Pneumatic Conveying

APPLICATION SHEET

BASIC PRINCIPLES OF PNEUMATIC CONVEYING A properly designed air flow system to transport bulk material from one point to another is often the most practical and economical means. Pneumatic conveying systems usually require less plant space, can be easily automated, and can be readily installed. Hauck’s Turbo Blowers are one of the most mechanically efficient air movers for this type of application. In addition, the inlet suction pressure and/or outlet pressure are typically more constant for varying air flows which means that the Turbo Blower can provide more constant “pick-up” pressures. Materials may be conveyed with a blower system either by the direct method, where material passes directly through the blower, or by the indirect method, where the blower furnishes the flow of air for conveying but the material does not pass through the blower. Refuse exhaust systems for woodworking machines, polishers, and grinders generally use the direct system. Refer to figures 1, 2, and 3 for illustrations of these types of systems. Most pneumatic collection and conveying systems incorporate some type of hood or inlet device to properly pick up the material to be conveyed. Some state or local codes dictate the design criteria. In all cases the hood design should minimize turbulence, offer the lowest possible entrance losses and mechanically prevent the entrance from choking off and stopping the air flow. If the material enters the pressure side of the system, an air lock or a venturi device should be installed to prevent a blow back. See figure 5 for the amount of suction required at the system inlet to pick up various materials. Refer to figures 6 and 7 for inlet velocity considerations. When using Hauck’s Turbo Blower Systems, it is very desirable to incorporate a flexible inlet connector to eliminate any stresses on the inlet cone. Also a BVA butterfly valve should be installed in the outlet to ensure that there is enough resistance to prevent overloading of the motor. Hauck Turbo Blower can not be operated with a “free flow” condition. Care should be taken in the sizing of the air piping system to prevent “surging”. This phenomenon can occur with piping on the negative side of the blower as well as on the positive side. Since the purpose of any pneumatic conveying system is to move material suspended by air, the ratio of material to air (by weight) is very important. A conservative design approach is to keep the ratio of material-to-air below a 1:2 proportion. For example, if it is required to convey 1800 Lbs/Hr of sawdust through a 6” diameter duct, 800 cfm of air flow at 4073 FPM velocity would be required.

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HAUCK MANUFACTURING CO., P.O. Box 90, Lebanon, PA. 17042 717-272-3051 04/94

Example:

Fax: 717-273-9882

GJ74FA

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1800 Lbs/Hr sawdust = 30 Lbs/Min 30 Lbs/Min x 2 (ratio factor) = 60 Lbs/Min 60 Lbs/MIn ÷ 0.075 Lbs/scf air = 800 cfm 2 6” ID pipe = 0.1964 ft area 2 800 cfm ÷ 0.1964 ft = 4073 fpm Figure 4 provides conveying velocities for various weighted materials. Sufficient velocities must be maintained throughout the conveying system to prevent settling. All airborne materials, except the finest of dusts or fumes, can settle in the system or even in the fan itself. When the settling occurs in the horizontal plane it is called “saltation”. When it occurs in the vertical plane it is simply called “choking”. Saltation is probably the most difficult to avoid because even the smallest ridge or duct seam can initiate the settling. Whenever possible, a downward slope is advantageous to employ the aid of gravity to minimize potential build-up. In selecting a blower for this application, the required conveying velocity for the maximum material flow and total duct areas dictate the volume of air required. Just as designing around a velocity that is too low will impede the material conveying capability of the system, unnecessarily high velocities can be detrimental as well. System resistance increases as the square of the increase in velocity, so energy can be wasted in the form of fan sizing to overcome the resistance. Figure 8 shows the friction loss for various velocities and duct sizes. These resistances are based on standard air. There are no charts available which list the friction for the various material carrying air flows with varying percentage of carrying capacities. The best method of determining resistance of air/material mixture is through pilot plant testing or experimentation. Most writings on this subject however, seem to indicate that selection based on standard air provides satisfactory performance. Pneumatic conveying systems do have their limitations, such as material size and temperature. However, they still provide many benefits. In addition to being very economical, they are also useful in controlling or minimizing product loss, improving dust control, and thus improving overall plant conditions.

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Fig. 1 Direct System for Conveying

Fig. 2 Draw-Through System

Fig. 3 Screw Feed System MATERIAL AND DUST HANDLING

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PICK UP SUCTION REQUIRED Figure 4 CONVEYING VELOCITIES MATERIAL

PNEUMATIC CONVEYING Low pressure — is used in applications for handling light , bulky materials like wood chips and dust, wool, grains, ash etc. Selection Example: Assume conveying 3000# of sawdust in an 8 hour day. 3000 ÷ 8 = 375#/Hr ÷ 60 = 6.25#/min. 3

Per Fig. 5 table, dry sawdust weight = 12#/ft . Per fig. 4 curve material weighing 12#/ft

3

requires 67 cfm/# & 3750 FPM conveying velocity. Per fig. 5 chart 2½ “ w.c. suction

Ashes, coal Barley Beans Bran Buffing Cement Cinders Cork Corn Cobs Corn, ear Corn Meal Corn Shelled Cotton, (dry) Feathers, (dry) Fruit, Dried Grinding Dusts Hair Lime Malt Mineral Wool Paper, scraps or cuttings Rags, dry Sand Sawdust (dry) Shavings, wood (light) Shavings, wood (heavy) Tan Bark Wheat Wool (dry)

required at pick-up. 6.25 x 67 = 418.75 cfm required. Determine S.P. loss is system & collector + ½ suction = total S.P. Select fan at 418.75 CFM @ total S.P. @ O.V.

Figure 5

close to conveying velocity.

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HOOD DESIGN DATA

Approx. Weight per Cu. Ft. [Lbs] 30 38 28 16 100 46 14 25 56 40 45 5 5 30 30 5 30 35 12 20 30 105 12 9 24 20 46 5

Required Pick up Suction [in. w.c.] 3 3½ 4 2 2½ 5 4 1½ 2½ 4½ 3½ 3½ 2 1½ 3 2 1½ 3 3 2 3 2½ 5 2½ 2½ 3 2½ 4 2

GJ74FA Page 5 Plain Openings Air will move in all directions towards openings under suction. Flow contours are lines of equal velocity in front of the inlet hood. Streamlines are lines perpendicular to the velocity contours. The tangent to a streamline at any point indicates the direction of air flow at that point. Figure 6 illustrates air flow in front of a circular inlet. The equation of flow before free hanging inlet hoods, round inlet hoods, and rectangular hoods which are essentially square, is: V=

Q 10 X 2 + A

Where: V = Centerline velocity at X distance from hood, fpm. X = Distance outward along axis in ft. (Note: accuracy of equation is assured only for conditions where X is within 1½ D.) Q = Air flow, cfm. A = Area of hood opening in square feet D = Diameter of round hoods or side of essentially square hoods As is demonstrated by the above equation an d figure 6 below, there is a rapid decrease in velocity with increasing distances from the hood, varying almost inversely with the square of the distance. Where distances of X are greater than 1½D, the velocity decreases less rapidly with distance than the above equation indicates.

% of Diameter

% of Diameter

Plain Opening Fig. 6

Flanged Opening Fig. 7

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Friction of Air in Straight Ducts for Volumes of 1000 to 100000 Cfm Figure 8