Dust Control Basics...First in a Series
Date Posted: March 15, 2001
This series is based on a pre-conference workshop held prior to the start of GEAPS Exchange 2001 in March in Phoenix, AZ. This first part is based on remarks by Delmar Mains, dust control projects engineer for The Boone Group, Boone, IA (800-265-2010/www. boonegroup.com).
The first step toward learning how to manage a grain facility dust control system is to learn some terms commonly used in the dust control industry and some of the physical principles governing air carrying particulate matter through an enclosed pipe or duct.
Some Basic Formulas
The two most common measurements used in dust control systems are:
• CFM – cubic feet per minute, a measure of a volume of air through a duct.
• FPM – feet per minute, a measure of the velocity of air through a duct. Many of the calculations used in designing or troubleshooting a dust control system involve the cross-sectional area of a duct. Most of the ducts used in the grain industry for dust control are round. The formula for finding the cross-sectional area of a round duct is:
(Pi) x (radius) x (radius)
Pi is a constant number, approximately 3.14. The radius is half the diameter of the duct. Say you have a round duct, 28 inches in diameter. To determine the cross-sectional area, calculate:
(3.14) x (14 inches) x (14 inches) = 615.44 sq. in.
Most formulas used in designing dust control systems utilize feet, not inches. To convert this number into square feet, divide by 144: 4.27 sq. ft. Occasionally, rectangular ducting is used in dust control systems. The formula for determining cross-sectional area in rectangular ducts:
(height) x (width)
For example, take a rectangular duct measuring 24 inches x 26 inches:
(24 inches) x (26 inches) = 624 sq. in.
To get square feet, divide by 144: 4.33 sq. ft.
Determining Velocity and Volume
You need a number of things to determine the velocity of air and particulate matter moving through a duct:
• Laminar flow, also known as “non-turbulent flow,” is a gas or fluid moving in parallel layers. The laminar flow of air in a duct usually is measured at five to 10 pipe diameters downstream from a transition point such as a change in the diameter of a round duct or an elbow.
• Measuring laminar flow requires a pitot tube, an inexpensive device available from most industrial distributors. A pitot tube is a double-walled, L-shaped tube with holes in the outer wall. The shorter end of the tube is inserted into the duct and pointed upstream. The longer end is connected to a manometer or MagnehelicTM gauge, to measure velocity pressure.
• A Magnehelic gauge, available from Dwyer Instruments, Michigan City, IN (219-879-8000/www.dwyer-inst.com), registers velocity pressure and gives a readout in “inches of water.”
• Inches of water (WC) is a measure of pressure expressed as a weight per area. One inch of water is the weight of a 1-inch-high column of water covering one inch square of area. In dust control applications, it is the pressure that 1 cu. in. of water exerts on 1 sq. in. of area. One cubic inch of water weighs 0.03606 lbs. Therefore 1 in. WC = 0.03606 psi (pounds per square inch). Thus 27.7 in. WC exerts 1 psi of pressure.
•In dust control applications, inches of water are used to measure pressure losses in air conveying ducts, static pressure in fan applications, and pressure drop across filter bags.
• As air moves through a duct and strikes a solid unmoving object, say the end of a pitot tube, it generates a pressure against that object. This is the velocity pressure.
Converting Velocity Pressure to FPM
As air moves against the mechanism of a pitot tube, the Magnehelic gauge reads the pressure generated in inches of water. To convert the velocity pressure of air to fpm, the formula is:
(The square of the velocity pressure in inches water) x (4,004, a constant number)
For example, take a 14-inch-diameter round duct. A Magnehelic gauge measures 1.15 in. WC inside the duct. To find the velocity of air in fpm:
(1.15 inches) x (1.15 inches) x (4,004) = 5,295 fpm.
The fpm, in turn, is used to calculate the cfm, or volume of air moving through the duct. In this last example of the 14-inch-diameter duct, first calculate the duct’s cross-sectional area:
(3.14) x (7) x (7) / 144 = 1.07 sq. ft.
Second, use this formula to calculate cfm:
(Velocity) x (cross-sectional area)
In this example:
(1.07 sq. ft) x (5,295 fpm) = 5,666 cfm
Pressure Loss in Round Ducts
Duct manufacturers make available charts showing pressure loss over a given distance for the various diameter ducts they sell. Pressure loss is a measurement of the pressure required to move a given volume of air through a given length and diameter of round duct. These charts are based on this formula:
• “2.74” is a constant number for a particular type of duct commonly used in grain dust collection systems, with seams at a given distance.
• “Velocity” is in fpm.
• “1.9” is an exponent to the velocity in fpm.
• “Length” is in feet.
• “501,187” is the number 1,000 raised to the 1.9 power.
• “Diameter” is the diameter of the duct in inches.
• “1.22” is an exponent to the diameter.
Here’s an example of how to use this formula to calculate pressure loss in a duct. Take a 5-inch-diameter duct that is 60 feet long. Using a pitot tube and Magnehelic gauge, you find that the duct has an air velocity of 4,589 fpm. Applying the formula:
(4,589)1.9 = 9,062,840
(5)1.22 = 7.12
(2.74) x (9,062,840) x (60)
(501,187) x (7.12) x (100)
Run it through your calculator, and you get a pressure loss of 4.18 in. WC.
Here are some rules of thumb for pressure loss in ducts:
• A 10% increase in velocity results in a 20% increase in static pressure.
• The smaller the duct size, the greater the pressure loss.
• The longer the duct run, the greater the pressure loss.
• Elbows and transitions add to the pressure loss.
• A 90-degree elbow 10 inches in diameter has the same pressure loss as 20 feet of 10-inch-diameter straight pipe.
• Pressure loss where the air enters a hood also must be accounted for in a ducting system. While this can vary according to the hood design, a rule of thumb commonly used in dust control system design is to use 1 in. WC.
You need to know several things to make a proper fan selection.
• You need to know your application. Different types of fans are used for clean vs. dust-laden air and for different types of dust.
• You need to know the cfm and static pressure required in order to determine proper fan size, rpm, and horsepower.
• You’ll need to see the fan curve, available from the fan manufacturer. The fan curve shows how it will perform at a given rpm.
With most fans, the greater the static pressure, the less air it will move. There is a maximum static pressure a fan will produce at a given rpm. If the baghouse system demands a greater static pressure, the fan has reached its peak on the curve and will go to the “back side of the curve,” where its performance is very unpredictable.
For this reason, Fan Curve A on page 70 shows a poor choice, because it is in the unpredictable range. It’s not certain that it will produce the 20,000 cfm for which it is designed. In fact, it is not certain it will move any air at all.
Fan Curve B depicted on this page is a better choice, because if the static pressure requirement were to go as high as 12 in. WC, the fan will still move about 16,500 cfm, sufficient for many grain elevator applications.
Ed Zdrojewski, editor