Fin efficiency in Type1223new is calculated using an analytical solution for circular plain fins. This can, however, overestimate the fin efficiency for high performance heat transfer surfaces (fins with waves, slits and louvers) [16]. If the overall heat transfer coefficient is determined using the same fin efficiency calculation as in developing of a heat transfer correlation, no error will be generated. Therefore, the fin efficiency calculation according to Schmidt [15] (taken from [9]) was implemented in the model, which was used in developing of the implemented correlations.

1.2. Liquid-side correlation

For a liquid-side heat transfer correlation a simplified Gnieliski correlation Eq. (1) was implemented, which corresponds to the chosen airside heat transfer correlations [10-13].

_k_](Rep -1000)Pr(f,/2) D. J1 + 12.7f2 (Pr2/3 -1)

(1)

u. =

This simplified correlation is only valid for turbulent and transitional flow. At small Re numbers (< 4000) heat transfer can be considerably over predicted with this correlation.

1.3. Heat transfer rate calculation

The overall heat transfer resistance is defined from the following relationship

1 ua	1
(3)

V

The authors of the airside heat transfer correlations applied s-NTU relationships for cross-counter flow heat exchangers from ESDU [17]. These relationships are used in the model instead of those in Type1223new for counter-flow heat exchanger. The number of heat transfer units, the capacity flow rate ratio C* and the efficiency s are defined as:

* C C* = (5), max	Q
NTU = UA/Cmn (4),	s =

Knowing NTU and C*, the efficiency can be calculated using a corresponding s-NTU relationship. The total heat transfer rate is determined as:

Q =sQ =sC ■ (t — T ) (7)

max min in, air in, liq