Keywords: logarithmic mean temperature difference, LMTD method, generalised mean temperature difference, GMTD method, overall heat transfer coefficient, OHTC, supercritical carbon dioxide, heat exchangers, Nusselt number, thermal design
A generalised mean temperature difference method for thermal design of heat exchangers
A Generalised Mean Temperature Difference (GMTD) method instead of the Logarithmic Mean Temperature Difference (LMTD) method for heat exchangers is proposed. Unlike the LMTD method, the GMTD method allows the performance analysis of heat exchangers with distributed physical properties. GMTD is calculated by a new method to take fluid temperature profiles as a function of heat load. Mathematical proof is made that the formula of the LMTD can be derived from that of the GMTD, in the case of constant physical property. The GMTD method is applied to experiments of a hot water supplier with supercritical carbon dioxide as a heating media that works through its pseudo-critical point where specific heat behaves anomalously. The empirical correlations of the local Nusselt number and pressure loss coefficient based on the GMTD gives an accurate prediction of the overall heat transfer coefficient, with a 5% error rating, whilst an error two times larger appears in the case of the LMTD.