11-09-2014, 02:51 PM
WASTE HEAT RECOVERY
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Introduction
Waste heat is heat, which is generated in a process by way of fuel combustion or
chemical reaction, and then “dumped” into the environment even though it could still be
reused for some useful and economic purpose. The essential quality of heat is not the
amount but rather its “value”. The strategy of how to recover this heat depends in part on
the temperature of the waste heat gases and the economics involved.
Large quantity of hot flue gases is generated from Boilers, Kilns, Ovens and
Furnaces. If some of this waste heat could be recovered, a considerable amount of
primary fuel could be saved. The energy lost in waste gases cannot be fully recovered.
However, much of the heat could be recovered and loss minimized by adopting following
measures as outlined in this chapter.
Heat Losses –Quality
Depending upon the type of process, waste heat can be rejected at virtually any
temperature from that of chilled cooling water to high temperature waste gases from an
industrial furnace or kiln. Usually higher the temperature, higher the quality and more
cost effective is the heat recovery. In any study of waste heat recovery, it is absolutely
necessary that there should be some use for the recovered heat. Typical examples of use
would be preheating of combustion air, space heating, or pre-heating boiler feed water or
process water. With high temperature heat recovery, a cascade system of waste heat
recovery may be practiced to ensure that the maximum amount of heat is recovered at the
highest potential. An example of this technique of waste heat recovery would be where
the high temperature stage was used for air pre-heating and the low temperature stage
used for process feed water heating or steam raising.
Ceramic Recuperator
The principal limitation on the heat recovery of metal recuperators is the reduced life of the
liner at inlet temperatures exceeding 1100o
C. In order to overcome the temperature limitations of
metal recuperators, ceramic tube recuperators have been developed whose materials allow
operation on the gas side to 1550o
C and on the preheated air side to 815o
C on a more or less
practical basis. Early ceramic recuperators were built of tile and joined with furnace cement, and
thermal cycling caused cracking of joints and rapid deterioration of the tubes. Later
developments introduced various kinds of short silicon carbide tubes which can be joined by
flexible seals located in the air headers.
Earlier designs had experienced leakage rates from 8 to 60 percent. The new designs are
reported to last two years with air preheat temperatures as high as 700o
C, with much lower
leakage rates