Law, Mujumdar - Energy Savings in Drying of FVFwhere humid air is recycled to extract its moisture and thereafter heated up to reduce
its relative humidity. Heat pump dryers generally have high performance (COP ranging
from 5 to 7). Likewise, recycling of exhaust superheated steam in superheated steam
drying can also increase the energy efficiency significantly.
7.4.2. Use of Phase Change Material (PCM)
PCM can be used to store waste heat in the exhaust air. It is a good thermal energy
storage agent which can be placed in exhaust duct of a batch dryer to capture the resi-
due heat of the exhaust air. The PCM then changes from solid to liquid (in molten form)
after receiving thermal energy. The heat stored in the PCM is then recovered later by
passing ambient air over the exchanger with molten PCM. Supplementary heat can be
supplied if the heat transfer from the PCM does not meet the total heat requirement of a
drying system. This idea requires further exploration.
7.4.3. Control of dryer
Good control strategy can be applied in a drying system to reduce energy consump-
tion and reduce operating costs. It has been reported that optimization of grain drying
could reduce the cost significantly by optimally tuning the air flows (Ryniecki & Nellist,
1991a) as well as optimal tuning the heater power (Ryniecki & Nellist, 1991b). In addi-
tion, installation of an on-line optimal controller in a grain dryer was reported to reduce
drying time by 18% which in turn saved 6.4% in fuel consumption, and 1.3% in total
(Mc Farlane & Bruce, 1996). The application of a model based predictive controller in a
beet sugar processing facility could reduce the energy costs by 1.2% (equivalent to
£18,900/year) and decrease the downstream energy cost by £14,000/year (CADDET,
1997 , 2000a, 2000b). Further to this, installation of a PI controller in a rotary dryer has
been reported to decrease energy consumption by 7% compared to manual control
(Iguaz et al., 2002).
Dufour (2006) gave a comprehensive dryer control strategy. There are two types of
control strategies, namely open loop and close loop. Open loop control strategies include
model-based optimal control, genetic algorithm which is a data-based method; whereas
close loop control strategy includes PID control, model-based optimal control, model-
based predictive control, expert system and fuzzy logic control.
Readers may refer various works published elsewhere, for instance the application
of Multiscale Computational Model to reduce energy consumption in wood drying:
(Perre ́ et al., 2007).
7.4.4. Development of mathematical modeling
The development of mathematical modeling of a drying system or a dryer may help a
design engineer to design an energy efficient dryer by simulating the profiles of mois-
ture content, temperature, determination of heat and mass transfer as well as specific
energy consumption. Table 7. 2 shows the recent findings on using mathematical model-
ing to improve energy efficiency of a dryer.