HX coolant too fast to cool?
HX coolant too fast to cool?
(OP)
Is it possible to have coolant go thru an engine too fast and have the coolant not pick up enough heat so the engine overheats?
I think it would be like a simple heat exchanger.
Is it possible to have the coolant go so fast that the heat exchange is lessened?
I think it would be like a simple heat exchanger.
Is it possible to have the coolant go so fast that the heat exchange is lessened?





RE: HX coolant too fast to cool?
How fast are we talking about?
rmw
RE: HX coolant too fast to cool?
It's more likely that something else is wrong; your exchanger may be grossly fouled.
TTFN
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RE: HX coolant too fast to cool?
Heat transfer takes place at a rate which has time as a factor. If the time of the water in contact with the hot surface is inadequate, less heat transfer takes place. The rate is also affected by factors like velocity and turbulence. The higher the velocity, the more turbulence you have which improves the heat transfer rate. (The reason you sling your finger through the air when you burn it - speeding up the cooling rate.)
But they are not linear. Doubling the water flow through the engine (velocity) does not necessarily mean that you double the heat transfer rate or the duty.
So at some point, theoretically you can blast the water through so fast (if physically possible with your pumping system) that it picks up virtually no heat. Easy to theorize, hard to do.
So, unless someone has sneaked out there and added several pumps to your water circuit, I would look for other more realistic factors, starting with fouling.
rmw
RE: HX coolant too fast to cool?
Started off as a debate about an engine overheating but now has engineer A (me) saying it is theoretically possible to have velocity so fast in a closed system that the cooling capacity can be reduced.
Engineer B says that as mass increases then the heat transfer increases and therefore there can not be a reduction in heat transfer no matter how fast the coolant is going thru the system.
The heat transfer would be radiation plus convection plus conduction.
I origianlly thought this would be a very easy question to answer but now I am learning my heat transfer knowledge is lacking.
Anybody got a formula or model or difinative answer to say either way?
Just in theory
Thanks
RE: HX coolant too fast to cool?
I have to go with Engineer B on this one. Here's why:
Let's assume that the amount of heat that you need to remove from the engine is a constant, say 50,000 BTU/hr.
If the cooling through the engine is say 5 gpm, that gives you a temperature rise through the engine. Using water properties(to simiplify the math), that works out to a 20 degree rise.
Now, if we double the water flow rate to 10 gpm, the temperature rise is only 10 degrees. This affects two things. If we also assume the inlet water temperature is a constant, this increases the temperature difference between the coolant and the engine. It also greatly increases the heat transfer because the film coefficient of the water is increased (or the film resistance is reduced).
The same thing happens in the radiator.
In the raal world, the system will achieve its own equilibrium, but is affected by whatever controls are used such as thermostats.
A secondary consideration is the coolant itself. Pure water is a great coolant, but causes problems with corrosion and freezing. Pure antifreeze doesn't freeze, but is a not a very good heat transfer fluid, because the viscosity is high, the specific heat is low, and the thermal conductivity is low, all compared with water. That's why most systems use a mixture of approx 50% of each. Its freezing point is very low, it boiling point is high, and it contains some corrosion inhibitors.
Regards,
Speco
RE: HX coolant too fast to cool?
Most, if not all, of those numbers are a function of velocity.
In a real engine block, none of these numbers are a constant as the coolant passes through the block.
So the definitive answer is: It depends.
RE: HX coolant too fast to cool?
Just as I was going to post to engineer "B" and eat crow telling him that he is right and I am wrong, you offer me a glimmer of hope that it is in theory possible.
That is what I was thinking that in a real world engine block that there would be so many calculations to make that it would be easier to test then calculate a certain condition.
Shear, pressure drop, laminar flow, turbulent flow, surface film, vapour bubbles, viscosity, cavitation,.....and likely more variables that I have not heard about.
It would be unpredictable chaos inside and engine block.
Now I don't know what to do.
I guess it is not that important right now.
But, how important is the pursuit of knowledge to me?
How important is it to me to admit I might be wrong?
I think I will post engineer "B" that I can not find any proof of my theory but that I still think it can happen.
I figure he will claim victory.
I will continue my research when I have more time.
Thanks for any and all input for and against.
Any future facts would still be welcome.
RE: HX coolant too fast to cool?
Things are not homogeneous and isotropic, either, it's a simplifying assumption.
RE: HX coolant too fast to cool?
When you move fluid through a duct there is head loss due to friction. This energy is dissipated as heat. The head loss increases with increasing velocity, so maybe at high enough flow rates the coolant would heat up enough due to this viscous dissipation that it would not be able to take out as much heat.
RE: HX coolant too fast to cool?
RE: HX coolant too fast to cool?
"I came, I saw, I made it better."
-Ode to Industrial Engineers
Will ChevronTexaco Corp.
RE: HX coolant too fast to cool?
Process 1 - Coolant picks up heat in engine engine
Process 2 - Coolant loses heat in radiator
So far, I think all the focus I think is on process 1, where the effect of increased coolant flow is clearly beneficial (neglecting friction) if we look at system 1 in isolation (assume constant supply supply temperature at the inlet of the engine.)
If we look at process 2 in isolation, increased flow means increased radiator outlet temperature (assuming constant radiator inlet temperature).
Putting them together, one would certainly intuitively think that increased flow helps. Because we are not looking at maximinizing heat transfer per unit mass of coolant, we are looking at maximizing heat transfer per time. Faster flow should help provide a better thermal link between the two systems (engine and air).
Nevertheless, there are non-linear effects to be considered as alluded above, and sometimes those defy intuition. It is not 100% transparent to me that it is always a benefit to increase flow (even with friction neglected).
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RE: HX coolant too fast to cool?
Nu= Re^.8*Pr^.4
hD/k=(rho*V*D/u)^0.8*(Cp*u/k)^0.4
V- velocity
rho =density
u=viscosity
D duct diameter
k thermal conductivity
At a minimum of logic, if you make V large enough, h will become almost proportional to V^.8 and (Tw-T) will approach (Tw-T0), so you have
Q =P*V^0.8*(Tw-To)
P= constant
Which mathematically grows without bound but practically, is limited by the limits of the correlation and pumping power, but in any event could be much larger than any of experience..
Tw= engine block temperature
T0 = initial cooant temperature
x = position along duct
T = temperature of coolant at any position along x
Now , the more interesting proposition is:
Does the heat transfer to the coolant always increase with increasing V? i.e.
Assuming that only u and k are sensitive to Temperature, I get
h=A*V^.8*k^(.6)*u^(-.4)
A =proportionality constant
It turns out that the product terms on the RHS are increasing functions of T, so we can write the inequality owing to the fact that the coolant temperature T is increasing along x
h>A1*V^.8
Now writing the energy inequality
rho*area*cp*V*dT/dx=hL*(Tw-T)> A1*V^.8*L*(Tw-T)
L= circumference of duct
rearranging using another constant, B and since Tw is a constant we may write
-Vd(T-Tw)/dx>B*V^.8*(Tw-T)
Now since - rho*cp*V*A*d(Tw-T)=q is the flux*dx from the engine block to the coolant we can change this to
-dq/dx>c*V^-.2*q
-dq/q>c*V^-.2*dx
where c is another constant
Integrating from x=0 to x and q=q0 to q yields
q>qo*exp^-c(V^-.2)*x giving the flux at any position
To get Q, the overall heat transfer we must integrate once more to get
Q>q0*integral[exp^-c(V^-.2)*x] dx=qo*V^.2*{1-exp^-c*( V^-.2)*x}/c
Since q0=h0*Tw-To)> A1*V^.8*(Tw-To) and combining exponents
Q> A1*V^.8*(Tw-To)*{1-exp^-c*( V^-.2)*x}=
Q> A1*V*(Tw-To)*{1-exp^-c*( V^-.2)*x}
Finally we must prove that dQ/dV>0. i.e. that a positive change in V yields a positive change in Q.For this we go inside the
Integral [exp^-c(V^-.2)*x] dx
and differentiate with respect to V
yielding
integral[0.2*c*V^-1.2*exp^-c(V^-.2)*x] dx
It is seen that the integrand is always positive and therefore the result is positive proving
dQ/dV>0
DISCLAIMER:
I won't take this to the bank as is, since I am prone to making errors ( hopefully not in judgment). I mostly used water as the coolant, although the temperature behavior of most of the liquids I looked at fit the assumptions.
RE: HX coolant too fast to cool?
Moreover, the overall system is limited by the thermal resistance to the ambient air, so there may be a net change of zero.
TTFN
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RE: HX coolant too fast to cool?
Moreover, the overall system is limited by the thermal resistance to the ambient air, so there may be a net change of zero. "
We'll just blow the radiator fans faster, or buy an infinite radiator.
RE: HX coolant too fast to cool?
RE: HX coolant too fast to cool?
Even if you consider a theoretical system were fluid flows in at one constant temperature and flows over the engine and out then there will be a bounding point where the rate of heat transfer from the engine to the fluid reaches a maximum. As heat transfer coefficient goes to infinity the delta T between fluid and surface of engine goes to 0.
RE: HX coolant too fast to cool?
I wonder if anybody has access to any HX, engine, boiler tests where the flow of process or coolant was analyzed.
Now I guess the question would be if the high flow rate would cause a more turbulant flow and therefore less chance of air bubbles against the cylinder wall, or more?
RE: HX coolant too fast to cool?
The only place that cavitation could routinely occur is in the water pump itself, on the impeller blades. Any place where the engine is supposed to be transferring heat to the coolant would not be designed to promote cavitation.
TTFN
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RE: HX coolant too fast to cool?
I'd check that the water pump is actually moving coolant through the engine block. The pump itself? Stuck thermostat? Fouled radiator? Might even be a head gasket leak into one or more of the cylinders. That would give you a low coolant level, with no sign of an external leak.
RE: HX coolant too fast to cool?
RE: HX coolant too fast to cool?
So, sorry to advise the OP, he loses to engineer B.
RE: HX coolant too fast to cool?
Remember Q=UAT, but this is all covered in the Crane Tech. Notes or the Compressed Air Data Book.
RE: HX coolant too fast to cool?
While not an example of a high flow lowering the cooling capacity, it is an example of how attempting a high flow can decrease cooling capacity.
rp
RE: HX coolant too fast to cool?
As near as I can tell, the OP was speaking to a hypothetical, yet physically realizable scenario.
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RE: HX coolant too fast to cool?
Higher flow rate means higher LMTD and higher mass flow rate, resulting in higher Q, since Q = m cp LMTD. And, since Q = U A LMTD and LMTD and U are both increasing with increasing flow rate, then your engine block is cooler, not hotter. Stop dwelling on residence time....
RE: HX coolant too fast to cool?
RE: HX coolant too fast to cool?
RE: HX coolant too fast to cool?
If the engine overheats due to fouling, then does increasing the flow ALWAYS result in lowering the jacket temperature.If you prove that proposition then you proved that the heat transfer rate increases.
So without any heavy math, and with the better basic assumption that the engine puts out heat flux, say q BtU/HR-FT^2 and the heat is removed in a simple heat exchanger, a tube with area A and perimeter l, length, L
Writing 2 equations
1) rho*V*c*dT/dx=q*l whose solution is
T=T0+q*l*L/(rho*V*c)
2)h*(Tw-T)=q
Solving 2) for Tw after substitution for T, we get
3) Tw=q/h+T0+q*l*L/(rho*V*c)
where
T0= entrance water temperature
Tw =jacket temperature
rho= density
c= specific heat
V = velocity
Since h has been shown to be proportional to V^.8 (see my posts above)then 3) looks like
4) Tw=q/aV^.8+q*l*L/(rho*V*c)+T0
a= proportionality constant
Now looking at equation 4),it is clearly seen that increasing V results in a decrease of the first 2 terms and the last term, a constant, is the input temperature from the radiator.
Therefore Tw must be lowered by increasing V.
RE: HX coolant too fast to cool?
RE: HX coolant too fast to cool?
Can you add a coolant-to-air heat exchange into your model?
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RE: HX coolant too fast to cool?
Good question.
My answer responds to the OP question of an "open loop" heat exchanger, but if you want to make it a real life situation then you close the loop with a radiator that is removing the same amount of total engine heat. In that case, as you know, a thermostat maintains control of the temperature entering the radiator, say 180 F, so the flow to the radiator and its temperature profile inside the radiator is independent of the fouling problem at the engine,which only causes the jacket Temperature to rise to accommodate the fouling. If you now increase the flow into the jacket, then the amount of increased flow rate would ends up bypassing the radiator,without affecting the flow through the radiator
So bottom line-- the exit temperature at the engine is always 180 and , of little consequence, but worth noting, owing to the increased flow, the entrance temperature to the jacket is somewhat increased because the delta T across the jacket is reduced.
Since.the maximum walll temperature is at the exit jacket wall where the water temperature is maximum.and at that point, you have q, the same flux I used previously, we can write
q=(Tw-180)/h
[BTW, we could have arrived at this without all the discussion following the the fsct that the water exits the engine at 180 F]
where h is proportional to V^.8 ; this leads to the obvious conclusion that the maximum wall temperature is reduced.
Seems that the actual closed loop case is easier to prove than the open loop.
RE: HX coolant too fast to cool?
This question was raised with EARLY Ford V8 passenger cars.
Stated. "If you remove the thermostat in the engine, the system WILL overheat."
As I was 15 at the time I remembered the statement well.
Did the Fords have radiators & fanblades maxed out??
Cocktail engineering of the overheating was. "Water did not stay in contact with the radiator long enough."
Rich
RE: HX coolant too fast to cool?
Water is in contact with the heat exchange surface 100% of the time. If it flows at a high rate a given unit of water will not heat-up as much as slower water so the water temperature will be lower. This increases the heat transfer rate per unit area of heat transfer surface. So per unit time the amount of heat transferred goes-up.
RE: HX coolant too fast to cool?
Brian Bobyk - Hoerbiger Canada
RE: HX coolant too fast to cool?
1. tube and tubesheet erosion.
2. excessive cooling of the oil side which could bring oil temp too low and increase viscosity to a point where pumping loses were excessive.
We once had an oil cooler on the North Slope of Alaska that was acting up. The customer called and said that his oil side pressure drop was too high. We told him to pinch off his cooling water, hence raising the oil out temp (but still acceptable temp), decreasing the viscosity, and reducing his pressure drop.