Sub-freezing Vent temps... HOW?
 

Serious, honest, question.

Several owners are saying sub-freezing vent temperatures are not only possible, but desirable.

I am well aware that lots of BTUs are required for water to change state from liquid to a solid, ice. and that takes time even with the evaporator core well below freezing.

These are owners seemingly in the HOT and HUMID mid/deep south.

I have had instances of evaporator freeze up in several Lexus LS vehicles, 2 '92's and a '95, both happening on a climb to an elevated altitude, at a 2000-3000 foot level. Obviously some climatic anomaly with the Lexus system since the evaporator core remains LOCKED (verified in at least one instance) at 33dF.

So my question is: how is it possible for an air-cooled 911 A/C evaporator to NOT freeze up during a longish drive with >50% Rh, and the vent temperatures well below freezing, therefore the evaporator core even moreso.

An additional question: The T-stat on/off range/span/hysteresis is supposedly 26dF +/- 2dF, to 33dF +/-2dF. Obviously the average being below freezing. Again, how does, how can, this work absent evaporator freeze-up?

Note: I have read about numerous instances of owners reaching sub-freezing vent temperatures via lifting the capillary tube sensor partway out of the evaporator core, only to later discover that led to freeze up.

Conundrum?

Edmonton, Alberta, driving with the outside air knob in the full open position, in Feb does the trick. Otherwise, the A/C stuff that was in my car is at the recyclers bin. Windows make the temps just right!

Sorry, I should have made it more plain that I meant using the A/C system to get low vent temps.

:rolleyes:SmileWavy

Hi WWest,
With low humidity, sub freezing vent temp should be fine. Freezers work that way. The t-stat to keep from freeze up is only to deal with humidity. Some are adjustable, and you could have whatever vent temp you want within the capacity of the heat exchangers.
Dave

+1. I don't get it either.

Since you answered the question, can we now close the thread before it turns into a death spiral?

😜

Dave is sharp and I agree with your premonition.

I don't understand how the thermo knows the difference between sub-freezing temps and associated "freezing" humidity.

Thermo does something to abate.........? It knows frost from cold air?

All I am asking.

I've never met, owned, a freezer in constant use and without a defrost cycle that didn't require manual "defrosting" once or twice a year.

I would say these old cars don't have climate control computer . I can compare it with the 4 temp sensors in various locations of the.HVAC system of my other car, 89 Peugeot wagon that can keep 40 degree air coming in the middle of the summer. So, I use a small thermometer in the duct and if I see the temp much below 40 I adjust the thermostat. It is what it is, a totally manual system.
My 2 cents.

maybe the defrost cycle is what that switch provides

The more I think about that, it makes sense.

The latent heat of evaporation is 600 cal/gram. The specific heat of ice is .5 cal/gram. The latent heat of fusion (melting) is 80 cal/gram.

So once the sensor/evaporator get a coating of ice on it the temperature rapidly goes down below freezing but the fan is still blowing hot air over it sucking out 600 calories per gram until it melts. I bet it isn't ice for long.

I suspect that's why the factory placement for the T-stat capillary sensor tube is at the "warmest" point, last to freeze over, of the evaporator core.

My "vision" of a frozen over evaporator core doesn't allow for much, if any, "warming" airflow. Actual experience bears this out.

Ask any orchardist what the effects are of a coating of ice.

Modern day heat pump controls place a defrost control sensor in a area MOST likely to freeze over. Once that happens the sensor's temperature no longer reacts to the cooling refrigerant flow and a defrost cycle is triggered.

You are correct, in an odd way. Once the T-stat's capillary sensor declines to 24dF(worse case) the compressor clutch circuit opens and is not closed again until the sensor temperature rises to 35dF (best case). But then if we allow for a slight "coasting" period due to previously "stored" liquid refrigerant in the R/D the sensor area is still likely to decline another degree or more until the "store" is exhausted.

So if the evaporator core was tending toward freeze up, frost, rime ice, formation it would thaw during the compressor off period.

My problem is that the average control temperature would be sub-freezing and in a highly humid environment as is often encountered in the mid-south, south, a freeze up seems a virtual certainty. Especially so should the blower speed be reduced to moderate the cooling level rather than FIRST turning the T-stat control CCW.

Plus the above, 24dF T-stat control limit, does not account for the level of sub-freezing vent temperatures being reported.

Hmmmm...

I wonder...

Is the wider spacing ("poor" cooling efficiency) of the factory evaporator fins/tubes the reason the factory was able to use a sub-freezing "low" control point? With the wider fin spacing it would take a serious level, maybe an extraordinary level, of formation of frost or rime ice within those fins to mostly, or fully, block the "warming" system airflow. A frost or rime ice formation that did not fully bridge the space between the fins would most quickly thaw during the compressor off cycle.

But that does not answer my question, only gives it more "depth": Why would not the more tightly spaced, MORE efficient, cooling fins of these aftermarket evaporators have even a greater tendency to freeze up vs the factory cores.... Or do they??

Simple answer is yes, but it's due to running with somewhat undercharge which achieves dual functions. One is to get a cold evap, and two is to provide hi side head room, i.e. keep hi pressure from going too hi if in heavy traffic for example.
Ideally one would charge enough so that low side pressure corresponds to just over 32F, but with the 911 system that would put the hi into the too much pressure, definitely over if caught in traffic.
The down side of the undercharging setup is monitoring temps, since like I said before there is no climate control computer.
Around town on normal traffic this is no problem, there is no freeze up, but on the hiway it will freeze up after an hr or so if you don't realize vent is in the 30s and you don't turn up the thermostat.

In your '87 you can ameliorate, at least partially, your heavy traffic high pressure problem via connecting a high pressure or temperature detection system(***) to the engine heat sensor input to the cabin heat controller. The cabin heat blower will then activate automatically in low speed sop and go traffic thereby providing additional cooling airflow through the rear lid condenser on an as needed basis.

Side benefit is that some of the airflow "preheated" by the rear lid condenser is diverted around the engine.

***Trinary pressure switch or....

Senasys -

So you're saying that a person can purposely under fill the system in order to get lower evaporator core temperatures than the system, TXV, etc, would otherwise allow..??

But, as you say, and I agree, on the "flip" side of that one would need to pay reasonably close attention to the vent temperatures and raise the cooling setpoint should those indicate an impending freeze-up.

1. The "claimants" contend that no such adjustment is required, which I could understand in DRY climates, but "these" are well known to be oftentimes relatively, even extremely, high Rh climatic areas.

2. In any case shouldn't, wouldn't, a properly installed and calibrated T-stat accomplish exactly that? Disregarding, for the moment, the fin spacing issue.

With the above criteria met the evaporator core would not decline enough to trigger the opening of the compressor clutch circuit unless/until the cabin atmosphere had reached the top end of the Delta "T" span, the incoming air to the evaporator core is low enough that the system can/will/could/might lower the outflow into the danger zone, freeze up imminently pending.

So, working properly, as designed, fully charged with refrigerant, you would have SUPER cooling of the cabin right up to the point of impending evaporator core freeze up.

If not, what am I missing?

^^^this. You can get sub-freezing vent temps for a while, even in high-humidity environments. But a high heat load, plus positive airflow? Ice might form, but it'll melt almost immediately under the amount of thermal stress a high temp/high humidity location can provide. Now, in a high humidity lower heat area, maybe evap freeze-up is a concern for those who run the AC at full cold 100% of the time, but if you're going for cabin comfort, the thermostat will regulate the compressor such that evap freeze up doesn't happen.

If you're getting evap freeze-up, something is wrong with your system or set-up, OR you're trying to transport meat across state lines. :)

My point was, is, owners are making the claim that they can operate their A/C systems in HOT climates with sub-freezing vent temperatures, seriously so, unconditionally, without freeze-up.