NTUyksiköinä

NTUyksiköinä refers to describing the performance of heat exchangers using the dimensionless parameter Number of Transfer Units (NTU). In Finnish technical literature, the term is often written as NTU-yksiköinä.

The NTU method expresses heat exchanger effectiveness (epsilon) as a function of NTU and the capacity ratio C_r, enabling design and analysis independent of shell or tube arrangement. NTU is defined as NTU = U*A / C_min, where U is the overall heat transfer coefficient, A the heat transfer area, and C_min the minimum heat capacity rate between the two streams (C = m_dot*c_p). C_min = min(m_dot1*c_p1, m_dot2*c_p2). The capacity ratio is C_r = C_min / C_max, with C_max the larger of the two capacity rates.

Given C_r and NTU, the effectiveness epsilon can be obtained from standard relations: for parallel-flow exchangers, epsilon = (1 - exp(-NTU*(1+C_r))) / (1 + C_r); for counterflow exchangers, epsilon = (1 - exp(-NTU*(1 - C_r))) / (1 - C_r*exp(-NTU*(1 - C_r))). When C_r = 1, epsilon = NTU / (1 + NTU).

NTUyksiköinä is used in planning, evaluation, and optimization of heat exchange systems, including shell-and-tube and plate heat exchangers. It enables comparing different designs by a common parameter set, and is widely taught in mechanical and chemical engineering.