Performance characterisation of a seperated heat-pipe-heat-recovery-heat-exchanger for the food drying industry

Thomas, Nathan Shane (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: In light of the ever increasing demand for energy efficiency, waste heat recovery has become an important engineering design consideration. Heat-pipe-Heat-Exchangers (HPHE’s) are waste-heat-recovery-units (WHRU’s) that utilise heat pipes/thermosyphons that contain a working fluid as the heat transfer mechanism from the high temperature waste stream to the low temperature stream. To prevent cross contamination for the food industry, the exhaust and inlet streams are often far apart. However, performance correlations for separated-HPHE’s are difficult to find. For this reason, the thermal performance of an air-to-air separated-HPHE is investigated and characterised. The investigation involved the theoretical and numerical modelling of the separated-HPHE. The models were then compared to the experimental results for validation purposes. It is also important to use energy efficiently, hence the effect of air temperature and flow rate on the drying times of various materials were also investigated. To develop the HPHE model, outside and inside heat transfer coefficients for the HPHE are required. The outside heat transfer coefficients were obtained by passing hot air over a HPHE filled with cold water and of similar geometry to the HPHE’s used for the separated-HPHE. The inside heat transfer coefficients for the separated-HPHE were determined with R600a, R134a and R123 as working fluids. The experiments were undertaken at various temperatures and flow rates. The results showed that R600a works the most effectively in the temperature range considered and this is expected since R600a is less dense and has a higher latent heat of vaporisation than both R134a and R123. As an example, the R600a charged separated-HPHE yielded heat transfer rates in the region of 9352 W compared to the 7017 W and 4555 W yielded for R134a and R123 respectively at an air temperature difference of 27 °C and mass flow rate of 0.841 kg/s. The predicted inside heat transfer coefficients correlate the experimentally obtained heat transfer coefficients reasonably well. However, it is found that theoretical models correlated by previous researchers do not correspond to the predicted values obtained from the correlations found from the testing of the separated-HPHE. The differences are attributed to the poor manifold design and the fact that the researchers conducted their experiments on a single thermosyphon, whereas the entire heat exchanger was used in this case. The main objective of the thesis was because the as-tested separated-HPHE was shown to worked effectively (recovering up to 90 % of the of the dryer exhaust heat) for typical food industry drying temperatures of between 25-80 °C. Additionally, the theoretical simulation models for the HPHE was validated in as much that its energy saving performance was within  12 % of the as-tested experimental models; and thus it was demonstrated that substantial energy cost saving could be realised using standard heat exchanger manufacturing technology. If the heat exchanger is installed in a plant, charged with R600a and is operated with an inlet air temperature of approximately 80 °C and mass flow rate of 0.841 kg/s, the heat recovered is 13.828 kW in an environment of 13 °C. At these conditions, the potential payback period of installing the heat exchanger specified for this study is 3.22 years. It is recommended that notwithstanding accuracies of roughly 22 % obtained by the theoretically predicted correlations to the experimental work, the heat exchanger design should be optimised to allow better refrigerant flow and various performance parameters like liquid fill charge ratio and condenser/evaporator length dependencies should be further investigated.

AFRIKAANS OPSOMMING: As gevolg van die stygende noodsaaklikheid van effektiewe energie gebruik raak energie behoud en herwinning al hoe meer belangrik ingenieurs ontwerp oorwegings. Hittepyp-hitteruilers (HPHR‘s) is afval-hitte-herwinnings-eenheid (AHHE) wat hittepype vol koelmiddel bevat wat die hitteoordrag meganisme is vanaf die hoë temperatuur vloeistroom na die lae temperatuur vloeistroom. Om kruiskontaminasie te verhoed in die voedsel bedryf, is dit noodsaaklik dat die uitlaat en inlaat strome geskei is. Daar bestaan tans nie veel korrelasies vir geskeide-HPHR‘s. Vir hierdie rede word die termiese verrigting van ‘n geskeide-hittepyp-hitteruiler (HPHR) ondersoek. Die ondersoek bevat die toeretiese en numeriese modellering van die geskeide-HPHR. Die modelle word vergelyk met die eksperimentele resultate om hulle te valideer. Dit is ook noodsaaklik dat energie spaarsamig gebruik word en om hierdie rede word die effek van lug tempratuur en vloeitempo op die droogmaak tye van verskeie materiale ondersoek. Om die HPHR model te ontwikkel is dit nodig om die buite- en binne hitte oodragskoëffisiënte te vind. Die buite hitteoordragskoëffisiënte was bepaal deur warme lug oor ‘n HPHR te laat vloei wat geometries dieselfde is as die HPHR’s wat gebruik word vir die geskeide-HPHR. Die binne-hitteoordragskoëffisiënte was bepaal met R600a, R134a en R123 as koelmiddels. Die eksperimente was gedoen by verskeie temperature en lugvloeitempo's. Die resultate wys dat R600a die effektiefste werk by die temperature waarteen die eksperimente gedoen was. Dit was verwag as gevolg van die feit dat R600a ‘n ligter gas en hoër latente-hitte-tydens-verdampings eienskap het as albei R134a en R123. As voorbeeld, die geskeide-HPHR vol R600a het 9352 W herwin in vergelyking met 7017 W en 4555 W vir die R134a en R123 respektiewelik teen ‘n lug temperatuur verskil van 27 °C en ‘n lugvloeitempo van 0.841 kg/s. Die voorspelde binne-hitteoordragskoëffisiënte korreleer die eksperimentele waardes redelik goed. Dit was egter gevind dat die teoretiese modelle wat gekorreleer was deur vorige navorsers nie goed ooreenstem met die voorspelde waardes vir die geskeiede-HPHR nie. Die verskille word toegeskryf aan die swak spruitstuk ontwerp en die feit dat die navorsers hul eksperimnete op ‘n enkele hittepyp gedoen het, terwyl die hele HPHR gebruik was in hierdie geval. Die hoof objektief van die tesis was bereik deurdat die geskeide-HPHR wel effektief (so hoog soos 90 % van die uitlaat hitte) gewerk het tussen die temperatuur limiete van 25-80 °C wat tipies in die voelsel bedryf gevind word. Dus was dit bewys dat daar groot energiebesparings verkry kan word deur die installasie van die HPHR. Daarbenewens, die toeretiese modelle van die HPHHHR het die eksperimentele waardes tot binne 12 % voorspel. As die geskeide-HPHR op ‘n fabriek geinstaleer word met inlaat lugvloei kondisies van 80 °C en 0.841 kg/s kan 13.828 kW herwin word as die omgewingstemperaturr 13 °C is. Vir hierdie toestande is die potensiële terugverdieningstyd 3.22 jare. Dit word aanbeveel dat die HPHR se ontwerp geoptimiseer moet word vir minder vloei weerstand en dat die vloeivulverhouding en die verdamper-tot-kondensator-lengteverhouding verdere ondersoek vereies, aangesien die 22 % akkuraatheid tussen die teoretiese en praktiese metings te hoog is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98795
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