Heat transfer and fluid flow in spray evaporators with application to reducing refrigerant inventory

Abstract

<p>As the phase-out of chlorofluorocarbons (CFC's) approaches, there is a current need for the development of new vapor compression chillers which can operate with non-CFC refrigerants while still maintaining high operating efficiencies. Redesign of the traditional flooded evaporator used in refrigeration chillers so as to incorporate a spray evaporation capability offers both a potential for increased heat transfer performance and a reduction in refrigerant inventory for a given chiller capacity relative to that found with existing industrial units;This study is an evaluation of the spray evaporation heat transfer performance of refrigerants HFC-134a, HCFC-22, and HCFC-123 with commercially available copper alloy tubes. In addition, the effects of small concentrations of oil on the spray evaporation heat transfer process are also investigated;Two different spray evaporation heat transfer facilities were designed and constructed, namely, a multi-tube facility and a large scale bundle facility. Testing of HFC-134a was conducted on the multi-tube test facility with six different enhanced and low-finned surface copper tubes. These initial tests indicated that enhanced condensation surfaces were better suited for the spray evaporation environment than enhanced boiling surfaces. The pure refrigerant work was followed by lubricant effects testing with two different viscosity polyol-ester oils. It was found that oil concentrations through 5.0% of a 340 SUS polyol-ester oil yielded heat transfer performances greater than those measured in the pure refrigerant testing;Following the initial tests on the multi-tube test facility, large scale bundle work was conducted with all three refrigerants. The performance of HFC-134a was approximately 100 percent greater than that found with HCFC-123, and it was verified that pure HCFC-22 performed better than HFC-134a with two different tube surfaces. Small concentrations of a polyol-ester oil (through 2.5%) increased the spray evaporation heat transfer performance of HFC-134a with all surfaces evaluated. Refrigerant HCFC-22 received less benefit from small concentrations of an alkyl-benzene oil than that seen in the HFC-134a testing with the polyol-ester oil. In a similar film-feed supply rate range as that used in the high-pressure refrigerant testing, small concentrations of a napthenic mineral oil decreased the spray evaporation heat transfer performance of HCFC-123;Limited pool-boiling testing was conducted with pure HFC-134a for comparison with spray evaporation heat transfer results. It was verified that the spray evaporation heat transfer performance of a low-finned tube bundle was better than that found in flooded evaporator testing with the same bundle. More importantly, it was found that the spray evaporation heat transfer performance of an enhanced condensation surface bundle surpassed the pool boiling performance of an enhanced convective boiling surface bundle.</p>