Calculating U - in heat transfer

[kamal60]

Until now I look up a vendor catalogue and search for U for a heat medium and process conditions - its usually a range. I take the lowest and find the corresponding area required for heat transfer. Is this the best way of doing it? Load of thanks

 

[insult2injury]

By using the lowest possible U-value, you may be severely oversizing your equipment if the actual U-value is towards the mid to upper end of the manufacturers range.

I2I

 

[kamal60]

In most cases my transfer area is not so far away from the vendor's - there are tons of software and calculations but what engineering judgement should one take - what ahould be the approach?
thanks

 

[WebWizz]

If you want to calculate the U value, a free webcalculator is available at

http://www.engineeringpage.com

choose heat exchanger and pick the calculation that you want.

Regards

 

[rmw]

http://www.processassociates.com

click tools has a good and thorough method of calculating U values.

rmw

 

[25362]

What about factors such as type of exchanger, pressure and temperature ranges (process and mechanical effects), geometry details, fluid allocation ("tube" or "shell" side), flow arrangement (e.g., parallel, crossflow or counterflow), number of passes, possible fouling effects, safety and corrosion considerations, unit physical location, etc.?

Selecting a unit just on the estimated U values could only serve to make ballpark preliminary cost estimates, surely not to design and rate it.

 

[kamal60]

Checked out both the sites engineeringpage.com and processassiciates.com - wealth of info - load of thanks. 25362 is right there is a lot more to a heat exchanger sizing than just 'U'. But those considerations are the job of a heat exchanger vendor. A process design engineer does not go into such considerations (mostly - because clients are not willing to pay for that work).

Sicne starting this thread I figured out that U is inverse of resistance. When asked to confirm a heat exchange 'A' I would take the lowest of the U and calculate 'A' to see if it is in the ball part of lowest U - this ensures in a worst case scenario that exchanger will still perform.
thanks

 

[insult2injury]

Just be sure you are using U-values quoted for the particular heat exchanger geometry you are evaluating and not general ranges for all heat exchangers. Also, make sure the U-values have fouling factors included. Using those two stipulations, you will be designing for a worst case scenario. Keep in mind that you will never be able to achieve an optimal or efficient design this way.

I2I

 

[UmeshMathur]

Another point to note concerning exchanger oversizing: if this is done indiscriminately, the film coefficients on both sides can drop off significantly compared to the values for a properly sized exchanger. In addition, fouling can be a lot worse as the fluid velocities could also be a lot lower. So, the extra surface - which costs you a bundle - often ends up buying you very little in return.

 

[niyeki]

First select you U value and make your calculations for the exchanger untill you end up with the velocity on the shell and tube side. If the velocity you end up is withi the limits the starting U is fine else revise the U untill getting the right velocity (stay between 1,5 to 3 m/s)

regards

 

[chemeng1357]

you may use practical values for U
from LUDWIG handbook as a guide
then calculate heat transfer coefficients of each side
then overall U from HEAT TRANSFER handbooks
with consideration of accuracy of relations
and fouling.

 

[valentino15]

For calculating U-value you must have the phsycal properties of the medium or gas. For detailed information over thermal designing refer to heat exchanger design handbook T.Kuppan and Handbook heat transfer Adrian Bejan.