Development of a range of air-to-air heat pipe heat recovery heat exchangers

[摘要] ENGLISH ABSTRACT: As the demand for less expensive energy is increasing world-wide, energy conservation isbecoming a more-and-more important economic consideration. In light of this, means torecover energy from waste fluid streams is also becoming more-and-more important. Anefficient and cost effective means of conserving energy is to recover heat from a lowtemperature waste fluid stream and use this heat to preheat another process stream. Heatpipe heat exchangers (HPHEs) are devices capable of cost effectively salvaging wastedenergy in this way.HPHEs are liquid-coupled indirect transfer type heat exchangers except that the HPHEemploys heat pipes or thermosyphons as the major heat transfer mechanism from the hightemperature to the low-temperature fluid. The primary advantage of using a HPHE is that itdoes not require an external pump to circulate the coupling fluid. The hot and cold streamscan also be completely isolated preventing cross-contamination of the fluids. In addition,the HPHE has no moving parts.In this thesis, the development of a range of air-to-air HPHEs is investigated. Such aninvestigation involved the theoretical modelling of HPHEs such that a demonstration unitcould be designed, installed in a practical industrial application and then evaluated byconsidering various financial aspects such as initial costs, running costs and energysavings.To develop the HPHE theoretical model, inside heat transfer coefficients for the evaporatorand condenser sections of thermosyphons were investigated with R134a and Butane astwo separate working fluids. The experiments on the thermosyphons were undertaken atvertical and at an inclination angle of 45° to the horizontal. Different diameters wereconsidered and evaporator to condenser length ratios kept constant. The results showedthat R134a provided for larger heat transfer rates than the Butane operatedthermosyphons for similar temperature differences despite the fact that the latent heat ofvaporization for Butane is higher than that of R134a. As an example, a R134a chargedthermosyphon yielded heat transfer rates in the region of 1160 W whilst the samethermosyphon charged with Butane yielded heat transfer rates in the region of 730 W at23 °C . Results also showed that higher heat transfer rates were possible when thethermosyphons operated at 45°. Typically, for a thermosyphon with a diameter of 31.9 mmand an evaporator to condenser length ratio of 0.24, an increase in the heat transfer rateof 24 % could be achieved.Theoretical inside heat transfer coefficients were also formulated which were found tocorrelate reasonably well with most proposed correlations. However, an understanding ofthe detailed two-phase flow and heat transfer behaviour of the working fluid insidethermosyphons is difficult to model. Correlations proposing this behaviour were formulatedand include the use of R134a and Butane as the working fluids. The correlations wereformulated from thermosyphons of diameters of 14.99 mm, 17.272 mm, 22.225 mm and31.9 mm. The evaporator to condenser length ratio for the 31.9 mm diameterthermosyphon was 0.24 whilst the other thermosyphons had ratios of 1. The heat fluxesranged from 1800-43500 W/m2. The following theoretical inside heat transfer coefficientswere proposed for vertical and inclined operations (READ CORRECT FORMULA IN FULL TEXT ABSTRACT)φ = 90° ei h = 3.4516x105Ja−0.855Ku1.344φ = 45° ei h = 1.4796x105Ja−0.993Ku1.3φ = 90°ll lci l lvh x kg1/ 3 2.0524.61561 109Re 0.364ν ρρ ρ− ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦φ = 45°ll lci l lvh x kg1/ 3 1.91623.7233 10 5Re 0.136ν ρρ ρ−⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦The theoretically modelled demonstration HPHE was installed into an existing air driersystem. Heat recoveries of approximately 8.8 kW could be recovered for the hot wastestream with a hot air mass flow rate of 0.55 kg/s at an inlet temperature of 51.64 °C andoutlet temperature of 35.9 °C in an environment of 20 °C. Based on this recovery, energysavings of 32.18 % could be achieved and a payback period for the HPHE was calculatedin the region of 3.3 years.It is recommended that not withstanding the accuracies of roughly 25 % achieved by thetheoretically predicted correlations to that of the experimental work, performance parameters such as the liquid fill charge ratios, the evaporator to condenser length ratiosand the orientation angles should be further investigated.

[发布机构] Stellenbosch University