Performance of Plate Heat Exchangers Used as Refrigerant Liquid-Overfeed Evaporators

Abstract

The heat transfer and pressure drop characteristics of plate heat exchangers were measured, when used as refrigerant liquid over-feed evaporators. The three units all had 24 plates but with different chevron-angle combinations of 28°/28°, 28°/60°, and 60°/60°. R134a flowing upwards was used as the refrigerant, in a counter-current arrangement with water flowing on the other side. Heat transfer and pressure drop measurements were made over a range of mass flux, heat flux and corresponding outlet vapour fractions. The effect of system pressure on the evaporator performance was not evaluated due to the small range of evaporating temperature. Experimental data were reduced to obtain the refrigerant-side heat transfer coefficient and frictional pressure drop. The results for heat transfer showed a strong dependence on heat flux and weak dependence on mass flux and vapour fraction. Furthermore, the chevron angle had a small influence on heat transfer but a large influence on frictional pressure drops. Along with observations that were obtained previously on large ammonia and R12 plate evaporators, it is concluded that the dominating heat transfer mechanism in this type of evaporator is nucleate-boiling rather than forced convection. For the two-phase friction factor, various established methods were evaluated; the homogeneous treatment gives good agreement.

Dynamic modeling of flow boiling within plate heat exchangers for heat pump and refrigeration applications

“…A general correlation for nucleate boiling by Cooper (1984) will be used later for comparison purposes. The Cooper correlation is independent of steam quality but is reported to estimate BPHE average htc well when multiplied with a factor of 1.5 (Palm and Claesson 2006) or 1.7 (Huang 2010). Different sets of measurement data allow calculating the heat transfer coefficient for chevron type plate heat exchangers for diverse geometries and boundary conditions.…”

“…The modeling approach using the finite volume method is validated with single-phase heat transfer data from Huang (2010). Two correlations are used.…”

“…Correlation and fitting factors are taken from Huang (2010) and were obtained by fitting the correlation to data for the heating of cold water with the same BPHE which will later be used as evaporator. As an alternative approach, the correlation of Martin (2010) was implemented, which is also used for modeling the heating of subcooled liquid or superheated steam.…”

“…Table 3 gives an overview of the boundary conditions of the evaluated experimental data. The complete measurement data sets for inlet temperature, volume flow and inlet pressure can be found at Huang (2010). Figure 3 displays a parity plot, comparing simulated and experimental heat transfer coefficients and heat flow in the case of refrigerant R134a and β=28 °.…”

“…The modified Amalfi, et al (2015) (A) and Donowski and Kandlikar (2000) (C) have moderate prediction quality for the specific data evaluated. The experimental values of Huang (2010) were determined using the logarithmic mean temperature difference. The fluid properties used in his calculations were from ASHRAE Handbook 2005 at saturation temperature.…”