Lets learn from each other!
I built an ambient air cleaner/purifier system like you are talking about a couple years ago for use in my 24’x24’ garage. I've had great luck with it so far but am now looking to update it some since I have a much bigger shop now and need more filtration. I am a welder and use it to get rid of the smoke, gases, fumes, dust, etc.
I have researched many sites and many existing machines and come up with what I think is a great solution but am looking for someone with more knowhow to tell me if my calculations are correct. It can be very daunting trying to understand all that is involved in creating an efficient air cleaner/purifier. There are so many calculations and variables involved; it is hard for me to wrap my head around it all.
I will post some pics later. For starters I built a box with a 13.5”, 1/10th HP, 1320CFM attic fan. The fan is mounted in the top facing the ceiling. This way the clean air is distributed using the "Coanda Aerodynamic Principle." The air hits the ceiling and is distributed across the shop ceiling and away from the intake/return filters. I made my box so that all four sides have filters. Leaving a space between the fan and the filters assures that the entire area of the filters will be used. Good filters require significant backpressure, which means tight sealing to prevent bypassing. There are currently 4 stages of filtering. Each side of the box has four 20”x20” filters in series. That is a total of 16 filters. I do not remember the micron rating or the cost for all of the filters.
These are the filters:
4" merv 12 pleated filter with 226.66 Square Ft. of filter media, and an initial resistance of 0.15 @ 300 FPM,
1" merv 9 carbon activated filter with 33.33 Square Ft. of filter media, and an initial resistance of 0.218 @ 300 FPM,
1" merv 8 electrostatic washable filter with 33.33 Square Ft. of filter media, and an initial resistance of 0.32 @ 300 FPM,
1" merv 2 cheap Fiberglass furnace filter prefilter, (I will not use this type again, it doesn’t do enough good to justify using it. I am replacing it with 1” MERV 4 Cut-N-Fit Blue Natural Fiber furnace filter with 33.33 Square Ft. of filter media, and an initial resistance of 0.125 @ 300 FPM.
I plan to make a 5th washable prefilter out of fine mesh fiberglass window screen, just to catch the large debris.
I know that this is a lot of filters, but it is my understanding that you want to have a low static pressure for the machine to work properly. And in my opinion, the more filtering, and the more types of filtering, the better.
My combined total Square Feet of filter media for the 16 filters is = 1306.6
My total initial resistance of filter media for the 16 filters is =? I am not sure how to add these up!
The idea behind a proper air filtration system is to exchange the greatest amount of air in the shortest duration while at the same time keeping the velocity slow enough so as not to disturb settled dust in other parts of the space. Another important issue is to avoid negating the 'arrestiveness' of the filter by having too much air being drawn through the filter and thereby drawing the dust through the filter. Arrestiveness is defined as the ability to retain particulate without it being drawn through the filter. So, the idea here is to move enough air to trap the dust at the source by using the proper amount of airflow and, of course, using the proper filters.
In the grand scheme of air handling, any fan is able to exchange a certain volume of air in a certain amount of time and is expressed in (CFM) or cubic feet per minute.
All of the air which enters the air cleaner/purifier system enters through the return air filter. The filter must be theoretically sized according to the cfm (cubic feet per minute) requirement of the system. A filter that is too small will cause a number of problems. The filter will clog very rapidly if undersized and reduce air flow. Air velocity also becomes critical with reduced size and dirt will pass through instead of staying in the filter. The system is trying to draw in the designed air quantity and if the filter will not allow flow, air will be drawn in from any possible crack. Every system, no matter how air tight you construct it, will have small cracks through which dirty, warm air will enter the system as static pressure increases. A large surface area filter will negate the effect of these cracks by having a lower static pressure and lower air velocity.
The airflow resistance of a filter and the fan pressure required to overcome it depend on how fast the air is moving and how long and narrow the paths are. Friction along air paths creates resistance to airflow. Fans must develop enough pressure to overcome this resistance and move air through the filter.
Airflow resistance and fan pressure are usually expressed in inches of water column (in. water, or in. H2O). This term comes from gages called u-tube manometers that are sometimes used to measure pressure.
Propeller fans normally can't generate more than about 2 in. water pressure. They are most commonly used for exhausting air from attics or overhead spaces, or general air circulation.
Tube-axial and vane-axial fans are the most common types used for HVAC. They are relatively inexpensive and fairly efficient when static pressure is less than about 4 in. water. The main disadvantage of these fans is that they are very noisy.
The maximum allowable filter velocity is 300 feet per minute (fpm) on disposable filters. For best results, the recommended minimum filter surface area is 2.00 cubic feet per minute (cfm) per square inch of filter area. Using 2.00 cfm per square inch the velocity of air across the filter will not exceed 300 fpm.
Example: 2000 (CFM) ÷ 200 in. water = 1000 square inches of filter media.
Because of the way fan impellers (blades or rotors) are designed, the amount of air they can move decreases as the pressure they are working against increases. The airflow vs. pressure information for a particular fan is called the fan performance data. Performance depends on the size, shape, and speed of the impeller, and the size of the motor driving it. Performance differs widely among brands and models, even for fans with the same size motor.
Access to fan performance data is essential for selecting fans and for determining airflow provided by existing fans. Most manufacturers sell fans that have been tested using procedures specified by the Air Movement and Control Association International, Inc. (AMCA). The manufacturers can provide you with performance data in the form of tables or graphs. Avoid fans for which AMCA data is not available.
Calculate total airflow needed
The first step in selecting a fan is to determine the total airflow it must provide.
Estimate static pressure
The next step in selecting a fan is to estimate the pressure the fan will be operating against.
Estimating fan power requirements
Fans are usually described by the horsepower (hp) rating of the motor used to drive the impeller. It's helpful when selecting fans to estimate the power requirement first so you know where to start looking in the manufacturer's catalog.
Fan motor size depends on the total airflow being delivered, the pressure developed, and the impeller's efficiency. Impeller efficiencies generally range from 40% to 65%. If we assume an average value of 60%, we can use the following formula to estimate the fan power requirement.
Fan power (hp) = airflow (cfm) x static pressure (in. water) ÷ 3814
Example, Fan power = 7325 cfm x 2.4 in. water ÷ 3814 = 4.6 hp.
Selecting the best fan available
The most critical factor is whether the fan can provide enough airflow at the expected operating pressure. Start by looking at performance data for a fan having a motor rated just under the power value you calculated. If this fan provides more than enough airflow, look at the next size smaller. If your first pick is too small, try the next size larger.