Heat Transfer rate in HE 1

[YoYo2000]

I have some general questions regarding heat transfer rate.

Assuming we have a shell and tube heat exchanger, with the hot fluid going through the coils and cooling water through the shell. As per my limited understanding, if we have turbulent flow of hot fluid going through the coils, this increases the heat transfer rate due to 'eddie transports' as opposed to laminar flow which would solely rely on thermal conductivity of the fluid to transport heat.

When we say heat transfer rate increases, does this the amount of heat transfer is higher? Does this mean the heat exchanger outlet temperature of the hot fluid will be lower in turbulent flow than laminar..and obviously higher in the exitng cooling water?

In a tube in tube HE, should attempt to achieve turbulent flow in both tubes to transfer more amounts of heat?

Thanks

 

[macmet]

Yoyo,

Check out this link.

http://www.advantageengineering.com/fyi/156/advantageFYI156.php

The more mixing the better for heat transfer. Depending on your application it may be better to go to a PHE.

 

[TD2K]

The governing equation for heat transfer is Q = U A dT

Q is the heat transferred
U is the overall heat transfer coefficient
A is the area available for heat transfer
dT is the temperature difference between the two fluids, the log mean temperature difference is commonly used for heat exchangers, corrected for specific exchanger geometries.

The heat gained by one stream is equal to the heat lost by the other stream. Heat losses to the atmosphere are typically ignored. That's important to remember when trying to solve the above equation.

Looking at the equation, if you double U for example then you can solve for the new outlet temperature of the hot and cold fluid side. The outlet of the hot fluid will be less than it was and the cold fluid will be warmer.

You can easily set up a spreadsheet using the above equations. That will allow you to change different variables and see what the effect is.

 

[willard3]

You're going to need to take a course in heat transfer.....this isn't something you can learn on a web forum.