1. 7TH SEMESTER ,MECHANICAL ENGINEERING BRANCH
WASTE HEAT RECOVERY
DEBASHREE SENGUPTA
7TH SEMESTER ,MECHANICAL ENGINEERING BRANCH

2. TOPICS OF DISCUSSION 1.INTRODUCTION 2.TYPE OF WASTE HEAT RECOVERY
3.WASTE HEAT RECOVERY DEVICES
4.ASSESMENT OF WASTE HEAT RECOVERY
5.CONCLUSION

3. INTRODUCTION WASTE HEAT
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 thus loss is minimized

4. WASTE HEAT SOURCE AND QUALITY
S. No
Source of Waste Heat
Quality of Waste Heat 1
Heat in flue gases
The higher the temperature, the greater the potential value for heat recovery 2
Heat in vapour streams
As above but when condensed, latent heat also recoverable 3
Convective & radiant heat lost from exterior of equipment
Low grade – if collected may be used for space heating or air preheats 4
Heat losses in cooling water
Low grade – useful gains if heat is exchanged with incoming fresh water 5
Heat losses in providing chilled water or in the disposal of chilled water
High grade if it can be utilized to reduce demand for refrigeration
Low grade if refrigeration unit used as a form of Heat pump 6
Heat stored in products leaving the process
Quality depends upon temperature 7
Heat in gaseous & liquid effluents leaving process
Poor if heavily contaminated & thus requiring alloy heat exchanger

5. HIGH TEMPERATURE HEAT RECOVERY
Types of Devices
Temperature (0C)
Nickel refining furnace
1370 – 1650
Aluminum refining furnace
650 –760
Zinc refining furnace
760 – 1100
Copper refining furnace
760 – 815
Steel heating furnace
925 – 1050
Copper reverberatory furnace
900 – 1100
Open hearth furnace
650 – 700
Cement kiln (Dry process)
620 – 730
Glass melting furnace
1000 – 1550
Hydrogen plants
650 – 1000
Solid waste incinerators
Fume incinerators
650 – 1450

6. METAL REFINERY FURNACE

7. CEMENT KILN

8. MEDIUM TEMPERATURE HEAT RECOVERY
Types of Devices
Temperature (0C)
Steam boiler exhaust
230 – 480
Gas turbine exhaust
370 – 540
Reciprocating engine exhaust
315 – 600
Reciprocating engine exhaust (turbo charged)
230 – 370
Heat treatment furnace
425 – 650
Drying & baking ovens
230 – 600
Catalytic crackers
Annealing furnace cooling systems

9. GAS TURBINE PLANT

10. CATALYTIC CRACKER

11. STEAM BOILER PLANT

12. LOW TEMPERATURE HEAT RECOVERY
Source
Temperature 0C
Process steam condensate
55-88
Cooling water from: Furnace doors
32-55
Bearings
32-88
Welding machines
Injection molding machines
Annealing furnaces
66-230
Forming dies
27-88
Air compressors
27-50
Pumps
Internal combustion engines
66-120
Air conditioning and refrigeration condensers
32–43
Liquid still condensers
Drying, baking and curing ovens
93-230
Hot processed liquids
32-232
Hot processed solids
93-232

13. OPEN HEARTH FURNACE

14. Commercial waste heat recovery Devices-
RECUPERATORS-metallic radiation, convective, radiation/ convective hybrid, ceramic
REGENARETORS
HEAT WHEELS
HEAT PIPE
ECONOMIZER
PLATE HEAT EXCHANGER

15. RECUPERATOR CERAMIC RECUPERATOR Heat exchange between flue gases
and air through
metallic/ceramic walls
Ducts/tubes carry
combustion air for
preheating
CERAMIC RECUPERATOR
Less temperature limitations:
Operation on gas side up to 1550 ◦C
Operation on preheated air side to 815 ◦C
New designs
Air preheat temperatures <700◦ C
Lower leakage rates
Inlet air from atmosphere
Outside ducting
Tune plate
Preheated air
Centre tube plate
Exhaust gas from process

16. METALLIC RADIATION RECUPERATORS
Simplest recuperator
Two metal tubes
Less fuel is burned
per furnace load
Heat transfer
mostly by radiation

17. CONVECTIVE AND HYBRID RECUPERATORS
CONVECTIVE RECUPERATORS
The hot gases are carried through a number of parallel small diameter tubes, while the incoming air that is to be heated enters a shell surrounding the tubes and passes over the hot tubes one or more times in a direction normal to their axes
HYBRID RECUPERATORS
Combinations of radiation & convection
More effective heat transfer
More expensive but less bulky than simple metallic radiation recuperators

18. RECUPERATOR

19. CONVECTIVE RECUPERATOR

20. REGENERATORS AND HEAT WHEELS
Large capacities
Glass and steel melting furnaces
Heat transfer in old regenerators reduced by
Dust & slagging on surfaces
heat losses from the walls
HEAT WHEELS
Porous disk rotating between two side-by-side ducts
Low to medium temperature waste heat recovery systems
Heat transfer efficiency up to 85 %

21. HEAT PIPE PERFORMANCE AND ADVANTAGE Transfer up to 100 times more
thermal energy than copper
Three elements:
- sealed container - capillary wick structure
- working fluid
Works with evaporation
and condensation
PERFORMANCE AND ADVANTAGE
Lightweight and compact
No need for mechanical maintenance,
input power, cooling water and lubrication
systems
Lowers the fan horsepower requirement
and increases the overall thermal efficiency
of the system
Can operate at 315 ◦C with 60% to 80%
heat recovery
Process to space heating- Transfers thermal energy from process exhaust for building heating

22. PLATE HEAT EXCHANGER Run around coil exchanger
Parallel plates forming a thin flow pass
Avoids high cost of heat exchange surfaces
Corrugated plates to improve heat transfer
When directions of hot and cold fluids are
opposite, the arrangement is counter current
Run around coil exchanger
Heat transfer from hot to colder fluid via heat transfer fluid
One coil in hot stream
One coil in cold stream

23. PLATE HEAT EXCHANGER

24. WATER TUBE BOILER
Water tube boiler: hot exhaust gases
pass over parallel tubes with water
Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from gas turbines, incinerators, etc pass over a number of parallel tubes that contain water. The water is vaporized in the tubes and collected in a steam drum from which it is drawn off for use as heating or processing steam.

25. ECONOMIZER Used when the medium containing SHELL AND TUBE ECONOMIZERS
Utilize the flue gas heat for pre-heating
the boiler feed water
1% fuel savings if
60 ◦C rise of feed water
200 ◦C rise in combustion air temp
SHELL AND TUBE ECONOMIZERS
Used when the medium containing
waste heat is a liquid or a vapor that
heats another liquid
Shell contains the tube bundle,
and usually internal baffles to
direct the fluid
Vapor contained within the shell

27. ASSESMENT OF WASTE HEAT RECOVERY
Saving money by recovering heat from hot waste water:
Discharge of the waste water is kg/hr at 75◦C
Preheat kg/hr of cold inlet water of 20◦C
A heat recovery factor of 58%
An operation of 5000 hours per year
The annual heat saving (Q) is:
Q = heat content in kCal
V = the flow rate of the substance in m3/hr
 = density of the flue gas in kg/m3
Cp = the specific heat of the substance in kCal/kg oC
T = the temperature difference in oC
Cp (Specific heat of flue gas) = 0.24 kCal/kg/oC
Q = V x  x Cp x T
Q = m x Cp x T x 

28. HEAT SAVING CALCULATION EXAMPLE
m = kg/hr = x 5000 kg/yr = kg/year
Cp = 1 kCal/kg ◦C
T = (75 – 20) ◦C = 55 ◦C
 = Heat Recovery Factor = 58% or 0.58
CV of Oil = 10,200 kCal/kg
Equivalent Oil Savings = / = L
Cost of Oil = USD/L
Monetary Savings = USD/Annum
 Q = x 1 x 55 x 0.58
= kCal/year

29. CONCLUSION Benefits of Waste Heat Recovery
Benefits of ‘waste heat recovery’ can be broadly classified in two categories:
Direct Benefits:
Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected by reduction in the utility consumption & costs, and process cost.
Indirect Benefits:
a) Reduction in pollution: A number of toxic combustible wastes such as carbon monoxide gas, sour gas, carbon black off gases, oil sludge, Acrylonitrile and other plastic chemicals etc, releasing to atmosphere if/when burnt in the incinerators serves dual purpose i.e. recovers heat and reduces the environmental pollution levels.
b) Reduction in equipment sizes: Waste heat recovery reduces the fuel consumption, which leads to reduction in the flue gas produced. This results in reduction in equipment sizes of all flue gas handling equipments such as fans, stacks, ducts, burners, etc.
c) Reduction in auxiliary energy consumption: Reduction in equipment sizes gives additional benefits in the form of reduction in auxiliary energy consumption like electricity for fans, pumps etc..

30. THANK YOU