Download waste heat - Allied Alloy Products

W ASTE H EAT
Waste Heat" may be defined as "Heat that is rejected from a process, at a temperature
enough above the ambient temperature to permit the plant engineer to recover
additional value from it".
Waste Heat Energy may broadly be categorised in following three ranges (In Degrees
Fahrenheit)
I.
High Temperature Range
Above 1200
II.
Medium Temperature Range 450 to 1200
III.
Low Temperature Range
Below 450
SOURCES OF WASTE HEAT:The combustion of fuel produces flue gases in the high temperature range. The
maximum theoretical temperature that we come across usually is 3000°F. Hereunder
temperature of waste gases from a few of industrial processes is being given:
 Glass Melting Furnace
1800°F to 2800°F
 Steel Heating Furnace
1700°F to 1900°F
 Cement Kilns (Dry Process) 1250°F to 1350°F
 Heat Treatment Furnaces 800°F to 1200°F
 Gas Turbine Flue Gases
700°F to 1000°F
 Diesel Engine Exhaust
600°F to 1100°F
 Drying & Baking ovens
450°F to 1200°F
 Steam Boiler ovens
450°F to 900°F
 Hot Processed Solids
200°F to 400°F
 Process Steam Condensate 130°F to 190°F
 Air Compressors
80°F to 120°F
HOW TO USE WASTE HEAT?
The most appropriate means of utilising waste heat is by transferring the heat from one
fluid stream to another (e.g. from flue gases to feed-water or combustion air), that is
why the device that transfers the heat is termed as "Heat Exchanger".
In few cases, exchangers are given a particular name to enable them to identify their
specific application, such as Economiser, Recuperate, Air-Pre heater etc.
Just for reference, high temperature gases from glass melting furnace can be used to
generate steam in Waste Heat Boilers. This steam can be used for running a Back
Pressure Steam Turbine to generate electricity. Exhaust steam from this can be used
for process heating. Medium temperature exhaust gases can be used to preheat the
feed or boiler make up water by using Economisers or to preheat the combustion air.
Still further, low temperature gases may profitably be used for preheating feed stocks
in industrial processes by using finned tubes or "Tube-in-Shell" heat exchangers or may
be used for space heating.
Waste Heat
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HEAT RECOVERY SYSTEMS: PRINCIPLES: Economy is a crucial factor in designing any Steam Generator or Heat Recovery
System. Like any other functioning entity, such systems also operate under the law of
Diminishing Returns.
Temperature of flue gases drops while heating the surface of boiler which in turn
absorbs the heat. A level soon arrives beyond which it becomes uneconomical to
absorb heat through this heating surface at a saturation temperature. This is the limit
from where an Economiser or heater must start absorbing the heat.
Operation Engineers should never forget the fact that, "there always is most
economical temperature for flue gases above & below which steam production costs
tend to increase". Any superfluous reduction in flue gas temperature would eventually
increase the capital investment and other charges so much that these may offset any
possible gain in efficiency. This stresses the necessity of selecting or designing any
Heat Recovery Equipment very carefully. Otherwise any such addition may actually
reduce the effectiveness of the already available heating surface.
For smaller, low pressure boilers, an Economiser is a more economical proposition as
compared to Air Heater. For outputs more than 50,000 Lbs steam/hr however an Air
Heater becomes a batter choice.
In nutshell, the temperature to which flue gas can be cooled while passing through
equipment is determined by the following facts: 1.
2.
3.
4.
Amount of heat than can be absorbed by such equipment
Temperature of incoming feed water or combustion air.
Dew point of flue gases.
Exist temperature below which any gain is off-set by costly equipment.
APPLICATION: The arrangement of heat recovery systems must consider economy of design as well as
accessibility for their operation and maintenance.
Waste Heat
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SPECIFICATION OF WASTE HEAT RECOVERY EQUIPMENT: The equipment that is used to recover waste heat can range from something as simple
as pipe or duct to something as complex as a Waste Heat Boiler. The authentic Data
prepares ground for the design of equipment having optimum effectiveness and low
operational & maintenance cost.
 Temperature of Waste Heat Fluid
_____________________°C
 Flow rate of Waste Heat Fluid
__________________ M³/Hr
 Chemical Composition of Waste Heat Fluid
_______________________
 Name of Waste Heat Fluid
_______________________
 Acceptable Pressure Drop of Waste Recovery Stream
_______________________
 Acceptable Temperature Drop of Waste Recovery Stream_____________________°C
 Temperature of Fluid to be Heated
_____________________°C
 Flow Rate of Fluid to be Heated
__________________ M³/Hr
 Chemical Composition of Fluid to be Heated
_______________________
 Name of the Fluid to be Heated
_______________________
 Acceptable Pressure Drop on Fluid to be Heated side
________________ Kg/Cm²
 Acceptable Temperature rise of Fluid to be Heated
_____________________°C
 Waste Heat Available
_______________Hours/Day
_______________Days/Year
_____________Months/Year
 Others
_______________________
_______________________
Waste Heat
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