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Effects of airflow induction on heat transfer and energy consumption while freezing passion fruit pulp in stacked boxes

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Abstract

The objectives of this work were to characterize the energy consumption and the heat transfer process by the determination of the convective heat transfer coefficient (h) of passion fruit pulp contained in high-density polyethylene (HDPE) boxes and frozen in two conditions: without and with airflow induction, which was achieved through the installation of obstacles. To determine the convective heat transfer coefficients, HDPE boxes containing passion fruit pulp (contained in polyethylene bags) were interspersed with boxes containing metal tanks filled with low freezing point solutions. Three types of solutions were used: ethylene glycol, propylene glycol, and ethanol. The airflow induction under the stacks of passion fruit pulp provided higher h values than without airflow induction. The calculated average values and standard deviation were 6.340 ± 0.87 W/m2 °C, respectively, without airflow induction and 8.419 ± 1.39 W/m2 °C with airflow induction. The average reduction of the freezing time was 25 % for the boxes located at the top and 20 % in the base of the stack. This proved that directing the airflow under the stacked product promoted more uniform and efficient heat transfer. The analysis of the electrical parameter measurements revealed an approximate decrease of 16.7 % in energy consumption due to the reduction of the freezing time, without compromising the quality and operation of the electrical system. This practice was shown to be viable for small producers and agribusinesses that desire reductions in processing time and energy consumption and, consequently, the overall cost of the final product.

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Abbreviations

A :

Heat transfer area (square meter)

AD:

Active demand (kilowatt)

AE:

Active energy (kilowatt hour)

AEM:

Active energy/month (kilowatt hour)

AP:

Amount of product/batch (kilogram)

Bi:

Biot number

Bi s :

Biot number = h2L/k

CD:

With air induction

Cp:

Specific heat (kilojoule per kilogram per degree Celsius)

CF:

Charge factor

D :

Diameter (meter)

e :

Error (percent)

E :

Equivalent heat transfer dimensionality

h :

Convective heat transfer coefficient (watt per square meter per degree Celsius)

k :

Thermal conductivity (watt per meter per degree Celsius)

m :

Mass (kilogram)

MAD:

Maximum active demand (kilowatt)

NBM:

Number of batches/month

PF:

Power factor

PPM:

Physical production/month (kilogram)

RE:

Reactive energy (kilovolt-ampere-reactive-hour)

SC:

Specific consumption (kilowatt hour per kilogram of pulp)

SD:

Without air induction

T :

Temperature (degree Celsius)

t :

Time (hour)

X :

Length in the coordinate system (meter)

Y :

Height in the coordinate system (meter)

Z :

Width in the coordinate system (meter)

Z n , Z m :

Roots of a transcendental equation of the type C = αtanα

Z nm :

Defined by Eq. (12).

β i :

Ratio of dimension to characteristic dimension, i = 1, 2

Δ:

Difference

∞:

Air cooling

Air:

Air

AN:

Dimensionality analytically derived

Eff:

Effective

eq:

Equivalent

etg:

Ethylene glycol

eth:

Ethanol

exp:

Experimental

f:

Freezing

max:

Maximum

pred:

Predicted´

prop:

Propylene glycol

slab:

Slab

sol:

Solution

tq:

Tank

0:

Unfrozen

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Acknowledgments

The authors wish to thank the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG-Brazil), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq-Brazil) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brazil) for financial support for this research.

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Authors and Affiliations

  1. Department of Food Science, Laboratory of Food Refrigeration, Federal University of Lavras, P.O. Box 3037, 37200-000, Lavras, MG, Brazil

    Natália Cristina Belchior, Tales Márcio de Oliveira Giarola & Jaime Vilela de Resende

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  1. Natália Cristina Belchior
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  2. Tales Márcio de Oliveira Giarola
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  3. Jaime Vilela de Resende
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Correspondence to Jaime Vilela de Resende.

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Belchior, N.C., de Oliveira Giarola, T.M. & de Resende, J.V. Effects of airflow induction on heat transfer and energy consumption while freezing passion fruit pulp in stacked boxes. Energy Efficiency 7, 777–790 (2014). https://doi.org/10.1007/s12053-014-9253-y

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