Why Doesn't My Air Purifier Have a Suction Feeling at the Intake?

[kyphysics]

Hey, everyone

I just bought an air purifier yesterday (the Honeywell 50250 model) and have a question about whether or not it's normal what is happening when I turn it on. I followed the instructions exactly in terms of assembly (it was really only putting on the pre-filter - no other assembly was needed). This is the Amazon page for it as a visual reference:

https://www.amazon.com/Honeywell-50...id=1529975926&sr=8-3&keywords=honeywell+50150

At the top of the filter is a "ring" around the perimeter that vents air. I can feel air pushed out of that ringed vent. On the sides of the purifier (360 degrees) are lots of slits (behind them is the pre-filter and filter). I am assuming that the purifier sucks air in through the sides with slits and releases it from the top. However, I don't feel any "suction" or "vacuuming" effect when running my hand along the sides of my machine. Is this normal for air purifiers? If you don't feel any suction/vacuuming feeling, how does air get sucked into it? As mentioned, I definitely feel lots of air blowing out of the top, so I'm puzzled and wondering if my machine is working properly (in case it's a defective one that needs returning).

Do your air purifiers have a "suction/vacuuming" effect or feel to them where they in-take the air?

Thanks for your help and feedback!

Edit: Russ has given me some feedback privately, which has been useful, but this is just for further discussion and educational value, as I know nothing about how they work and am also curious about others' experiences using them (particularly, if you have a model that does have a suction/vacuuming sensation).

[edit: quotes by mod]

 

[kyphysics]

I wanted to clarify that when I run my hand across the sides (where the slits are), there is maybe a very, very minute difference in air flow pressure that I feel vs. just having my hand away from the machine and in the air (or randomly placed anywhere else). But that very minor difference is nothing like a vacuum sucking in air that you can feel on your skin if you were to place your hand over a vacuum's air intake area. You can feel a vacuum sucking your skin inwards. Nothing like that happens on the air purifier's air intake slits. It's just a very (almost imperceptible) slight feeling of air sort of flowing by it...so faint that I likely wouldn't notice if not consciously trying to sense it.

If this is normal, is this standard for most purifiers? Does anyone actually have one that does have a "sucking" feeling around the intake areas?

 

[marcusl]

Yes it’s normal. The exhaust comes from a tiny annular area while intake is spread over an enormous area so the intake flux (air flow per square cm) is small. Wet your finger and hold it next to the intake—it will feel cold on the side that air is coming in from so you’ll know it’s sucking. The blast of exhaust air is your proof that it’s working.

 

[russ_watters]

There are actually two separate effects here that contribute to the difference between the inlet and the outlet. @marcusl is correct about the first one, but I'll expand a little:

Since pressure acts in all directions and the back of a fan "pulls" air in by reducing pressure, air comes into the fan from all directions. But when air leaves the front of the fan, it is largely its own momentum that keeps it going in a cohnerent stream. That's why you feel so much difference between the front and back of a standard axial floor fan.

An additional issue with an air purifier is that filters require a low velocity across them in order to work properly both for proper filtration and lower energy use. As a result, the velocity at the inlet (because the filter is right behind it) is much lower than the velocity at the outlet (because there is no reason to oversize it). It should be obvious from looking at it that the inlet surface area is larger than the outlet surface area, which causes the difference in velocity. And what we feel on our hand in front or behind a fan is due to velocity.

 

[russ_watters]

But that very minor difference is nothing like a vacuum sucking in air that you can feel on your skin if you were to place your hand over a vacuum's air intake area. You can feel a vacuum sucking your skin inwards. Nothing like that happens on the air purifier's air intake slits.

This is a totally separate question from why the velocity is/feels higher in the outlet from the inlet. The answer to this one is that air purifiers and vacuum cleaners have fundamentally different purposes, despite being fundamentally similar devices. Vacuum cleaners use high [negative] pressure because need high velocities to pick up objects off the floor. Air purifiers are removing particulate suspended in the air, so they don't need to do any extra work to draw the particles into the device.

 

[kyphysics]

Yes it’s normal. The exhaust comes from a tiny annular area while intake is spread over an enormous area so the intake flux (air flow per square cm) is small. Wet your finger and hold it next to the intake—it will feel cold on the side that air is coming in from so you’ll know it’s sucking. The blast of exhaust air is your proof that it’s working.

Appreciate the explanation and I shall try the wet finger tip when I get home, marcusl.

Another indication it's working that I found online is to look at the filter over time. If it's got lots of dirty "gunk" on it, then you're obviously drawing in particles. But, of course, our's is only a week old, so we can't see anything yet.

 

[kyphysics]

There are actually two separate effects here that contribute to the difference between the inlet and the outlet. @marcusl is correct about the first one, but I'll expand a little:

Since pressure acts in all directions and the back of a fan "pulls" air in by reducing pressure, air comes into the fan from all directions. But when air leaves the front of the fan, it is largely its own momentum that keeps it going in a cohnerent stream. That's why you feel so much difference between the front and back of a standard axial floor fan.

An additional issue with an air purifier is that filters require a low velocity across them in order to work properly both for proper filtration and lower energy use. As a result, the velocity at the inlet (because the filter is right behind it) is much lower than the velocity at the outlet (because there is no reason to oversize it). It should be obvious from looking at it that the inlet surface area is larger than the outlet surface area, which causes the difference in velocity. And what we feel on our hand in front or behind a fan is due to velocity.

Appreciate the feedback - both private (and the other thread about how air purifiers suck - lol) and public, Russ.

Sorry for the late re:. As I explained to another poster privately, I was dealing with a nasty (still ongoing) infection all week and been feeling miserable.

I really liked your front and back of a fan explanation above. That's actually very correct. I guess I never realized how big of a difference it can be in terms of the feeling you get from intake vs. outgoing air. It's not just the difference, but the scale/magnitude of the difference that's pretty amazing. One is barely perceptible and the other is like a giant wind and very forceful.

I guess for practical purposes it's good the intake isn't that powerful, as you'd literally have a crazy vacuuming effect of everything in your room and have stuff flying around and getting sucked like a black hole into the fan or air purifier area. So, in that sense, it's nice it's subtle!

Not sure I completely understood the filter comments, but will search that and think about it more when I get home.

Are you still using a Honeywell now, Russ (I recall you bought one for your gf in the other thread)?

 

[rbelli1]

so they don't need to do any extra work to draw the particles into the device.

Also you will not want an air purifier to sound like a vacuum cleaner. Low noise is a high priority in their design thus a low fan speed.

BoB