Page 409 - Handbook of Energy Engineering Calculations

OPERATION

A counterflow shell-and-tube heat exchanger has one shell pass for the
heating fluid and two shell passes for the fluid being heated. What is the
actual LMTD for this exchanger if T = 300°F (148.9°C), T = 250°F
1
2
(121°C), t = 100°F (37.8°C), and t = 230°F (110°C)?
2
1

Calculation Procedure:

1. Determine how the LMTD should be computed
When the numbers of shelf and tube passes are unequal, true counterflow
does not exist in the heat exchanger. To allow for this deviation from true
counterflow, a correction factor must be applied to the logarithmic mean

temperature difference (LMTD). Figure 3 gives the correction factor to use.

2. Compute the variables for the correction factor
The two variables that determine the correction factor are shown in Fig. 3 as
P = (t – t )/(t – t ) and R = (T – T )/(t – t ). Thus, P = (230 – 100)/(300 –
2
1
2
1
1
2
1
1
100) = 0.65, and R = (300 – 250)/(230 – 100) = 0.385. From Fig. 3, the
correction factor is F = 0.90 for these values of P and R.