Tubular Heat Exchanger Rating Calculator
Quickly rate an existing shell-and-tube heat exchanger using the Kern method. Calculate heat duty, LMTD, F-factor, film coefficients, and compare calculated U vs. assumed U.
This rating tool evaluates the thermal performance of a specified shell-and-tube heat exchanger. By inputting fluid properties, flow rates, and exchanger geometry, the calculator determines the actual heat transfer coefficient (Ucalc) and compares it to your assumed or required value to determine if the exchanger is adequately sized.
Tube-Side Fluid
Geometry
Formulas Used (Kern Method)
Heat Duty:
\[ Q = w \times c_p \times (t_2 - t_1) \]
LMTD Correction Factor (F):
\[ P = \frac{t_2 - t_1}{T_1 - t_1}, \quad R = \frac{T_1 - T_2}{t_2 - t_1} \]
Overall Heat Transfer Coefficient:
\[ \frac{1}{U_{calc}} = \frac{1}{h_o} + R_{d,o} + \frac{D_o \ln(D_o/D_i)}{2k_w} + \left(\frac{D_o}{D_i}\right)(R_{d,i} + \frac{1}{h_i}) \]
Understanding Heat Exchanger Rating
Rating an existing heat exchanger involves determining its actual thermal performance (Calculated U) and comparing it to the required or assumed U. If the calculated U is greater than the assumed U, the exchanger has positive overdesign (margin) and is adequately sized for the service.
Limitations of the Kern Method
- Assumes perfect crossflow in the shell side, ignoring bypass and leakage streams (which the more advanced Bell-Delaware method accounts for).
- Best suited for preliminary rating or services with low to moderate viscosity.
- Assumes constant fluid properties across the exchanger length.
Frequently Asked Questions
Typically, an overdesign margin of 10% to 25% is considered acceptable. Less than 10% may risk underperformance due to fouling, while over 30% may indicate an oversized, unnecessarily expensive exchanger.
An F-factor below 0.8 indicates significant deviation from true counterflow, leading to temperature crosses or inefficient heat transfer. It is generally recommended to use multiple shells in series if F drops below 0.8.