Energy Conservation Through Recirculation of Hot Air in Solar Dryer

Abstract

The use of pneumatic conveying system is quite new in solar drying system. The study of the application of pneumatic conveyor in solar drying system experiments using single unheated and heated pipe was conducted. It was found that the power requirement to convey rough rice was about 1.02 W/kg/h less than that used in the industry. The experiments also have found an expression of two-phase pressure of air and grains similar to Ergun’s correlation. Testing with both inclined and vertical drying chamber of a recirculation-type solar dryer indicated that with the initial load of between 104 and 380.5 kg and initial moisture content on wet basis of 27.3–31%, less than a day of drying time is required. The drying efficiency and the specific energy of the solar dryer with inclined drying chamber were on an average less than the vertical type. The drying capacity (initial load) of the inclined drying chamber was larger than the vertical chamber type.

Nomenclature

A :

area (m²)

A :

coefficient of Eq. (15)

A :

geometric coefficient in Eq. (2)

A ₁ :

coefficient of Eq. (16)

A ₁ :

cross section of pipe below the hopper (m)

A _o :

cross section area of hopper on top of grain column (m²)

A _p :

cross section area of pipe (m²)

B :

coefficient of Eq. (15)

B ₁ :

coefficient of Eq. (16)

C :

coefficient of Eq. (15)

C ₁ :

coefficient of Eq. (16)

C1:

discharge coefficient (−)

C _pi :

initial heat capacity of the grains (kJ/kg °C)

C _pf :

final heat capacity of the grains (kJ/kg °C)

C _pw :

specific heat of water vapor (kJ/kg °C)

C _pl :

specific heat of liquid (kJ/kg/°C)

C _f :

friction coefficient (-)

CV _g :

calorific value of LPG (MJ/kg)

D :

coefficient of Eq. (15)

D :

pipe diameter (m)

D ₁ :

coefficient of Eq .(16)

D _h :

pipe diameter below the hopper (m)

D _v :

mass diffusivity (m²/h)

E :

coefficient of Eq. (15)

E ₁ :

coefficient of Eq. (16)

F :

coefficient of Eq. (15)

G :

coefficient of Eq. (15)

g :

acceleration due to gravity (m/s² )

Go :

mass flux of air under superficial air velocity (kg/m² h)

W _a :

mass flow rate of the air (kg/s)

G _a :

mass flux of air (kg/m₂ s)

W _p :

mass flow rate of the grains (kg/s)

G _p :

mass flux of grains (kg/m² s)

G _r :

mass flux of air within the shell (kg/m²)

h :

heat transfer coefficient (W/m² °C)

h _cv :

volumetric heat transfer coefficient (W/m² °C)

H :

pressure head (Pa)

H :

absolute humidity (kg of moisture/kg of dry air)

H ₁ :

height of pipe below the hopper (m)

H ₀ :

height of grain column in hopper (m)

H _h :

height of grain column in hopper (m)

ΔHfgw :

latent heat of pure water (kJ/kg)

ΔHfg :

latent heat of evaporation of grains (kJ/kg)

I _rad :

solar irradiation (W/m²)

k :

the drying constant (1/hr)

L _p :

pipe length (m)

L _pc :

pneumatic conveyor pipe length (m)

L _d :

length of drying chamber solar collector (m)

M :

moisture content (% db)

$\overline{M}$ :

average moisture content (% wb)

m _pf :

final mass of grains (kg)

m _pi :

initial mass of grains (kg)

Me :

the equilibrium moisture content (% db)

P :

pressure (N/m²)

Po :

pressure at conveyor inlet (N/m²)

P _L :

pressure at conveyor outlet (N/m²)

Pw :

power of blower (W)

Qa :

volumetric flow rate of air (m³/s)

r :

radius of grains modeled as spherical body (m)

R :

coefficient of Eq. (15)

R ₁ :

coefficient of Eq. (16)

R _h :

relative humidity (%)

S _E :

specific energy (MJ/kg of water evaporated)

t :

time (h)

t _D :

drying time (h)

t _T :

tempering time (h.)

t _D :

travel time of grains within solar collector drying chamber (h.)

tpc :

travel time of grains within pneumatic conveyor (h.)

Ta :

temperature of the air (°C)

T _g :

temperature of the grains (°C)

Ua :

overall heat transmission from the shell (w/m² °C)

Uv :

volumetric heat transmission from the shell (W/m³ °C)

Vo :

superficial velocity of the air (m/s)

Vp :

velocity of rough rice (m/h)

W :

initial mass of the grains (kg)

Wg :

consumption of LPG (kg)

Wa :

mass flow rate of air (kg/s)

Wp :

mass flow rate of rough rice (kg/s)

Xi :

initial moisture content of the grains (% wb)

Xf :

final moisture content of the grains (% wb)

y :

distance traveled by grain from pneumatic conveyor inlet (m)

1.1 Greek Letters

ε:

void ratio (−)

γ:

ratio between mass flow rate of air and that of rough rice (−)

μ:

absolute viscosity of the air (kg/m s)

ρ_p :

bulk density of the grains (kg/m³ )

ρ_a :

density of the air (kg/m³)

η_D :

drying efficiency (−)

1.2 Subscript

a:

air

D:

collector drying chamber

i:

inlet, initial

o:

outlet

p:

rough rice, pipe

p_c :

pneumatic conveyor

T:

tempering