Law, Mujumdar - Energy Savings in Drying of FVF
the developments and advancements which are related to energy efficiency of industrial
drying can be classified into the subtopics given in Figure 7. 2.
Figure 7.2. Methods to improve energy efficiency of a dryer7.4.1. Heat Recovery
Recovering waste heat in the exhaust air can be realized by heat recovery or heat
Recovering waste heat in the exhaust air can be realized by heat recovery or heat inte-
gration. Heat recovery is simpler as it only involves heat exchanger for heat transfer be-
tween the exhaust air and the inlet air; or a mixing T and purging system for recycling
exhaust air directly with the inlet air.
Both direct and indirect heat recycle methods enable the recovery of part of the en-
thalpy of the hot and humid exhaust air to maximize the energy efficiency. It is especially
important for drying processes that are operated at high air velocity and have short con-
tact time between drying medium and drying material. This is due to the fact that the
contacting efficiency is generally low.
Heat integration is typically complex and therefore may increase the complexity of
start-up and shutdown operations; it also limits the flexibility of the process. Generally
heat integration gives higher energy efficiency as compared to conventional heat recov-
ery methods. Kemp (2005) gave a few examples of using heat integration to maximize
energy efficiency and commented that this technique is especially useful for minimizing
energy consumption of a drying operation in sites that have combined heat and power
(CHP) system. Further, the effectiveness of heat integration is affected by initial product
moisture content which is dependent on the upstream processes rather than the dryer
itself. Therefore, the entire process should be looked into when heat integration is per-
formed.
With reference to heat recovery, there are a few factors that one needs to consider:- Costs associated with recycle ducting and reduction in driving potential
- Problems of condensation if the exhaust air is near to the saturation point
- Dust in the exhaust air when it is directly recycled may cause surface abrasion,
and pose dust explosion hazard
Heat pump assisted drying recovers latent heat and thus gives higher energy effi-
ciency. Details of heat pump dryer can be obtained elsewhere in this book and refer-
ences mentioned in the introduction of this chapter. It is operated in a closed system