Asit Kumar Mishra
@akm76.bsky.social
Engineer, Researcher. Working towards healthy indoor climates in sustainable buildings. Putting occupants first.
In another scenario, if the first filter has an efficacy of just 15% and allowed a flow of 1200 cubic feet per minute, it would give us 180 cubic feet of clean air, as opposed to the 199 from the second one. 20/
October 22, 2023 at 9:40 AM
In another scenario, if the first filter has an efficacy of just 15% and allowed a flow of 1200 cubic feet per minute, it would give us 180 cubic feet of clean air, as opposed to the 199 from the second one. 20/
The big difference is the bigger flow allowed by the first guy more than makes up for its lower effective filtration. That is why we want to check CADRs and not just effective filtration. 19/
October 22, 2023 at 9:40 AM
The big difference is the bigger flow allowed by the first guy more than makes up for its lower effective filtration. That is why we want to check CADRs and not just effective filtration. 19/
So, every minute, the first guy delivers 1000 cubic feet air, 70% likely to be clean. So that is about 700 cubic feet clean air every minute. The second guy delivers 199.8 cubic feet clean air every minute. 18/
October 22, 2023 at 9:40 AM
So, every minute, the first guy delivers 1000 cubic feet air, 70% likely to be clean. So that is about 700 cubic feet clean air every minute. The second guy delivers 199.8 cubic feet clean air every minute. 18/
Hypothetical time. Say, there is a filter which picks out stuff you don't want with 70% probability. Another, filters the stuff out with 99.9% probability. Every minute, the first lets 1000 cubic meters of air pass through, the second lets 200 cubic meters of air pass through 17/
October 22, 2023 at 9:40 AM
Hypothetical time. Say, there is a filter which picks out stuff you don't want with 70% probability. Another, filters the stuff out with 99.9% probability. Every minute, the first lets 1000 cubic meters of air pass through, the second lets 200 cubic meters of air pass through 17/
The CADR tells you the overall impact, taking into account how good the filter's efficiency is in the size range of concern and the flow rate through the filter. 16/
October 22, 2023 at 9:39 AM
The CADR tells you the overall impact, taking into account how good the filter's efficiency is in the size range of concern and the flow rate through the filter. 16/
That sounds complicated. Which is why we talk about something called clean air delivery rate. Depending on the size of the pollutant of concern, a CADR can be calculated, following some standards (ahamverifide.org/ahams-air-fi...) 15/
Air Filtration Standards
ahamverifide.org
October 22, 2023 at 9:39 AM
That sounds complicated. Which is why we talk about something called clean air delivery rate. Depending on the size of the pollutant of concern, a CADR can be calculated, following some standards (ahamverifide.org/ahams-air-fi...) 15/
So, based on the pollutant size of concern, we decide on a filter type. There will be a range of filters in this group. They may have different flow restrictions. The compromise we need to strike is the bet value of efficiency times flow rate. 14/
October 22, 2023 at 9:38 AM
So, based on the pollutant size of concern, we decide on a filter type. There will be a range of filters in this group. They may have different flow restrictions. The compromise we need to strike is the bet value of efficiency times flow rate. 14/
Let me take it to an extreme. You could have an impermeable plastic film - that will give you 100% efficiency but 0 flow rate. We certainly do not want that. 13/
October 22, 2023 at 9:38 AM
Let me take it to an extreme. You could have an impermeable plastic film - that will give you 100% efficiency but 0 flow rate. We certainly do not want that. 13/
We need to strike a balance of an optimal flow rate and filtration efficiency. 12/
October 22, 2023 at 9:37 AM
We need to strike a balance of an optimal flow rate and filtration efficiency. 12/
There is air in the room, there are pollutant sources, the air cleaner is continuously passing the air through a filter, getting stuff out of air and doing it again while the sources continue put stuff into the air. 11/
October 22, 2023 at 9:37 AM
There is air in the room, there are pollutant sources, the air cleaner is continuously passing the air through a filter, getting stuff out of air and doing it again while the sources continue put stuff into the air. 11/
And this is not like filtering water - its not like you are going to let the air pass through the filter and then collect in a large tank and breathe some of it in whenever you feel out of breathe 10/
October 22, 2023 at 9:37 AM
And this is not like filtering water - its not like you are going to let the air pass through the filter and then collect in a large tank and breathe some of it in whenever you feel out of breathe 10/
At the same time, higher rated filters (filters with better efficiency) need to be more restrictive to do their job. Means, they let less air flow through them than a comparatively lower rated filter if the driving force is the same 9/
October 22, 2023 at 9:37 AM
At the same time, higher rated filters (filters with better efficiency) need to be more restrictive to do their job. Means, they let less air flow through them than a comparatively lower rated filter if the driving force is the same 9/
To be fair, even your handkerchief will filter some 0.3 micron particles. That does not mean you will use it to protect against asbestos dust. Things have to be "fit for purpose" 8/
October 22, 2023 at 9:36 AM
To be fair, even your handkerchief will filter some 0.3 micron particles. That does not mean you will use it to protect against asbestos dust. Things have to be "fit for purpose" 8/
At the same time, if you are interested in particles around 1 micron size, you would not even think of getting a MERV 8 filter. Not that they would not filter anything at 1 micron. Just that the values are not of practical interest and MERV 8s don't get rated at that size 7/
October 22, 2023 at 9:36 AM
At the same time, if you are interested in particles around 1 micron size, you would not even think of getting a MERV 8 filter. Not that they would not filter anything at 1 micron. Just that the values are not of practical interest and MERV 8s don't get rated at that size 7/
Similarly, for MERV 13, particles between 0.3 and 1 microns, have about a 75% chance of getting trapped. Again, not a definitive NO or YES. More than an even chance, so not too shabby. 6/
October 22, 2023 at 9:36 AM
Similarly, for MERV 13, particles between 0.3 and 1 microns, have about a 75% chance of getting trapped. Again, not a definitive NO or YES. More than an even chance, so not too shabby. 6/
For example, have a look at the performance of different MERV rated filters, MERV 8 - 16 (hpac.com/industry-per...) For MERV 16, you can't say 1 micron particles will be trapped - they almost certainly will be, but not a definitive YES. 5/
October 22, 2023 at 9:36 AM
For example, have a look at the performance of different MERV rated filters, MERV 8 - 16 (hpac.com/industry-per...) For MERV 16, you can't say 1 micron particles will be trapped - they almost certainly will be, but not a definitive YES. 5/
When it comes to taking tiny particles out of air, it is no longer a YES or NO thing. It gets all probabilsticky around there. 4/
October 22, 2023 at 9:35 AM
When it comes to taking tiny particles out of air, it is no longer a YES or NO thing. It gets all probabilsticky around there. 4/
However, when it comes to filtering stuff out of air, the action is very little like a sieve. There are a range of mechanisms that lead to particles in air being filtered and straining (the sieve like action) has a very minor role 3/
October 22, 2023 at 9:35 AM
However, when it comes to filtering stuff out of air, the action is very little like a sieve. There are a range of mechanisms that lead to particles in air being filtered and straining (the sieve like action) has a very minor role 3/
If you have a sieve which allows anything smaller than 4 mm to pass through, it is like a YES-NO question, either you go through or you do not. 2/
October 22, 2023 at 9:34 AM
If you have a sieve which allows anything smaller than 4 mm to pass through, it is like a YES-NO question, either you go through or you do not. 2/