Page 446 - Handbook of Energy Engineering Calculations

Determine the heat transferred, the required heat-exchanger surface area,

pressure-drop through the exchanger, and the final dimensions of the heat
exchanger.

Calculation Procedure:

1. Find the heat-transfer rate and log mean temperature difference
Figure 13 shows several possible arrangements of spiral-flow heat
exchangers. Using the same relation as in the previous procedure, we have

heat-transfer rate, Btu/h = (flow rate, lb/h)(inlet temperature – outlet
temperature)(specific heat, Btu/lb .°F)(1.8 conversion factor for temperature).
Or, heat-transfer rate = (6225)(200 – 120)(0.71)(1.8) = 636, 444 Btu/h (671.4

kJ/h). Then, the log mean temperature difference, LMTD, T = (60 – 49.4)/ln
M
(60/49.4) = 54.5°C (129.9°F).

2. Find the surface area required for this heat exchanger
For the first trial, the approximate surface area for this exchanger can be

computed using an assumed overall heat-transfer coefficient, U, of 50 Btu/h
2
2
2
ft .°F (8.8 W/m .°C). Then, A = 636,444/(50)(54.5) (1.8) = 129.75 ft , say
2
2
130 ft (12 m ).
2
2
Because at 130 ft (12 m ), this is a small heat exchanger, we will assume
a plate width of 24 in (60.9 cm). Then, the plate length, L = 130/(2)(2) = 32.5
ft (9.9 m). Assume a channel spacing of 3/8 in (0.95 cm) for both fluids.

3. Find the Reynolds number of the flow conditions
The Reynolds number for spiral flow can be computed from N = 10,000
Re