Heat exchanger physics question
 

To increase effectiveness of a heat exchanger, whether it be an oil cooler or a closed cooling exchanger, which will be a bigger influence? Increasing the size (say 25%) or switching from a 2 pass to a 4 pass (of raw water) on the same exchanger?
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More surface area to cool, the better the cooling.

you'd really have to determine whether your existing cooler was getting heat saturated. You have four deltas to contend with.

Delta 1 would be the temp rise of the cooling water in vs out.
Delta 2 would be the temp drop of the medium needing cooled in vs out.
Delta 3 would be the temp diff of the incoming water vs the incoming oil (if it was an oil cooler)
Delta 4 would be the temp diff of the outgoing cooling water vs the outgoing oil temp.

Sometimes you have a surface area issue, sometimes you have a flow volume issue. You need all 4 deltas to know.

Yes, though, the more surface area, the more heat transfer takes place. You almost always get more results out of more area. If, however, your flow volume is too low, added area won't do nearly as much. Your three main variables are: coolant flow, oil flow, transfer/surface area. They all contribute.

Lesser variables also contribute, such as heat conductivity of the tubing used, the internal and external cross section of the tubing used, the flow path through the unit, and all the other stuff...

mcollinstn!

I did not expect you to pipe in here but if definitely is up your alley.

Turns out I have been asking the same question different ways.

Without setting up a true laboratory setting where your deltas (changes) can become known values, let me give you my real world scenario. I use the big Teague coolers with raw water coming in after the strainers. My oil temps spike after a long hard run but come back down pretty quickly when cruising.

I have a stepped bottom, high X-dimension and sportmaster shortys. Others (with similar rigging) have found that too much air enters the pick ups at high speed, reducing cooling efficiency. My raw water pressure stays at 15lbs and my closed cooling ranges from 170-180. The heat exchanger is much bigger for the closed cooling but has the same copper nickel tubing inside a brass housings.

The next step in my real world application is to place cone top lids on the strainers, with a check valve and overboard transom dump. Hopefully this will eliminate air (steam) in the raw water side, improving the oil cooler's ability to cool the oil.

#1: what sort of SPIKE in oil temps are you seeing?

mkay. yeah, you can be getting air thru the cooler and losing efficiency, but you're still getting decent total water volume else your closed cooling loop would be showing an overheat tendency as well.

Are you splitting after the strainers and sending water one way thru the oil coolers and another way thru the engine H/E ? Or are you running thru the oil cooler and THEN into the engine h/e?

Describe your raw water path, including any splits and dumps and restrictors, etc.

Normal strainers will pass air pretty easily since the in and out are both located at the top of the strainers. Industrial air/water separators use a cyclonic action to sling the water to the outside, and they extract from the center in order to purge/separate the air. I think your top purge idea for your strainers will ASSIST in removing air, but I think a lot of it will still go thru the system. BUT some air is probably okay as long as you aren't getting pockets trapped. I'm not sure how many passes your raw water makes in your oil cooler, but if there are any places where air can "pocket" then add brass fittings and run small diameter hoses from them to tee into the main outflow. That will make sure nothing in the cooler is sitting in an air pocket. Orienting the coolers on an angle will aid in shifting the air to one area where it can be better extracted. You can also "cheat" the internal dividers to purge air from early chambers to the outbound chamber - there is a slight pressure differential from high to low as you move from the inlet to the outlet, so any bleed passage you cut will move air towards the outlet side...

you might be chasing the wrong demon.
post your water flow and let's go from there.
MC

on your motors, is it possible that the places on the block you are taking and returning oil from the cooler could possibly be on the same circuit, and thus allowing oil to partially bypass the cooler? Some BBCs need to add a restrictor in the main supply to keep this from happening.

I'm not looking at an oil flow diagram of your motors, and I don't know where you are taking and returning the oil, but this is something that has bitten people before..

bypass springs can also play a role...

OK here goes;

Oil is run through Eickert/Hardin filter(HP6)/thermostat mounts. I run an external Aivaid oil pump and plum it directly into the old Filter mount with the Aiviad supplied adapter. It is the only way in. Even though it is a 14 qt. wet sump pan, everything is external.

Water; in from the sportmaster shorties, Into the MM strainers through a 6" PS cooler then into the the teague oil cooler. Then to the Eickert/hardin raw water pump then on to the coolant heat exchanger then out to the Stellings headers. I essentially followed the layout from Cigarette but with everything very upsized. I got rid of a lot of extraneous hose in the process.

All raw water is 1 1/4" ID. All oil is 12AN and all coolant is 16AN.

Oil runs 220-230 at cruise and moderate speeds, RPM from 3500 to 4500. Going up in rpm gradually brings oil temps up to 270 sometimes 280 on long hard runs (5500-6200rpm). I run Mobil V-twin oil.

When oil gets that hot, I back off to 3500 to 4000 rpm and it rapidly drops back down to 220 to 230.

Hope I got all the variables you asked for.

Oh yeah,

Strainers load from the top, exit from the bottom.

Both heat exchangers are what I believe are called a double pass. In and out on the same end, making a 180 turn on the other end.
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good config on the strainers. if you do put an air bleed on the top of the strainer, be sure to put a spring loaded checkvalve in line with it so you don't suck any air into the cooling system at idle.

With the cooling water layout you describe, i'm gonna say that air in the cooler is not going to be a huge part of the problem. If you CAN, it would be a little helpful to angle the cooler so that the in/out end is higher than the closed end at running speeds, and you may still want to drill an air bleed hole (1/8" diam) between the in/out divider beneath the cap. This will ensure any air passes straight from the inlet side to the outlet side without collecting in the cooler anywhere.

Now, I'm still fixated on your oil system.
See if this is right:

Oil is pulled thru a "normal" pickup in a wetsump pan.
It passes thru the old oil pump housing, and to the factory oil filter boss on the block? (is the original oil pump there, does it have rotors in it? is it replaced by an adapter and long pickup?)

Okay, then you have an aviaid adapter screwed onto the oil filter boss and it sends oil OUT to the external pump.

It goes from the external pump to the Hardin filter/thermostat mount.

It loops out thru the cooler and back to the mount.

It leaves the mount and goes back to the aviaid adapter on the filter boss on the block.

Is this correct?

Where is your oil pressure regulator? Are you using the one in the engine block filter boss or are you using an external one?

I'm still wondering if excess pressure is being diverted somewhere that keeps some of it from flowing thru the cooler.


WHERE are you taking your oil temp reading? Where is your sender? I'd love to see what the outlet temp is of the oil right when it leaves the cooler, in comparison to the pan temp.

IF all of the oil is coming directly from the pan, through the pump, through the filter/tstat, and looping thru the cooler, then back into the block gallery, then we go back to looking at a larger cooler.

Right now, I want to completely rule out that oil is being diverted somewhere before the cooler. I'm wondering it either a bypass in the filter/tstat is diverting oil from the cooler, or something between the pump and the filter/tstat is diverting oil and keeping it from going thru the cooler.

In a perfect world, you would run pickup directly to pump. Pump directly to filter. Filter directly to tstat. The tstat would direct oil either thru the cooler or to a bypass line that tees back on the other side of the cooler. Then you go from the junction (tee) directly to the pressure regulator. 65psi oil is fed to the block gallery, and any excess oil is dumped back into the pan. The block has all of the pressure reliefs or bypass springs plugged closed.

I'm not sure your system is functioning like this. Places for oil to be diverted: in the block, either at the filter boss or in a gallery. at the aviaid adapter. at the remote filter mount.

One thing is for sure.

If you have an oil temp of over 240 at the oil cooler outlet, then you need a bigger cooler.

I guess this is the easiest thing to check. Drop a tee on the outlet and screw in an oil temp sender and run the snot out of the boat and when the oil temp gauge on the dash shows 270, check to see what the outlet temp is on the cooler.

i'm thru rambling.
mc

Nope, you figured it out exactly. It is essentially a single stage external pump on a wet sump pan. And you described the routing exactly how I have it.

Ford Blocks don't have bypass's sprung or otherwise. The original wetsump pump feed is tapped and plugged. The aiviad adapter blocks off the outgoing oil allowing only incoming oil at the block's filter boss.

The pressure regulator is on the pump I have them at 80 psi cold and 65 hot. I have temp senders in the pan and in the thermostat housing next to where the incoming oil from the cooler is. Since I have two engines, I run one off the pan and one of the T-stat housing. They are different, but only about 10 degrees when I am running hard and almost none when cruising (at least too little to read on the gauges).

These are the biggest non-custom oil coolers I am aware of. Teague swears they are good with supercharged power far exceeding mine.....but they run raw cooling on their big stuff and the overall engine temps are lower. I also don't know if they run t-stats for the oil but I would guess they do.

two things strike me as unusual. First, the water temps don't go crazy even when I know the oil is screaming hot and the coolant exchangers get their raw water after the oil coolers.

Second is just how damn fast the oil temps drop when I cut the rpms down (and of course speed). Watching the gauge you would think the water wasn't on and just got going again.

On Serious offshore, MERPerformance was describing a cooling issue with raw water (if I recall on a TS Top Gun) and they ported and polished the intakes on the drives and vented the strainer to help. That is what got me thinking here.

I have the motors out for a non related repair. I took advantage of the down time to return the strainers to MM for retrofitting vented lids. I can also drill the vent hole between the two passes on the brass end to keep the air out there. If these don't help, I can port and polish my sportmaster pick up holes. If all fails, I can have American Industrial build me bigger oil coolers. They built the teague coolers and I had them build my coolant exchangers. As you know, these exchangers are very heavy even when dry, I can see why racers don't want them.

ok. when you say the regulator is on the oil pump, does it recirculate oil back to the pump inlet or does it dump excess oil back through a separate bypass line? If it recirculates back to the pump inlet, you may not be getting the oil volume you need thru the cooler. That's a maybe. The way around this would be to have the regulator AFTER the cooler, with a separate dump back to the pan. Also, I'm not familiar with the routing of the oil thermostat. The oil Tstat is on the filter block or is it on the cooler? At any rate, for right now let's stop focusing on the oil flow (although low flow thru the cooler will behave just like yours is).

Let's look at water flow.
You say you have 15psi of water pressure. Is this reading taken from the intake manifold crossover, or right after the raw pump, or what? I would be very interested to know what your water pressure is at the strainer. Sometimes a raw pump can act as a restrictor.

If you have more water pressure before the raw pump than you have after it, then you are limiting your water flow thru the cooler. If that is the case, a pressure dump between the oil cooler and the raw pump would allow you to take advantage of the extra flow that's currently being restricted.

On the other hand, your raw pump may not be a restriction at all. You may have even more pressure after the raw pump, and you might have a restriction somewhere else in the loop limiting flow.

Fluid flow (gas or liquid) is dynamic and changes due to different circumstances.

Porting your pickups properly will increase the water delivered to the inlet side of the raw pump. With the outboards I used to play with, before low water pickups, we put plates over the side inlets and used a die grinder to make a "ledge" to direct the water towards the pickups. Worked great, even with pretty high transom heights. With low water pickups, outboards on boats that required positive trim would sometimes lose water pressure at speed. With them, pulling out the ole die grinder and changing the profile ahead of the pickup holes would restore water flow. So on your drives, deepening the troughs ahead of the pickup holes will load more water into them. Also you can drill onsize holes at the pickups and tap short lengths of stainless tubing into them to serve as "scoops" (I'd have to draw you a picture, this doesn't come across right with just the words..). Once again, if the raw pump is a restriction, then you'll get no benefit unless you have a pressure dump to allow the flow to go thru the cooler.

It's all a matter of pressure readings taken at different locations to determine what is happening at speed.

mc

It is a recirculating pump. But when I examine the inside, the inlet and outlet are on the main body, on either side of the two pump gears and the pressure regulator is in the very back of the pump. I have a tough time visualizing how it reduces flow.

Oil Flow(just to be sure) Pan, pump, T-stat filter combo, (cooler If in loop) and back to engine boss. I am going to run out the shop and see if the remote filter boss has a bypass in the schematic.

The oil T-stat is in the remote filer boss assembly. I have added a picture of that. One of which you see the routing of the raw water line; Strainer, PS cooler, Oil cooler, Pump, coolant cooler, header dump.

I can visualize the tubes you added to the pick ups.

I honestly don't know what my water pressure is at the strainer. I measure it at the last component to use raw water, the Coolant heat exchanger on the raw water side. just before it dumps into the headers and out the boat.

Someone just P/M'd me and stated they solved the problem going dry sump. I hope he pipes in with more details. :drink:

When your regulator diverts flow from the outlet of the pump to the inlet of the pump, you are fractionalizing the volume of oil being pumped. In other words, the pump is actually pumping say X gallons of oil in a given time, but less volume than that is making it to the next component in the loop, because pumped oil is being rerouted back to the pump inlet.

Properly designed drysump systems flow (and cool) a huge volume of oil, yet supply the block with only what it needs. Your setup should be able to perform adequately for your application.

Let's derail this part of the discussion and go back to basics. After you run hard for a weekend, have you gotten an oil analysis done? Is there any chance you are scuffing at max power? Oil heat comes from several areas: Heat from viscous friction, heat from the sliding contact parts in the motor (piston, cylinders, cam/lifters), from the heads (combustion heat), and from VALVETRAIN. Valvesprings operating at the upper deflection limit of their design spec will generate incredible heat at high revs. There is actually a spike in the heat curve of spring material when a certain cycle speed (RPM) is exceeded.

There are variables in play all over this situation, and everything interrelates.

I'm just not that familiar with Ford lube passages, and I don't know if you are flowing enough oil to take advantage of the cooler --- wait, you already said that one of the motors takes the temp reading right after the cooler. Is that correct? If that's right, and there's only 10 degrees diff between the oil cooler exit temp and the pan temp, then let's stop worrying about oil flow and go back to water flow. A low oil flow condition would show a much colder oil cooler exit temp than pan temp.

I apologize for the rambling thoughts, but you're getting it as it comes off my head, not after several draft revisions.

gotta go for now

I appologize for hijacking the thread but what is the max oil temperature that you consider acceptable during extended WOT running (1.) in the pan and (2.) coming out of the oil cooler?

I do get skirt scuffing once I exceed 6400 rpm so I stay below that. They have redesigned my blocks since and have longer bores to keep the skirt from scuffing at the bottom of the stroke. I also have a generous oil flow up top to keep the valvetrain cool. I have added drain back lines to assist flow back. The pans are 15 quart pans with several baffles with lids and a windage tray built in.

I found a picture of the two heat exhangers before painting, not that the picture informs you of how many BTUs are transferred but to show how big they are relating to one another. The small one is the teague coooler.

I also loaded the schematic for the T-stat/filter Boss. The only By-pass I can see would be in the filter itself.