SIMPLIFIED PNEUMATIC CONVEYING DESIGN GUIDE
Aman Kapur Class XII-A Delhi Public School, Mathura Road, New Delhi
ABSTRACT
I discovered my passion for pneumatic conveying when I was just about seven or eight years old. I used to see my mother vacuum the whole house and always wondered how does it actually work and where does all the dust vanish once it got into the storage compartment of the vacuum cleaning machine. This interest evolved further during my visit to a local hospital to meet my grandmother who was being treated for some illness. While settling bills I got the opportunity to see a live tube conveying system and was fascinated by the way such a complex technology was made to work in such a simple way. Bills, reports, samples etc. from every department were being sent to the centralized bill processing center and other departments at the click of a button. My interest in the technology and my good luck helped me complete my internship at New Logic Automation, a leading manufacturer of Pneumatic Conveying Systems in India during the summer of 2016. I got an opportunity to learn about the design, development, manufacturing, assembly, testing & installation of actual pneumatic conveying systems for conveying sugar, flour, gram flour etc. The design of Pneumatic Conveying Systems is a very complex exercise since it involves a lot of variables including diverse material properties. I decided to work on the same with an objective to come out with a Simplified Pneumatic Conveying Design Guide. I compiled this research paper under the mentorship of Mr. Sagar Mallah, Director, New Logic Automation and shall always remain thankful to him for all his time & guidance. This Simplified Pneumatic Conveying Design Guide has been compiled with an objective to help any professional (having little or no knowledge about Pneumatic Conveying) conclude on basic system design parameters.
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PNEUMATIC CONVEYING DESIGN GUIDE
As per IS:8647-1977 Pneumatic Conveying is defined as the art of transporting dry bulk materials through a pipeline by using either a negative or a positive pressure air stream. The principal limiting factor in the use of pneumatic conveyors is usually the material to be conveyed, Materials to be conveyed should be dry and relatively freeflowing. Friable materials as a rule should not be conveyed pneumatically, except when partial degradation of the material being conveyed is inconsequential to end use of the material. Without booster stations, vacuum systems are practical up to 500 m in length and pressure systems up to 2 km or more.
The following are the main conveying systems: a) Low Pressure System, 1) Positive pressure system, 2) Negative pressure system, 3) Combined negative-positive pressure system, b) Medium Pressure System, and c) High Pressure System,
Low Pressure System — Low pressure systems are ideal for in-plant use to convey non-abrasive or mildly-abrasive dry pulverized materials. Air pressure used is normally limited to 760 mmHg (1 atmosphere), and the air supply is provided by a positive displacement lobe type of blower. Because of the low pressure, these systems are restricted to a relatively short distance and small flow rate.
Positive pressure system is normally used for conveying material from one source to several remotely located discharge points. A typical positive pressure system is illustrated in Fig. 1
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Negative pressure system is normally used for conveying material from several sources to one remotely located discharge point. A typical negative pressure system is illustrated in Fig. 2.
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A combined negative-positive pressure system is generally used for conveying materials from several sources to several discharge points. This type of system is quite versatile and is commonly used for conveying materials from railway container wagons and/or storage bins to different points of use. A typical negative-positive pressure system is illustrated in Fig. 3.
Medium Pressure System - In medium pressure system, the material is first compacted in a variable pitch screw before it is mixed with compressed air in the mixing chamber of the screw type pump. This system thus requires a screw directly coupled with motor in addition to a compressor for providing air at pressures ranging between 760 to 2280 mmHg (1 to 3 atmospheres). Air pressure which can be used in this system is limited because of the necessity of providing a seal between the screw and the mixing chamber. This type of pump conveys continuously. A single stage rotary compressor is generally connected directly with the screw type pump for supplying the required compressed air. High Pressure System - High pressure system uses dense stream conveying with low ratio of air to material resulting in lowest particle degradation when handling friable materials. Because of higher air pressure used, smaller pipelines can be used in this system. 5
High pressure conveying pumps are batch type units which use compressed air only when conveying. These pumps are used for continuous, intermittent or batching operations. In a continuous process operation, a surge hopper is provided above the pump for collecting the material during the discharge cycle of the pump. A typical high pressure system is illustrated in Fig. 4.
For high pressure conveying system a double stage compressor is normally used for supplying compressed air at pressures ranging between 3040 to 5320 mmHg (4 to 7 atmospheres). Pumping being a batch operation, an air receiver is required for the compressed air. The air receiver should be sized in such a way that full pressure in the air receiver is recovered in the time the pump performs the filling and discharging operation. The compressor delivers air continuously during the discharge cycle and should be completed by the time the pump is filled again with material.
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Factors for Design and Use The material to be conveyed should be known by all its characteristics including bulk density, particle size and shape, temperature, susceptibility to moisture, corrosiveness, nature of material, abrasiveness and moisture. Bulk Density - The mass per unit volume should be known under three conditions, namely, in poured condition, stored in bin and when under influence of aeration. The material may enter the system in the poured state, but when it is discharged, it may be in the aerated state, which means that more bulk is involved at the discharge than at the entrance. Volumetric capacity, as well as flowability is vitally affected by this phenomenon. Particle Size - The particle size of material to be conveyed may be classified as below: Very fine - All passing 150 micron sieve [see IS:460-1962 Test sieves (revised)"] Fine - All passing 1.00 mm sieve Granular - All passing 12.5 mm sieve Lumpy and irregular - More than 12 mm in any direction Irregular materials are those that are fibrous, stringy, etc Moisture Content - The moisture content of material should be known. The conveying rate of material reduces with increase in moisture. For satisfactory operation, free moisture should normally not exceed 1 percent by mass. Corrosiveness - The pH (a symbol denoting acidity on alkalinity) value of material should be known. This information is necessary for selecting the material of construction for conveying equipment and also the type of cloth to be used in dust collector. A highly corrosive material may require special materials of construction. Explosive or Combustible Nature of Material - Even though pneumatic conveying is amongst the safest material handling techniques, however in handling explosive or combustible materials, precautions in the system must be taken. Some of these materials may require inert gas to be used as conveying medium. Abrasiveness - Abrasiveness may be classified as follows: Class 1 2 3 4
Abrasiveness Non-abrasive Slightly abrasive Medium abrasive Highly abrasive
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Materials having hardness above that of 'highly abrasive' materials are mostly too abrasive to be handled in pneumatic conveyors. Duty Requirement - In conjunction with conveying rates, the severity of operation should also be determined. The construction of pneumatic conveyor can be linked to the degree of design and construction of speed reducer. Severity of operation can be classified as follows: Class 1 2 3 4
Hours of Operation per Week 160 80 40 20
With the above classification, the degree of sturdiness to which the conveyor must be constructed, can be determined. It will also tell the appurtenances necessary to meet the requirements of the operation. Elevation and Environmental Condition a) Elevation of the installation In relation to sea level should be known. Difference in air density is very pronounced in the operation of pneumatic conveyors and should always be considered. b) Environmental condition, that is, whether system is exposed to hazardous dusts and gases or corrosive elements should be known.
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Design Procedure - Based on the factors mentioned above, the Tables 1, 2 and 3 may be used for determining the type of conveying to be used.
TABLE 1: SYSTEM TYPE ACCORDING TO PARTICLE SIZE Particle Size Type Of System Low Pressure Medium High Negative Pressure Pressure Negative Positive - Positive Lumpy or Irregular X X X Granular
X
X
X
-
X
Fine
X
X
-
-
X
Very Fine
X
X
X
X
X
Note: X indicates suitability
TABLE 2: SYSTEM TYPE ACCORDING TO MATERIAL CHARACTERISTICS Material Type Of System Characteristic Low Pressure Medium High Negative Pressure Pressure Negative Positive - Positive Non - Abrasive
X
X
X
X
X
Abrasive, Slightly
X
X
X
X
X
Abrasive, Medium
*
*
-
X
X
Abrasive, High
-
-
-
X
X
Hygroscopic
X
X
-
-
-
Deliquescent
*
*
*
*
*
Combustible
X
X
X
X
-
Explosive
X
X
X
-
-
Acid
X
X
X
X
X
Alkaline
X
X
X
X
X
Toxic
-
-
-
-
-
Fragile
X
X
-
-
-
Corrosive
*
*
*
*
X
X X Thermoplastic Note: X indicates suitability. The asterisk mark (*) indicates that a pilot plant study will be necessary to establish the suitability of the material for pneumatic conveying.
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TABLE 3 SYSTEM TYPE ACCORDING TO MATERIAL Material Type Of System Low Pressure Medium Negative Positive Negative Pressure -Positive Alum X X Alumina, Floury X X X Alumina, Sandy X X Aluminium Hydrate X X Aluminium Oxide X Arsenic Oxide X Asbestos Dust X Barites X Bauxite X X Best Pulp, Dried X Bentonite X X X X Borax X X X Calcium Carbonate X X X X Calcium Phosphate X Carbon, Activated X X X X Carbon Black, Pelletized * * Catalysts, Petroleum X X Cellulose Acetate X X Cement, Portland X Cement, Raw Materials X Cereals X X Cerelose X X Caly, Kaolin X X X Coal, Pulverized X Cofee Beans X X Coke, Fines and Flour X X Corn Grits X X Cottonseed Meal X X Detergent Powders X X X Diatomaceous Earth X X X X Dolomite X Feed Ingredients X X Feeds, Soft X X Feldspar X X Fertilizers * * * Flaxseed X X flint X Flour, Wheat X X Fluorspar X Fly Ash X Fuller's Earth X X X Gluten Meal X X Grain, Whole X X Grain, Ground X X Graphite X X Gypsum X X X Lime, Hydrated X X X X
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High Pressure X X X X X X X * X X
Lime, Pebble Limestone, Pulverized Magnesium Oxide Malt, Brewers Milk, Dried Ores, Pulverized Petroleum Coke Phosphate Rock, Pulverized Polyethylene Pyrites Resins Rice Rubber Pellets Salt Salt Cake Sawdust Seeds Semolins (Suji) Silica, Pulverized Soap Ingredients Soda Ash, Lights Soda Ash, Dense Sodium Carbonate Sodium Phosphates Sodium Sulphate Strach Sugar, Granulated Talc Titanium Dioxide Wood Chips Wood Flour Zinc Oxide
X X X X X
X X X X X
-
X X X -
X -
X X X X X X X X X X X * X X X X X X * X X
X X X X X X X X X X X * X X X X X X * X X X
X X X X X X X -
X X * X X X X
X X X -
Note: X indicates suitability. The asterisk mark (*) indicates that a pilot plant study will be necessary to establish the suitability of the material for pneumatic conveying.
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Estimation of Pneumatic Conveying design parameters using Nomographs (for preliminary design). This method provides conservative approximations for conveyor size & power for a given product bulk density.
Step 1: Preliminary calculations are started with determination of equivalent length of the system under consideration. The equivalent length is the sum of vertical and horizontal distances along with allowance for all the pipe fittings used. Allowance for most common pipe fittings are as under: Long radius 90O elbow pipe: Equivalent length = 25ft ~7.6m 45O elbow pipe: Equivalent length = 15ft~4.6m Step 2: Next find the air velocity needed to convey the solids. This can be found using table for air velocity for various bulk densities, as under: Bulk Density 3
Air Velocity
lb/ft
Kg/m
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
160 240 320 400 480 560 640 720 800 880 960 1040 1120 1200 1280 1360 1440 1520 1600 1680 1760 1840 1920
3
Ft/min.
m/min.
2900 3590 4120 4600 5050 5500 5840 6175 6500 6800 7150 7450 7700 8000 8250 8500 8700 9000 9200 9450 9700 9900 10500
884 1094 1256 1402 1539 1676 1780 1882 1981 2072 2179 2270 2347 2438 2515 2591 2652 2743 2804 2880 2957 3118 3200
Table: Air velocities needed to carry solids of various bulk densities.
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Step 3: Assume a pipe diameter and draw a straight line on Nomograph 1 between the air velocity and the pipe-diameter scales. The same line when extended shall intersect the air volume scale at some point.
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Step 4: Once we know the air volume we can now go to Nomograph 2 and draw a straight line between air volume and system capacity under consideration. This line shall cut & give corresponding value of solids ratio. Incase value of solids ratio exceeds 15 repeat step 3 with a large diameter pipe.
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Step 5: Next go to Nomograph 3 and draw a line joining known value points of pipe diameter & air volume. This shall give us value of design factor, or P100 ie the pressure drop per 100ft (30.5m) (Point of intersection of center scale).
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Step 6: Go to Nomograph 4 and draw a line connecting known values of equipment length of system & design factor. This line may be extended to intersect the pivot line in the centre. Once we join this point on pivot line with point on solids ratio line, the intersecting point gives value of system pressure loss. Incase system pressure loss exceeds 10 PSI (70 kPa), repeat step 3, 4, 5 & 6 assuming a large pipe diameter.
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Step 7: Incase system pressure loss is ≤ 70 kPa, go to Nomograph 5 and draw a straight line connecting corresponding values of system pressure and air volume.
Step 8: The point of intersection of the system pressure loss & air volume shall give value of horsepower required. 17
In short the various Nomographs provide the following critical information for system design:
Nomograph 1. Velocity (m/min)
Pipe Diameter (mm)
Air Volume (m3/min)
Nomograph 2 Air Volume (m3/min)
Solids Ratio -
System Capacity (kg/hr)
Nomograph 3 Pipe Diameter (mm)
Design Factor -
Air Volume (m3/min)
Nomograph 4 Equivalent length - Design Factor (m) Nomograph 5 System Pressure Loss (kPa)
&
Horsepower (Metric)
System Pressure Loss - Solids Ratio (kPa)
Air Volume (m3/min)
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Working Example: Masala Conveying Bulk Density: 450 kg/m3 System Capacity: 1200 kg/hr Length of pipe (Horizontal + Vertical): 150 m No of Bends: 12 Nos. Calculations as per Nomographs: 1
Equivalent length of pipe = 150 m + (
2
Air velocity needed to convey the solids from the table for bulk density of 450 kg/m3 = 1487 m/min = 24.8 m/sec From Nomograph 1, assuming pipe diameter = 3” (76.2mm) air volume = 245 cfm = 416.43 m3/hr = 6.94 m3/min From Nomograph 2, corresponding value of solids ratio = 2.7 From Nomograph 3, Design factor = 60 From Nomograph 4, System Pressure loss = 7 PSI ≈ 48 kPa Horse Power from Nomograph 5 ≈ 11 HP
3 4 5 6 7
.
) = 241.2 m
Calculations as per conventional formulas: System capacity = 1200 kg/hr Consider material to air ratio: 2.5:1 Consider conveying velocity of 25 m/sec Quantity of air required = 1200/2.5 = 480 kg/hr M3/hr = 83.14 X ( Q = A x V i.e.
.
)X( =(
) = 411.6 m3/hr
) x 25
d= 0.076 m = 76mm = 3” Pipe Diameter System Pressure Loss, considering 1 PSI/100 ft =
= 7.91 PSI = 8 PSI
Roots Blower required 411.6 m3/hr @ 7.91 PSI Select Model Expo 1 (From Everest Blowers Standard Product Catalogue) Q (FAD At Inlet Of Blower) = 411.6 m3/hr (242.158 CFM) P (Differential Pressure across the blower) = 7.91 PSI N (Blower Operating Speed) = 3420 rpm BHP (Power Consumption) = 12.43 BHP Recommended Motor = 15 HP/2 Pole Calculations from Nomographs & conventional formulas yield similar results, hence preliminary design calculations using nomographs may be considered for design data & power consumption approximations. 19
Bibliography 1. Pneumatic Conveying Systems – Design, Selection and Troubleshooting with Particular Reference to Pulverized Fuel Ash – David Mills & V.K. Agarwal 2. Pneumatic Conveying Of Solids – A theoretical and practical approach – R.D. Marcus, L.S. Leung, G.E. Klinzing and F. Rizk 3. Pneumatic Conveying Design Guide – David Mills 4. Pneumatic Conveying – A concise treatment of the principles, methods and applications of pneumatic conveyance of materials, with special reference to the conveying and elevating of heavy solid materials for engineers, works managers and students – Ernest George Phillips 5. IS : 8647 – 1977 (Reaffirmed 2006) – Design Criteria For Pneumatic Conveying Systems 6. Flow assessment of powders in pneumatic conveying : a bench top assessment – M.K. Desai – University of Wollogong 7. Pneumatic Conveying Systems – A. Bhatia – Continuing Education and Development, Inc. 8. The Heart Of Pneumatic Conveying Systems – Positive Displacement Blower Calculations – Roger E. Blanton 9. A View From India – Pneumatic Conveying Of Bulk Materials – G.D. Nigudkar 10. Material Conveying with Pneumatic and Vacuum Systems – Hank Van Ormer 11. Physical Properties and Conveying Characteristics of Corn and Barley Seeds using suction type Pneumatic Conveying Systems – H. Ghafori, A. Hemmat, A.M. Borghaee and S. Minaei 12. Design and Development of a Dense-Phase Suction Pneumatic System for Conveying Granular Materials in Agriculture – Hasan Ghafori, Abbas Hemmat, Ali Mohammad Borghaee, Saeid Minaei 13. Pneumatic Conveying Of Bulk Solids – P.W. Wypych – University Of Wollongong 14. Perry’s Chemical Engineers’ Handbook – Robert H. Perry, Don W. Green, James O. Moloney
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