Page 410 - Handbook of Energy Engineering Calculations
P. 410
FIGURE 3 Correction factors for LMTD when the heater flow path
differs from the counterflow. (Power.)
3. Compute the theoretical LMTD
Use the relation LMTD = (G – L)/ln (G/L), where the symbols for
counterflow heat exchange are G = T – t ; L = T – t ; ln = logarithm to the
2
2
1
1
base e. All temperatures in this equation are expressed in °F. Thus, G = 250 –
100 = 150°F (83.3°C); L = 300 – 230 = 70°F (38.9°C). Then LMTD = (150 –
70)/ln (150/70) = 105°F (58.3°C).
4. Compute the actual LMTD for this exchanger
The actual LMTD for this or any other heat exchanger is LMTD actual = F
(LMTD computed )= 0.9(105) = 94.5°F (52.5°C). Use the actual LMTD to
compute the required exchanger heat-transfer area.
Related Calculations. Once the corrected LMTD is known, compute the
required heat-exchanger size in the manner shown in the previous calculation
procedure. The method given here is valid for both two-and four-pass shell-
