Page 474 - Design and Operation of Heat Exchangers and their Networks

APPENDIX

Source codes used in the book

Example 2.1 Sizing an electrically heated tube
(MatLab code)

% Example 2.1 Sizing an electrically heated tube
% The compressed air at 1.5 bar with a normal volumetric flow rate of
% 1.2 Nm3/h shall be heated from 20°Cto80°C by the heating wire uniformly
% wrapped around the tube as it flows through the tube. The tube outside
% diameter is 25 mm, tube wall thickness is 2 mm, and thermal conductivity
% of the tube material is 15 W/mK. The tube temperature shall not exceed
% 200°C. Determine the length of the tube heating section.

d_o = 0.025; % outside diameter, m
d_i = 0.021; % inside diamter, m
lambda_w = 15; % tube thermal conductivity, W/mK
V_N = 1.2; % normal volumetric flow rate, Nm3/h
p_N = 1.01325; % normal pressure, bar
t_N = 0; % normal temperature, °C
t_in = 20; %inlet temperature, °C
p_in = 1.5; % inlet pressure, bar
t_out = 80; % outlet temperature, °C
t_max = 200; % maximum allowed temperature, °C
t_m = (t_in + t_out) / 2; % mean temperature, °C
rho_N = refpropm('D','T', t_N + 273.15, 'P', p_N ∗ 100, 'air');
% density at normal condition, kg/m3
[mu, lambda, cp] = refpropm('VLC','T', t_m + 273.15, ...
'P', p_in ∗ 100, 'air');
% viscosity, sPa; thermal conductivity, W/mK; isobaric heat capacity, J/kgK
Pr = mu ∗ cp / lambda; % Prandtl number
m = rho_N ∗ V_N / 3600; % mass flow rate, kg/s
G=m ∗ 4/pi / d_i ^ 2; % mass velocity, kg/m2s
Re = G ∗ d_i / mu; % Reynolds number
Q=m ∗ cp ∗ (t_out - t_in); % heat load, W
Nu_H = 4.36; % Nusselt number for constant wall heat flux
alpha_H = Nu_H ∗ lambda / d_i; % heat transfer coefficient, W/m2K

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