Exergy analysis of banana drying process via a closed-loop air source heat pump system

ABSTRACT

In this study, an air source heat pump drying system with an external condenser circuit connected in series was developed. The system was designed to enable the external condenser to be activated serially with the internal one via the solenoid valves controlled by the digital thermostat when the drying temperature of the cabinet reaches the desired level. Temperature control of the drying chamber was achieved by activating the external condenser along with the internal condenser. Exergy analysis was conducted for the process of drying banana slices via a closed-loop air source heat pump dryer (HPD) at various drying temperatures. Exergy efficiencies and losses for the HPD system were calculated. As a result, the exergy efficiency of the dryer was calculated to be between 75.93% and 80.95%, while the exergy efficiencies of the system and heat pump were found to range from 7 to 13.07%. Moreover, the expansion valve was found to have the highest exergy efficiency with 93.32%. The highest exergy losses were also found in the compressor and condenser with 0.557 kW and 0.366 kW, respectively.

KEYWORDS:

• Closed-loop
• heat pump
• drying
• bananas
• exergy

Disclosure statement

No potential conflict of interest was reported by the author(s).

Nomenclature

C_pspecific heat, J/kgK

exspecific exergy rate, J/kg

Exexergy rate, W

hspecific enthalpy, J/kg

Mmoisture

M_eequilibrium moisture content, g.water/g.dry matter

M₀initial moisture content, g.water/g.dry matter

MRmoisture ratio

M_tmoisture content at any time, g.water/g.dry matter

$\dot{m}$mass flow rate, kg/s

sspecific entropy, J/kgK

Ttemperature, K

Ppressure (bar)

$\dot{Q}$heat transfer rate, W

Ẇpower consumption, W

wmoisture content, g.water/g.dry air

Llatent heat, J/kg

Xmass fraction

HPDHeat pump dryer

PIDProportional integral derivative controller

RGas constant

Greek symbols

ηefficiency

φrelative humidity, %

Subscripts

1compressor inlet of refrigerant

2compressor outlet of refrigerant (internal condenser inlet)

3internal condenser outlet of refrigerant

$3^{\mathrm{\prime }}$external condenser outlet of refrigerant

4evaporator inlet of refrigerant

aair

Adryer inlet of drying air (condenser outlet)

Bdryer outlet of drying air

Cevaporator inlet of drying air

cdcondenser

chcarbohydrate

compcompressor

Devaporator outlet of drying air

Destrate of exergy destruction, kW

Econdenser inlet of drying air

evevaporator

evapevaporation

exexergy

expexpansion valve

extexternal

ffat

fifiber

h_lossheat loss

hpheat pump

ininlet

intinternal

mmean, mass (kg)

odead point

outoutlet

pproduct

prprotein

rrefrigerant

ssurface

syssystem

vvapor

wwater

oDead state

uUncertainty in the independent variables

U_FUncertainty

zFunction

FIndependent variables