3 replies to this topic

[David Southall]

It's a question of economics. Remember the basic heat transfer equation:

q = UAdT

Rearrange this to:


q/dT = UA

and you can see that as the temperature diffence tends towards zero, the area required tends towards infinity.

In practice, what this means is that a large temperature difference will give you a smaller capital cost (i.e. a smaller heat exchanger), but higher running costs due to the utilities required.

On the other hand, a very tight temperature difference will reduce your operating costs, but will result in a much higher capital cost.

For something like a plate-fin unit, you can feasibly design down to 1 - 2 degC temperature difference. For a shell and tube though, a good place to start would maybe be 10 - 30 degC.

The only way to know for sure is to design for a number of cases (i.e. various dT's and corresponding utility requirements), then compare the cost impacts with your project's requirements.

[missbk]

I am figuring out about this, too. I found that the minimum temperature approach is a parameter of the temperature difference between the lowest temperature of the hot fluid and the highest temperature of the cold fluid.
For example, you draw the diagram of heat movement and you'll see the minimum distance between the two line is the minimum temperature approach.
Of course, the closer the cold fluid approachs to the hot, your heat tranfer area must be the larger.
As a rules of thumbs, for 1 pass shell side and 1 pass tube side, this parameter should be chosen at 10 degree F. The above guideline is useful for this assumption.
I have to find out this because when I sizing a HE by Hysys, if there is just 2 fluid flows of four are known, I must have some parameter of the HE spec, then that's the Hot side approach, or the temperature approach.