2.7 Waste Heat Recovery.pdf

PREPARED BY K. MANOKARAN, LEAD AUDITOR, DNVPREPARED BY K. MANOKARAN, LEAD AUDITOR, DNV Page: 1

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IDENTIFY YOUR WASTE, SAVE ON ENERGY BILLS!


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What is waste heat? Waste heat is heat, which is Waste heat is heat, which is

generated in a process by generated in a process by way of fuel combustion or way of fuel combustion or chemical reaction, and then chemical reaction, and then dumpeddumped into the into the environment even though it environment even though it could still be reused for could still be reused for some useful and economic some useful and economic purposepurpose..


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BoilersBoilers KilnsKilns OvensOvens FurnacesFurnaces DG SetsDG Sets Thermic Fluid HeatersThermic Fluid Heaters Low temperature process streamsLow temperature process streams

! Quality

Heat Loss

Quantity


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Low grade useful gains if heat is exchanged with incoming fresh water.

Heat losses in cooling water.

Low grade if collected may be used for space heating or air preheats.

Convective and radiant heat lost from exterior of equipment

As above but when condensed, latent heat also recoverable.

Heat in vapourstreams.

The higher the temperature, the greater the potential value for heat recovery

Heat in flue gases.

QualitySource


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QualitySource

Poor if heavily contaminated and thus requiring alloy heat exchanger.

Heat in gaseous and liquid effluents leaving process.

Quality depends upon temperature.

Heat stored in products leaving the process

a) High grade if it can be utilized to reduce demand for refrigeration.b) Low grade if refrigeration unit used as a form of heat pump.

Heat losses in providing chilled water or in the disposal of chilled water.


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1000-1550Glass melting furnace

620- 730Cement kiln (Dry process)

900-1100Copper reverberatoryfurnace

760-1100Zinc refining furnace

650-760Aluminium refining furnace

1370 1650Nickel refining furnace

Temperature, oCTypes of Device


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Temperature, oCTypes of Device

425 - 650Annealing furnace cooling systems

425 - 650Catalytic crackers

425 - 650Heat treating furnaces

315-600Reciprocating engine exhausts

370-540Gas turbine exhausts

230-480Steam boiler exhausts


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3243Air conditioning and

refrigeration condensers

27-50Air compressors

66-230Annealing furnaces32-55

Cooling water from:Furnace doors

55-88Process steam condensate

Temperature, oCSource


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Direct benefits

Indirect benefits

Reduction in Pollution

Reduction in equipmentsizes

Reduction in auxiliaryEnergy consumption


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Understand the process

Sources and uses of waste heat Upset conditions occurring in the

plant due to heat recovery Availability of space Any other constraint, such as dew

point occurring in an equipment, etc.

Economic evaluation of WHR System


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Recuperator Regenerator Heat Wheel Heat Pipe Economiser Shell and tube heat exchanger Waste heat boiler Heat Pump Thermocompressor Direct contact heat exchanger


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In a recuperator, heat exchange takes place between the flue gases and the air through metallic or ceramic walls. Duct or tubes carry the air for combustion to be pre-heated, the other side contains the waste heat stream.


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The radiation recuperator gets its name from the fact that a substantial portion of the heat transfer from the hot gases to the surface of the inner tube takes place by radiative heat transfer

Figure 8.2 Metallic Radiation Recuperator


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The hot gases are carried through a number of parallel small diameter tubes, while the incoming air 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.


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For maximum effectiveness of heat transfer, combinations of radiation and convective designs are used, with thehigh-temperature radiation recuperator being first followed by convection type.


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In order to overcome the temperature limitation of metallic recuperators which is about 1000oC on the gas side, ceramic tube recuperators have been developed.

The materials of ceramic recuperators allow operation on the gas side up to 1300 oC and on the preheated air side up to 850 oC.


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The Regeneration which is preferable for large capacities has been very widely used in glass and steel melting furnaces.Important relations exist between the size of the regenerator, time between reversals, thickness of brick, conductivity of brick and heat storage ratio of the brick.


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Widely used in low to medium temperature wasteheat recovery systems.A disk rotatesbetween two side-by-sideducts: one a cold gas duct, the other a hot gas duct.As the disk slowly rotates, sensible heat (moisture that contains latent heat) is transferred to the disk by the hot air and, as the disk rotates, from the disk to the cold air.


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Heat pipe is a thermal energy absorbing and transferring system and have no moving parts and hence require minimum maintenance.

wall


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Widely used in a boiler system

1. To pre-heat the boiler feed water.

2. To pre-heat the combustion air.

By utilizing the heat in the flue gas.


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When the medium containing waste heat is a liquid or a vapor which heats another liquid, then the shell and tube heat exchanger must be used since both paths must be sealed to contain the pressures of their respective fluids.


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The cost of heat exchange surfaces is a major factor when the temperature differences are not large. In such case this is used. Hot liquid passing through a bottom port in the head is permitted to pass upwards between every second plate while cold liquid at the top of the head is permitted to pass downwards between the odd plates. When the directions of hot & cold fluids are opposite, the arrangement is described as counter current

Figure 8.10 Plate Heat Exchanger


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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 containing 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


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1. In the evaporator the heat is extracted from the heat source to boil the circulating substance;

2. The circulating substance is compressed by the compressor, raising its pressure and temperature; The low temperature vapor is compressed by a compressor, which requires external work. The work done on the vapor raises its pressure and temperature to a level where its energy becomes available for use

3. The heat is delivered to the condenser; 4. The pressure of the circulating substance

(working fluid) is reduced back to the evaporator condition in the throttling valve, where the cycle repeats.

The heat pump was developed as a space heating system The heat pumps have the ability to upgrade heat to a value more than twice that of the energy consumed by the device


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SUCTION STEAM L.P.

Discharge SteamM.P

MotiveSteamH.P

The thermo-compressor is a simple equipment with a nozzle where HP steam is accelerated into a high velocity fluid. This entrains the LP steam by momentum transfer and then recompresses in a divergent venturi.


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COLD WATER IN

VENT

L.P. STEAM

HOT WATER

Low pressure steam may also be used to preheat the feed water or some other fluid where miscibility is acceptable. This principle is used here.

2.7 Waste Heat Recovery.pdf

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