Lesson 8 - Heat Exchangers

Overall heat transfer coefficient is one of the major factors impacting on the design and sizing of heat exchangers. Below we will look at some typical values for overall heat transfer coefficient and the factors impacting on these.

The overall heat transfer coefficient is calculated by combining the heat transfer characteristic of each of the media that make up the combined heat transfer system.

Some of the factors that influence the overall heat transfer coefficient in heat exchangers are:

• Type of heat exchanger

• Characteristics of the heat exchanger components

• Properties of the product and utility fluid

• Flowrate of the product and utility fluids through the exchanger

• Whether product is being heated, cooled, condensed or evaporated

The tables below gives some typical U values for plate and shell and tube heat exchangers. Because the overall heat transfer coefficient can vary considerably based on the unique design features of each exchanger, these values give only a very general indication.

Table: Typical overall heat transfer coefficients for plate heat exchanger applications

Table: Typical overall heat transfer coefficients for shell and tube exchangers

When determining the overall heat transfer coefficient for shell and tube heat exchangers, compensation is made for the difference between the inner and outer diameter of the tubes. In plate heat exchangers this is not applicable as both sides of the plate have exactly the same area.

If you would like to learn more about how overall heat transfer coefficients are calculated you can read the following article.

http://thermopedia.com/content/1007/

Did you notice how much lower the U values for shell and tube exchangers are compared with plate heat exchangers? This highlights what we have already learned about plate heat exchangers:

In plate heat exchangers the high degree of induced turbulence across the plates generates a high overall heat transfer coefficient. This translates to better heat transfer rates than shell and tube exchangers of the same surface area.

In the lesson on Radiation and Combined, you learned how U can be modified to include r, a factor representing the impact of fouling of the exchanger surface.

• When designing heat transfer equipment, the U value allows for a reasonable amount of scale and fouling that would typically occur during normal operation.

• Increasing U ensures that even if the exchanger is fouled (to within a reasonable degree) it is sufficiently sized to meet the performance specifications. This avoids the need for the exchanger to be cleaned after every use.

• This however, means that a new or clean exchanger with no fouling, will initially exceed its performance specification because the actual overall heat transfer coefficient will be higher than design.

• However, gradually, as the exchanger fouls, its performance will drop. This decline will continue until the performance falls below the design specification, at which time the exchanger will need to be cleaned.

You are now more familiar with the overall heat transfer coefficient used in heat exchanger sizing, so let’s look further at the concept of log mean temperature difference.