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Air Conditioning

Parallel Flow Condensers

New Paradigm in A/C Repair

Just like all the other systems on modern vehicles, air conditioning systems are becoming more sensitive and shortcuts, whether designed to save you time or to save your customer money, have gone the way of the honeycomb radiator core. Back in the old days of A/C repair, if a car came in needing a compressor, then a compressor was the only part it would get. Never was it deemed necessary to replace a POA, STV, or thermal expansion valve. On the old Chrysler RV2 compressors, if the EPR valve went bad, we removed it, threw it in the garbage, replaced the gasket, and charged up the system. But those days are over!

Here in the 21st Century, when a compressor has a terminal failure, it's our mission to make sure that the entire A/C system is 100% free of contaminants. With the R12 systems, this merely involved replacing the receiver/dryer or accumulator and, when applicable, the orifice tube as well, and flushing out the serpentine condenser. But R-134a systems don't use serpentine condensers which poses yet another problem for consumers and technicians alike. The old style serpentine condenser with its tube-and-fin design was sufficient for cooling an R12 system but is not sufficient for the higher demands of cooling an R-134a system.

CONDENSER HISTORY
The old style A/C condensers are manufactured using a series of straight tubes, connected together by "loops" on each end. Each loop is 180º and connects the tubes so that they may go straight across the condenser next to each other, thus creating the "serpentine" effect. The earliest of these condensers were manufactured out of copper tubes with copper fins and typically used 3/8" tubes with 10-12 fins per inch. Eventually, condenser manufacturers discovered, along with the rest of the cooling industry, that, although more difficult (and more expensive) to make, aluminum provided better cooling. So they began to manufacture condenser tubes out of aluminum with copper fins. Eventually, aluminum condensers were made with aluminum fins and the size of the average serpentine condenser tube was cut down to 1/4".

At about the same time as the development of aluminum fins, the fin count on most condensers was raised to 15 or 16. Thanks to these advances, high side pressure readings dropped as did vent temperatures. The driving public was happy, sitting in their cars in August with the A/C at Artic Circle temperatures. Although these condensers were quite sufficient for an R12 system, they were not good enough for R134a systems.

HOW CONDENSERS WORK
Cooling is about contact, contact between the coolant and the tubes and between the tubes and the fins. The parallel flow condenser, developed by Modine in the late 1980s and now used by every auto manufacturer in their vehicles, was designed to provide a lot of contact between the coolant and the tubes.

Parallel flow (PF) condensers maximize the cooling ability of the Freon in two ways. The first is by turning the single "tube" into many tubes. On average, a PF condenser is 3/4" to 1 1/4" in width and consists of not one tube, but many (commonly 10 to 12) parallel tubes that, from the outside, look like a single tube. Instead of having one giant round tube where at least 75% of the Freon is not making contact as it flows through the condenser, a PF condenser's tubes are between 1mm and 2mm.

With openings this small, nearly all the Freon flowing through the condenser is in constant contact with the tube. The contact with the tube transfers the heat being carried by the Freon onto the tube.

The heat that is generated is then transferred to the fins and is subsequently dissipated by either the natural air flow (at highway speeds) or the cooling fan (or fans, depending on the vehicle).

The second manner in which PF condensers produce more cooling for the Freon is through higher fin counts. PF condensers really pack in the fins. The basic PF condenser has in the neighborhood of 18 to 24 fins per inch. More fins, as we all know, means more heat dissipation, lower high side pressures, less strain on the compressor, and a net result of better cooling for the motorist.

Parallel flow condensers are a great advancement for the mobile air conditioning industry, but they do have their drawbacks. The first is that they are far more difficult to repair than serpentine condensers. If a serpentine condenser has collision damage, all you need to do is get your torch out and repair the damage. If one of the end loops breaks, all you need to do is to sweat the old one out of the condenser and either aluminum weld or solder on a replacement loop. However, PF condensers aren't so easy to repair. When one of these comes in for repair after a collision, there will almost inevitably be more than one tube cut open, and they will always be twisted away too. Attempting to repair collision damage to a PF condenser is always a bad idea because, even if you are able to straighten the condenser enough to aluminum weld the outer walls of the tubes together, you will either have tubes leaking internally into each other and/or have tubes welded completely shut, cutting off significant amounts of flow. Also, as with all condensers, you should never repair a condenser that is porous or rotten, unless you happen to like comebacks, of course.

PARALLEL FLOW CONDENSERS AND BLACK DEATH
By now, we have all heard of "Black Death". Breaking it down to the absolute basics, "Black Death" is the result of a catastrophe within the compressor, resulting in compressor pieces of varying size and shape being spat out of the compressor throughout the entire A/C system. The term "Black Death" is a reference to what the chunks look like once they are trapped. In a system using a receiver/drier, the majority of these pieces will get trapped within the receiver/drier. In a Cycling Clutch Orifice Tube (CCOT) system, they will primarily be trapped at the orifice tube and accumulator. As far back as the 1980s, we learned that, in order to properly change the compressor without having to change it again every week until smashing our heads through a wall, we needed to change the parts that trapped the metal bits before those pieces wound up in our warranted new or rebuilt compressor. Yet sill there were unexplained comebacks. As it turned out some of the smaller particles made it through the filtration system and into the condenser. After the debris would once again become heated by the condenser, it would loosen up and the flowing Freon would carry the particles back to the compressor and kaBOOM...another comeback!

Of course, A/C flushing systems have been around for a long time and they work great in the relatively large tubes of a serpentine condenser. Once a serpentine condenser is flushed, it is almost assured that there are no particles hiding inside it and, used in conjunction with replacing the filtration system, renders foreseeable comebacks almost nonexistent. Although a perfect, inexpensive solution for R-12 condensers, a flush is not the solution for any parallel flow condenser whose system has suffered from the Plague. The miniscule tubes that make up the PF condenser trap small particles, and they trap them really well! This can create two separate problems. The first is that the particle(s) will prevent proper flow through at least one of the tubes, cutting cooling ability down by eight to twelve percent for each plugged tube. This in turn raises the pressure being exerted on the compressor, and raises vent temperature.

The second problem, if you haven't figured it out already, is that once that piece dislodges, it will eventually work its way into the compressor, and you will have a hot, irritated, grumpy customer standing in front of you on Monday morning after his trip to the beach was made quite unpleasant by the sound of four kids whining that they can't breathe and wish they'd stayed at grandma's house instead.

So where does that leave you when faced with a customer whose compressor lost its battle to time, pressure, and friction? If you don't want the warranty card to turn into a marriage certificate, you must replace the parallel flow condenser, along with the filtration system. And a good filter installed on the liquid line is not a bad idea either. When replacing a PF condenser, you should always use another PF condenser. The low prices of an aftermarket serpentine condenser (particularly those made of copper) may be tempting, but the fact is, as I have explained throughout this article, PF condensers will cool significantly better and keep the cold air coming from the vents cold! Telling a customer that they have to spend $1,300 to repair their A/C while the butcher shop down the block told them they can change the compressor for $600 may be difficult. But if you can educate your customers about the operation of their A/C system, it's a lot easier to sell the proper job.

by John McCabe, Automotive Cooling Journal, September 2002 with permission of Four Seasons/Trumark

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