Heat Transfer and Velocity

Try Perry, "The Chemical Engineer's Handbook", any edition, looking for heat transfer correlations. In an 8in pipe, the flow will be turbulent. The higher the velocity, the higher the heat transfer, though the effect is not linear. There is no truth in the rumour that the velocity can be too high for heat transfer, though it could be too high to be practical, like at the end of the duct where it comes hurtling out...

The limiting factor for heat transfer in your case will be the total surface area of the pipes, its material and the temp of ambient where heat is rejected.

ausvirgo, a GA here.

Like others have said, increasing velocity always increases heat transfer. So no problem there.

You are correct to suspect a velocity limit exists, but it's not what you suspected: with increasing velocity, friction losses increase and so does the pressure drop. You'll eventually have a problem blowing the air through the pipes (will need bigger fan, compressor etc; you'll also be generating friction heat inside the pipes- even if you don't mind that, it's never an efficient idea energy-wise). That's where the limit lies.

Consult a Moody chart for specific pressure drop calculations. Entrance and exit local pressure drops will also be important but will depend strongly on the local geometry. Again you can find charts for these, else just 'destroy' the entire dynamic pressure component (wost case scenario) in the Bernoulli equation.

Cheers

The heat transfer coeff. inside the pipe is propotional to Reynolds no. to the power of 0.8 or approx. velocity to the power of 0.8. However it is required to look at the overall heat transfer coeff. which includes heat transfer coeff. outside the pipe as well. It is not just what is happening inside the pipe but outside as well. Your question is not fully detailed and so I cannot give a more complete answer. This is a standard heat transfer problem and you could get easy references for it.

Hi,

If you've ever looked into the combustion chamber of a fire tube boiler or the smoke box of a railway loco you will have seen a very cost effective and efficient (in terms of space per unit of heat transferred)arrangement for getting a lot of thermal energy transferred to the secondary side media. You would also see that the tubes will vary from say 2'' to 4'' bore; nothing remotely approaching your 8'' variety . Interestingly the length of the tubes i,e distance between tube plates is not much different to your tubes. You will find when you start looking up the various references that by reducing the diameter you will improve the specific transfer rate however this ultimately becomes self limiting as tiny bores of that lenth would present an unacceptably high friction loss. Countless decades of practice has evolved into an effective solution!

The other thing to remember is the boundary layer condition which is a most effective insulating barrier to heat transfer, for this reason also the smaller bore tubes by working at a higher velocity benefit the disruption of this effect. Some manufacturers of heat transfer units offer what are sometimes called ''turbolators'' these are spirals or other assemblies inserted into the internal bore with the purpose of causing the hot gas stream to impinge more effectively on the tube wall disrupting the boundary layer.

So if you can reduce the diameter and increase the number of tubes to achieve the primary conditions that you need you will be able to get a better transfer without incurring excessive friction losses.

Adhesion of the molecules to the pipe wall form a barrier. As velocity increase this barrier increases forcing the hotter air to flow down the center of the pipe. Now the heat has to transfer thru the barrier. So yes there is a point at which the velocity of the air makes the system inefficient.

Ozzb: Do you have a reference for your comment "hot air flows down the center of the pipe due to very high velocity". This seems to be the effect with laminar flow and when a boundary thermal layer is set up. However, I agree with you but can't find a good reference demonstrating extremely high velocity relating to a lower heat transfer rate.

Its same i am searching for....

i am relating heat transfer and velocity and pressure drop

Hi

I am Soumen, I have same related problem.I have flue gas(from boiler) blowing through duct, I want heat transfer to air. I need hot air for other application.Probable idea is like that:

flue gas is blowing through 2" pipe( pipe with fin )

Air is blowing contacting pipe with fin.

Heat transfer required like flue gas to pipe wall & fin and fin to air.

Air will be blowing by blower.

Can anyone help to calculate the same.

Thanks

Provide the missing data like temp of the flue gas, ambient, quantity of flue gas and the velocity of the proposed blower and its size.

Hi

flue gas temp. is 300 deg C, ambient temp. is 30 deg C quantity of fule gas and velocity of the proposed blower and its size yet to find out from site condition.Approx air temp. should be 170 deg C

Dear Mr. Soumen,

Your reply to Mr.Capri (Post No.11) covers the data required but not in FULL.

You have to inform the Quantity (in VOLUME/Hr. or Kg/Hr.) of Air to be heated. From your data - the Flue gas Temp. is 300 deg.C, Air Inlet Temp. is 30 Deg. C, air to be heated to 170 Deg.C. The quantity of Air if in Volume, it is to be converted to Weight.

Then only we can calculate the HEAT EXCHANGER area required by using

HEAT GAINED BY AIR TO BE HEATED = W Kg/hr x Sp.Heat of Air x ( 170-30) K.Cal. = U X A X LogMeanTemp.Diff.

Value of U can be taken from te tables, ( for steel it is 5 BTU/Hr./Sq.Ft in FPS UNIT and equivalent unit to be used in METRIC or IS Unit) and LOG MEAN TEMPERATURE can be calculated, and substitute in the above Equation. Hence you can find out Area for the Heat Exchanger. Add 5% etc. for fouling etc.

DHAYANANDHAN.S

Dear Mr. Sloppy Joe,

The Heat Transferred depends upon the OVER-ALL HEAT TRANSFER CO.EFFICIENT (OHTC) denoted by U. There are Several Factors involved for the effective value of U and velocity is ONE FACTOR.

The value of U - the OHTC depends upon VELOCITY also and proportional to V^0.8. But increasing VELOCITY results in PPRESSURE DROP. Pl. refer Post No.5 here.

Referring to Post No.1, INCREASING VELOCITY need not necessarily result in TURBULENT FLOW. When VELOCITY is increased The REYNOLD'S NUMBER also INCREASES but still if it is less than 2200, the FLOW is NOT TURBULENT FLOW.

TURBULENCY depends upon REYNOLD's NUMBER and for TURBULENT FLOW REYNOLD's NUMBER SHOULD BE Minimum 2200 and ABOVE this. Flow with Reynold's Number upto 2000 is LAMINAR FLOW, Flow with Reynold's Number 2000 - 2200 is TRANSITION FLOW and above 2200 it is TURBULENT FLOW.

DHAYANANDHAN.S

Dear Mr. Sloppy Joe,

I have searched my old record books studied during 1973-74 and I found that HEAT TRANSFER is PROPORTIONAL TO V^0.8 where V is the Velocity - refer Unit operations Book by Authors McCab and Smith,

DHAYANANDHAN.S