Design and Performance Analysis of Plate Heat Exchangers for Heat Pumps using Pure and Mixed Refrigerants

Roberta Mancini

Research output: Book/ReportPh.D. thesis

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Abstract

Optimal heat exchanger (HEX) design and accurate estimation of its performance is of paramount importance to improve the energy eÿciency and the economic feasibility of heat pump systems. A plate heat exchanger (PHE) is a multi-channel HEX configuration, which attracted intense industrial attention due to enhanced heat transfer in a relatively compact design compared to traditional configurations. Moreover, due to the possibility of setting the flow in a counter-current arrangement, PHEs constitute an ideal solution to match the temperature glide of zeotropic mixtures during phase change with the secondary fluid temperature profile, thus reducing the irreversibility due to heat transfer.
This PhD thesis presents detailed numerical models of PHE evaporators and condensers using pure and mixed refrigerants. The numerical models were implemented based on both one-dimensional and two-dimensional discretization of the HEX, with the aim of simulating channel-to-channel flow maldistribution. The PHE models were further integrated with design and o˙-design models of single-stage vapour compression heat pumps, in order to evaluate the impact of HEX design and performance on the thermodynamic and economic performance indicators of the cycle. Additionally, the presented models were validated against available experimental data for both pure fluids and zeotropic mixtures.
The developed modelling framework was subsequently applied to case studies in order to address research gaps in the field of PHE technology. First, a comparison of di˙erent correlations to compute the evaporation heat transfer coeÿcient of zeotropic mixtures was carried out, with the aim of deriving recommendations on the most suitable ones. Each correlation was applied to the PHE model to compute the local heat transfer coeÿcient, and the calculated heat flow rate was subsequently compared to experimental data for zeotropic mixtures of carbon dioxide (CO2)/propane at di˙erent mass compositions.
Second, guidelines for PHE evaporators and condensers design were derived. It was shown that condenser pressure drop solely a˙ects the economic feasibility of the heat pump, while an optimal trade-o˙ between heat transfer area and pressure drop of the PHE evaporator must be ensured to optimize the system thermodynamic and economic performance. Moreover, a methodology to derive simplified guidelines for the design of optimal PHE evaporators was formulated and applied to a case study of heat pump integration in a spray drying facility for waste heat recovery purposes.
Last, a study on the impact of flow maldistribution on the performance of PHE evapora-tors was carried out. Specifically, the impact of liquid/vapour non-uniformities at the evaporator inlet and the e˙ect of end plates were estimated on a PHE evaporator using two pure fluids and two zeotropic mixtures as refrigerants. The impact of maldistribution on the heat pump thermodynamic performance was subsequently evaluated by an inte-grated PHE - heat pump model. The evaporator degradation was thus translated into a reduction of the thermodynamic performance of the heat pump, negatively impacting the system operating costs.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages170
ISBN (Electronic)978-87-7475-578-4
Publication statusPublished - 2019

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