An Introduction to Air Pollution

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1 An Introduction to Air Pollution Chapter 6. Fabric filters 薛人瑋 Ph.D 2 nd October 2012

2 Fabric Filters Fabric filtration Air or combustion gases pass through a fabric Dust is trapped on the fabric Cleaned air exits the system Baghouses Rows of bags Inlet Exit Cleaning mechanism

3 Baghouse application and operation Baghouses- Compartmented: Shaker Reverse flow Noncompartmented: Pulse Jet Operation: Dirty air enters at low velocity Multiple filters (bags)collect PM PM falls to bottom Cleaned air exits

4 Advantages and disadvantages Advantages High efficiency for small particles Modular design Low velocity Low pressure drop Expense Large area Frequent cleaning/maintenance Operating temp limitations

5 Types of Baghouses Compartmented: Shaker baghouse Bags cleaned by oscillating framework Reverse air Clean air blown through bag is opposite direction Non-compartmented: Pulse jet Compressed air blown down bags for cleaning

6 Dust loading Fabric: filter material Woven fibers micron diameter Interstitial holes microns PM layer forms between fibrils Increased filtration efficiency Increased pressure drop S= filter drag V= filtering velocity

7 Theory Δ P Total pressure drop Δ P f Pressure drop due to the fabric Δ P p Pressure drop due to the particulate layer Δ P s Pressure drop due to the bag house structure

8 Darcy s equation ΔP f Pressure drop N/m 2 ΔP p Pressure drop N/m 2 D f D p μ V Depth of filter in the direction of flow (m) Depth of particulate layer in the direction of flow (m) Gas viscosity kg/m-s superficial filtering velocity m/min K f, K p Permeability (filter & particulate layer m 2 ) 60 Conversion factor δ/min V = Q/A Q volumetric gas flow rate m 3 /min A cloth area m 2

9 Dust Layer L Dust loading kg/m 3 t time of operation min ρ L Bulk density of the particulate layer kg/m 3 ΔP = ΔP f + ΔPp Filter Drag S = ΔP/V Areal dust density W = LVt S= k 1 +k 2 W

10 Permeability, K Permeability of filter material, (K 1 )( K e ) Extrapolated from test data Permeability of particulate layer, (K 2 )( K s ) Slope of test plot Determined from test data Fabric, dust Contributes to filter drag (S) as a function of areal dust density (W)

11 Filter drag model Filter drag: dependent on areal dust density (W) and fabric and dust layer permeability (K) S= filter drag, Pa-min/m or inches of water- min/ft W= areal dust density, kg/m 2 of fabric or lb/ft 2 of fabric L= dust loading, kg/m 3 or lb/ft 3 T= time of operation, minutes

12 Filter drag model Using the filter drag model to predict pressure drop (ΔP) after 60 min of operation Dust loading (L) = 15 g/m 3, V= 0.8 m/min Ke = 500 Pa-min/m, Ks= 3 Pa-min-m/g

13 Filter drag model Using the filter drag model to predict pressure drop (ΔP) after 60 min of operation Dust loading (L) = 15 g/m 3, V= 0.8 m/min Ke = 500 Pa-min/m, Ks= 3 Pa-min-m/g

14 Filter drag model Using the filter drag model to predict pressure drop (ΔP) after 60 min of operation

15 Dust Layer

16 DESIGN OF FABRIC FILTERS The equation for fabric filters is based on Darcy s law for flow through porous media. Fabric filtration can be represented by the following equation: S = K e + K s w Where, S = filter drag, N-min/m 3 S = P/V K e = extrapolated clean filter drag, N-min/m 3 K s = slope constant. Varies with the dust, gas and fabric, N-min/kg-m W= Areal dust density = L V t L = dust loading (g/m3), V = velocity (m/s) Both K e and K s are determined empirically from pilot tests.

17 Obtain Pilot Data to Determine ΔP versus loading

18 Problem Estimate the values of K e and K s for the filter drag model: Time (min) Filter P (Pa) Limestone dust loading L = 1.00 g/m 3 Fabric Area A = 1.00 m 2 Air flow rate Q = 0.80 m 3 /min

19 Solution Step 1: Calculate the air velocity Air velocity = 0.80 (m 3 /min)/1.00 m 2 = 0.80 m/min Step 2: S = P/V W = LVt Step 3: Determine K e and K s graphically K e = 470 N-min/m 3 K s = N-min/g-m

20 Reverse Air Fabric Filter

21 Example Problem

22 Solution

23 Solution

24 Pulse Jet Fabric Filter

25 Pulse jet design considerations Different filtering velocities No compartments Compressed air for bag cleaning Compressor power Pressure drop

26 Compressor power Major operating expense of pulse jet systems Compressor power (ω), kw: η = compressor efficiency γ = 1.4 (ratio of heat capacities C p /C v ) P 1, P 2 = initial and final pressures (abs), kpa Q 1 = volumetric flow rate at compressor inlet, m 3 /s

27 Compressor power Example: Find compressor power (ω), kw Flow rate (Q) = 20,000 cfm (9.5 m 3 /s) T = 50 C (323 K) P 1 = 1 atm (101.3 kpa) Air pulse (P 2 ) 100 psig (790 kpa) abs. Compressed/filtered air ratio = 0.6% Compressor efficiency (η)= 50%

28 Compressor power Compressor power (ω), kw: Flow rate (Q) = 20,000 cfm (9.5 m 3 /s) T = 50 C (323 K) Compressed/filtered air ratio = 0.6%

29 Compressor power Compressor power (ω), kw: Compressor efficiency (η)= 50% P 1 = 1 atm (101.3 kpa) Air pulse (P 2 ) 100 psig (790 kpa)

30 Fan Horsepower Flow rate (Q) = 20,000 cfm Assume 60% efficiency (η) for motor For ΔP = 17 inches w.g. BHP = 90 hp For ΔP = 3.4 inches w.g. BHP = 18 hp

31 Problem Calculate the number of bags required for an 8-compartment pulse-jet baghouse with the following process information and bag dimensions. Q, process gas exhaust rate 100,000 ft 3 /min A/C, gross air-to-cloth ratio 4 (ft 3 /min)/ft 2 Bag dimensions: bag diameter 6 in. bag height 12 ft

32 Solution

33 Solution

34 Solution

35 Solution

36 Shaker Baghouse

37 Hopper

38 Filtration time, t f Shaker and reverse-air baghouses Several compartments One compartment off-line for cleaning t f = filtration time, min N= number of compartments t r = run time, min t c = cleaning time, min

39 N=5, N-1=4 Filtration time, t f t f

40 Number of bags Example: Net cloth area = 8,000 ft 2 Select 3 (N) compartments N-1 = 2 (1 off-line for cleaning) 2 compartments on line to meet NCA Each compartment = 4,000 ft 2 4,000 ft 2 x 3 compartments = 12,000 ft 2 Bag size: 6 inch diameter, 8 feet long Bag area: πdh= π(0.5)(8)= 12.6 ft 2 12,000/12.6 = 952 bags

41 Filtering Velocity All (N) compartments on-line for Q = 20,000 cfm flow rate (Q N )through one compartment: N-1 compartments on line during cleaning flow rate through on-line compartments:

42 Filtering Velocity All (N) compartments on-line Filtering velocity (V N ) in one compartment (C): N-1 compartments on line during cleaning Design Filtering velocity (V N-1 ) in on-line compartments:

43 Pressure drop Max pressure drop (ΔP m ) occurs before next compartment to be cleaned (j) end of cleaning time for last compartment (j-1) at time t j (the time compartment j is on-line) t j t f

44 Pressure drop Calculating Max pressure drop (ΔPm) t f = 60 min, t c = 4 min, t r =? t f

45 Pressure drop Calculating Max pressure drop (ΔPm) During t j, the cloth in compartment j has accumulated areal dust density (W j ) Given dust loading (L) of 10gr/ft 3

46 Pressure drop Calculating Max pressure drop (ΔP m ) Given K e = 1.00 in wg-min/ft, K s = in wg-min-ft/gr During t j, the filter drag (S j ) in compartment j is

47 Pressure drop Calculating Max pressure drop (ΔPm) During t j, the actual filtering velocity (V j ) in compartment j is calculated Ratio of V j to V N-1 Total Number of Compartments, N f N = V j /V N

48 Pressure drop Calculating Max pressure drop (ΔPm) Finally, the maximun pressure drop can be calculated

49 A Compartment of Bags

50 Use Pilot Data to Design a Multicompartment Baghouse

51 Use Pilot Data to Design a Multicompartment Baghouse

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