*2.2. Thermal Design Parameters*

The design process of HPHE starts with the specification of assumed design parameters which result from HPHE type and its utilization. HPHE will be utilized as an air-to-air recuperator for air conditioning systems. This heat exchanger type implies the following design parameters:


HPHE will be used in very small air conditioning systems (ventilation of up to 160–180 m2 spaces). Both airstreams' volumetric flow rates are taken as typical for small systems: *V* = 300 m3/h.

Computational model. The output parameter, which is the main indicator of the efficiency of heat transfer, is HEX temperature effectiveness:

$$
\varepsilon = \frac{t\_{\text{li}} - t\_{\text{ho}}}{t\_{\text{li}} - t\_{\text{ci}}} \tag{26}
$$

where *thi*—hot air stream inlet temperature, *tho*—hot air stream outlet temperature, and *tci*—cold air stream inlet temperature. A hundred percent recuperation efficiency would occur if the hot stream temperature would drop at the outlet to the level of cold stream inlet temperature.

The goal of optimization is to design HPHE with thermo-hydraulic characteristics (thermal effectiveness, pressure drop) similar to or exceeding most of the existing HPHE reported earlier in the literature review. Comparative parameters are:


The geometry of the finned tube was chosen to ensure the ease of HPHE prototype manufacture. It was selected from the bimetallic high-finned tubes manufacturer catalog— CEMAL company [24]. Heat exchanger channel height was *L*<sup>1</sup> = 0.245 m (a consequence of finned heat pipe geometry), and width *L*<sup>3</sup> = 0.245 m was a result of the assumption of a square channel. Geometrical parameters of the finned aluminum tube applied on the standard copper tube: ø 22 × 1 mm are given in Table 2 and presented in Figure 6. Fins were cold-rolled on the outer aluminum tube.


**Table 2.** Geometrical parameters for the high-finned tube.

**Figure 6.** Finned surface heat pipe heat exchanger dimensions.
