×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

HX coolant too fast to cool?

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?

RE: HX coolant too fast to cool?

Possible; yes.  Probable; doubtfully.

How fast are we talking about?

rmw

RE: HX coolant too fast to cool?

Not even possible, I think.  A fast coolant would simply maximize the temperature delta across the exchanger, therefore, there some be even more cooling than with a slower, hence, hotter coolant.  A trivial check is to see what your coolant temperature is when the engine is overheating.

It's more likely that something else is wrong; your exchanger may be grossly fouled.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: HX coolant too fast to cool?

My first post assumed you were talking about the water going through the engine.

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?

(OP)
This is in theory only.
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?

franklee,

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?

The heat transfer from the block wall to the coolant is a function involving a bunch of dimensionless numbers named after dead guys - like Prandlt, Nessault and Renyolds.

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?

(OP)
Oh great MintJulep!
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?

As Engineers, we always assume things are steady state and they're not......it's a simplifying assumption. Combustion in an otto cycle is not steady state and neither is the cooling.

Things are not homogeneous and isotropic, either, it's a simplifying assumption.

RE: HX coolant too fast to cool?

Here is a thought:

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?

Heat transfer will never go down with an increase in coolant velocity. Frictional heating is a separate issue and not really a practical consideration except for energy efficiency (i.e., don't use a 100 hp pump where one hp will do).

RE: HX coolant too fast to cool?

The only way friction would come into play as a temperture rise and the collant velocity rises is if you were pumping a slurry or something with a very high viscocity.

"I came, I saw, I made it better."
-Ode to Industrial Engineers
Will ChevronTexaco Corp.

RE: HX coolant too fast to cool?

If I understand correctly, there are two separate heat exchange processes:
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).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: HX coolant too fast to cool?

From the well known "dead men's"  correlation
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?

That may be true on the hot side, but the reverse should be true on the cold side.  The faster the coolant flow, the harder it will be for the coolant to lose heat to the external exchanger.

Moreover, the overall system is limited by the thermal resistance to the ambient air, so there may be a net change of zero.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: HX coolant too fast to cool?

"That may be true on the hot side, but the reverse should be true on the cold side.  The faster the coolant flow, the harder it will be for the coolant to lose heat to the external exchanger.

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?

We are talking about forced convection. The Nusselt number increases as fluid velocity increases and so does heat transfer coefficient: no way to reduce heat transfer with increasing flow velocity. Just my (and probably Wilhem Nusselt) opinion.
 

RE: HX coolant too fast to cool?

I have to agree that when you consider the closed system of the engine and the radiator, that if the flow around the loop speeds up you'll reach a maximum heat transfer point were increasing the velocity of the fluid will not increase the heat transfer from the engine. Since at a certain point the heat picked up by the fluid cannot be removed fast enough at the radiator.

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?

(OP)
I was thinking that it would have more to do with the fluid moving thru the engine block so fast that low pressures would build up around the jacket areas and form a boundary of air bubbles in certain areas where there in next to no heat transfer thru the bubble film.

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?

You seem to be talking about cavitation, which would not even be possible in a normal engine, since the fluid flows would need to be substantially higher than their normal rates, and the general geometries are not conducive to cavitation.  Cavitation cannot occur solely due to fluid flow rate.

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

FAQ731-376: Eng-Tips.com Forum Policies

RE: HX coolant too fast to cool?

I doubt that the flow being too great is your problem. If it really is that high, I'd be worried about erosion damage.

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?

The heat removal (heat load) from the engine establishes the coolant flow rate. The geometry (sections) and the flow rate determine the fluid velocity. Increasing the coolant velocity (i.e. making the flow more turbulent) just promotes the heat transfer process (with all the remaining boundary conditions kept consistent). The problem could be just the opposite of too high coolant velocities. What I mean is the presence of stagnation areas in the circuit where the coolant velocity are too low. A poor design (geometry) could led to this scenario which implies a worse performance of the cooling system.

RE: HX coolant too fast to cool?

The problem  the OP posted is theoretical and does not speak to the practicality of requiring a larger radiator to transfer the heat since the coolant temperatures exiting the jacket would be lower.
So, sorry to advise the OP, he loses to engineer B.

RE: HX coolant too fast to cool?

It's all about mass flow rate and temp. rise/fall between inlet and outlet. Essentially, higher flow, less temp rise/fall, lower flow more temp. rise/fall. The objective becomes to optimize the heat exchange rate and the size of the heat exchanger to accomplish the req'd. heat removal or addition to the system.

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?

One issue absent from this disucssion is that if rubber hoses are used to connect the radiator to the pump, high flow rates can create a vacuum at the pump inlet high enough to cause the hose to colapse, decreasing the flow.  

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?

That would only occur if the pump wasn't at, or near, the lowest gravitional point in the system, and if the fluid was't glycol/water and could vapor lock.  And the typical water pump is designed and placed so something like that can't happen.

As near as I can tell, the OP was speaking to a hypothetical, yet physically realizable scenario.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: HX coolant too fast to cool?

You guys.....
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?

MrBTU, If we hold "A" constant, "U" constant  and increase flow so that "T" or "lmtd" decreases, then "Q" must decrease. Right? ;)

RE: HX coolant too fast to cool?

But U increases with increased flow rate....

RE: HX coolant too fast to cool?

The problem as stated could be rephrased to say;
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?

Perhaps, we need to run a lab bench test and check our assumptions...  ;)

RE: HX coolant too fast to cool?

zekeman - I suspect your conclusion is correct. However, you are modeling the engine-to-coolant heat transfer only... under assumption that the inlet temperature is constant, correct?

Can you add a coolant-to-air heat exchange into your model?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: HX coolant too fast to cool?

Electricpete,

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?

Again, the answer to the very simple original question is simply "no".

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?

Some industrial nat gas engines have a minimum recommended external restriction to prevent "coolant flow rates [from becoming] excessive."  The consequences of "excessive" flow are not given though.

Brian Bobyk - Hoerbiger Canada

RE: HX coolant too fast to cool?

In my 25 years with heat exchangers, the implications of too high coolant flow are:
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.   

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources