A/C Evaporator intake theory
 

On my 1973 Targa, I have the dealer installed VW of America A/C. On these installations the evaporator is vertical in the footwell and the whole surface is used as an intake. On all the factory installs the evaporator is horizontal with a reduced air intake.
It would seem the VWoA setup would be less restrictive, apart from being easier to clean.
If so why do the factory installs limit the air intake? Does this not reduce air volume? I have always felt that air volume is more important than air velocity to properly cool a car , or even a home. Plus the noise should also be less.
I'd love to see what Griffiths's opinion on this would be.

The Porsche system operates in recirculate mode always. In theory, once the cabin interior temperature, "atmosphere" and surfaces are cooled down to a satisfactory level the blower speed can be significantly reduced.

So the only time any level of airflow restriction is of consideration is during initial cooldown when high levels of airflow are desired.

Speaking which, "outflow" restriction is just as important as inflow and you don't tell us how your's is configured.

The question remains unanswered, where, what is the cause of the shortfall of the factory A/C system..??

My suspecion, unfounded as it may be in some minds, is the lack of condensor capacity/efficiency. If using sufficient condensor capacity would result in the evaporator being capable of being kept cooled to the 34F target throughout the cooldown period I suspect nothing else would need "enhancing".

The data that folks give regarding SLHX installs gives me pause when this topic comes up. If condenser capacity is at or near max, SLHX installs on these cars should not provide any benefit at all - in fact, it would be detrimental to the system as a whole, since only 60-80% of the KCal tranfer are actually being transfered to the high pressure fluid. The rest being lost to the outside world, I suppose. In any case, adding one of these devices *should* cause vent temps to *increase* if the condenser capacity is maxed out - there are only so many KCal that can be transferred, right?

This implies to me that the evaporator is the place where the heat exchange is poor. Airflow + evaporator design/location/size might make for poor transfer, even though the compressor and condensers are doing an adequate job. Since there is only so much room for an evaporator, it might actually be an inherent design compromise for this car, no matter what tech is used of the evaporator itself.

Something to think about.

I think that everything but the drier and compressor is a weak link... and the weakness is fully exposed when you "fix" the weak links one at a time. Throw a couple fender condenser's in and you still dont get enough cold air in the car on a super hot day. Add a Keuhl fan and its not as cold but the volume is better. add an evap without condenser capacity and the air at any speed is inadequately cold. I think that these systems are best when the systems are built with similar capacity in each component. Thats why Griff wants to add condenser capacity, evapo capacity, and air volume and air flow tweaks all at the same time. To say its just condenser is over simpliftying it, which is why SPAL fans do no good at 92 degrees, 90% hiumidity on a triple black car after it has sat in the sun for a couple hours....

1). I don't think there is any doubt that insufficient condenser capacity is the cause of the cooler problem.
2). My question has to do with volume of airflow through the evaporator, why did the factory prefer horizontal placing instead of a fully exposed vertical evaporator like my VWoA?
3). My vents are the VWoA vents which I think are pretty efficient.

In my experience, all the stock 911's through the Carrera's suffer from inadequate air flow through the evaporator and out through the dash or under dash vents. This is why there are aftermarket fans and controls and better/additional vents marketed. Reducing pressure drop for the air, improving the fans, eliminating air leaks and reducing inducted air bypassing the evaporator air side heat exchanger surfaces all improve matters as does thermally insulating the evaporator housing. The evaporator housings (including old aftermarket units like VW and Kccool) generally have a poor fit to the evaporator heat transfer assembly; some air readily bypasses around the fins and coils instead of through them. This can be addressed by packing air conditioning service rated foam rubber around the perimeter of the evaporator heat transfer. Removing evaporators for cleaning (especially the air side) is helpful; some have the air side partially plugged with mold, dirt and lint. Care should be taken removing the evaporator to avoid braking off the integrally molded condensation drain tube.

Regarding the pro-cooler I think it comes down to this:

During the initial cabin cooldown period the evaporator will be highly efficient due to the large disparaty between the incoming evporator airflow being quite high, well above the human comfort level. In that case very little refrigerant cooling capacity will be leftover for the pro-cooler use.

Now, as the cabin cools down to a reasonable comfort level, less and less of the refrigerant cooling capability can be used within the evaporator. Now, if this were a simply capillary/restrictive expansion valve/technique, that "leftover" refrigerant cooling capacity would be available to the pro-cooler.

But.

Our Porsche factory systems use a variable expansion valve that does not allow the refrigerant to "escape" from the evaporator until the full capability of its cooling capacity, HEAT transfer efficiency (cabin inlet airflow is not at a desireable comfort level, say 72F) is EXHAUSTED.

So, now, yes, there will now be leftover cooling capacity, for any transfer to "objects"/liquids above ~72F. but what good will be served?

Given the above operational scenario of what viability will be the pro-cooler since it only comes into play/use once the cabin is cooled to a satisfatory level.

To Jim Sims,
That is my question, in the dealer installed units cleaning is easy because the whole face of the evaporator is accessible for cleaning. All cabin air goes through it as it is well sealed against the bulkhead. On top of this there should be less intake restriction so there would be no possibility of cavitation by the fan. So why does the factory use a vertical installation that is impossible to clean and has a small intake.
I don't know what the pro cooler discussion has to do with my question.

RSTarga,

My son has a unit like yours in his '73 and I also have one in my '76 and they appear to have less induction loss (compared to the factory) if the correct amount of the firewall has been removed - my '76 "dealer installed" merely had four very rough, round holes made by a hole saw - I opened these up by removing the interior ligaments. I located a complete unused NOS kit from the mid-70's and we installed it (less the deck lid condenser and the York compressor) in my son's car per the instructions - a significant, roughly rectangular, air flow window was made in the passenger side foot well firewall.

I have also installed, from "scratch" (cutting the required hole in the firewall), a "factory" system evaporator in a '74. I too was surprised at the small triangular shaped inlet with rodent screen and fan speed control resistor wires preheating the inducted air:D. However air is also inducted from the tunnel so there is more inlet area in the factory system that is readily apparent.

My speculation is that Porsche wanted to minimize the loss of stiffness in this important transverse bulkhead - hence the small and triangular shaped air induction opening.

Testing both systems for pressure drop/air flow would be the only way to be sure or maybe a CFD analysis.

I really do not think Porsche took air conditioning seriously in the 911's until after the Carrera's.

Although the front inlet of the evaporator is well exposed in these vertically oriented units the back is not accessible and inducted material will likely collect there and eventually require evaporator removal for complete cleaning.

Cheers, Jim

With LH drive models, the Behr evaporator box (factory air) located in the smuggler's box is not 'restrictive', there are 2 air intakes (driver's and passg's foot boards).

Discussing the LH drive version, its dilemma is air must rise upward which makes draining of condensed moisture difficult when the fan is running; leading more easily to icing situations.

The other issue with the LH drive Behr system is lack of sufficient air outlets: speaking of years 78-85. In 86 the side vents were enlarged. You can add a center Kuehl vent or Tri-Kuehl vents to these models. Prior to 78 the Behr knee pad 6 louver panel is perfect for the shorter dash models.

So then vertical evaporators are easier to drain and not as prone to icing?

No, horizontally oriented evaporators drain easier, the problem in Porsche's case is that the fan pulls air upward through the vanes preventing, somewhat, the downward, and OUT, flow of droplets of condensate.

Vertically oriented is not so good since the condensate must flow down the face of the evaporator giving the fan more time, opportunity, to "wick" it into the cabin atmosphere.

Inspecting one of the vertical evaporators under discussion (I have a spare used unit on my parts shelf) I note that condensate drains downward throughout the entire evaporator tube/fin array not just down the front face (water/mold stains show this). Further, since the air flow is horizontal through the tube/fin array any water entrained in the air flow will be pulled out the back of the array where it will impinge and collect on the back housing wall which is sloped downward and forward to the drain holes. These particular vertical units should clear condensate reasonably well out the bottom of the unit.

I've certainly never had a problem with a plugged drain on this unit, whereas on my 91 Turbo I've had to blow the drains out several times due to clogging.
Plus this car never smells musty since it is easy to clean the evaporator and isn't in some dark hidden housing where mold loves to grow.