Law, Mujumdar - Energy Savings in Drying of FVFprove the thermal efficiency, one needs to recover the waste heat in the exhaust air to
further reduce the outlet air temperature before it is discharged into the atmosphere.
Since drying is not a steady state process, outlet air temperature, amount of mois-
ture removed vary at different drying time; thus energy efficiency varies throughout the
entire drying process. In this regard, instantaneous energy efficiency (given in eq 3) can
be measured at a particular drying time and the average efficiency can be calculated
based on the instantaneous energy efficiency from equation (4). Energy efficiency (eq 1
and 2) is useful when comparing the efficiency of different types of dryer whereas in-
stantaneous energy efficiency is useful when analyzing a drying process or a dryer con-
figuration.
Instantaneous energy efficiency, εE
휀퐸=퐸푛푒푟푔푦^ 푟푒푞푢푖푟푒푑푖푛푝푢푡 푒푛푒푟푔푦^ 푓표푟^ 푒푣푎푝표푟푎푡푖표푛 푎푡 푡푖푚푒 푡^ 푎푡^ 푡푖푚푒^ 푡 (7.3)
Average energy efficiency within time = 0 until time t
휀��퐸�= 1 푡∫ 0 푡휀퐸(푡)푑푡 (7.4)
Drying efficiency on the other hand evaluate the percentage of energy transferred to the
dryer for the evaporation of moisture. It is defined as:
휀퐷=퐸푛푒푟푔푦(푖푛푝푢푡 푒푛푒푟푔푦푟푒푞푢푖푟푒푑 −푒푥^ 푓표푟ℎ푎푢푠푡^ 푒푣푎푝표푟푎푡푖표푛 푒푛푒푟푔푦) 푎푡^ 푎푡 푡푖푚푒푡푖푚푒 푡푡 (7.5)
휀��퐷�=^1 푡∫ 0 푡휀퐷(푡)푑푡 (7.6)
significant with reference to the input energy, drying efficiency is not a good indicator to
reflect the energy efficiency; simply because the energy efficiency is low although the
drying efficiency might be high. As such, both energy efficiency and drying efficiency
may provide insight on energy aspect and efficiency aspect of a drying system. Table 7. 1
shows the various scenarios with reference to energy efficiency and drying efficiency
variations and area for improvement with regard to the respective scenario.
Table 7. 1. Area for improvement with reference to low energy efficiency and low drying
efficiency
Energy efficiency Drying efficiency Area for improvementLow Low Loss of energy to the atmosphere. May con-
sider insulation to improve drying efficiency.Low High Loss of energy in exhaust air. May consider
to recover waste heat7.4. METHODS TO IMPROVE ENERGY EFFICIENCY
Over the past few decades, tremendous developments and improvements in drying
technology have been reported in the literature. Readers may refer to review papers on
the latest development and advancement in this area (Mujumdar & Pasos, 2000; Mu-
jumdar, 2001; Mujumdar, 2006; Mujumdar, 2007; Law et al., 2008). Suffice it to say that