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 In-tank Coolers - What You Want to Know
Narsa System Reports
Volume 14, No. 2 May/June 2000

Answers to your toughest cooler questions — plus other related issues

When you take off the radiator outlet or inlet tank, there may be an oil cooler inside one of them. Oil coolers are primarily used for cooling automatic transmission fluid, although in some (admittedly rare) cases it’s for engine oil. A few logical questions:

  • If the transmission failed, should you replace that cooler?
  • What should you do if engine or transmission oil has been running too hot and you want extra cooling?
  • Is there a meaningful difference among brass, aluminum, and stainless steel construction, between “concentric tube” and plate designs.

The subject of in-tank coolers is an ongoing discussion between NARSA and its members. In this report we’ll cover these questions, and also review other issues that have been raised.

There are four types of in-tank coolers produced today (see FIGs. 1 and 2):

Concentric Tube - There are types in this design, both of which are brazed construction. One is made of brass, one of aluminum. As the name implies, there’s a tube within a tube, and there’s a “turbulizer” (turbulator) between them to improve heat transfer. The brass type has been around a long time and the manufacturing technique is well-established. The turbulizer is wrapped around the inner tube, inserted with the inner tube into the “jacket” that is the outer tube. Brazing rings are added, drop in the fittings and send it through. The aluminum concentric is not in the aftermarket yet — it’s strictly original equipment — but is being used both by Ford and Chrysler, so you can expect it soon. It provides a weight advantage for the OE, but that’s not a concern for the aftermarket now. Metallurgically, the brass is 100 percent compatible —- it can be installed in a copper- brass, aluminum or plastic tank. The aluminum can’t be used with copper-brass.

Plate-type - Two are also of this design — one of stainless steel with fittings and turbulizers of cold-rolled steel, the other entirely of brazed aluminum. The number of plates varies by heat transfer requirement — could be just one or as many as 18. The number of plates is determined by the size of the tank and the need for heat transfer. Long Manufacturing currently supplies three “standards,” a 1-3/4 x 11-1/2 inch model with one to seven plates; a 2-1/2 x 8-1/2 inch model with one to six plates; and a 3 x18- incher with 5-18 plates.

Rich Vernon of Long Mfg. states that there is virtually no difference between the heat transfer of the two (“the performance curves are almost on top of each other”). But like the brass, the stainless steel is compatible across the board.

The OE manufacturer sets the requirements for the oil cooler — specs for adequate heat rejection and low pressure drop of the oil flowing through, plus high quality and durability standards.

In the days of tall downflow radiators, with large tanks and lots of airflow, “We could put in there a pretty good size concentric oil cooler, and there was a large core face for good heat transfer,” Vernon noted. “Today, vehicles have sloped front ends to improve aerodynamics — much more restrictive, with borderline airflow. The size of the tanks decreased as the radiators had to fit into less space. If the concentric tube designs were not changed, he said, they’d have come down in size. A smaller oil cooler would result in an increase in the pressure drop between inlet and outlet,” and that would result in transmissions starving for fluid. Overheating and clutch slippage would occur, and there would be more transmission failures.

There’s an obvious incentive to use a concentric tube oil cooler — it’s a compact design and a lot less expensive than a plate type, even in aluminum. But the plate type has more surface area for its size, transferring more heat, making it more space-efficient. The price penalty is significant, so where possible, the concentric tube cooler gets the nod.

One thing that helps keep these in production: a major change was made to lowpressure- drop turbulizers (See FIG. 3 on next page). They’re made of a thinner material, shaped for more cross-section, taking up less space between the tubes. This improves heat transfer and fluid flow.

“If you have to replace it, you should be sure you’re getting another that’s also got a low-pressure-drop turbulizer,” Vernon said. If you install a replacement with a conventional turbulizer of the same overall size, you could starve the transmission for fluid. You can’t see inside, so you have to deal with a reputable supplier.

Cleaning a Concentric. What about cleaning the existing concentric? If you’ve got the oil cooler out because you’re repairing the radiator, it is good practice to clean it at the same time. Some NARSA members said they were satisfied with ultra-sonic cleaning. One uses his Vibro-Sonic machine, which he feels does a better job (like others who have been using this type, with its air-pressure-produced agitation).

In the case of a transmission failure, Vernon asked, “After you clean, how do you test? How do you know if you have all the metal particles out of the turbulizer?” (particularly the new low-pressure-drop designs).

The answer is, unless you have a laboratory- type flowmeter, you cannot tell. If there is any migration of metal particles during vehicle operation, a problem might surface at some time. Do you want to take the chance?

One NARSA member said he had been doing it for a long time — judiciously picking out the occasions based on what had happened to the vehicle — and hadn’t had comebacks. But it is a gamble, and a new cooler is relatively cheap insurance compared to the price of the automatic transmission.

CAUTION: if you clean the oil cooler in the same tank as the radiator, you risk getting coolant in the cooler. You’ve absolutely got to get it all out. As noted in the November/December 1998 issue of NARSA Service Reports, just a trace amount of antifreeze in the transmission fluid is enough to ruin the friction material on clutch plates and cause swelling or cracking of many of the rubber seals. If the cooler is for engine oil, anti-freeze/coolant in engine oil is also trouble, possibly causing piston rings to stick.

If you place the entire radiator in the cleaning tank, do you plug the cooler lines first? Anyone who doesn’t is asking for trouble.

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