The calculation of overall heat transfer coefficient U using the equations previously presented can be rather tedious. Heat transfer specialists have computer programs to calculate this value. There are some quick approximation techniques. Table 2-8 comes from the Gas Processors Suppliers Association’s Engineering Data Book and gives an approximate value of U for shell and tube heat exchangers.
It can be seen from the table that exchanging water with 100-psi gas gives a low U value. Thus, a high surface area is required. Exchanging water with 1,000 psi gas gives a much higher U value and less surface area can be used in the exchanger. If water is being exchanged with water, a very high U value is achieved. Water with air gives a very low U value. Oil with oil is much less than water with water, because of the viscous nature of the oil.
The approximate U values in the table do not differentiate between tube-side and shell-side fluids. Which fluid is on the inside of the tubes and which is on the outside does make a difference to the U value. This is beyond the accuracy of the table.
Figure 2-11 gives values of U for exchange from a water bath to a natural gas stream in a coil. Figure 2-12 is a nomograph for crude oil streams heated with a water bath.