2. WHAT IS MY SPECIFIC REQUIREMENT?
Regardless of the apparent similarity of your system requirement to that of others, there are nevertheless, unique differences between each and every system. Dust collector, fan, plant layout, percentage of machines in operation, growth considerations; all influence your specific application. Your most practical and economic choice of product and system will reflect these parameters.
3. WHAT DOES 'CFM' MEAN?
Now we get to the heart of the system. The 'CFM' requirement is the first of three parameters that must be taken into consideration in every dust control design. CFM, standing for cubic feet per minute, is the measure of the air required to carry the dust produced by the specific machine. In reality, the composition of air to dust is approximately 95% air and 5% product. Practical values have been established for all common dust and machine applications. For example, a 12" table saw requires 550 CFM, a 16" planer requires 1100 CFM, and so on. Charts for various machine applications are readily available.
4. HOW DO I KNOW WHAT SIZE OF DUCT TO HOOK UP TO EACH MACHINE?
If the duct size is too small, air velocity will have to increase dramatically to satisfy the CFM requirement for the specific machine. This will result in additional duct friction and energy requirements. If the duct size is too large, the air flow will slow down to the point where the dust particles are no longer conveyed in the ductwork, filling up the pipes. There is a correct conveying velocity for each type of dust. For sawdust and shavings we use 4000 FPM (feet per minute) in the branch lines, and then reduce this to 3500 FPM in the mains. For metalworking dusts these values need to be larger; 4500 FPM in the branches and 4000 FPM in the mains. For light dusts and fumes, we can decrease these values. Therefore, (looking at Question 3), for a table saw application, requiring 550 CFM at a conveying velocity of 4000 FPM, we need a 5" duct size. Or for the planer, requiring 1100 CFM at a conveying velocity of 4000 FPM, we need a 7" duct size. The following chart gives branch duct sizes for a conveying velocity of 4000 FPM:
5. HOW DO I DETERMINE WHAT HORSEPOWER IS REQUIRED?
The dust collector fan, driven by a motor, supplies the energy to move the air through the system. The energy required is expressed as "pressure loss" or "pressure drop" and is a measure of the frictional pressure losses through the system. These include:
a. entrance losses - energy required to accelerate the dust particle into the duct system. This is sometimes called "hood loss".
b. duct friction losses - energy required to convey the dust particle through the ductwork into the collector.
c. internal dust collector losses - energy required to draw or push the air through the collector or filter, and d. discharge ductwork, or return-air ductwork friction losses. This includes any frictional losses forcing the clean return-air through a duct and/or silencer.
As mentioned, this energy loss, expressed as a pressure drop in inches of water column, (or metrically, in Pascals or Kilopascals), is the second parameter that must be considered in any duct control design. Hence the expression, CFM at System Pressure drop, allowing us to select the fan size, RPM, and horsepower requirement.
6. WHAT IS CLOTH RATIO?
Cloth ratio is the third and the most neglected parameter that must be considered in the design. It is the amount of CFM per sq. ft. of filter cloth that the fan has to move. This is a complex analysis and involves such factors as: type of cloth material, method of baghouse regeneration, physical and chemical properties of the dust, application of the system, temperature, particle size, etc. This parameter is often ignored for the following reasons: