Lab Fans, Fume Hood Blowers, & Ducting
Special Considerations for Your Fume Hood Blowers & Exhaust System:
- What type of blower do I need?
- What size blower do I need? What factors determine the size that I need?
- Blower Terminology
- What type of ducting do I need to consider?
- Who will install my lab blower/ducting?
- What are the electrical requirements to consider?
- How much ducting will I need?
- Do I need a centrally controlled system for all my hoods?
1. What type of blower do I need?
The types of fumes you exhaust from your fume hood are directly related to the type of blower you’ll need. Many standard airfoil bypass fume hoods require a simple epoxy coated steel blower. This is the least expensive type blower you can buy, but it’s not right for every application. If you are working with any type of explosive materials, you will want to use an explosion proof blower, usually fiberglass or plastic blowers. Explosion proof/spark resistant blowers usually have a totally enclosed motor so that any potential fumes that go through the air stream do not interact with the electrical current or spark of the motor itself. The fan impellers on this type of blower are also non-sparking. If your application is located in an area near a coast and you’re concerned about saltwater corrosion, go with all plastic blowers or PVC blowers. You won’t have to worry about replacing it nearly as fast as a steel coated blower.
2. What size fume hood blowers do I need? What factors determine the size that I need?
Getting the right size blower is essential to the successful operation of your fume hood. The chemical hoods manufacturer will dictate the amount of air you need to move in order to operate the fume hood at 75 feet per minute (fpm), 100 fpm, 125 fpm or 150 fpm. The manufacturer will state the airflow that’s needed, stated in cubic feet per minute (cfm), at a designated level of static pressure, stated in inches. Static pressure is best described this way:
S.P. Static Pressure. Resistance to air flow measured in inches of water gauge (WG). The static pressure is the total of the negative pressure on the suction side of the plastic blowers and the positive pressure on the discharge side. Resistance is caused by fume hoods, ductwork, elbows, stacks, filters, or anything else that impedes the free flow of air.
An example of what a fume hood might require would be something like this:
At 100 fpm, fume hood #1 requires 1400 cfm @ 0.37” SP
So you would use this information as a basis for beginning your search for plastic or PVC acid resistant laboratory blowers. Any elbows, long duct runs (over 25 feet) and the natural pressure loss of a duct system will add to the actual airflow requirement of your system. The info above is only for the chemical hoods at their face. A simple rule of thumb is to size the blower up a little bit (10-15%); you can always use a damper to reduce the airflow if it’s too great. If you need additional information or want to ask a question about how to size your plastic blowers or duct system, please contact LDS by email; we’ll be happy to assist you.
3. Fume Hood Blowers Terminology
CFM: Cubic Feet per Minute The amount of air moving through a system. CFM = FPM x AREA in square feet.
S.P. Static Pressure Resistance to air flow measured in inches of water gauge (WG). The static pressure is the total of the negative pressure on the suction side of the blower and the positive pressure on the discharge side. Resistance is caused by fume hoods, ductwork, elbows, stacks, filters, or anything else that impedes the free flow of air.
RPM: Revolutions per Minute. Rotational speed of the blower impeller.
FPM: Feet per Minute Velocity of the airflow measured at a certain point in the system. FPM = CFM divided by Area in square feet.
VELOCITY: Speed of the airflow measured past a certain point, measured in FPM. Velocity across the face of the fume hood is the most important factor in hood exhaust design. VELOCITY x FACE AREA (in square feet) = CFM required.
BHP: Brake Horsepower required to turn the blower at a given RPM at a given CFM and static pressure. Usually determined from fan performance charts.
4. What type of ducting do I need?
What applications determine which one I need?
PVC is, by far, the most prevalent type of duct material used in labs because it resists all types of corrosive and acidic fumes and won’t corrode in high moisture environments. Stainless steel can be used also in high moisture environments, but it will corrode in some acidic environments. Sometimes, though, the specification may call for stainless steel. Consult your engineer or architect about this specification to find out if there are viable alternatives. Most lab duct is either straight or preformed elbows. Transitions are available for both stainless steel and PVC blowers, to move from one size to a larger or smaller size. In-line damper systems are available also. Flexible stainless and PVC are available if the application demands it. Normally, we do not recommend galvanized sheet metal ducting as it will corrode fairly quickly. Trying to replace corroded ducting is much more expensive after it’s already installed compared to spending a little more up front for the right material.
5. Who will install my lab blower/ducting?
LDS can install your plastic, PVC and steel blowers and ducting system, but normally this is best when left to a certified and trained HVAC contractor that you choose. The reason is that the HVAC contractor will know how to wire the blower so that it complies with local and national electrical codes and standards. These codes vary by state and municipality, and your local contractor will know best how to comply. If you choose for LDS to install plastic, PVC and steel blower and ducting system, we would ask that your electrician complete the hookups and test the system. Normally, lab blowers are mounted on a rooftop.
6. What are the electrical requirements to consider?
The electrical requirements are driven by the needs of the blower, the availability of electrical power and the distance between the blower and the fume hood(s). Some larger blowers operate more efficiently on 240v power. As long as the electrical power is available on the rooftop, you can request that the blower motor be 240v compatible. Most laboratory blowers, however, use standard 115v power. Again, consult your electrician or HVAC contractor for more information.
7. How much ducting will I need?
This is all based on the layout of the chemical hood and lab blower using straight duct runs and elbows. You can, of course, shorten the length of duct by using flex duct. Flex duct also serves to absorb vibration from the blower system and will not transfer vibration to the fume hood. Normally, ducting comes in 10-foot sections and can be coupled together with PVC couplings. Try to mount the PVC, plastic or steel blower as close to the fume hood as possible so as to reduce the amount of duct and thus the amount of pressure loss in the duct system itself.
8. Do I need a centrally controlled system for all my hoods?
Many blower systems used in larger labs require a central control to maximize the efficiency of the airflow and maintain consistent temperatures within the lab. This is especially true in multi-hood installations. When more fume hoods are in use, the amount of conditioned air is increased and vice versa. Using an intricate system of airflow monitors and motorized dampers, a centrally controlled system works to keep you comfortable in any situation and season.