heat exchanger theory

[Bruce King]

This issue came up several years ago where I worked.
Since we have a lot of brainy people on here I thought it would be interesting to toss this out here.


We had several flight simulators with 250 gal. hydraulic systems.
The movement of the sim (18-20k lbs) on six servo actuators created a lot of heat within the hyd fluid.

The hydraulic pack for the 1500 psi motion system included three 50hp motors but the heat was primarily from the operation of the motion.

The hyd fluid (synthetic oil) was pumped through a heat exchanger that had cooling water flowing through the other part of the heat exchanger.

The cooling water was not always as cool as it should be, the cooling towers did not work well in hot humid weather and we were loading the system up with several sims.

The hydraulic fluid should stay around 95f to 105f if my memory is good.
The cooling water in the summer was usually between 80f and 98f.

The cooling water had a mechanical thermostically controlled flow valve on it that was adjustable. They did not really work too well so we tweaked them sometimes when things got too hot but this did not always help on hot days.

Then someone commented that one of the sims overheated and shutdown, the fix was to slow down the cooling water flow to that heat exchanger.

This sounded wrong at first and then after much thought and discussion, a few of us decided it might be possible that the water was just not absorbing the heat well when it was passing through so fast.

We argued and argued, testing was hard because the motion usage (load), the cooling water temp and the weather changed every hour.

So we started reading, one guy brought in some books on thermodynamics and we read, argued, read and argued. Those books just did not have the whole answer to this actual scenario. We were all flight simulator engineers which involves many things, electronics, avionics, electrical, instrumentation, hydraulic, software, computers so we could not really concentrate on becoming experts in any one field.


I did find this statement in one book that was in a section concerning a metal plate between the hot fluid and the cold fluid "the transfer of heat to water is a slow process"


So we argued some more, I called two heat exchanger type engineers and explained the whole thing to them and how I felt that the water could be flowing too fast to pick up the heat.

Here is their actual answers over the phone:

Eng 1: You are exactly right.

Eng 2: I don't know, but am leaning towards thinking you are wrong.

So the argument was never really settled.

Anyone know of a document that explains heat transfer to water through a metal plate or have any insight?

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[wayne soper]

When an internal coil on a boiler gets clogged with mineral buildup the water flow has to be slowed or reduced to alow proper absorbsion of heat into the water. Same theory. Normal application. Why is life so simple when your looking in someone elses window?

[John Arnold]

All I know is - an auto parts guy once told me my van engine was overheating because I had removed the thermostat - the water was circulating too fast to absorb heat from the engine - which is complete and utter horse hockey.
Turned out the radiator was clogged up. Any backyard mechanic knows that removing the thermostat will make the engine run too cool.
My way of looking at it is this: the heat exchanger doesn't care, or know, how fast the water is moving. It only "cares" what temperature the water is. The faster the water moves through, yes, the less heat it will pick up per volume of water, but a larger volume of water is moving through!
I'm not an engineer and I don't play one on web forums, but that's my theory and I'm stickin' to it.

[Jerry Peck]

Nothing to back it up except personal experience, but I think Eng 1 is correct. But it is not as simple as Eng 1 put it either.

There would be an optimum flow for any given temperature of water. Cooler water could flow faster than not-as-cool water and still transfer the same amount of heat in the same amount of time.

Ever touch a really hot pan or handle? The longer your skin is in contact with it the greater the heat transfer, the greater the pain. A really quick touch and you 'didn't feel a thing', a slightly slower touch and 'ouch, that's hot', hold it a little longer and 'dam that sucker's f*&king hot!' - you get the idea.

Yes, Eng 1 is on the right track. (Far be it from me to profess that this is "the correct answer" as, I stated, I have "Nothing to back it up except personal experience".

[Jim Luttrall]

Nothing to back it up, but I think you need to look at the turbulence of the water. Flow too fast and you actually reduce the contact of the fluid to the heat exchanger since water will form air pockets. Also, if you speed the water up in the cooling tower you might be at its limit and actually be returning the water slightly hotter than on a slower pass. Bottom line is you guys needed colder water.

BTW, on the removing thermostats from engines subject. There is one scenario when removing the tstat will make the engine overheat and lose water. On a hot day with the tstat removed and you kill the engine, the residual heat in the engine will boil the water increasing the pressure above the pressure cap setting and loose water.
Put the tstat back in and the problem goes away since now there is cooler water to quench the residual heat in the engine just on the other side of the tstat even when the water pump is not running.
The rest of the time the engine will run cooler with the tstat out.

[Richard Moore]

Also with absolutely no science to back it up...

It does seem that there has to be an optimum point beyond which additional flow would return no gain.

I could see moving water too fast through a very simple, straight shot, heat exchanger may not allow the majority of the water to "roil" and come in contact with the hot parts. Not sure this is the best analogy; but kind of like a fast moving river where the faster current in the middle never touches the banks. However, I'm not sure (there's an understatement!) whether that would negatively affect the actual amount of cooling or just be a waste of the excess water. Through in a few bends in a more complex heat exchanger and that "theory" seems less likely.

And then there's the fact that it sounds like a closed system with the cooling towers a part of the equation. I have no idea how those work, but could the extra flow be affecting their ability to cool the coolant?

[Jerry Peck]

I was thinking about this some last night (as I previously stated - I don't know nothin' 'bout it) and Richard's post with his example of the river is quite like I was thinking.

Not sure this is the best analogy; but kind of like a fast moving river where the faster current in the middle never touches the banks.

The thin film of water in actual contact with the sides of the heat exchanger, is that water actually moving? Actually moving at the speed of the other water if actually moving?

Is it possible that the thin film of water on the surface of the heat exchanger does not move or flow much, and that it serves as the 'heat transfer media' between the flowing water 'in the middle of Richard's river example' and the heat exchanger, with the heat being exchanged through the cooling water as the center of that water moves, leaving a 'somewhat stationary film of water' in contact with the heat exchanger?

Either way (the film moves or does not move), it would be like an evaporator coil (I would think anyway) in that 'too much too fast' transfers less than 'just enough just fast enough', and that 'too little too slow' is also not good.

We all know that forcing too much air (too much too fast) over an air handler coil results in less cooling, right?

[Brandon Chew]

I can't explain the physics behind it without doing more research but I do know that the efficiency of heat exchangers depends upon the mass flow rate of the two fluids involved (gas-gas, liquid-liquid, or liquid-gas systems). A given heat exchanger has a "sweet spot" -- get the fluids moving either too fast or too slow and the efficiency of the exchanger will drop off. I bet it has to do with the amount of turbulent vs laminar flow and that Reynolds numbers are involved.

[Ken Amelin]

Heat exchangers perform differently due to many variables, including velocity of water across through the exchanger.

Each manufacturer will rate their exchanger for heat transfer in btuh, gpm of fluids and temperature differentials of both fuild absorbing the heat and the fuild rejecting the heat. They also perform differently at various ranges of fluid operating temperature.

Most shell and tube heat exchanges are designed to perform on a 10 degree delta t. It seems to be the economical price point and optimum pressure drop for pumping horsepower.

Plate type heat exchangers can be selected to have a much lower differential, but as performance increases so does cost and size of the exchanger. Plate type heat exchanges typically have a lower pressure drop as the fluid velocity is slower across the plates.

If you're using a evaporative open type cooling tower as the cooling fluid, the tower can be sized for your specific heat rejection requirements but the operating temperature range of the cooling water will be within approx 5-10 degrees of the "wet bulb" temperature.

BOTTOM LINE - reducing the velocity through the heat exchanger will result in a larger delta T, on the water cooling tower side, but it may not improve total heat transfer in BTUH which may not improve the delta T on the hydraulic fluid temp. Each exchanger has a separate performance curve.

[Bruce King]

good info, this all makes it understandable why it takes so many years to see real improvements in many products, it just gets complicated fast.


I should have mentioned also that the cooling tower loops and another heat exchanger with the inside loop. The inside closed loop had chemicals mixed in with it. The heat exchanger I mentioned earlier was on just one sim, each sim had at least one, the building equip had two.

When things got really hot, we dumped a lot of city water into the cooling towers and let the overflow go down the drain. The water bill really showed this little trick but it was all we could do. The big self contained A/C units in the computer rooms also were on the cooling loop.

One sim had a water chiller on it at one time, then we moved it without moving the chiller and had to put 4 regular heat exchangers on it.

[Ed Voytovich]

As a student of building science, I think this is about the three methods of heat transfer: conduction, convection, and radiation--and it's about how long those transfers take. It's also about thermodynamics. Heat goes from areas of high concentration to areas of lower concentration. Every time. But as a home inspector, I say for the record: refer to an expert in heat transfer.