Slipstream - February 2004

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Searching for New Cool in Older Porsches by Bill Middleton As anyone who has owned and older 911 type Porsche knows in any even moderately warm climate, the design of the air conditioning system was not something in the forefront of priorities with Porsche engineers. For one thing, these cars are designed and built at latitudes far above Texas, the southern US, and most places that get any form of summer. For another, the typical sports car line of thinking is that any item that robs horsepower from the engine simply to provide creature comforts is an item to be only grudgingly considered. After all, the windows roll down, don’t they? The result was an attempt at an air conditioning system that, on mild days in the Northeast, Midwest, or any latitude much above the Mason-Dixon line will perform reasonably well. Anywhere south of these idyllic places with mild summers, forget it! To begin with, in order to not have to create new space in the already legendary framework and body sheet metal of the 911, Porsche had to do some very creative locating of components. The cooling coil, along with its fan, was too big of an assembly to easily locate under the dash or anywhere in the passenger compartment, where short duct runs to the vents would require it to be. As a result, it wound up in the ‘smugglers box’, a.k.a. the early gas heater, location under the trunk deck. The condenser, a necessary evil, was not easily locatable in any air stream at the front of the car without either reconfiguring the front sheet metal (God forbid!) or placing it in a delicate position under the belly pan, exposing it to damage. Needing forced air flow at idle as well as while in motion, the only choice left was in the engine decklid, using the cooling fan for the engine as draw for air through the coil. A smaller secondary condenser was also located in some editions behind the front valance panel with a pitifully small blower forcing whispers of air over it, and in some models inside the front fender well instead of behind the front valance. With the compressor in a standard location mounted direct to the engine, and the balance the components scattered about the vehicle like stray puzzle parts, the real challenge began – plumbing all this together. With the condenser on a moveable panel – the rear decklid – and the balance of the components at some distance from others, the length of tubing and fittings required to connect all this together bears no small resemblance to the transfusion apparatus from a Frankenstein movie. Add to all these lengths of hose an o-ring fitting at each connection, and the total number of potential leak points increases to equal some bad autocross lap times. The result of all this was a system consisting of an evaporator not much bigger than a ladies shoebox, a condenser located in a very compromising position for something that is supposed to be rejecting heat, not absorbing it from the hot engine directly below it, and over 40 feet of rubber hoses with more fittings than the number of candidates in the California recall elections. This is supposed to keep us cool in a Texas August? I don’t think so! In order to really understand the problems, let’s first look at what an air conditioner requires to work well, then look at the design – and the resultant issues that need to be overcome to get REAL cool from older Porsches. An air conditioning system is nothing more than a heat transfer device – it absorbs heat from the interior of the car, and disposes of it outside. In its most basic terms, it does this by circulating a heat transfer medium – the refrigerant – in a closed system consisting of a coil where heat is absorbed and a coil where heat is rejected to the atmosphere. Photo by Bill Middleton While the full system is much more complicated than this, the basic functionality performs in this manner. In order to satisfy the performance needs of the basic system, you need the following: 1. Sufficient surface area and air flow over the heat absorbing (evaporator) coil to effectively absorb the heat of the car interior 2. Sufficient surface area and air flow over the heat rejection (condensing) coil to effectively dispose of BOTH the heat absorbed from the car interior, AND the heat generated by the system that circulates the refrigerant (heat of compression). 3. Sufficient refrigerant in terms of capacity and flow rate to effectively and quickly transfer heat from one coil to the other. The condenser was placed above the cooling fan under engine decklid While Porsche did an excellent job of providing for more than sufficient refrigerant flow and capacity – the standard compressor capacity equates to about a 3 ton residential air conditioning unit – they severely violated the size needs of both the heat absorbing and heat rejecting coils. By so doing, it’s a lot like putting a lawnmower carburetor on a big block V-8 – lots of potential, but highly limited capacity due to critical components being significantly undersized. Thus begins a long and exhausting search for the “fix” – one that took lots of reading, lots of asking questions of various suppliers, vendors, other owners, and some mechanical engineer friends. The end result, installed last summer, has to date been a most satisfactory outcome of this work. Following is a summary list of SOME of the options currently available. I will discuss the pros/cons of each of them, and which I chose, later. Some omitted from the list were not omitted because they have no merit – they were simply omitted because, for my uses, they did not apply for various reasons. 1. Add an additional ‘static’ condenser under the belly of the car. 2. Add an additional blower assisted condenser in the fender well 3. Add electric cooling fans to the existing condenser 4. Replace the evaporator coil with a newer, more efficient design 5. R & R the evaporator fan and hose distribution system 6. TOTAL system change-out: compressor, evaporator, condenser 7. Other mechanical capacity improvements – sub coolers, capacity regulators, etc. 22
As each purveyor of these options will tell you, their ‘fix’ is the best, does the job, and will “completely” cure the problem. Knowing that the problem is systemic and not isolated to only one component, I seriously doubted these claims of ‘one component’ fixes. While curing the condenser surface area problem will indeed resolve the ability of the system to better reject heat, what good is that if the evaporator coil and blower are still so inefficient they can’t effectively absorb the interior heat? Similarly, what good a cooling coil capable of freezing you out of your living room if the condenser can’t dispose of the heat from a shoebox? Below are summarized the opinions, results of queries of others, and reviews given of the above options. 1. A static condenser may really add to heat rejection capacity, but without forced air flow it’s not much good at long idles in traffic and, if mounted under the belly pan of the car, becomes more of a heat absorber than a heat rejecter on really hot days idling in traffic over 140 degree pavement temperatures. Further, placing ANY type of tube and fin coil on the belly of the car exposes it to damage, potential leaks, dirt that decreases efficiency quite quickly, and reduction in ground clearance by nearly an inch. 2. An auxiliary condenser in the rear fender, like the static condenser above, adds a lot to heat rejection, but at certain times has the potential of picking up superheated air being exhausted from the engine cooling system, and also absorb rather than reject heat when at a standstill – remember the heat you can feel rolling out of the rear fender wells at idle from a full hot engine? It, too, is subject to getting dirty quite quickly and losing efficiency, but is more protected than the ‘belly pan’ static condenser and, with an auxiliary electric fan, works better at idle – but still has its drawbacks. 3. Just adding a fan to the existing condenser helped, but still the size was too small no matter how much air you pushed over it, and adds some heat to the engine since it is increasing the heat drawn off the condenser and putting that larger quantity of heated air directly down into the engine compartment. 4. While replacing the evaporator is helpful, by itself it solves only part of the puzzle especially on REALLY hot days in traffic. 5. As with 4. above, this solves only a small part of the problem – but as part of a larger overhaul, helps. 6. A complete system change kit, while having several merits, had some drawbacks – first of all, cost – well up into the $2000 and better range just for all the components. Second, while addressing the evaporator sizing issue, it did not (in my opinion) effectively address the condenser heat rejection capacity issue. 7. Install a subcooler, auxiliary refrigerant exchange device, or other mechanical controllers. Hands down, something had to be done about increasing condenser efficiency, and the evaporator had to be changed out and evaporator air flow improved. The evaporator change and improvements to cooling air path was a simple decision – Griffiths offers a complete kit, fully ready to install. Check out their website at www.griffiths.com for more details. Solving the condenser issue was another story – lots of options, lots of claims – few facts to go on. I had problems with either add-on condenser coil. In both cases, it required adding a delicate coil in a compromising position relative to potential for damage and dirt, and further the install positions were not optimal for heat rejection in slow or stopped traffic. While the added fan on the stock condenser definitely helped, it wasn’t enough – I needed something else. Photo by Bill Middleton Enter the idea of the sub cooler. In a nutshell, in most auto refrigeration systems, there is significant wasted refrigerant effort. The example of this is evident in the temperature of the returning refrigeration line going back to the compressor – on any given day, this line will be cold and sweaty, and may even frost over or freeze. That cold, sweaty line is giving up lost refrigerant effect to make that line cold and sweaty – thus the idea of somehow capturing this lost refrigerant effect. Most auto systems are designed with a fair amount of oversize in the compressor capacity. The oversizing is usually done to insure the compressor can maintain some manner of cooling at lower engine speeds. In this case, why not use this lost refrigerant effort to make the system more efficient? To utilize this lost cooling, it is necessary to pass the returning refrigerant containing the remaining cooling effort in close proximity to the refrigerant going to the cooling coil from the compressor. By utilizing a double walled vessel, a fair amount of this ‘extra cooling’ is transferred into the refrigerant headed for the cooling coil – and thus this lost cooling effect gets put right into the passenger compartment. I found this concept not only attractive from an efficiency point of view – anything that more efficiently uses something already there without heavy modifications is better than adding on. Further, there was some significant research, done at Texas A & M, to back up the concept of the sub cooler idea. For further details, you can view the manufacturers website at www.procooler.com In the end, as with any performance improvement, small additive improvements can mean BIG gains in performance. Adding a constant electric fan to the rear condenser (be sure to relay it, NOT drive it directly off the compressor clutch power wiring!), installing a more efficient evaporator, and most significantly installing the sub cooler, dropped discharge air temperatures in the range of 35 to 40 degrees with cabin temperatures at 95 degrees. Actual passenger compartment air temperatures in August heat dropped from only 10 to 15 degrees cooler than exterior temperature to well over 25 degrees cooler than exterior temperatures. To further document where the efficiency was gained I monitored temperatures with calibrated thermistors at 12 points along the system during operation, and found that over 50% of the efficiency gain was at the sub cooler assembly. The sub cooler installed in place of stock receiver/drier, in the driver-side front fender well 23
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Slipstream - February 2004

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