Pneumatic Conveying Principles BSH&E SEMINAR

Transcription

1 Pneumatic Conveying Principles

2 Definition What is Pneumatic Conveying? A method for conveying bulk material using gas (usually air) The material can be blown (pressure systems) or sucked (vacuum systems) from its source to its destination Conveyance Material Characteristics Air Velocity Air Pressure Air Temperature

3 Basic Concepts Expansion of gases Air Pressure vs. Volume Air Pressure vs. Altitude Air Pressure vs. Temperature Velocity of gases in a system Pressure Drop Friction State Diagram

4 Expansion of Gases V 1 P 1 = V 2 P 2 (@T = Constant) [Boyle s Law] Pressure VP = P 1 > P 2 Gases Expand at Lower Pressures VP = 240 Volume

5 Air Pressure vs. Velocity Air Pressure vs. Velocity V = ρ x A x v OR v = V / (ρ x A) 1 L d 1 Tapered pipe d 2 1 P 2 < P 1 L 1 L 2 d 1 Stepped line d 2 2 2

6 Pressure Drop Contributions Force to overcome friction drag across ΔL P + ΔP ΔP is: ΔL L 1 L 2 ΔP Directly proportional to velocity squared Directly proportional to the conveying distance Inversely proportional to the diameter of pipe P D

7 Pressure Drop Contributions Δp T = Δp acc = Δp g = Δp s = ΔH g = ΔH s = Total pressure loss in the system Force required to accelerate the solids Frictional pressure loss of the gas Frictional pressure loss of the solids Elevation pressure loss of the gas Elevation pressure loss of the solids ΔP misc = Pressure loss from other equipment

8 State Diagram for Pneumatic Conveying Systems Conveying pressure ΔP Dense Phase Unstable area Dilute Phase Conveying gas velocity, ve

9 Pneumatic Conveying Classification

10 Classification Depending on the ratio of material to air, pneumatic conveying is classified as Dilute or Dense Phase

11 Dilute Phase

12 Dilute Phase High air to solids loading ratios (>2.0) Minimum conveying air velocity (~762 to 1828 m/min) Characterized for high velocity conveying 976 to 2450 m/min Operating pressures in range of 0.25 to 1 bar (positive) or negative pressures of 0.25 to 0.45 bar

13 Dilute Phase Vacuum

14 Vacuum System Zero Pressure High Vacuum = Low Pressure

15 Vacuum Systems Advantages At the pick-up: Less $$ (no Aerolock) Lower head height Easier for multiple sources (just need diverter valve or hose switch) Avoids dust at the source Prevents dust leakage Uses cool, ambient conveying air

16 Vacuum Systems Disadvantages At the Destination More $$ (the receiver must withstand high vacuum) A destination Aerolock is prone to wear Vacuum is less efficient; line sizes are bigger The blower is bigger (more $$) because it has a higher inlet air flow than a pressure blower

17 Dilute Phase Pressure

18 Pressure System

19 Pressure Systems Advantages Pressure systems are more efficient Lines sizes, blowers, and destination filters are usually smaller and cost less $$ At the destination: Easier for multiple destinations (just need diverter valve or hose switch) Less $$ (the bin vent filter doesn t see much pressure or vacuum) Less head height

20 Pressure System Disadvantages At the pick-up: Need Aerolock: more $$ and more head height A source Aerolock is prone to wear Dust and air leakage above the Aerolock must be considered Any leak in a pressure system will cause material to escape A pressure blower heats the conveying air (can be corrected with an aftercooler)

21 Dilute Phase Comparison Pressure vs. Vacuum

22 Pick-up Pressure Systems Pick-up has to overcome a pressure differential Airlocks (rotary valves) are common devices that overcome pressure differential Vacuum Systems Pick-up does not need to overcome a pressure differential More simple devices, often with no moving parts, can be used

23 Convey Line Pressure Systems Any leakage risks product blowing out of the convey line (high pressure to low pressure) More expensive connections / flanges required for high pressure systems Most typical dilute phase systems use regular pipe/tube and couplings Vacuum Systems Any leakage pulls air into the convey line (high pressure to low pressure)

24 Destination Pressure Systems Receiving vessel can be at atmospheric pressure Filter area can be smaller than a similar sized vacuum systems Vacuum Systems Receiving vessel is under vacuum, needs to be strengthened Receiving vessel has to overcome a pressure differential to discharge the material

25 Pressure Drop Pressure Systems Maximum pressure drop is limited by the gas source not the laws of physics Typically pressure drop on positive pressure systems is much higher than vacuum systems Vacuum Systems Maximum pressure drop is between atmospheric pressure and absolute vacuum Typical vacuum systems have a pressure drop of 10 Hg

26 Pressure drop Vacuum system pressure drop limited by laws of physics Pressure drop across pressure system Total possible pressure drop across vacuum system Atmospheric Pressure Absolute Vacuum

27 Truck & Railcar Unloading Systems

28 Railcar Unloading Dual Blower System Conveying rates up to 45,000 kg/hr Carbon steel & stainless steel construction Food, plastic & chemical industries

29 Vacuum Sequencing

30 Vacuum Sequencing Systems Most cost-effective form of pneumatic conveying Used for LIW refill Use standard pre-engineered components

31 LWF Refill Sequence of Operation Pump active Secondary filter Receiver 1 Discharge valve closed Sequence valves closed Receiver 2 Bypass valve in vent position

32 Step 1 Pump active Discharge valve closed Sequence valve 1 open Sequence valve 2 closed Bypass valve in fill position

33 Step 2 Pump active Discharge valve closed Receiver 1 filled Sequence valves closed Receiver 2 empty Bypass valve in vent position

34 Step 3 Pump active Discharge valve open Sequence valve 1 closed Sequence valve 2 open Discharge valve closed Bypass valve in fill position

35 Step 4 Pump active Discharge valve closed Sequence valves 1 & 2 closed Receiver 2 filled Discharge valve closed Bypass valve in vent position

36 Step 5 Pump active Discharge valve closed Sequence valve 1 open Sequence valve 2 closed Discharge valve open Bypass valve in fill position

37 Weighing & Scaling

38 Pneumatic Scaling Systems Aeropass Scaling Filter receiver scaling Weighing at the source Vacuum sequencing scaling

39 Aeropass Scaling System Vacuum or Pressure Conveying rates up to 14,000 kg/hr Ideal for use in low-headroom applications above a mixer No filter is required at each scale hopper destination All air is filtered back at the source Bin Vent Filter

40 Aeropass Scaling System Pressure

41 Scaling (Weighing) Systems Multiple Destinations Limited Room Above Process Purging Accuracy Low Maintenance at Destination

42 Filter Receiver Scaling System Conveying rates to 14,000 kg/hr Recipe, formula and inventory capabilities Gain-In-Weight Mode available Loss-In-Weight Mode is optional Vacuum or Pressure

43 Complementary Unit Operations During pneumatic conveying other unit operations may be used Weighing Mixing Drying Sifting Metal detection Screening Grinding, shredding (size reduction) Inventory control