Energy and Exergy analyses of the Drying Process of Ghost Chilli and Sliced Ginger
6.3 Energy Analysis
The energy analysis of the drying system was performed on the basis of the general mass and energy conservation equations. The data obtained from the experiments were used to perform the energy and exergy analyses. The basic data requirements are the temperature of the air
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streams at different locations, the mass flow rate of air, the energy inflows and outflows of the drying system, and the mass of moisture removed from the product. The energy inflows of the drying system are the solar and electrical energy. The energy outflows of the drying system are heat losses from the solar air heaters, heat losses at different sections of the connecting ducts, heat losses from the walls of the drying chamber, and heat loss in the exhaust of the drying chamber. During the energy analysis, the flow in the drying system was assumed to be steady.
The general mass and energy conservation equations for the steady flow open system can be expressed as follows (Cengel and Boles, 2010).
i o
m m
(6.1)2 2
( ) ( )
2 2
o i
o o o i i i
v v
Q W
m h Z g
m h Z g (6.2)where 𝑚̇𝑖, 𝑚̇𝑜 = mass flow rate of air at the inlet and outlet, respectively ℎ𝑖, ℎ𝑜 = enthalpy of air at the inlet and outlet, respectively
𝑣𝑖, 𝑣𝑜 = velocity of air at the inlet and outlet, respectively
𝑍𝑖, 𝑍𝑜 = height of the inlet and outlet from reference plane, respectively 𝑄̇ = heat transfer rate
𝑊 ̇= work transfer rate
𝑔 = acceleration due to gravity
In the drying chamber, the effects of the kinetic and potential energy were neglected for the following reasons.
The dryer was horizontal,
The inlet and outlet of the solar air heaters were located at the same height from the ground, and
The change in velocity between the inlet and the exit was negligible.
Therefore, the mass and energy conservation equations for the SAH can be expressed by Eqs.
(6.3)‒(6.5).
( )
in ls a o i
Q Q m h h (6.3)
The heat input (𝑄̇𝑖𝑛) to the solar air heater is given by Eq. (6.4)
in a c SAH
Q IA (6.4)
i o a
m m m (6.5)
where 𝑄̇𝑙𝑠 is the heat loss in the solar air heater, 𝑚̇𝑎 is the mass flow rate of air, ℎ𝑖 and ℎ𝑜 are the specific enthalpies at the inlet and outlet of the solar air heater, respectively.
The right hand term of Eq. (6.3) is known as the useful heat gained by the SAH, and it can be obtained by measuring the inlet and outlet temperatures of the air heaters and the mass flow rate of air. The useful heat gained of the SAH – 1 can be evaluated by Eq. (6.6) (Midilli and Kucuk, 2003; Celma and Cuadros, 2009).
1 ( 1 1)
uSAH a pa oSAH iSAH
Q m C T T (6.6)
where 𝑇𝑖𝑆𝐴𝐻−1 and 𝑇𝑜𝑆𝐴𝐻−1 are the inlet and outlet air temperatures of the first solar air heater, respectively, and 𝐶𝑝𝑎 is the specific heat of air.
The instantaneous first law efficiency of the SAH is defined here as the ratio of the useful heat gained to the solar radiation incident on the absorber surface of the heater. Although, it is generally defined as the ratio of the useful heat gained to the solar radiation incident on the air heater plane (Benli, 2013; Bahrehmand and Ameri, 2015, Omojaro and Aldabbagh, 2010).
Therefore, it is expressed by Eq. (6.7) for the SAH - 1.
1 1
1
1
( )
a pa oSAH iSAH
SAH
a c SAH
m C T T
IA
(6.7)
The heat losses from the SAH - 1 can be calculated by Eq. (6.8).
1 1 1
lsSAH a c SAH uSAH
Q IA Q (6.8)
Similarly, the useful heat gain, the instantaneous thermal efficiency, and the heat losses of the SAH - 2 can be expressed by Eqs. (6.9), (6.10) and (6.11), respectively.
2 ( 2 2)
uSAH a pa oSAH iSAH
Q m C T T (6.9)
2 2
2
2
( )
a pa oSAH iSAH
SAH
a c SAH
m C T T
IA
(6.10)
2 2 2
lsSAH a c SAH uSAH
Q
IA Q (6.11)115
where 𝐴𝑆𝐴𝐻−1 and 𝐴𝑆𝐴𝐻−2 are the areas of the SAH - 1 and SAH - 2, respectively, 𝑇𝑖𝑆𝐴𝐻−2 and 𝑇𝑜𝑆𝐴𝐻−2 are the inlet and outlet air temperatures of the SAH - 2, respectively.
The size of the two air heaters were the same, and the inlet temperature of the SAH - 1 was assumed to be equal to the ambient temperature and therefore,
1 2
SAH SAH SAH
A
A
A
(6.12)And
T
iSAH1 T
amb (6.13)The overall efficiency of the SAHs was determined by Eq. (6.14).
2 1
1 2
( )
( )
a pa oSAH iSAH
SAH
SAH SAH a c
m C T T
A A Iα
(6.14)
The specific energy consumption (SEC) is defined as the ratio of the total energy input to the drying system to the total moisture removal from the product. It was computed by Eq. (6.15) reported by Fudholi et al., 2014a.
t v
SEC p
m (6.15)
The total energy input (𝑝𝑡) is the sum of the solar energy incident on the surface of the solar air heaters and the electrical energy consumed by the blower. It was calculated by the following equation.
( 1 2)
t SAH SAH bl d
p A A Ip t (6.16)
where 𝑝𝑏𝑙 is the power consumed by the blower.
The total mass of the moisture (𝑚𝑣) evaporated from the products during the drying period (𝑡𝑑) was calculated as follows (Ayensu and Asiedu-bondzie, 1986).
( )
100
p i f
v
f
m M M
m M
(6.17)
Where, Mi and Mf are the initial and the final moisture contents of the product, respectively.
The thermal efficiency which is defined as the ratio of the actual temperature drop to the maximum possible temperature drop of the drying air in the drying chamber was evaluated by the following equation (Kudra, 2004; Saravacos and Maroulis, 2011).
id od
dc
id amb
T T T T
(6.18)
where 𝑇𝑖𝑑 and 𝑇𝑜𝑑 are the inlet and outlet air temperatures of the drying chamber, respectively and 𝑇𝑎𝑚𝑏 is the ambient temperature.
The overall thermal efficiency of the drying system is defined as the ratio of the energy required to evaporate the moisture from the product to the total energy input into the drying system. It is expressed as follows.
v fg ds
t
m h
p (6.19)