Type1223new uses airside heat transfer correlations of Elmahdy and Biggs [5]. The latter, however, is restricted to coils with circular or continuous plain fins and to the coil dimensions used in the experiment (9 samples with plain fins and 12 finned tube heat exchangers). In order to extend the validity range and the generality of the model, other correlations [6-14] were identified in the literature, which had been generated from larger databases, sometimes including experimental data from previous reports. Unfortunately, different correlations cannot be directly compared in terms of heat transfer coefficients, because different data reduction methods[9] were used, which makes it difficult to choose the “best” one.

Jacobi et al. [14] reviewed many correlations and recommended the correlations of Wang et al. [7] for dry surface and Wang et al. [8] for wet surface for plain fin round-tube heat exchangers. Nevertheless, the more recent correlations of the same authors [10, 11] have been implemented in the model, which include data of previous reports (in total 74 samples with dry surface and 31 samples with wet surface).

Plain fins are, however, not the current industry standard. Most fins have either waves or louvers for heat transfer augmentation. The heat exchanger used in the test plant in Bishkek has herringbone wavy fins. Therefore, the heat exchanger model was extended for herringbone/wavy fins by adding the correlation of Wang et al. [12] for dry surface (61 samples in the data base) and Pirompugd et al. [13] for wet surface (18 samples in the data base). If necessary, other correlations, e. g. for louvered fins, can be easily implemented as well. The switch between fin types and their respective correlations is done by a model parameter FinType (e. g. 0 — plain fins, 1 — wavy fins).

It has to be noted, that the same equations for liquid-side heat transfer, fin efficiency and s-NTU relation have to be used to calculate the overall heat transfer rate as in the data reduction method for developing the airside correlation. All implemented airside heat transfer correlations of the model have the same data reduction method and its equations will be presented in the following paragraphs.