Heat Exchanger Effectiveness NTU Calculator

The effectiveness-NTU (epsilon-NTU) method is used to analyse heat exchangers when outlet temperatures are unknown. Effectiveness epsilon = Q_actual / Q_max, where Q_max = C_min * (T_hot_in - T_cold_in). NTU = UA / C_min, where U is the overall heat transfer coefficient, A is the heat transfer area, and C_min is the smaller of the two fluid heat capacity rates C = m_dot * Cp.

For a counterflow heat exchanger with C_r = C_min/C_max not equal to 1: NTU = [1/(1-C_r)] * ln[(1 - C_r * epsilon)/(1 - epsilon)]. For C_r = 1 (balanced): NTU = epsilon/(1 - epsilon). Counterflow achieves the highest effectiveness for a given NTU and is the preferred configuration for energy recovery applications.

NTU (Number of Transfer Units) measures the thermal size of a heat exchanger. NTU = 1 means a modest heat exchanger. NTU = 3 is a large, efficient unit. Higher NTU yields higher effectiveness but with diminishing returns: going from NTU = 1 to NTU = 2 gives a large effectiveness gain, but going from NTU = 5 to NTU = 6 gives almost nothing in a balanced counterflow exchanger.

The capacity rate ratio Cr = C_min / C_max = (m_dot * Cp)_min / (m_dot * Cp)_max. For Cr = 0 (condensers and evaporators where one fluid changes phase), effectiveness = 1 - exp(-NTU) for all flow arrangements. For Cr = 1 (balanced flow), counterflow and parallel flow behave very differently: counterflow approaches epsilon = 1 at high NTU, parallel flow is capped at epsilon = 0.5 as NTU approaches infinity.

UA = Q / LMTD (log mean temperature difference). For a counterflow exchanger: LMTD = (dT1 - dT2) / ln(dT1/dT2), where dT1 = T_hot_in - T_cold_out and dT2 = T_hot_out - T_cold_in. Manufacturers often provide UA directly, or it can be estimated from overall heat transfer coefficient U (W/m^2.K) and heat transfer area A (m^2).