil the oil flowing with the refrigerant. When there is a lack of refrigerant, the lubrication of the compressor is lowered as well. At some point a lack of lubrication can make a compressor fail.

I stand my ground firmly. My position is even backed up by the esteemed Wally P.

dprantl

Here's my stance:

- I'm not disputing that a system's evaporator can freeze up when the refrigerant level is low. This can and does happen, although it should be stopped by the freeze switch.
- Yes, there is a low-pressure safety switch on car A/C systems. This has nothing to do with the discussion.
- If there was no reason to use the A/C system below 50 deg F, Porsche would not deem it necessary to design the defrost position on the climate control which automatically runs the compressor.
- Fact: I just checked out a BRAND NEW BMW 335 that a friend bought. It was ~70 deg outside and humid. I thought of our discussion so I started the car, set the fan speed to medium, turned on the A/C and watched the compressor pulley. So guess what happened ~10 seconds after turning the A/C on? Vent temps were ice cold and compressor cycled... hmm, why is that? Low refrigerant level on a car that just rolled out of the showroom? All my cars do this after a recharge of the proper amount of refrigerant by weight specified by the manufacturer. Like I said, my wife's 968 does it as well with the factory R134A fill. I have all the service records and know the previous owner and the A/C has never been touched. We can talk about theoretical facts all day long, but watching a car run as the factory designed it are the facts that are important to me.

Dan
'91 928GT S/C 475hp/460lb.ft

jcorenman

John, I agree with a lot of what you wrote, but I must respectfully disagree with your comments on low charge, and the freeze switch.

The circulation of freon is primarily controlled by the expansion valve, which regulates the flow of liquid freon into the evaporator according to high-side pressure and low-side temperature. In warmer climates (e.g. 90-95F and up) the airflow through the evaporator keeps the core above 32F, but in cooler climates (including cold-weather defrost duty) the evaporator core temp will fall below 32F and, absent a freeze-switch, it will freeze.

The effect of low charge is that there isn't enough freon to keep liquid in the receiver/dryer, and gas rather than liquid flows to the expansion valve. The cooling effect is less, not more. The suction-side pressure will indeed be lower than expected, but not because of colder temperatures-- there isn't enough freon so the A/C pump is simply sucking a vacuum. The freon temperature/pressure is only valid if there is liquid freon evaporating in the evaporator.

The best guide to proper charge level is to check the sight glass on top of the receiver/dryer. If you watch it from start-up it will be initially clear (gas), then foamy (liquid and gas mixed), then a few bubbles (mostly liquid) and should finally clear (all liquid). All this should happen in a few seconds. It's fine if there are a few bubbles going by, but it should be mostly liquid. (The sight glass will likely be dirty, might still be masked off from painting, or might even have been painted over).

John, your point about oil circulation is right on, that's why the compressor will shut down with low pressure (as well as high). But that's also why the system is designed to cycle rather than operate continuously with very low flow, if the outside temps are cool. It's quite possible to design a system where the expansion valve can regulate the evap temp to avoid freezing, but the freon flow would be too low for proper compressor lubrication. So it runs at higher flow rates, and cycles.

There's been lots of discussion of freeze switches in the past, here's a comment by Wally that seems to address this question:
https://rennlist.com/forums/2594133-post6.html

Cheers,

WallyP

I think that I was wrong - it isn't the first time, and it won't be the last!

Both John and Jim make very valid points, and I think that both are correct in most regards.

The 928 refrigeration system is nothing special (unlike the HVAC control system, vacuum system, etc.).

The expansion valve is a thermostatic expansion valve. It should control the flow of refrigerant to maintain an evaporator temp of just above freezing. The only time that the evaporator will go below freezing on a properly functioning system is when the refrigerant level gets so low that the wide-open expansion valve still can't keep the temp up.

If, in high ambient temperatures, you are getting below-freezing air from your system, either something is wrong with the expansion valve or the system is undercharged so much that the expansion valve can't correct the pressure in the evaporator.

In low ambient temps, the condensor works so effectively that more of the refrigerant is condensed to liquid, and the effect becomes the same as a serious undercharge. This effect is heightened if the system is slightly undercharged to begin with.

I suspect that a fully charge system will rarely cycle because of the freeze switch, but it probably can happen during cold-weather defrost operation, where the A/C is automatically used. As John says, this is a safety operation, but as Jim says, it probably will happen in cold weather. I don't think that it should happen in a properly charged system in hot weather.

BTW - the sight glass is pretty well useless if the system contains anything other than R-12.

dr bob

I've been watching this discussion closely but refrained from commenting. Until now. John's analysis of how the system should operate is exactly correct. For a stationary refrigeration unit in relatively steady-state operation. In those systems, cooling is carefully controlled by throttling the condenser cooling water flow, and things like suction throttling valves are often used in older systems. In the hunt for energy efficiency, a system is loaded just enough to keep the evaporator from icing. A target of 38º evaporator discharge is qute common.

Step out of the utility room and into the parking lot, where the duty is much different. A dark car with no window tint sits in the sun all afternoon. Driver opens the door and give the car a few seconds to ventilate, then climbs in, starts the engine and punches the AC button. Evap inlet temp is over 100º. Driver wants the coldest discharge temp possible. System starts out with 60º discharge temp in recirc mode (fresh air flap is closed). As the air inside the car starts to cool, so too does the evap discharge temp. Inside of the car is still pretty hot and the sun is still beating down, so it takes a while to get the car cooled. Plus the driver is stuck in a monumental traffic jam so compressor speed and condenser airflow are not ideal for maximum cooling capacity. Colder is better.

What's the point? Auto systems operate in tremendously variable conditions. My system is theoretically undercharged slightly (about 5% by volume) (R-134a) to make absolutely sure that the head pressure stays within reason on 100º days in slow traffic. The slight undercharge does make the possible evap temps slightly lower than a 'full' charge might, by about 5º. Still, evap discharge temps are below 20º in steady-state operation, 60 MPH airflow and 1500 RPM engine speed. It bounces off the freeze switch. I notice that the temp varies between 35º and 39º when cycling. Hmmm, I think it's working OK, especially considering all the different conditions under which it operates. Operating with a larger charge mass would reduce the cycling OK, but it would also increase the time needed to cool the heat-soaked car. Having the car cycle at 40º-45º might be OK, I guess. But why when I can have it chill the car much faster?

The 'perfect' freeze control would have the feeze switch bypassed for the first 15 mins or so of system operation, causing maximum cooling capacity to be available. After that time, cycling the freeze switch would be used to de-ice the evaporator as needed. It might also reduce the frozen-fingers discomfort factor.

Here in desert-dry SoCal, I keep the freeze switch jumpered. I forgot to remove the jumper for the trip to OCIC, and a few times had to manually cycle the compressor off to thaw out the evaporator in Texas humidity. Meanwhile, cruising across the Mojave Desert in 100º+ ambient, I set the temp slider to 70º and the system controlled perfectly at that temp. The fresh air vent cycled once in a while, and I could hear the blend door motor moving as it worked to balance heat loads in the car. I'm starting to preach like MK does, as if my way is the only way, but I did want to share the results for this one car. At least a few others in this area have done the same thing with similar improvements, so it's not a fluke.

Some car systems are using a suction-pressure switch to cycle the compressor. Those tend to be fixed-orifice systems. Is that the best way? It works for many American cars. Our cars use a freeze switch instead, allowing lower suction pressures to do more cooling when needed, and cycling the compressor only when there is an actual risk of evaporator icing. The expansion valve plays a huge part in how well the 928 systems work. The valve is fooled slightly with the slight undercharge. Downside is that it is also fooled big-time if there's air in the system. But that's another issue that deserves its own thread.

dprantl

Thanks Dr. Bob. How it works in a house is not at all how it works in a car. I knew you would be able to explain it better than me

Dan
'91 928GT S/C 475hp/460lb.ft

Dozman

Running out of time today, too many things to do...

Dan every car that rolled of the assembly line with an a/c is engineered to operate during defrost due to this is meant to dehumidify.

I will post more later.

Jim, if the refrigerant charge is low, what happens to the suction pressure? It lowers, thus temperature pressure have a direct relationship to each other. This meaning if the suction pressure for R134a drops from 30 psi/35 degrees to say a low charge or restriction or refrigerant staying in the condensor... to say 28 psi or lower/which is 32 degrees or colder. What will happen to water dropping on an evap that is at freezing temp? yes, it will freeze. One can not just put logic on refrigerant unless you understand the pressure temp. relationships. A correctly charged system operates as designed, an under charged system at times the TXV will see vapor instead of sub cooled liguid. If gas is seen the pressure is lower than sub cooled liquid correct? then the temperature would also be lower. Unles you have a different P/T chart than what is well published and understood in Physics.

(Too many variables, but glad to keep talking about all of them that are brought up)

The low pressure switch function is not to keep liguid refrigerant in the reciever/dryer. It's function is to keep the compressor from damaging itself due to low ref. charge.
The compressor DOES not suck vapor, it compresses refrigerant vapor gas to a denser gas, then the condensor condenses the compressed hot gas to liquid and sub cools the liquid... A car's compressor does not create a vacum, that is one reason for a low pressure switch. A vacum can damage a comp. If the pressure goes below low pressure, the LP switch will cut power to the compressor, keeping the compressor from damage. Once again the saftey's keep the compressor from damage, not to operate the compressor system... The COMPRESSOR never operates or should never operate in a vacuum. If it did, where would the lubrication come from? If a compressor operates in a vacum, where is the refrigerant and cpompressor lubrication? A vacum of 0 psig on the low side with a compressor running with R134a would create an evaporator saturation pressure/temp of -15 degrees. Say the pressure of the low side of the compresor is operating in a vacum of say 5" of mercury, the corresponding evaporator saturation pressure/temp would be -22 degrees. The 928 compressor NEVER operates in a vacum.

Simply put, the 2 members who in the start of this thread are under charged. If they both feel 32 degree air or cooler, then warm air. The evaporation saturation pressure/temp is too low. Then they see a large amount of water pool than normal, meaning ice is forming on the evap and thawing draining to the ground.

What I state is not just theory, but actual workings....I may not be an instructor nor want to in refrigeration, but the facts still stand. The 2 members a/c systems are under charged.

For fact many factories under charge the a/c systems of refrigerant before they roll of the line to save ounces of refrigerant times how many cars. This makes one way the manufacturer's cut corners to make $$. This is also a known fact.

Lets post numbers from the problematic cars, as in low pressure, high pressures, ambient outside temp, inside cabin air temps coming out of the vents. Yes, i know the car should be moving to make absolute valid measurements. If one wants to do this, use your fluke meters, and buy longer thermocouples to reach inside your car as you drive.

It is also a known fact a condensor is typically engineered to do reject 125% heat load than what the evaporator removes. It is also known the 928 condensor is over sized too, which is a benefit.

dprantl

Seriously? So maybe they also put in a little less oil and coolant to save money too?

I've charged over a dozen cars with the factory specification of refrigerant by weight with a scale that's accurate to 0.1 oz and have observed all of them bouncing off the freeze switch minutes after the recharge unless ambient temps are very hot (systems leak tested with N2 at 100psi for hours without pressure drop and vacuumed with a proper pump for over an hour with a new expansion valve and drier always). So it's safe to assume these car manufacturers specify to "undercharge" their systems? Keep in mind that today's parallel-flow auto condensers are many times more efficient than older models. It is entirely possible for these condensers to dissipate enough heat to cause the expansion valve to run out of it's low-end adjustment limit.

In the end, when I'm charging my car, I want it to perform for me the best it can. Like Dr. Bob said, when I sit in my car and the inside temp is 120 deg F and outside it's 100, I want the system to be able to blow as cold as it can and cool down the interior of the car as fast as possible. If in that state the system is as you describe "undercharged", then that's how I want it. Wouldn't you? I also would like to save some pennies on gas while cruising (or get that extra 5hp during WOT) when the compressor is cycling off the freeze switch. It's a win-win situation.

I think I'm done, I kinda feel like this is a bench racing vs. real racing discussion. You can calculate all you want, but the real world results are all that matter. Also, if the two members are having ice form on their evaporators, they most *certainly* have a problem with their freeze switches or have had them bypassed (or both). Whether or not they have undercharged systems or not is not definitive, and depending on how undercharged, it may not even be desirable to add refrigerant unless you like warmer A/C air, counter to the purpose of the system.

Dan
'91 928GT S/C 475hp/460lb.ft

Dozman

dan I agreed to disagree with you previously. I have no ill will. I am just backing up what I believe, and you do. I am done as well.

You did ask what would I want. I will continously charge my a/c to spec. I am glad you are happy with how you charge your cars. Everyone has opinions, and thats a good thing. opinions make for interesting conversations.

Of course I do have ample supply of R12. My current car was changed over to R134a by the PO. But this winter I will be converting back to R12, since it has better cooling characteristics.

j.kenzie@sbcglobal.net

John,
How do clutch-cycled systems work? My impression is that the 928 uses a thermostatic expansion valve to regulate evaporator temp/pressure and the freeze switch is a second layer of control, mostly to keep the evaporator from freezing. Do clutch-cycled systems on other cars rely solely on a thermostatic switch to turn the compressor on and off via the clutch.
Thanks

WallyP

Most fixed-orifice systems (which include most late-model American cars) use a pressure switch to control the clutch - again, in an effort to keep the evaporator just above freezing.

Dozman

Thanks Wally.

Jim there are sooo many variables that can creep up, and try to explain in an a/c system.There's not enough time in a day. I have a life and don't spend my day on the net, yet alone just on Rennlist.

I have in the past enjoyed all my sharks a/c system operating correctly without the evap freezing up, and to the best of my knowledge, not cycling on a freeze stat. Although I did not install any type of monitor software on my sharks nor installed red stem headsup devices (small electrical devices that pop up a red pin to tell you that a fault occured, to simplifiy the explanation of the device). But I am happy with how my a/c systems operate. I never noticed the air temp being cold then warm. Just as cold as it's suppose to be. Matter of fact I usually ran with a small hand held temp meter connected via a thermocouple measuring the air temp from the vents.

Not into argue, nor explain any small question or variable anyone can think of. I was trying to explain what the problem was with the posters a/c system per his descriptions of symptoms.