Combustion Air Preheater
The most common way of increasing
heater efficiency and capacity.
Major Components of Air Preheating
System
o Air Preheater
o Flue gas ducts
o Air ducts
o ID Fan
o FD Fan
o Dampers
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AIR PREHEAT SYSTEMS
| Flue gas temperature
leaving convection
| Flue gas temperature
leaving stack
| Combustion air
temperature entering
APH
| Combustion air
temperature leaving
APH
TYPES OF AIR PREHEATERS Flue gas
duct
y Recuperative
|Cast Iron
|Plate
y Regenerative
y Indirect APH
Hot air
duct
RECUPERATIVE
AIR PREHEATER
| Separate passages for Flue gas in
flue gas and air
Flue gas out
By-Cast Air Preheater
CAST IRON AIR PREHEATER
| Fins are
provided on
both sides of
tubes in -HT
tubes
| Fins are
provided on
one side (
flue gas) to
control tube
metal
temperature
-LTL tubes
By-Cast Air Preheater
PLATE TYPE AIR
PREHEATER
| Compact construction
| Light weight
| No leakage
| Easy maintenance
| Wide variety of
configurations
Courtesy
NAT
Rotary Regenerative Air Preheater
o Metallic elements
o Regenerative
o Rotary
Courtesy ABB Air Preheater
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ROTARY REGENERATIVE AIR PREHEATER
| Metallic elements rotate in a
casing.
| Hot flue gas flow on one side
and heats the elements while
air to be heated flows on the
other side.
| Cylinder rotates and heat is
transferred from heated
elements to air.
Courtesy ABB Air Preheater
ROTARY AIR
Rotary air
PREHEATER
preheater
ASSEMBLY
Courtesy ABB Air preheater
Air Preheaters
o Plate type air preheaters installation
on coker heaters
Plate air
preheaters
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INDIRECT AIR PREHEATERS
| Separate tube banks in
the flue gas and
combustion air stream.
Heat Recovery
Stack
Unit
Process In
| Connected together by
piping and a heat Process
Out
transfer fluid flowing
through the exchanger.
| Fluid absorbs heat
Burner
from the flue gases and Hot Oil Air
gives it to the
Tank
combustion air.
Hot Oil
ID Fan Pump
AIR PREHEATER OPERATION
| Air doors in the duct
open automatically
providing fresh air.
| Natural draft
operation
INDUCED DRAFT FAN
| A single fan is
sufficient.
| If fan fails, stack
damper opens and
heater reverts to
natural draft
operation.
FORCED DRAFT FAN
| Key element in the
design of APH system.
| Fan failure results in
air supply failure
| Back up is to provide
natural draft
operation at full/
reduced loads.
| Alternative is to
provide two 50% fans.
FAN CONTROL
| Fan performance is
generally controlled
by two major ways:
y Damper Control
| Inlet
| Outlet
y Speed Control
| Variable frequency
| Hydraulic coupling
DAMPERS IN FIRED HEATER AND APH
SYSTEM
| Dampers are needed
in the
y Forced draft system
y Induced draft system
y Stack
y Combustion air bypass
y Emergency air doors
y Burners
Combustion Air
o APH Bypass Damper
o Control flue gas temperature leaving APH
o Multi-blade louver damper
o Emergency Air Doors
o Low leakage dampers
o Quick response
o Isolation
o Burners
o Multi-blade low leakage damper to control and isolate
air flow
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Stack
o Multi-blade louver damper
o Quick response
o Tight shutoff
o Control during natural
draft operation
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FANS
| FD Fan
y Inlet
| Radial vane damper or inlet box
damper
y Outlet
| Guillotine blind for isolation for
personnel safety
| Multi-louver damper for control of
air ( balancing)
| ID fan
y Inlet
| Box damper for control
| Guillotine for personnel safety
y Outlet
| Guillotine for personnel safety
COLD END CORROSION
| Cold air bypass
| Cold air preheating
| Hot air recirculation
| Use of low alloy
corrosion resistant
steel
| Use of ceramic coated
heat transfer surface
| Use of special
tubes/glass tubes
Air Preheater Sizing
o Heat duty
o Pressure drop
o Air
o Flue gas
o Air leakage
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Quick Calculations
Check your APH Performance
o Flue gas specific heat (avg.) = 0.28
o Combustion air specific heat (avg.) = 0.24
o Ratio of the flue gas /air sp heat = 1.15
o Ratio of flue gas/ air mass flow rate = 1.05
o Ratio of flue gas to air temp diff = 1.2
o Check your flue gas temperature drop
o Check your air preheat temperature rise
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Effect of APH on Fired Heater
Performance
o Heat duty split
o Radiant heat duty goes up
o Radiant heat flux
o Increase
o Tube metal temperature
o Increase
o Film temperature
o Increase
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