Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: As the demand for less expensive energy is increasing world-wide, energy conservation is
becoming a more-and-more important economic consideration. In light of this, means to
recover energy from waste fluid streams is also becoming more-and-more important. An
efficient and cost effective means of conserving energy is to recover heat from a low
temperature waste fluid stream and use this heat to preheat another process stream. Heat
pipe heat exchangers (HPHEs) are devices capable of cost effectively salvaging wasted
energy in this way.
HPHEs are liquid-coupled indirect transfer type heat exchangers except that the HPHE
employs heat pipes or thermosyphons as the major heat transfer mechanism from the high
temperature to the low-temperature fluid. The primary advantage of using a HPHE is that it
does not require an external pump to circulate the coupling fluid. The hot and cold streams
can 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 an
investigation involved the theoretical modelling of HPHEs such that a demonstration unit
could be designed, installed in a practical industrial application and then evaluated by
considering various financial aspects such as initial costs, running costs and energy
savings.
To develop the HPHE theoretical model, inside heat transfer coefficients for the evaporator
and condenser sections of thermosyphons were investigated with R134a and Butane as
two separate working fluids. The experiments on the thermosyphons were undertaken at
vertical and at an inclination angle of 45° to the horizontal. Different diameters were
considered and evaporator to condenser length ratios kept constant. The results showed
that R134a provided for larger heat transfer rates than the Butane operated
thermosyphons for similar temperature differences despite the fact that the latent heat of
vaporization for Butane is higher than that of R134a. As an example, a R134a charged
thermosyphon yielded heat transfer rates in the region of 1160 W whilst the same
thermosyphon charged with Butane yielded heat transfer rates in the region of 730 W at
23 °C . Results also showed that higher heat transfer rates were possible when the
thermosyphons operated at 45°. Typically, for a thermosyphon with a diameter of 31.9 mm
and an evaporator to condenser length ratio of 0.24, an increase in the heat transfer rate
of 24 % could be achieved.
Theoretical inside heat transfer coefficients were also formulated which were found to
correlate reasonably well with most proposed correlations. However, an understanding of
the detailed two-phase flow and heat transfer behaviour of the working fluid inside
thermosyphons is difficult to model. Correlations proposing this behaviour were formulated
and include the use of R134a and Butane as the working fluids. The correlations were
formulated from thermosyphons of diameters of 14.99 mm, 17.272 mm, 22.225 mm and
31.9 mm. The evaporator to condenser length ratio for the 31.9 mm diameter
thermosyphon was 0.24 whilst the other thermosyphons had ratios of 1. The heat fluxes
ranged from 1800-43500 W/m2. The following theoretical inside heat transfer coefficients
were proposed for vertical and inclined operations (READ CORRECT FORMULA IN FULL TEXT ABSTRACT)
φ = 90° ei h = 3.4516x105Ja−0.855Ku1.344
φ = 45° ei h = 1.4796x105Ja−0.993Ku1.3
φ = 90°
l
l l
ci l l
v
h x k
g
1/ 3 2.05
2
4.61561 109Re 0.364
ν ρ
ρ ρ
− ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦
φ = 45°
l
l l
ci l l
v
h x k
g
1/ 3 1.916
2
3.7233 10 5Re 0.136
ν ρ
ρ ρ
−
⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦
The theoretically modelled demonstration HPHE was installed into an existing air drier
system. Heat recoveries of approximately 8.8 kW could be recovered for the hot waste
stream with a hot air mass flow rate of 0.55 kg/s at an inlet temperature of 51.64 °C and
outlet temperature of 35.9 °C in an environment of 20 °C. Based on this recovery, energy
savings of 32.18 % could be achieved and a payback period for the HPHE was calculated
in the region of 3.3 years.
It is recommended that not withstanding the accuracies of roughly 25 % achieved by the
theoretically predicted correlations to that of the experimental work, performance parameters such as the liquid fill charge ratios, the evaporator to condenser length ratios
and the orientation angles should be further investigated. / AFRIKAANSE OPSOMMING: As gevolg van die groeiende aanvraag na goedkoper energie, word die behoud van
energie ‘n al hoe belangriker ekonomiese oorweging. Dus word die maniere om energie te
herwin van afval-vloeierstrome al hoe meer intensief ondersoek. Een effektiewe manier
om energie te herwin, is om die lae-temperatuur-afval-vloeierstroom (wat sou verlore
gaan) se hitte te gebruik om ‘n ander vloeierstroom mee te verhit. Hier dien dit dan as
voorverhitting van die ander, kouer, vloeierstroom. Hittepyp hitteruilers (HPHR’s) is laekoste
toestelle wat gebruik kan word vir hierdie doel.
‘n HPHR is ‘n vloeistof-gekoppelde indirekte-oordrag hitteruiler, behalwe vir die feit dat dié
hitteruiler gebruik maak van hittepype (of hittebuise) wat die grootste deel van sy
hitteoordragsmeganisme uitmaak. Die primêre voordele van ‘n HPHR is dat dit geen
bewegende dele het nie, die koue- en warmstrome totaal geïsoleer bly van mekaar en
geen eksterne pomp benodig word om die werkvloeier mee te sirkuleer nie.
In hierdie tesis word ‘n ondersoek gedoen oor die ontwikkeling van ‘n bestek van lug-totlug
HPHR’s. Hierdie ondersoek het die teoretiese modellering van so ‘n HPHR geverg,
sodat ‘n demonstrasie eenheid ontwerp kon word. Hierdie demonstrasie eenheid is
geïnstalleer in ‘n praktiese industriële toepassing waar dit geïvalueer is deur na aspekte
soos finansiële voordele en energie-besparings te kyk.
Om die teoretiese HPHR model te kon ontwikkel, moes daar gekyk word na die binnehitteoordragskoëffisiënte
van die verdamper- en kondensordeursneë, asook R134a en
Butaan as onderskeie werksvloeiers. Die eksperimente met die hittebuise is gedoen in die
vertikale en 45° (gemeet vanaf die horisontaal) posisies. Verskillende diameters is ook
ondersoek, maar met die verdamper- en kondensor-lengteverhouding wat konstant gehou
is. Die resultate wys dat R134a as werksvloeier in die hittebuise voorsiening maak vir
groter hitteoordragstempo’s in vergelyking met Butaan as werksvloeier by min of meer
dieselfde temperatuur verskil – dít ten spyte van die feit dat Butaan ‘n hoër latente-hittetydens-
verdampings eienskap het. As voorbeeld gee ‘n R134a-gelaaide hittebuis ‘n
hitteoordragstempo van omtrent 1160 W terwyl dieselfde hittebuis wat met Butaan gelaai
is, slegs ongeveer 730 W lewer by 23 °C. Die resultate wys ook duidelik dat hoër hitteoordragstempo’s verkry word indien die
hittebuis bedryf word teen ‘n hoek van 45°. ‘n Tipiese toename in hitteoordragstempo is
ongeveer 24 % vir ‘n hittebuis met ‘n diameter van 31.9 mm en ‘n verdamper- tot
kondensor-lengteverhouding van 0.24.
Teoretiese binne-hitteoordragskoëffisiënte is ook geformuleer. Dié waardes stem redelik
goed ooreen met die meeste voorgestelde korrelasies. Nieteenstaande die feit dat
gedetailleerde twee-fase-vloei en die hitteoordragsgedrag van die werksvloeier binne
hittebuise nog nie goed deur die wetenskaplike wêreld verstaan word nie. Korrelasies wat
hierdie gedrag voorstel is geformuleer en sluit weereens die gebruik van R134a en Butaan
as werksvloeiers in. Die korrelasies is geformuleer vanaf hittebuise met diameters van
onderskeidelik 14.99 mm, 17.272 mm, 22.225 mm en 31.9 mm. Die verdamper- tot
kondensor-lengteverhoudings vir die 31.9 mm deursnit hittebuis was 0.24 terwyl die ander
hittebuise ‘n verhouding van 1 gehad het. Die hitte-vloede het gewissel van
1800-45300 W/m2. Die volgende teoretiese geformuleerde binne-hitteoordragskoëffisiënte
word voorgestel vir beide vertikale sowel as nie-vertikale toepassing (LEES KORREKTE FORMULE IN VOLTEKS OPSOMMING)
φ = 90° ei h = 3.4516x105Ja−0.855Ku1.344
φ = 45° ei h = 1.4796x105Ja−0.993Ku1.3
φ = 90°
l
l l
ci l l
v
h x k
g
1/ 3 2.05
2
4.61561 109Re 0.364
ν ρ
ρ ρ
− ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦
φ = 45°
l
l l
ci l l
v
h x k
g
1/ 3 1.916
2
3.7233 10 5Re 0.136
ν ρ
ρ ρ
−
⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦
Die wiskundig-gemodelleerde demostrasie HPHR is geïnstalleer binne ‘n bestaande
lugdroër-sisteem. Drywing van om en by 8.8 kW kon herwin word vanaf die warm-afvalvloeierstroom
met ‘n massa vloei van 0.55 kg/s teen ‘n inlaattemperatuur van 51.64 °C en
‘n uitlaattemperatuur van 35.9 °C binne ‘n omgewing van 20 °C. Na aanleiding van hierdie
herwinning, kan energiebesparings van tot 32.18 % verkry word. Die HPHR se
installasiekoste kan binne ‘n berekende tydperk van ongeveer 3.3 jaar gedelg word deur
hierdie besparing. Verdamper- tot kondensator-lengteverhouding, vloeistofvulverhouding en die oriëntasiehoek
vereis verdere ondersoek, aangesien daar slegs ‘n akkuraatheid van 25 % verkry is
tussen teoretiese voorspellings en praktiese metings.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16389 |
| Date | 12 1900 |
| Creators | Meyer, Meyer |
| Contributors | Dobson, R. T., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xxi, (various foliations) : ill. |
| Rights | University of Stellenbosch |
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