# Error: No. of variables exceed no. of equations (how to count variables)

조회 수: 4(최근 30일)
Muhammad Zain Haider Ali 2015년 6월 19일
댓글: Andreas 2015년 10월 15일
Here is my code for a counterflow water/water heat exchanger based on LMTD method.
The error which I am receiving is that no. of variables are exceeding the no. of equations. But I can't think of any equation which I missed as it is a very simple model. Can someone take a look at it and suggest any equation which I have missed.
________________________________________________
component counterflow_HX
% Heat Exchanger
% Ideal Counter flow heat exchanger based on LMTD method
nodes
A1 = foundation.thermal_liquid.thermal_liquid; % A1:left
B1 = foundation.thermal_liquid.thermal_liquid; % B1:left
B2 = foundation.thermal_liquid.thermal_liquid; % B2:right
A2 = foundation.thermal_liquid.thermal_liquid; % A2:right
end
parameters
Cp = { 4.2, 'J/g/K'}; % Specific heat
UA = { 0.34, 'J/s/K'}; % Product of heat transfer coefficient & area
end
variables
M1 = { 0.09, 'kg/s' }; % hot side mass flow
M2 = { 0.275, 'kg/s' }; % cold side mass flow
Phi1 = { 0, 'J/s' }; % hot side duty
Phi2 = { 0, 'J/s' }; % cold side duty
end
branches
M1 : A1.mdot -> B1.mdot;
M2 : A2.mdot -> B2.mdot;
Phi1: A1.Phi -> B1.Phi;
Phi2: A2.Phi -> B2.Phi;
end
equations
let
p_A1 = A1.p;
T_A1 = A1.T;
p_A2 = A2.p;
T_A2 = A2.T;
p_B1 = B1.p;
T_B1 = B1.T;
p_B2 = B2.p;
T_B2 = B2.T;
LMTD = ((T_A1 - T_B2) - (T_A2 - T_B1))/ (log ((T_A1 - T_B2) / (T_A2 - T_B1)));
in
Phi1 == M1 * Cp * (T_A1 - T_B1)
Phi2 == M2 * Cp * (T_A2 - T_B2)
Phi1 == Phi2
p_A1 == p_B1
p_A2 == p_B2
Phi1 == UA * LMTD
end
end
end
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Andreas 2015년 10월 15일
Could you share the working code?

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### 채택된 답변

Drew Davis 2015년 6월 22일
It appears you are missing equations for the "M1" and "M2" variables. See this example.

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