PhD, Faculty of Engineering and the Built Environment, University of the Witwatersrand / In this study the heat transfer and frictional pressure drop performance
characteristics of plate heat exchangers (PHE’s) used as refrigerant liquid overfeed
evaporators were investigated. PHE’s have been gaining new applications in
the refrigeration industry, especially as evaporators, during the last few decades,
but the available information in the open literature for operation in this mode is
rather limited. This study aims to extend the knowledge of PHE evaporator
performance and to develop a model for use in evaluating heat transfer and
pressure drop over as wide a range of operating conditions as possible.
A laboratory experimental facility was constructed and the thermal-hydraulic
characteristics of three middle-size industrial PHE’s were measured. The
exchangers all had 24 plates of the same size but with different chevron angle
combinations of 28°/28°, 28°/60°, and 60°/60°. Two sets of tests were carried out
with the three units: single-phase performance tests with water, and evaporator
performance tests with R134a and R507A, for which the exchangers operated as
refrigerant liquid over-feed evaporators. The tests with water served to provide
accurate water-side heat transfer information for the evaporator performance
analysis which is the primary purpose of this study. In the evaporator
performance tests, refrigerant flow boiling heat transfer and two-phase pressure
drop data were obtained under steady conditions, over a range of heat flux from
1.9 to 6.9 kW/m2, refrigerant mass flux from 5.6 to 31.4 kg/(m2s), outlet vapour
quality from 0.2 to 0/95, and saturation temperatures from 5.9 to 13.0 °C.
Additional field data of thermal performance were collected on an ammonia and a
R12 PHE water chiller, operating as thermo-siphon evaporators at their design
working conditions.
All experimental data were reduced and analyzed to obtain the refrigerant-side
heat transfer coefficients and frictional pressure drops in the PHE evaporators.
The heat transfer results showed a strong dependence on heat flux and a weak
dependence on mass flux, vapour fraction and the chevron angle. Along with the
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observations from the ammonia and R12 evaporators, it is concluded that the
dominating heat transfer mechanism in this type of evaporator is nucleate boiling
rather than forced convection. In contrast to the heat transfer characteristics, the
refrigerant two-phase frictional pressure drop was found to be strongly influenced
by mass flow rate, vapour fraction and also the chevron angle. An almost linear
increase of the frictional pressure drop with the homogeneous two-phase kinetic
energy per unit volume was observed for both refrigerants.
Based on the experimental data, correlations were developed for predicting the
refrigerant boiling heat transfer coefficient and two-phase frictional pressure drop
in PHE liquid over-feed evaporators. Two correlations were developed for
boiling heat transfer, one of these reflecting the h-q relationship in pool boiling,
the other with all constants and exponents determined by regression analysis. The
mean absolute errors are respectively 7.3% and 6.8% for these correlations. For
two-phase frictional pressure drop, data were correlated using two established
methods, namely the homogeneous and the Lockhart-Martinelli methods, with
means absolute errors of 6.7% and 4.2%, respectively. The homogeneous model
showed a slightly higher discrepancy with the experimental data but is likely to
be more physically sound for PHE evaporators, and is much simpler to apply.
Validation of these correlations with other data has been difficult due to the
shortage of published information. For other refrigerants operating at comparable
conditions, these correlations should serve as a guide, while more accurate design
or evaluation may need to be based on further testing.
The performance analysis carried out in this study was based on systematic
experimental investigations and field tests on industrial PHE units. Correlations
were developed covering a rather extensive range of flow parameters, plate
geometry and various refrigerants. Such correlations have not been reported
previously for PHE liquid over-feed evaporators. The results simplify the
performance analysis of PHE evaporators and provide a reliable thermalhydraulic
model of PHE liquid over-feed evaporators, which can be used for
system modeling of water-chilling machines employing this type of evaporator.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/9779 |
| Date | 16 May 2011 |
| Creators | Huang, Jianchang |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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