Performance characterisation of a separated heat-pipe-heat-recovery-heat-exchanger for the food drying industry
This paper presents the performance evaluation of a separated-HPHRHE for use in the food processing industry. 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 as-tested separated-HPHRHE was shown to have 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 HPHRHE 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. 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 such as liquid fill charge ratio and condenser/evaporator length dependencies should be further investigated.