15 replies to this topic

[pradeep4u]

Hello! I have doubt on two topics
 
1)  Turbulence or retention time of fluids which affect heat transfer more.
2) why do increasing velocity of cold fluid in   tubes not increase heat transfer rather decrease it whereas same is opposite when we increase  velocity of hot fluid?

[S.AHMAD]

Dear sir, could you please attached your calculation showing item 2?

[shan]

Basic heat transfer Equation: Q=U*A*LMTD

 

As demonstrated in the equation above, neither turbulence or retention time is directly involved in the heat transfer calculation. 

 

However, turbulence (Reynolds Number) is the important factor for the film coefficient and film resistance, which are included in the equation of overall heat transfer coefficient U. 

 

Retention time is reflected by the length of a heat exchanger shell and tubes, which is, in term, one of determine factor to the heat transfer area A.

 

You have to define the fluid flow regime variation limit and the retention time change range to discuss which variable has more impact to the amount of heat transferred.

[pradeep4u]

S.AHMAD:

 

No sir, I do not have calculation.  But I practically have experienced in oil coolers when we increased flow of hot oil that the oil temperature  decreased but when we increase cold water flow rate then we experienced that the temperature did not decrease.  In fact,  the Delta temperature between the hot oil temperature and the cold oil temperature  decreased.

[pradeep4u]

Shan:

 

Sir are u talking about laminar or turbulent flows?  Retention time I understand is Time the fluid spends in the heat exchanger.  Am I wrong?  Plz correct me if I am wrong.  Faster fluid flow spends lesser time in heat exchanger  so has less retention time while slower fluid flow spends more time in the heat exchanger.  So I think it has more retention time.

[shan]

Retention time is NOT a variable to determined the heat transfer rate based on basic heater transfer equation Q=U*A*LMTD. 

 

This means that even if the fluid stays at the identical retention time in a heat exchanger with different heat transfer areas, such as one single large diameter tube vs. multiple small diameter tubes with the same tube length and overall velocity, the heat transfer rates are different.

 

Actually, a heat exchange will trans more heat for a faster flow fluid than slower flow fluid because of better U value.  Faster flow fluid may have less temperature change through the heat exchanger but more volume passes through the heat exchanger.  Overall exchanger Q is larger. 

 

As another example, when the cold side temperature reaches the same temperature as the hot side (LMTD=0) or temperature crossing occurs inside heat exchanger, no matter how long the retention time is, there is no heat transfer at all.

 

In the case you disagree my point of view, please present your heat transfer equation with retention time as a variable.

Edited by shan, 07 December 2015 - 10:34 AM.

[S.AHMAD]

Pradeep4u:

 

It is good idea if you can do some calculation based on the plant data.  When the hot stream flowrate is increased, in theory the outlet temperature of both stream shall increase this is explained in the heat transfer correlation.

 

For the hot stream:

Qh = WCp(T1-T2)

For the cold stream 

Qc = wcp(t1-t2). 

Qh = Qc

and also the equation Q = UALMTD

 

the three basic equation must be satisfied that is

Q = Qh =Qc

 

In the calculation you must determine the outlet temperature that satisfy the three basic heat transfer equation. Use EXCEL SOLVER add-in to solve for T2 and t2. Alternative, give me the plant data, I can do it for you, haha but not foc....however for the sake of education to other embers I am happy to do it foc

 

 

S.Ahmad

[S.AHMAD]

I am sorry.... the cold stream equation should be:

 

Q_C = WC_P (t₂  -t₁).. since t₂ is greater than t₁....

[Lucian Gomoescu]

Hello everyone,

I have some observations:

@ Shan:
Temperature crossing means that cold fluid temperature at a position xc from the cold inlet gets higher than hot fluid temperature at a possition xh from hot inlet. Yes, HIGHER, which is thermodinamically possible because these temperatures ARE NOT AT THE SAME POINT IN SPACE. So, a temperature crossing doesn't mean LMTD=0, it only diminishes  MTD in case of a non 100% countercurrent HX.

@ pradeep4u:
1. Regarding the practical two cases that you described, i think that you told only half of the story.
Case A: Hot oil flow ++  ==> Hot oil Tout --
Case B: Cold water flow ++ ==> (Hot oil Tin - Hot oil Tout) --

Some details are missing.

Case A:
The cold water T in/out and flow rate are as in case B before flow increment?

If yes, than a possible explanation is the following: the htc on oil side is << the htc on water side and, when the flow rate was increased, the overall htc increased at such a value that overcomed the smaller retention time of oil in hot space.

 

But still this is a curious situation to me because, in general, the flow rate increment determines a higher outlet temperature for the hot fluid (htc ~ velocity^n, where n <1 assuming forced convection, while the heat capacity rate increases with velocity^n=1).

Case B:
Did you mean that (Hot oil Tin - Hot oil Tout) decreases compared to its value in case A or its value before water flow rate increment ?

If first option is the one you meant then it may have the same explanation as for case A (the water htc increment has a minor effect on ohtc, also the effect on MTD may be very small because water Tout-Tin << oil Tin -Tout, i only suppose it.

If second option is the one you meant than i really don't have an explanation at all.

2. I understand your point regarding the antagonic effect of flow rate on retention time of fluid and its film htc. In almost all cases, the retention time  effect on fluid outlet temperature (keeping the other space conditions unchanged) prevails. In other words, a fluid flow rate increment determines a fluid |Tin-Tout| decrease, in most of the situations.
 

3. Above i have not talked at all about heat transfer increasing/decreasing (understood as HX duty), but about fluid Tin-Tout which was the only one mentioned in the practical cases.

 

4. If we talk about flow rate increment effect on HX duty then it has only positive effects on it (higher film htc, higher MTD).

Lucian

[pradeep4u]

@lucian 

 

              I am saying that the temperature of the hot fluid increases when the cold fluid flow rate increases  so temperature difference of  inlet and outlet temperature of hot fluid will be less.  Then how can heat transfer rate increase?  Whereas when hot fluid flow rate increase then hot fluid outlet temperature decrease.  So temperature difference will be more

 

Regards

Edited by Art Montemayor, 10 February 2016 - 06:56 PM.

[pradeep4u]

@lucian

              I want some advice from u  regarding my doubt ex.presed in my previous post.

 

Regards

[pradeep4u]

   I am saying that temperature of hot fluid increases when cold fluid flow rate increases  so temperature difference of  inlet and outlet temperature of hot fluid will be less.  then how can heat transfer rate increase.  Whereas when hot fluid flow rate increase then hot fluid outlet temperature decrease.  So temperature difference will be more

 

Regards

[breizh]

Hi Pradeep ,

Thanks to share your data if you want to get a  meaningfull answer  ( temperature profile , flowrate cold and hot , type of HX , data sheet ).

 

Hope this helps.

 

Breizh

[Ganesh Buddha]

when the cold fluid flow is increased the heat transfer can reduce. this phenomenon is totally possible...

 

this is possible in case of viscous hot fluid. increase in cold fluid reduces the hot fluid flow at the hot fluid wall, resulting in lesser heat transfer from hot to cold fluid.

 

whereas increase in hot fluid velocity induces higher heat transfer...

[pradeep4u]

Dear Ganesh

 

                     How increase in cold fluid flow will decrease hot fluid flow in  hot fluid wall.  If lesser viscous fluids like  on tube side cold water is there and  hot  oil on shell side  then  can  it  happen?

[pradeep4u]

Hi Pradeep ,

Thanks to share your data if you want to get a  meaningfull answer  ( temperature profile , flowrate cold and hot , type of HX , data sheet ).

 

Hope this helps.

 

Breizh

Dear Sir

               I am sorry that I have no data sheet but  I  practically experienced this situation.  I also read somewhere that increasing cold fluid flow rate will decrease  heat transfer.

 

Regards