Down draft benches are a common type of dust collection equipment used for sanding, grinding polishing and other fabrication techniques. Incorporated into the equipment is a work bench which has a grate to hold the material being machined. Air is pulled downward away from the worker, through the grate, filtered and then released back into the ambient air.
There are many configurations and options available for down draft benches. Dual-sided benches, side-panels, ceiling panels and lights are some of the most common options. They also come in many different sizes that you can choose from depending on your application.
High constant velocity at the work deck is one of the most important factors when choosing a down draft bench. If the equipment is not producing enough airflow at the work deck, the process dust may not be fully contained. Additionally, the further the process is away from the deck, the lower the capture velocity. Obstructing or altering the airflow by the process is also a consideration that needs to be addressed when assessing your requirements of a down draft bench. For instance, the worker simply standing at the bench can disrupt the ambient airflow and create small eddies of swirling air. Additionally the product being worked on itself can create these eddies. Picture a large rock in a stream and notice the rippling that occurs on the down-stream side of the rock. The more rocks, the more disrupted the water becomes. The force of the air movement into the deck of a down draft bench must be adequate to eliminate the risk of contaminated air being circulated into the breathing zone of the worker.
In recent years, dust collection companies have introduced the addition of return air flow ducts, commonly referred to as “regain air” to down draft tables and containment booths. The Idea is that by returning a portion of the exhaust air directly toward the table, airflow will be increased forcing material toward the work deck. This appears to make sense, and has quickly become a great sales tool for companies looking to increase their bottom line. Unfortunately, if we take a closer look at what is actually happening, this “add-on” will quickly become a dinosaur as it has no effect on truly increasing the performance of the down draft bench and actually inhibits the bench from doing its job as initially designed.
The main function of the down draft bench is to protect the worker from hazardous dust. Smooth and consistent downward flow on a broad scale is the most effective way to keep dust particles away from the worker. The more fluctuations in air movement, the more ineffective the bench becomes. By increasing airflow toward the bench at the distance of typical return air flow ductwork, pressure is increased in the vicinity of the exhaust; however the velocity at the work deck does not change. The fan is going to pull the same amount of air that it was designed to. This is because down draft benches operate at very low water pressure. The result is that pressure is increased above the worker which means that it will be decreased elsewhere. For example, if the equipment is pulling x amount of air evenly through the deck so that ambient air from the facility is entering the deck at the same speed directly in front of the worker as it is to the sides, this is optimal. Increasing airflow from the top (or from any direction), will allow less air will be pulled from somewhere else. Most likely airflow will be decreased near obstructions, like in front of the worker. Any disruption of air in the area of lower pressure such as the unfortunate placement of HVAC ducts or a forklift whisking by could be all the force that is needed to leach contaminated air into the facility or introduce contaminated air into the breathing zone of the worker.
Typical filtration of an 80/20 poly-cotton blend filter commonly used in down draft benches is initially 80% down to 1 micron. As the filter is used and accumulates dust particles, this efficiency rises. Sometimes a HEPA filter is added to the exhaust of the equipment to increase the filtration efficiency. Wet dust collectors vary in efficiency with the most efficient being the orfice / impingement design which filters 95% of dust down to 3 micron. Both of these filtration processes exhausting into ambient air or ducted outside of the facility bring down the parts per million of dust to an acceptable level within the facility. As an old saying goes in the business: “Dilution is the Solution to Pollution”. The problem with return air is that 50% of this exhaust is pushed back directly toward the worker and their breathing zone. This increases the ppm of harmful dust surrounding the worker exposing them to a potentially higher than acceptable levels of process dust. Additionally, the most likely particles to make it past the filtration process are the smallest and easiest to inhale and absorb by the body. These particles are so fine they can be suspended in air indefinitely. The more time the worker is in this contaminated zone, the higher the exposure to an unacceptable concentration of harmful dust.
Often you will find down draft benches advertised by CFM, but this is misleading at best. Just because a bench is 5000 CFM doesn’t mean that the velocity of air at the work deck is adequate. Depending on the area of open space of the deck, the velocity of air movement (referred to as FPM or feet per minute) can vary greatly. It is this measurement that allows you to determine if your bench is pulling with enough force to protect the worker. FPM of benches can range widely from 100 FPM up to 500 FPM. To increase FPM, some companies will sell a 2 foot by 4 foot bench, but decrease the actual area of grating, where air is being sucked in, so 8 square feet of bench actually only has 4 square feet of open grating or ventilated area. This, of course, decreases the effectiveness of the bench because the airstream is not broad enough to encapsulate the dust. Also, inspect the material covering the deck. This can be utilized to decrease the amount of ventilated area as well. Deck material can cover up to 80% of the deck to help increase FPM. For example, an 8 square foot bench (4×2) has a FPM of 300 and runs at 500 CFM. The ventilated area is only 4sq ft. If we increase the ventilated area to 7sq ft., and utilize almost all the bench, the CFM of the blower would have to almost double to provide the same velocity at the deck. Take note of this relationship to help compare the benches you are considering.
If you are in the market for a down draft table, look for a company that is open about the amount of ventilated area within the work deck compared to the actual size of the deck. Make sure that there is sufficient air velocity at the deck (FPM) to adequately capture dust particles from the process. Remember, the further away from the work deck your process is, the higher FPM you should be looking for. Protecting your employees is always your first concern, so don’t forget to pass on a company offering return air flow.