Law, Mujumdar - Energy Savings in Drying of FVF
these two types of moisture is dependent on different mechanism. Hence a technique
that is efficient in removing surface moisture might not be suitable for removing internal
moisture.
Conventional dryers tend to apply constant operating conditions where operating
temperature and hot air flowrate are kept constant throughout the entire drying process.
High temperature and fast flowrate may enhance the rate of moisture removal, however,
these two parameters do not play an important role in removing internal moisture espe-
cially toward the end of drying process. Keeping the operating temperature and flowrate
as high as the initial stage is merely a waste of energy, the internal moisture transport is
governed by internal diffusion. In this case, external temperature and air flowrate does
not promote the diffusion of internal moisture.
Recent developments in drying technology has revealed that intermittent drying,
pulsating drying, multi-variable strategy and etc. can be applied to save energy. Inter-
mittent drying enables active drying to be conducted periodically where the process is
idle in between two active drying. During inactive drying, internal moisture slowly mi-
grates from the internal to the surface of the drying material. The subsequent active dry-
ing remove higher amount of surface moisture which in turn enhances the drying rate
Moreover, the operating cost is reduced significantly as drying is idle in intervals which
save noticeable amount of energy.
7.3. ENERGY EFFICIENCY
Specific energy consumption Es, which is normally expressed in GJ/t water evapo-
rated, is the measure commonly used to quantify the thermal efficiency of a dryer.
There are a number of energy efficiency indexes to measure the energy performance
of a dryer. Kudra (2004) pointed out that there are inconsistencies in the definition of
various terms that are used to measure energy efficiency, drying efficiency and etc.
among users and researchers. This chapter adopts the definitions given by Kudra (2004).
Energy efficiency (η) measures the percentage of total input that is used for the eva-
poration of moisture.
휂=퐸퐸푒푣
푖푛
(7.1)Here Eev is the energy that is used for moisture evaporation and Ein is the total energy
supplied to the dryer.
If heat capacity of air is constant, e.g. low humidity and low temperature convective dry-
ing, energy efficiency can be approximated by thermal efficiency (ηT).
휂=푇푇^11 −−푇푇^20 (7.2)
Here T 1 is the inlet temperature, T 2 is the outlet temperature and To is the ambient tem-
perature.
Since the outlet temperature, T 2 is impossible to reach the ambient temperature, To;
thus the thermal efficiency according to eq (2) will never be 100%! Therefore, to im-