Application of the Local Assumption for the Design of Compact Heat Exchangers for Boiling Heat Transfer

The design technique of interest here is the use of the local assumption, namely, that the local heat transfer coefficient during flow boiling is uniquely fixed by the local metal-to-liquid, ΔT, and the local fluid velocity. The object of this paper is to show the performance of two new compact heat exchangers that are specifically designed for boiling duty by this technique. These exchangers, from different manufacturers, were brazed aluminum, crossflow devices having core sizes of about 8 × 8 × 8 cm. The equivalent diameter of the flow passages on the boiling side, based on the wetted perimeter, was 0.167 cm. Offset fins gave excellent mixing of the boiling fluid, so homogeneous flow was assumed to prevail. Tests were made with Refrigerant-113 (R-113) at atmospheric pressure in these exchangers installed as thermosiphon reboilers. Heat was provided by condensing steam. The measured mass velocity of the R-113 was from 14 to 750 kg/s m², the inlet velocity was 0.008 to 0.45 m/s, the calculated homogeneous exit velocity was 0.5 to 20 m/s, the calculated metal-liquid, ΔT, varied from 15 to 120 K, and the heat duty varied from 5.5 to 57 kW. On a volumetric basis, this upper duty is 120,000 kW/m³, a remarkably high duty for the exchange of heat between two fluids. For both exchangers, the agreement between predicted and measured duties was satisfactory as long as no dryout occurred. When dryout occurred, flow oscillations were observed, and the observed heat duty was as much as 40 percent below the predicted value.