Hi,

increasing the gas velocity has two different effects: first as you said reduces the residence time, which tends to decrease the outlet gas temperature. Secondly, increasing the gas flow rate results in an increase in the Reynolds number, which consequently improves the heat transfer process.

If the gas flow is laminar at your lower flow rate but becomes turbulent at the higher one, then it seems to me like it could end up having a higher exit temperature. This is because the turbulent regime has much better mixing and heat transfer than the laminar regime. But, if both flow rates are within the same regime, I think the decreased residence time of higher velocities may outweigh the enhanced mixing caused by more turbulence. I'd calculate or test it at both flow rates if it is critical though.

Dear Ramendra Pandey

In heat exchanger which fluid cooled and which one heated? And please read ASHRAE equipment and fundamental heat exchanger section or condenser and evaporator.

Best regard

If the major aim is to increase the heat transfer, it is correct the temperature drop increase with increasing the turbulence while the pressure drop is increased rapidly with increasing the velocity so it is more convent torelate the enhancement in heat transfer with the pressure drop in a factor and it is sometime known as Thermal Enhancement Factor

View from another angle: When passing a gas through a heat exchanger, part of its hydraulic energy is lost to overcome the hydraulic resistances on the road. As the gas flow is greater, the greater the energy loss. Through the heat exchange part of this "lost" hydraulic energy returns to the gas and increases its temperature.

Thanks everyone for your responses..

The tube side fluid is getting heated while the shell side fluid is getting cooled. The flow is in laminar regime for the flow rates used. Is local turbulence a possibility due to surface roughness?

If you create a simple simulation model on the unit on an electronic spreadsheet, you will get your answer in less than an hour. Residence time in the HX is not relevant. The exit temperatures of both streams depend upon the heat transfer coefficients, and the total HX area available. Please refer to the effectiveness-NTU method described in your college heat transfer text book.

Dear Jimmy,

As far as I know, the available contact area only determines the residence time. It's just another way of saying the same thing. The argument is not about estimating the exit temperatures of both streams which as you mentioned can be done using a simple spreadsheet. As I mentioned in my query, in my system the exit gas temperature rises with increasing gas flow rates which is counter-intuitive based on any standard literature on heat exchangers. The only way it's possible is if the Nusselt number correlations were to scale as Re^x where x>1 which I have not yet seen in any literature.

What the evidence suggests is that the heat transfer coeff increases faster than the effect of reduced residence time. Remember that there could also be be significant density effects due to pressure if the fluids are gases. You should confirm that the measured shell side exit temp is also falling (assuming constant flow rate and inlet temp), because the heat balance MUST be maintained. But in a word, the temp on tube side can go either way, depending on the other parameters. It is not at all counter-intuitive to me. That is why I recommend to construct a simulation model instead of guessing.

it is possible to recirculate a fluid........, but this is a different situation