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Fans and air cleaners (2)
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Fan Laws• Fan laws are very useful for finding operating points
of a system (condition 1 → 2)• #1 is starting point; #2 is new point• 3 basic fan laws in terms of rotation rate (RPM)
• We can derive other system relationships from these:
⎟⎟⎠
⎞⎜⎜⎝
⎛=
1
2
1
2
RPMRPM
2
1
2
1
2⎟⎟⎠
⎞⎜⎜⎝
⎛=
RPMRPM
PP
S
S
3
1
2
1
2⎟⎟⎠
⎞⎜⎜⎝
⎛=
RPMRPM
BHPBHP
2
1
2
1
2⎟⎟⎠
⎞⎜⎜⎝
⎛=
PP
S
S
3
1
2
1
2⎟⎟⎠
⎞⎜⎜⎝
⎛=
BHPBHP
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Fan PT and horsepower
• Fan PT is a measure of energy input, so we can use it to estimate horsepower
• Typical eta (η) values for fans are ~0.5 - 0.6
⎟⎟⎠
⎞⎜⎜⎝
⎛⋅⋅
=η6356
, QfanPBHP T efficiency=η
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Examples
• Fan operating at Q=8000 CFM and Ps=2”H20 find Ps and RPM for Q=10,000 CFM
• Estimate BHP assuming η=0.55 at 10,000 CFM for a fan PT of 5” H20
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Examples• Fan operating at Q=8000 CFM and Ps=2”H20 find Ps
and % RPM change for Q=10,000 CFM
• ANS: Ps2=3.125 • RPM2/RPM1=1.25 or 25% increase
• Estimate BHP assuming η=0.55 at 10,000 CFM for a fan PT of 5” H20
• BHP~ 14.3
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• Need height and exit velocity• Goose neck inlets• Intakes in clean area
Stacks and Intakes
Outlet velocity too lowHeight too low
Gooseneck
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Stacks
• If h=building height– Min stack ~ 1.3 h – Needed to avoid wake cavity
• Exit velocity– Recommended ~ 3000 FPM or 1.8*(WS95) where
WS95 is the 95%tile of local wind speed
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Weather Stacks
Drain
Drain
Offset Stack
D+1
D
6"4'
min
1.75D
Offset ElbowsDischarge
4D
1"
Weather caps deflect contaminant down to the roof, have significant losses, and do a poor job of keeping out the rain.
The offset stack should be used whenever possible since it takes the stack off of the fan. Also, open drain hole in fan.
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Recirculation
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Air Cleaners
• Air cleaners often are an important part of the system design– Primary air pollution control device– Stack emissions must meet permit requirements
• Air Cleaners often have significant pressure drop– May be the single biggest loss in the system– Need to know how loss scales with Q– May alter humidity / density
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Reasons for selecting a cleaner
• Toxicity of the material discharged• Amount of material discharged• Value of the material discharged• Abrasive or corrosive material (protect fan!)• Air quality control requirements
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Selecting a cleaner-desirable features
• Clean the air stream to desired levels• Low costs (tco) and min space• Constant cleaning efficiency with changes in:
– Flow rate, age, concentration, etc.• Low down-time for servicing/cleaning• Min disposal problem & low employee hazard
to maintenance workers
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Air cleaner efficiency
• Efficiency defined by contaminant removal• ε = 1 – (mass discharge rate/mass input rate)
if Q is constant⎟⎟⎠
⎞⎜⎜⎝
⎛⋅⋅
−=inletinlet
outletoutlet
CQCQ1ε
inputremoval
CCC
CC
inlet
outletinlet
inlet
outlet =−
=⎟⎟⎠
⎞⎜⎜⎝
⎛−=
)(1ε
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COLLECTORS - SELECTION
• GASES or PARTICLES?– PARTICLE SIZE– RECOVERY NEEDS
• WHAT % REMOVAL?• HIGH TEMPERATURE?• CORROSIVE /SPECIAL AIRSTREAMS?
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Air-Cleaner Losses
Filter loss linear with QDynamic losses Q squared
Cyclone
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Comparison of Air Pollution Control Devices
A Setling ChamberB Simple CycloneC High efficiency cycloneD Electrostatic PrecipitatorE Spray Tower Wet ScrubberF Venturi ScrubberG Bag Filter
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Effectiveness of Air Pollution Control Devices
0.00001 0.0001 0.001 0.01 0.1 1 1 10 100 1000 10,000
Pollutant diameter ( m)
Electrostatic precipitator
Bag filter
Adsorber
Scrubber
Incinerator
Cyclone
Settling chamber
Wet scrubber
μm
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Efficiency of Particulate Control DevicesEfficiency of Particulate Control DevicesDevice Min.size
μm Efficiency(% mass)
Advantages Disadvantages
Gravity chamber
>50 < 50% Low ΔP loss Simple, low cost
Large space needed Low efficiency
Cyclone 5-25 20-90% Compact, med ΔP Simple, low/med. cost
Sensitive to Q High headroom (tall)
Wet collectors Spray tower >10 < 80%
Cyclonic >2.5 < 80% Cross-flow >2.5 < 80%
Venturi >0.5 < 99%
Both Gas & particle removal, cools & cleans high temp gases, works for corrosive gases/mists, low explosion risk
Corrosion, disposal of wastewater, freezing in cold temp., low efficiency for fine particles, visible plume in some conditions
Precipitator >.01 >99% Removes small particles, wet or dry operation, low ΔP, few moving parts, high temp. operation (300-400C)
High initial cost, sensitive to Q and loading, high voltage safeguards needed
Fabric filter <1 >99% Dry collection, decreased performance is noticed, removes small particles
Sensitive to Q, gases must be <450C, affected by condensation & chemical attack
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Air Pollution Control Technologies• Control of Particulate Emission
- Settling- Cyclone separation- Wet scrubbing- Baghouse filtration- Electrostatic precipitation
• Control of Vapor-phase Emissions– Wet scrubbing– Activated carbon adsorption– Incineration
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PARTICLE SIZE VS COLLECTOR CHOICE
SIZE - M METHOD
~ 100 settling
> 1 impact fabric
< 0.5 diffusion
0.01 - 5 electrostatic
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Stokes’ Law for Particle Settling
v gd
Cp
d a=
43
ρρ
dC Q
g L WD a
p=
0 75 2
2 2
. ρρ
Can be used for > 50-100 μm size particles
V = terminal settling velocity
g = gravitational acceleration
ρp= particle density
ρa= air density
Cd= drag coefficient
d = particle diameter
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Controlling Particulates
• Can use several types of treatment– settling chambers
• based on Stokes’ Law• practical lower limit is 50-100 um
– cyclones• lower limit = 10 um
– bag house filter• removes very small particles
– scrubber– electrostatic precipitator
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Horizontal Settling Chamber
• Cut diameter ~ 100 um• Much blows across if no impaction plate• Suitable for product removal, gross dust pre-cleaning
Dust Collection Hoppers
GasInlet
GasOut let
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CYCLONES
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Cyclone cut-off
•Advantages & Disadvantages
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Particulate Control: Cyclones
• can be used for ~5-100 μm size particles• simple economical unit
– no moving parts– relies on inertial effects
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Cyclones Efficiency• dpc is the diameter of the particle collected with 50%
efficiency• The efficiency of collection for particle j with
diameter dpj is:h=1/[1+(dpc /dpj)2]
• The overall efficiency is:h0=Σhjmj
• Hj is the mass fraction of particles in the jth size range
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Standard Cyclone EfficiencyIf dpc is 6.3 um , the cyclone will collect particles of ~32 um with 100 % efficiency; But those of 10 um with only about 60% efficiency; and PM2.5 with about 10% efficiency
Particle collection efficiency vs size for conventional cyclones
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Cyclone
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Multi-Cyclones
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Multiple Cyclones
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Pressure Drop and Costs
• Pressure drop important consideration, increases with with the ratio of inlet to outlet area (~HW/Dentry
2)ΔP ~ proportional to Q2
ε changes with Q• Cyclone costs can be estimated by inlet area
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FILTERS
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Particulate Control: Baghouse Filter
• particle size smaller that 10μm• similar to conventional home
vacuum cleaner• cannot be used for
• wet air systems• corrosive gases• gases above 260oC
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Types and Principles• Vacuum Cleaner Principle• Types: By Direction of Flow; By Cleaning
Mechanism• Advantages: Efficiency, Applications, Pressure loss
relatively low• Disadvantages: Foot Print, Type of Gas effect on
Fabric, Gas Conditions Effects on Fabric, Fire or Explosion Hazard, Effect of Gas Moisture
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Design Considerations
• Selection of Fabrics• Bag Arrangement• Fan Location• Costs: Case/Enclosure a function of size (cloth
area) and Metallurgy • Typical Costs for Baghouses by type
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Compartments
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Fabric Filters - Baghouses
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Fabric Filters - Baghouses
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PRECIPITATORS
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Particulate Control: ESP
• high efficiency, dry collector of particulates• high electrical direct current current potential
(30-75 kV)
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Principle of ESP Operation
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Electrostatic Precipitators
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Electrostatic Precipitators
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Wet ESP
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Wet ESP
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Gas Removal
• Adsorption Systems– consist of a bed of adsorbing material
• activated carbon
– adsorbing material is housed within a pressure vessel through which the contaminated air passes
– pollutants are transferred from air to adsorber
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Activated Carbon Adsorber
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Gas Removal
• Incineration or flaring– used to oxidize:
• carbon monoxide• organic air pollutants• organics containing chlorine, sulfur, and nitrogen
to carbon dioxide and water– direct flame combustion and catalytic combustion
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Incinerator
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Gas Removal
• Wet Scrubbers– transfer pollutants from air to water phase– further treatment may be necessary– pollutants must be highly soluble in water– for less soluble materials, a chemical may be
injected in the water• flue gas desulfurization
– SO2 in the flue gas may be removed by reacting it with a solution of lime or lime stone in water
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Particulate Control: Scrubber
• can be used where– air is wet– corrosive – hot– where baghouses can not be used– for even higher efficiencies, a combination of a
venturi scrubber and cyclone and can be used
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Theory
• Advantages: Can handle flammable Gases, simultaneous gas and particle removal, neutralization, cools and humidifies
• Disadvantages: corrosion problems, liquid waste (problems: recovery, disposal), in cold weather freezing protection needed
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Design Equations• Based on Penetration, i.e. complement of removal• Spray Chamber Equations• Venturi Scrubber Equations• Pressure loss• Efficiency and Energy Expenditure: Contacting
Power • Mist Elimination• Costs a function of capacity and type
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Ventury Scrubber
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Ventury Scrubber
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Vertical Spray Chamber
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Vertical Spray Chamber
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Horizontal Cross Flow Scrubber
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Horizontal Cross Flow Scrubber
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Cyclone Spray Chamber
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Humidification
• Example: A stream of 100,000 acfm at 150F, 1 atm and 20% humidity is scrubbed. Estimate temperature, flowrate of the scrubbed gas and make up water.
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Psychrometric Chart
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End here