PNEUMATIC CONVEYING FOR FOOD ENGINEERING FE 467 FOOD PLANT DESIGN
Prof. Dr. Mustafa BAYRAM University of Gaziantep/Faculty of Engineering Department of Food Engineering Gaziantep/TURKEY Rev 2- Nov 13, 2013 Fe 467 Prof. Dr. Mustafa BAYRAM 1 email@example.com
• The pneumatic transport: According to;flow regimes: classified into two flow regimes: • dilute (or lean) phase • dense phase. According to pressure; also two types dilute pneumatic available : – Positive type pneumatic conveying, – Negative type pneumatic conveying
Dense phase flow; • characterised by low gas velocities (1-5 m/s), • high solids concentrations (greater than 30% by volume) • high pressure drops per unit length of pipe (typically greater than 20 mbar/m).
Dense phase Pneumatic conveying
Dilute phase transport: • characterised by high gas velocities (greater than 20 m/s), • low solids concentrations (less than 1% by volume) • low pressure drops per unit length of transport line (typically less than 5 mbar/m).
Dilute phase pneumatic transport is limited to; • short route, • continuous transport of solids at rates of less than 10 tonnes/hour and is the only system capable of operation under negative pressure.
Design Approach to Dilute Phase Pneumatic Conveying
Steps in Dilute Phase Conveying Design 1. Material and Gas properties (density, size shape, viscosity, etc.) 2. Specify desired conveying rate 3. Estimate pipe diameter (with a little practice you get a feel for what works) (e.g. 10, 12, 15 cm etc.) 4. Calculate saltation velocity (USE THE FORMULA …) (in general: >10, 15, 20 m/s) 5. Check loading (mass solids/mass gas) • If > 10 to 15 then need larger diameter pipe • If < 1 then need smaller diameter pipe
6. Calculate pressure drop • May require iteration • If too large, may need to gradually increase pipe sizes (telescoping) • Do not let velocity drop below saltation velocity 7. Size blower
CALCULATION FORMULA AND TECHNIQUES Saltation velocity
Components sum to total pressure drop Blower Hopper Filter 4. 2. 3. 1. 5. 1. Blower, silencer, inlet filter 2. Acceleration 3. Horizontal pipe section 4. Pipe bends Other 5. Vertical pipe section Components Cyclone Dust collector ( ) Filter DP = – DP +DP +DP +DP +DP acc horiz bend vert filter Valves = DP blower
Acceleration • Gas (outside of blower) and particles must DP = DP +DP accelerate up to accel a-gas a-solids operating velocity r V 2 w V g g s p V = + g – Conveying gas velocity rg – Gas density 2gc Agc V – Particle velocity 2 p r V æ æV öö A – Pipe cross sectional area = g g ç1+2mç p ÷÷ w – Conveying rate (mass/time) 2g ç çV ÷÷ s c è è g øø r V A = p p w s m – Solids loading = w g
Must relate V and V p g • V – V = slip velocity g p • Empirical relation by V Hinkle (1954) p = 1-0.044d 0.3r0.5 – d = particle diameter V p p p g (meters) – Particle density in kg/m3 • In English units V – d in feet p = 1-0.123d 0.3r0.5 p 3 V p p – density in lbm/ft g
Bends • Major source of wear/attrition • Pressure loss associated with re- acceleration of gas and solids • Bends usually specified by a ratio of the bend radius to pipe diameter R/D • Typical in conveying R/D = 6 to 12 R D
Bends • Chambers and Marcus (1986) • R/D >= 6, B = 0.5 2 rV • R/D = 4, B = 0.75 DP = B(1+m) g g bend • R/D = 2, B = 1.5 2gc w s m = w g
Horizontal Pipe • Pressure loss associated L rV 2 with frictional losses by gas ( ) g g and solids DP = 4 f +lm horiz z • L = length of horizontal D 2gc section w s • = solids friction factor m= l w z g •Use experimental data if available
Solids Friction Factor • For d > 500 microns p 0.1 -0.3 -0.86 0.25 æ D ö l = 0.082m Fr Fr ç ÷ z s ç ÷ d è p ø • For d < 500 microns p æ ö0.1 -0.3 -1 0.25ç D ÷ l = 2.1m Fr Fr z s ç ÷ d è p ø • where V2 U2 m = ws Fr = g Frs = t Ut = Terminal wg gD gD Velocity
Vertical • Pressure loss accounts for frictional losses plus static head (gravity) L r V2 g ( ) g g 0 DP = 4f +l m +r Dz vert z D 2gc gc 0 ( ) Bulk density r =er + 1-e r g p w e = 1- s Void fraction of gas phase Ar V (recall for fluidized beds p p voidage is typically 0.4 to 0.7)
Working Principle of Bag filter Air along with dust particles under suction or pressure enters the lower portion i.e. hopper of the bag filter. The air travels through the filter bag, which retains the dust particles on surface of the bag, and the clean air passes out through bags and plenum to the outlet of Bag filter. Dust is collected on the outside the bag filter Accumulation of dust on bags causes an increase in the differential pressure across the filter bags. Compressed air is pulsed by a timer actuated series of normally closed pulse valves at preset intervals causing the valves to open. A momentary rush of high- pressure air (4-5 bar) flows from the compressed air header to the blow tube and is expelled from the blow tube through nozzles at a high velocity (primary air flow). Air from each nozzle induces a secondary airflow. The combined effect of the primary and induced secondary air causes an instantaneous pressure rise on the clean side of the filter bags, causing a reverse flow air through the filter bags, thus dislodging the dust particles held on the outer surface of the bags. By this mechanism, the dust collected is released from the bags and falls into the hopper & the differential pressure is controlled across the Filter Bags. From this hopper it is discharged through suitable device i.e. Rotary valve. Since only fraction of the total filter area of the bag filter is cleaned at any given time, continuous flow through the bag filter at rated capacities is assured.
For project • Find the permeability of bag filter • Determine the size of each bag • Calculate the area and number of bag • Calculate the pressure drop