Laser Exhaust System Basics

Copyright © 2010 by Davis Multimedia, Int'l. All Rights Reserved.
As Printed in July 2010, Volume 36, No. 1 of The Engravers Journal
 
 

 

Connection to the laser showing aluminum flex pipe attached using metal hose clamps.  

   We have all seen—and sometimes smelled—when a laser is working. We have seen the smoke when engraving wood and picked up on the unique scent as well. Modern laser engraving systems are very safe and productive when correctly installed and operated, however, a key component to the safe operation of a laser system is the exhaust ventilation system.
   During the engraving and cutting operation of a laser, smoke and particulates are generated as the engraving material is being vaporized by the laser. A laser system is designed to facilitate the efficient removal of the smoke and particulates by drawing them out of the enclosure using an exhaust system.
   Many of the materials processed by the laser “out-gas” fumes and odors that are neither pleasant nor safe for the human respiratory system. These out-gassed fumes from engraving or cutting are safe, however, when vented properly using a properly designed exhaust system. A correctly working exhaust will keep the inside of the laser cleaner and the air around the laser safe to breathe. On the other hand, using an improperly designed system can create an unpleasant work environment and expose the operator to low levels of the airborne fumes. You will find that some common sense, the correct materials and a good basic design make building an excellent exhaust system both easy to do and affordable.
Configuring Your Exhaust
   The first step is to consult your laser manufacturer’s manual to determine the airflow and pressure needed in configuring the exhaust system for your laser. These specifications are given in cubic feet per minute (CFM) for airflow and in inches of water or static pressure for pressure. A typical exhaust specification for a 12"x24" table laser system would be 500 CFM at 6" static pressure.
   Keep in mind that this air volume and pressure specification is needed at the exhaust outlet of the laser system. The number of bends in and the length of the duct will reduce the airflow and pressure. As a rule, keep the number of bends to an absolute minimum and the length of the duct as short as possible to ensure proper airflow and pressure.
Proper Duct Basics
   Since safe operation is paramount, we want to use the correct materials which will give us both a safe and long-lasting exhaust system. The main duct should be constructed of 4" or 6" metal duct. One of the most popular and cost-effective laser duct materials is often called galvanized steel “heat pipe,” the same type of metal duct used in most forced-air heating and cooling systems. Available from home improvement stores such as Lowes and Home Depot, these duct pipes are quickly cut with tin snips and can be easily assembled using various adjustable “elbows” and other fittings.
   Most laser systems have a 4" diameter port, so a 4" duct is adequate, but if the duct’s total length is more than 35', a 6" diameter duct will be more efficient and offer more airflow. Reducing the amount of friction the air has in traveling through the duct is ideal, so configure your duct to be as short as possible with as few bends and elbows as practicable. If in doubt about the optimum duct size, I would recommend using the next larger size, e.g. 6" vs. 4" diameter.
   When choosing the type of duct to use, use only metal duct. In particular, never use any type of plastic or rubber duct material! Why is metal duct required? The metal pipe allows you to electrically ground the entire duct system from the laser all the way to the exhaust port exiting from the building.


 
 
  Wall mounted exhaust fan with remote control switch.  

   The problem with using PVC pipe and other nonconductive duct materials is that a moving stream of airborne particle matter builds up a charge of static electricity, which can eventually cause a static electric charge. A static spark in the presence of certain concentrations of airborne particles can cause either a fire or, in extreme cases, even an explosion. This is very dangerous and must be kept in mind when designing any type of ventilation system.
   Metal duct, by nature, will dissipate static electricity that otherwise can build up, so having both the duct and the fan motor electrically grounded will add an extra level of protection. This should be done using a wire connecting the duct system to a source of electrical grounding like a metal conduit, a cold water pipe or a metal rod driven into the ground. A proper grounding wire would be a green insulated wire or bare wire attached by a screw on the duct and the grounding connection.
   It is recommended that each seam and joint of the metal duct be sealed to eliminate any air leaks. Sealing the duct can be done one of two ways, and I would recommend using either of them. The first is using a caulk-like sealing compound that comes in a tube and is applied before assembling the duct work. Home improvement centers sell this product, typically located near the duct parts. The sealant dries hard but stays flexible to provide a positive seal.
   Another way to seal the duct is to use a special aluminum tape. Do not use fabric tape or duct tape even though it may sound like the right thing to use. Aluminum tape is specifically designed for sealing metal duct and is available at your local home improvement center. Apply the tape around all of the seams, including any elbows.
   When attaching the metal duct pipes, it is always best to use self-tapping, sheet metal screws to secure the sections together. No predrilling is required when using these screws. A duct system that is screwed together will be more secure and less likely to leak for many years to come. It also reinforces the grounding connection by providing metal-to-metal contact between pipe sections. This connection, combined with proper taping, will make the duct joint stronger and keep any fumes where they belong—inside the duct!
   A small piece of flexible duct is also recommended to go between the laser system and the rigid metal duct. This allows a slight amount of movement of the laser when needed for service access. It also keeps vibration from the exhaust fan from travelling up the duct to the laser. This duct also makes connecting to the laser a breeze. Just slide the flexible duct over the exhaust port on the laser system and tighten the hose clamp.
   Using the correct type of flexible duct is critical. Use only one made of aluminum for this part of your exhaust system. Never use any kind of plastic flexible hoses, especially those that have a coil on the inside. A flimsy, plastic, dryer-vent type hose will collapse under the vacuum pressure and reduce the airflow drastically, not to mention posing a fire hazard if you ever develop a fire inside your laser.
   It is also important that you keep the amount of flexible duct used in your system to an absolute minimum to keep the airflow as efficient as possible and avoid any type of unnecessary turns and curls.
   The following statistics will give you an idea of the effect of airflow in the flexible hosing and elbows of your duct system. In terms of airflow restriction, 5' of flexible hose will deliver the same amount of air as 15' of straight pipe. Likewise, the installation of two 90-degree elbows within the pipe run will reduce the airflow as much as 12' of straight pipe. So, the trick in designing a properly engineered duct system is allowing for not only the length of the duct run, but adding extra capacity to compensate for flow restrictions caused by elbows, curves and flex pipe runs built into the system.

 
Connection to the laser showing wall mount bracket, aluminum flex pipe and a hose clamp.   A laser vent grounded to a cold water pipe using a pipe clamp and a piece of wire connected to the duct.

What About the Fan?
   Of course, the fan itself is an integral part of an exhaust system. The CFM and pressure specifications we mentioned earlier are important when we choose the size and type of fan for our exhaust system.
   The type of fan we should use is an impeller unit with, typically, a 1/2, 3/4 or 1 horsepower electric motor to power it. Choose a fan that is rated for continuous duty and has bronze or ball bearings in the motor. An exhaust fan with these ratings will give you many years of dependable service.
   Since the length and size of the duct and the number of bends in the run will reduce the amount of airflow and pressure, it is important to choose the correct size exhaust fan. Calculating the exact airflow will require an HVAC engineer or using the more common method of slightly oversizing the fan size to give you adequate flow and pressure. A simple rule of thumb is, if you have a 20"x24" table laser system and a duct run of 20' or less, a 1/2 horsepower fan that is rated for 660 CFM at 4" static pressure will be enough to meet the system manufacturer’s specifications. If you have a longer duct run or more than three or four bends in the duct, you will need to go to a larger size fan and possibly a slightly larger duct. Consulting an HVAC company may be a good idea, just to be sure.
   An important design criterion is the location of the exhaust fan along the duct. Keep in mind that the electrical motor and the fan blade combined can make a droning noise which might be in excess of 50 decibels. So, mounting the fan in an outside location is best whenever possible. If you cannot locate the exhaust fan outside, I would recommend moving it as far away from the laser system as possible.
   Locating the exhaust fan at the far end of the duct adds an important safety bonus in addition to a lower noise level. With the fan located at the far end of the duct, it is creating a vacuum pressure that essentially sucks the air out of the duct. One safety advantage is that if there is a break in the seal that creates a small air leak, room air will leak into the duct rather than dangerous fumes leaking out into the work environment. Configuring the fan at the far end of the duct will also reduce the odor and fumes that otherwise might drift into the work area, making it safer in the long term.
   However, having the fan located at the extreme end of the duct does cause a small logistical problem with switching the fan on and off. The best way to solve this issue is to have an electrician install an electrical outlet near the fan to plug the fan into and run a wire to a wall switch near your laser, which allows you to conveniently turn the vent system on and off as needed.
   Another solution would be to use a remote controlled switch device, like you have for your television, with a receiver connected to the on/off switch on the fan. This remote device must be able to handle the current of the exhaust fan, so be sure to get an industrial, heavy-duty-rated remote switch that will be up to the task. Much like your TV remote, remote switches are battery operated, so they can be placed within easy reach of your laser system.
Make-Up Air
   One important design criterion for any heating, cooling or ventilation system is called “make-up air” or “replacement air.” This is based on the idea that whenever you pump air into a sealed room (air pressure) without providing a means for the same volume of air to escape, the result is to “pressurize” the atmosphere in the room. Or, if your laser vent system sucks air out of the sealed room, that creates a vacuum, unless you provide a source of make-up air to replace the air you are evacuating. This serves the same purpose as having air returns in each room of a house with a forced-air heating or cooling system.
   A source of make-up air is important in any laser vent system. Without adequate replacement air, system efficiency can drop and, in extreme cases, laser users have reported symptoms of nausea or light headedness from breathing laser fumes due to the inability of the vent system to remove the fumes from a room operating under vacuum pressure.
   Keep in mind that make-up air requirements can be substantial. For example, if your ventilation system is rated at 500 CFM, the replacement air is the volume of air that is contained in a room roughly 8'x8'x8'. We are talking about evacuating that much air per minute.
   A temporary make-up air system can be as simple as opening a window in the work area, although this may not be very practical in a commercial shop. Users often provide replacement air by running a length of duct from the outdoors to (near) the laser. A number of commercial systems incorporate heat exchangers to “temper” the air temperature closer to room temperature as it enters the shop.


 
 
  Close up of an exhaust duct that shows a wall mount bracket and a pipe joint that has been attached using three sheet metal screws and sealed with aluminum tape.  

   Some users recommend a simple, do-it-yourself test for the adequacy of your make-up air apparatus. The test involves having someone stand near the fan (so he or she can hear the fan) and closing off all the doors and windows to the laser room. Next, open at least one door and/or large window allowing the free flow of air into the room. If you can hear the fan begin to labor as it changes its pitch when the room is fully closed, that’s an indication of inadequate make-up air, which is a potentially dangerous condition. However, this do-it-yourself test is not 100 percent reliable, so consulting an HVAC professional is recommended.
Temperature Controlled Rooms
   Heating, or in some cases cooling, a room which has a laser engraver installed may be a challenge, especially if you are exhausting the temperature-controlled air at a rate of around 500 CFM. The laser draws air into the enclosure and then the exhaust fan pulls it outside. If running the laser exhaust puts an extra strain on your heating/cooling system, one solution is to attach a fresh-air duct to the inlet of the laser engraving system. This type of duct will require some fancy work to hook up to the laser, but it will allow the laser system to draw air from the outside, which will then be exhausted back outdoors. It would be best to leave this type of special vent fabrication to your HVAC expert.
The End of the Duct
   Don’t forget that the end of the exhaust duct, called the stack, is just as important as all the other design factors. It is very important to vent the fumes from the laser up and away from your building, so the stack should rise 2' to 3' over the highest part of the roof. The stack, which is often overlooked but is very important in venting fumes away from the building, is topped off with a cap which keeps water, birds and other foreign matter from getting into the exhaust duct. Having no stack or one that is too small may cause the fumes to be drawn back into the building by an air conditioner or fan system, resulting in a strong odor and an unhealthy work environment.
Good Exhaust = Happy Laser
   A good laser exhaust system is one that is quiet, removes smoke and odors, helps in reducing sparks and flaming and creates a safe working environment. The result of having such a system will be an improvement in engraving and cutting quality, with both a decrease in smoke residue on your work pieces and the need for cleaning inside the laser system. Building a quality exhaust system is also a vital safety precaution for operating a laser system. In short, taking the time to develop and install a simple, durable, high-quality exhaust system will provide you with many years of fume-free air and trouble-free service.
References:
Designing Your Dust Collection System to Meet NFPA Standards
By Gary Johnson
http://www.nordfab.com/view/as
sets/literature/GaryJohnsonArticle.pdf
Air Handling Design Guide
By Air Handling Systems
http://www.airhand.com/downloads/Air_Handling_Design_Guide.pdf
OSHA Dust Control Systems
http://www.osha.gov/SLTC/silicac
rystalline/dust/chapter_3.html


 

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