SW Oil Cooler: Which is Inlet?

[Noah]

Might be a dumb question: I have a SW-8432K drawn cup oil cooler with both fittings on one end of the cooler. How do I know which is the inlet and which is the outlet? Is there a difference?

[Bubblehead]

I have never heard of their being a difference with the oil cooler, but once it's mounted they seem to work better with the oil from the engine going to the bottom and returning to the engine from the top.

None of the literature I have from SW mention any set inlet or outlet.

[Noah]

Thanks John, guess I'll just hook it up in whatever direction seems to work better for the hoses - appreciate the response!

[logansc]

You may hear some of the guys who've been at this awhile tell you that it is "possible" to get an air pocket in your oil cooler if you don't feed it from the bottom and empty it from the top. My SW had both inlet and outlet on one of the long ends, but I wanted, for space reasons, to mount it vertically. I ended up swapping it from both being on the bottom to both being on top as part of my trouble-shooting process for high oil temps. Turned out primarily to be an airflow issue so I don't know that eventually mounting it with both at the top actually made any difference. Not one that I could identify in terms of improved cooling numbers, anyway. Fanatically sealing the the baffles, trimming the front plates, smoothing the "ramp" under the upper cowl, etc. on the other hand, made a big difference.

Regards,

Lee...

[Bubblehead]

Lee - I was suspicious about the air bubble issue too. My oil cooler was in the normal position on the back of the baffle behind #4. I could not see that it would matter. After remounting my oil cooler after working on the baffles I made a flight in the morning and established a baseline, then landed and swapped lines so "in" was on the bottom. The OAT was maybe 5 degrees hotter for the second flight, but oil temp had dropped 10 degrees or so.

I cannot prove there was an air bubble, or that it would not have worked it self out anyway, but do know swapping the lines lowered temps.

[Noah]

I would expect there to be a difference in performance by switching the lines. If your cooler is mounted on the aft baffle, then part of it is likely "partially obscured" by the heating fins, so that the incoming air to that part of the cooler is a bit warmer than the rest. I believe that this would make a difference in the overall delta T of the cooler. It seems likely to me that the most efficient operation would occur where the incoming oil sees the coolest fins first (large delta T = large Qdot), followed by the warmer fins behind the cylinder head just prior to the oil reaching the outlet. Does this agree with your experiment John?

[Bubblehead]

Noah, It would seem that way, but it was just the opposite in my case. The hot oil is going into the bottom of the cooler and flowing out the top, so it is going closest to the cylinder first. I think your logic is right even if the observation does not agree.

With the hot oil going in to the bottom where the warmer air is, there would still be some delta T, and as the oil flows up, it is getting to cooler and cooler air that is moving faster and faster so even though the oil is cooler delta t is being maintained and mass flow rate is increasing. The other way the cooler oil is getting to warmer air so heat xfer is reduced. Only the top of the cooler is effective with top-down flow, but the whole cooler is effective with bottom-up flow.

That is just a guess! Without a way to measure at least oil temperature and one of the air temperatures (in or out) I have no way of really knowing what is happening.

FWIW I have a 1/4" spacer between the baffle and the oil cooler. It seems like whenever air flow is involved it is tough to predict performance.

[skylor]

Originally Posted by Noah

IIt seems likely to me that the most efficient operation would occur where the incoming oil sees the coolest fins first (large delta T = large Qdot), followed by the warmer fins behind the cylinder head just prior to the oil reaching the outlet. Does this agree with your experiment John?

Actually, in 2-fluid heat exchanger terms what you just described is more or less equivalent to a parallel flow heat exchanger where the hottest oil is cooled by the coolest cooling medium, and the cooler oil is cooled by warmer cooling medium (i.e. the cooling medium has already been warmed by the its pass through the heat exchanger). The thermal experts and engineering texts say that the more efficient heat exchanger design is the "counter-flow" design where the hot inlet oil is cooled the the warmed, exiting cooling medium and the cooled exiting oil is cooled by the colder cooling medium at the heat exchanger inlet. The reason this tends to be more efficient is because there is a more constant delta-T throughout the entire heat exchanger resulting in a larger average delta-T. Another way to think about this is that in the parallel flow heat exchanger the temperatures of the oil and the cooling mediums converge as they pass through the heat exchanger. The coldest that the oil can ever get is the exit temperature of the warm cooling fluid, whereas in the counter-flow heat exchanger the oil exit temperature can theoretically be cooled towards the lower cooling fluid inlet temperature.

In the case of the baffle mounted oil cooler, I would expect the more efficient scenario to be where the hot oil enters that the part of the cooler closest to the cylinder fins, and the cooled oil exits the upper, cooler part of the oil cooler.

Skylor
RV-8QB
Almost Done (been saying that for 3 years now!)

[osxuser]

Inlet at the bottom for all the reasons mentioned, but mostly the air bubble.

/Thread.

[Noah]

Well THAT is very interesting indeed, Skylor! Thanks for the education in heat exchanger theory. And cool that John's experience supports this argument. Now I will need to go study my installation to see how I hooked it up. I always regret not taking more heat transfer courses!

Incidentally, I was trying to figure out the oil path thru the SW cooler. As mentioned earlier, my cooler has both inlet and outlet at one end of the cooler (the bottom). I have assumed that the path through the cooler looks like this (except rotated 90 degrees clockwise). Is this correct?

INLET
[________________>__________________
_______________<___________________]
[________________>__________________
_______________<___________________]
[________________>__________________
_______________<___________________]
[________________>__________________
_______________<___________________]
[
OUTLET

I seem to recall seeing the term "single pass" oil cooler which I assume this is. This term implies there must also be multi path oil coolers, which I assume consist of plenums at each end, with parallel paths? Like this:

INLET...........................OUTLET
|___________>___________|
|___________>___________|
|___________>___________|
|___________>___________|
|___________>___________|
|___________>___________|
|___________>___________|
|___________>___________|

I also assume that the single pass would allow a larger delta T, and thus be more desirable? Are both types in use? If one is clearly better, why would both be used? I also assume that the plenum type would be important to have the inlet on the bottom, but for the single pass type, it wouldn't matter (or can't matter, since both inlet and outlet are on the bottom already)