Unless this is homework, you should use a manufacturer selection tool or talk to them.

If in series, each fan gets he same airflow. So they should be he same or you make things complex or inefficient. Same for parallel operation.... 2 different fans would fight each other.

what is your exact use case?

It's a DIY spray booth, i've also added HEPA filter to exhaust. So i need both airflow and pressure. I don't have enough space to use of those grim fans. I have two 280 m3/h and one 150 m3/h fan, trying to make combinations to maximize airflow

Cv9191.

1) wrong way around. Series is the same flow but higher pressure, parallel higher flow assuming back pressure doesn't increase.
2 yes
3 dunno. Depends on the fan curve.

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The flowrate of the fans will vary with differential pressure so you need the fan curve and the system curve of your ducting.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

A HEPA filter doesn't filter VOC. Any normal filter would take out paint particles if that is your concern.

I don't recall how many fpm you need for a paint booth, but it is quite a lot. Multiply with the crosssectional area you stand in and you will see your puny fans won't be enough.

without fan curve this all is meaning less guessing. Depending on type of fan an inlet and outlet conditions, they also may create fan effect for each other if too close to each other. Fans in series are rare for a reason. Fans in parallel typically are the same size. For a paint booth, a single fan usually is large enough to not need multiple fans.

it is like you drive a car. but now you want to move more stuff or go faster. You should get a larger or a faster car. You kind of try to drive 2 cars in parallel, or have them tow each other to get faster.

Thanks for all your supports,
Firstly as i worked on many CBRN Protection projects i know that HEPA doesn't filter VOC, I have a fiberglass prefilter before fan which catches most of the particles that HEPA is considered for catcthing runaway aerosol particles to take odour away (i was wrong). Now i decided to use activated carbon to filter odour, i think it's the right way.

Now the things have changed for fans, i've installed my 280 m3/h fan, it draws air with only prefilter but doesn't seem adequate for me. I've ordered a 600 m3/h inline fan which i want connect it to ducting. I have no curves for both fan. By assumming these airflow values at zero pascal and system will run at zero pascal what do you think if i run them in series?

Without curves you can't say.

It is though quite likely that the 280 fan could actually restrict airflow by acting as a restriction. Nothing runs at 0Pa....

So adding another fan in series will increase airflow from the current fan, but probably not as much as the "capacity" of the bigger fan

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Inch and Novice have answered your question. Parallel fans should always be similar as the the one w/the most balls (that's a MT Tech technical term) will override the other one. Fans in parallel (assuming the same size), double the air flow while working against the same differential pressure. Fan's in series, work w/ the same airflow and boost pressure. You mentioned about two fans in parallel and adding a fan in series? Let's not Rube Goldberg this thing (I just dated myself). I've done stuff like that but it's not recommended.

As far as filters, VOC's can't be handled by particulate filtration. Maybe activated carbon or UV or bi-polar stuff. Crap, I used to know that stuff when I was younger. Haven't run into it for awhile.

You have rule #1 reversed. Parallel fans increase flow. Series fans increase pressure. However, you cannot simple double or you your case expect "n" times more.

Two fans in parallel will only double the flow AT THE SAME PRESSURE. However, you most likely will not be able to operate at the same pressure. If you are blowing into a duct system, your friction will increase with the additional flow, so you will not get double. Eventually, adding more fans in parallel will have NO EFFECT on flow.

Two fans in series will only double the pressure AT THE SAME FLOW. However, you will get more flow, so the pressure will not double.

This might help you understand a bit better: https://www.engineeringtoolbox.com/pumps-parallel-...