Filter 3x64i

FILTRATION

Reading: Chap 6

Fiber filter

Q: Do filters function simply as sieves (to collect particles larger than the sieve spacing)?

Packing density/solidity

fiber volume   1 - porosity total volume For fiber filter,  < 0.1 For woven filter,  ~ 0.3 10/03/2002

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Theodore & Buonicore, Air Pollution Control Equipment, CRC Press, 1988. 10/03/2002

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Shaker Baghouse

Shaker Cleaning Parameters Frequency Several cycles/s Motion type Simple harmonic or sinusoidal Peak 1-10 gravity acceleration Amplitude Fraction to a few inches Mode Off-stream Duration 10-100 cycles, 30 s to a few minutes Common 5, 8, 12 in bag diameter

Q: What are the common problems encountered? Theodore & Buonicore, Air Pollution Control Equipment, CRC Press, 1988. 10/03/2002

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Reverse-Air

Q: Pros and Cons? Cleaning dust on baghouse walls by traditional sledgehammering 10/03/2002

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Reverse-Air Cleaning Parameters Frequency

Clean a compartment at a time, sequencing 1 compartment after another; continuous or initiated by a max.-pressuredrop switch Motion Gentle collapse of bag (concave inward) upon deflation; slowly repressurize a compartment after completion of a backflush Mode Off stream Duration 1-2 min, incl. valve opening, closing & dust settling periods; reverse-air flow itself normally 10-30 s Bag diameter 8, 12 inch; length 22, 30 ft Bag tension 50-75 lb 10/03/2002

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Reverse-Jet

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Pulse-Jet

Frequency A row of bags at a time; sequenced 1 row after another; can sequence such that no adjacent rows clean one after another; initiation of cleaning can be triggered by max-pressure-drop switch or may be continuous Motion Shock wave es down bag; bag distends from cage momentarily Mode On-stream; in difficult-toclean applications such as coal-fired boilers, off-stream compartment cleaning being studied Duration Compressed air (100 psi) pulse duration 0.1 s; bag row effectively off-line Bag 5-6 in diameter

Q: How can the blown-away particles by the on-line cleaning process be collected? Q: Felted fabric or woven fabric? 10/03/2002

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Air/Cloth Ratio Q V A Filtration velocity (average velocity) Q: If thicker fabric is needed to sustain the high force, is its operating cost higher?

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Filtration Mechanisms • Diffusion (Lee & Liu, 1982) E D  2 .58 Pe 

1 Ku

Pe 2 / 3

Q: How does efficiency change wrt dp? Q: How to increase efficiency by diffusion?

df U0

Ku  

D 1 2

ln  

http://aerosol.ees.ufl.edu/respiratory/section04.html

Peclet number 3 4

 

2 4

Kuwabara hydrodynamic factor

Lee, K.W. & Liu, B.Y.H., Aerosol Sci. Technol., 1:47-61, 1982 10/03/2002

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• Impaction (Yeh & Liu, 1974) ( Stk ) J Q: How to increase impaction efficiency? EI  Q: How does efficiency change wrt dp? 2 Ku 2 2  d  U0 p p Cc U 0 Stk   df 18 d f dp 0 . 62 2 2 .8 J  ( 29.6  28 ) R  27.5R for R  0.4 R  df (J = 2 for R > 0.4)

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Yeh. H.C. & Liu, B.Y.H., J. Aerosol Sci., 5:191-217, 1974 Aerosol & Particulate Research Lab 9

• Interception (Krish & Stechkina, 1978) 2   1 R    1  2 ER   (1  )  (1  R )  2 ln(1  R)  1     2 Ku  2 2 1 R  

Fat Man’s Misery, Mammoth Cave NP

Q: How to increase interception efficiency? Krish, A. A. & Stechkina, I. B., “The theory of Aerosol Filtration with Fibrous Filters”, in Fundamentals of Aerosol Science, Ed. Shaw, D. T., Wiley, 1978. 10/03/2002 Aerosol & Particulate Research Lab

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• Gravitational Settling E G  G(1 R)

for U 0 and VTS in the same direction

2  d VTS p p Cc g G  U0 18 U 0

• Total Single Fiber Efficiency E  1  (1  ED )(1  EI )(1  ER )(1  EG )

 ED  EI  ER  EG • Total Filter Efficiency   4E H E  1  P  1  exp   (1   )d f 

   1  exp S f E   

Sf: Solidarity factor Q: How does the filter efficiency change wrt particle size? 10/03/2002

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Filter efficiency for individual mechanism and combined mechanisms 1.0

Efficiency

0.8 0.6 0.4

Interception Impaction Diffusion Gravitation Total

H = 1mm  = 0.05 df = 2m U0=10 cm/s

0.2 0.0 0.01

0.1

1

10

dp (m)

Q: Should we increase or decrease flow velocity in order to increase collection efficiency for (a) tobacco smoke, (b) cement dust? 10/03/2002

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• Problem Filter is made of fibers with diameter of 10 um, density of 2.35 g/m3 and solidity of 5% Filter must collect 0.5 um particles with 90% eff. Air velocity is 10 ft/min Question: How thick must the filter be?

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Parallel Flow Operation

Q: How do you determine when to clean? 10/03/2002

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Pressure Drop (Filter Drag Model) P  Pf  Pp  Ps 

K2

 K1V  K 2 LVt V Areal Dust Density W  LVt K1

Filter drag

P S V

 S  K1  K 2W

K1 & K2 to be determined empirically Pf: fabric pressure drop Pf: particle layer pressure drop Ps: structure pressure drop Q: What is the pressure drop after 100 minutes of operation? L = 5 g/m3 and V = 0.9 m/min. 10/03/2002

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Time (min) 0 5 10 20 30

P, Pa 150 380 505 610 690

60

990 16

Time to clean

t f  N (t r  tc )  tc Flow rate Q QN  N Q QN 1  N 1 Filtering velocity QN Q VN   AC NAC

QN 1 Q VN 1   AC ( N  1) AC 10/03/2002

Q: What are the parameters that affect our decision on the number of compartments to be used?

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Filter drag

Areal dust density

W j  ( N  1)(VN Lt r  VN 1 Lt c ) Actual filtering velocity

V j  f NVN 1

S j  K1  K 2W j Pressure drop

Pj  Pm  S jV j N

f N  V f / VN 1

3 4 5 7 10 12 15 20

0.87 0.8 0.76 0.71 0.67 0.65 0.64 0.62

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Ex. Calculate the max pressure drop that must be supplied for the following baghouse for a filtration time of 60 minutes: K1 = 1 inch H2O-min/ft, K2 = 0.003 inch H2Omin-ft/grain, tc = 4 min, 5 compartments, L = 10 grain/ft3, Q = 40000 ft3/min, Ac = 4000 ft2/compartment.

Other factors to be considered for selecting a filter? • Temperature & Humidity • Chemical nature of gas • Fire/Explosion • Bag/arrangement • Dust handling • Fan location 10/03/2002

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Advantages & Disadvantages of a Baghouse • Advantages:

• Disadvantages:

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