The Cutting Edge — October 2003

Product & Engineering Standards

How to Select & Buy a Previously Owned Dust Collector
By Peter Levitt, Sternvent Co. Inc. (plevitt@sternvent.com)

As a result of the sluggish world wide economy, there is an abundance of used wood working machinery, including dust collectors, which is being offered by manufacturing companies and machinery dealers. If you are in need of well priced cyclone, portable collector or bag house and have some available cash, now is the time to do your home work and make your purchase, before the economy recovers and starts humming again.

During the past few years, the Sternvent customer service department has seen a large increase in the number of inquires from people who need information about dust collectors we manufactured during the past forty years. Some of the people who contact us are about to purchase a previously owned collector. However the vast majority of people have already made their purchase. They are not happy campers when they learn that their new dust collector may not be properly sized for their needs, is not well suited for their machines or has a motor that that does not match their voltage. Before making any purchases, you must assess your needs and determine if your prospective purchase will meet those needs. It sounds simple enough, but somehow we get caught up with the seller’s sales pitch and purchase the item, before someone else does. This article will assist you in selecting the proper previously owned dust collector to fit your needs.

Click here for illustrations of various types of dust collectors.

The Basics
      1. What air flow, in cubic feet per minute (cfm), do I need?
This is determined by the amount of air flow that is required, at each machine, to capture the dust and chips, etc. and send it through the ductwork. It is not dependent on the size of the work area. Most wood working machines have a built in port or hood for attachment of the suction pipe. The air requirements and typical duct size for some machines are in Table 1. Usually, the air requirements for all of the machines are added together. This determines the cfm capacity for the dust collector. If only a few machines will operate at a time, then sometimes this cfm is reduced. For home shops this is acceptable. However, for commercial shops, one should consider the recommendation of the National Fire Protection Association (NFPA 664), that the dust collector be sized to operate all of the machines simultaneously.

Table 1

Exhaust Requirements
Equipment	Size	CFM	Connection
Table Saw	Up to 12” dia.	350 - 550	4” – 5”
Radial Saw	Up to 12” dia.	350 - 550	4” – 5”
Band Saw	½ “ wide blade	200 bottom	3”
		200 top	3”
Disc Sander	Up to 12” dia.	350	4”
Drum Sander	Up to 200 sq. in.	350	4”
Edge Sander	Up to 6” wide	350	4”
Belt Sander	Up to 6” wide	550	5”
Wide Belt Sander	Up to 36” wide	1000 each	7”
Moulder	Each Head	600-1000	5 – 6”
Joiner	Up to 6”wide	400	4”
Shaper	Up to 1 hp	350	4”
Planer	Up to 20” wide	800	6”
Router, CNC	Each Head	350	4”

       2. How do I size the duct work?
The air should be transported at a speed of 4,000 feet per minute (fpm). If the duct is too small, the friction of the air will not allow the required airflow that was determined in step 1 to be achieved. It will also result in excessive noise. If the duct is too large, then there will not be enough air to fill the pipe and the transport velocity will be too low and result in material blocking in the bottom of the duct and become a fire hazard. As branch ducts from the machines feed into the main duct, the duct increase in size, throughout the system, so the velocity of 4,000 fpm is maintained in all sections of the duct. Table 2 shows the required pipe size for various airflows. The friction loss of the duct is expressed in inches of water (“WG”) of static pressure (SP). This and other duct design information is available from Air Handling Systems (www.airhand.com), specialists in duct fabrication. As a starting point, a dust collection system for a shop with ten machines will typically have a duct resistance of 5-10” WG.

Table 2

Duct Sizes at 4,000 Ft/Minute
For CFM	200	350	800	1100	1400	1800	2200
Use Duct Size	3”	4”	5”	7”	8”	9”	10”
For CFM	2600	3100	3700	4300	4900	5600	6300
Use Duct Size	11”	12”	13”	14”	15”	16”	17”
For CFM	7100	7900	8700	9600	10600	11500	12500
Use Duct Size	18”	19”	20”	21”	22”	23”	24”

       3. What type of dust collector do I need?

• Enclosureless Positive Pressure
  Dust is stored in easy to dispose plastic bags. Inexpensive. For indoor installation. Air can be re-circulated. For up to 5,000 CFM, per NFPA 664.
• High Velocity Cyclone
  Low initial cost, but high-energy consumption. Typically installed outdoors. Ideal for wide range of particle sizes, especially planning shavings. A workhorse. Suitable for production operations. Air can be re-circulated, when after filter is included. Dust is stored in 55 gallon drums or dumpster. 2,000 – 12,000 CFM.
• Filter Bag with Motorized Shaker
  Moderate cost. Typically installed outdoors. Energy efficient, when furnished with a backward inclined fan. Suitable for intermittent operations. Compact & quiet. Dust is stored in 55 gallon drums. Air can be re-circulated. 2,000-8,000 CFM.
• Pulse Jet Bag
  High cost. Installed outdoors. Energy efficient when furnished with a backward inclined fan. Requires clean & dry compressed air to backwash the filters, which can be a problem in cold climates. Ideal for production operations. A workhorse. Tall. Dust is stored in 55 gallon drums or dumpster. Air can be re-circulated. 5,000-20,000 CFM.
• Pulse Jet Pleated Cartridge
  Moderate cost. Similar to pulse jet bag style, except filter pleats can become clogged with large dust particles. Shorter and more compact than bag style. Sometimes requires a cyclone as a pre-cleaner. 5,000-20,000 CFM.
• Reverse Air
  High initial cost. Energy efficient. Similar to pulse jet bag style, but does not use compressed air. Uses low pressure air to backwash the filters. For production operations. Typically used by furniture manufacturers. A workhorse. Tall. Dust is conveyed to a dumpster or truck. Air can be re-circulated. 10,000-50,000 CFM.

Inventory Assessment

1. Will dust collector be installed outdoors or indoors?
2. Will the air be re-circulated?
3. What location will minimize the ductwork, but allow for easy access for removing the dust?
4. How much waste will be produced in a day?
5. What size dust container will be required?
6. Will the waste be given to the garbage company? A farmer? Burned?
7. What are the requirements of your fire department? Building dept? Liability insurance company?
8. What codes do you need to comply with? Mechanical? NFPA? EPA?
9. Are fire safety devices required? Sprinkler head? Explosion vent? Spark detection? Abort gate?
10. Are permits required?

Sources for Previously Owned Dust Collectors
First check with your local machinery dealer. They know you and your operation and are nearby. Two excellent publications are Wood Quip (www.woodquip.com) and Classified Exchange (www.classifiedxchange.com). An Internet based dealer who specializes in used and new machinery is Ex-Factory (www.exfactory.com).

Before you make your purchase

• Ask the dealer or owner for a digital photo and serial number plate information.
• What will the transportation costs be?
• What is the motor voltage? Full load amps? Confirm that it matches your available voltage and amperage service.
• Does the price include a motor starter?
• Can I re-circulate the air back to my building?
• Contact the manufacturer and ask:

  1. Is this dust collector suitable for my operation?
  2. When was this unit manufactured?
  3. Do you still manufacture this model?
  4. What is the price of a new unit?
  5. Are replacement parts still available?
  6. What is the cost of replacement filters?
  7. What should the dust be stored in? Must it be 100% airtight?
  8. Can you send me a model drawing with the dimensions, air flow, static pressure and horse power?
  9. Does this unit comply with safety codes such as NFPA 664 “Standard for the Prevention of Fire and Explosions in Wood Processing and Woodworking Facilities”? Can the unit be upgraded with an explosion relief vent, etc., to comply?
  10. Are there safety labels that should be added to this unit?

The Bottom Line
If you are satisfied with the answers you receive from both the seller and manufacturer and the price seems right—BUY IT!

Peter Levitt has been the product manager of Sternvent Co Inc, a manufacturer of dust collectors, for over 20 years. He can be reached at plevitt@sternvent.com.

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