How to decide the exhaust air flow for a paint booth hood? What kind of exhaust fan is typically used? The ASHRAE mentions the ACGIH.  What's that?

Paint booth hoods are like kitchen exhaust hoods in that you are designing for a capture velocity over the work area.  Typically these have one open end and the flow rate will be the face area x the capture velocity.  Smacna duct design manual describes how to design the ducts, likely sized for minimum velocity.
 

I am guessing that ACGIH is the American Council of Government Industrial Hygenists. They publish a book on Industrial Ventilation.

Paint booth ventilation is such a common application that the problem is addressed by a number of Ventilation textbooks. I think the ASHAE manual does not go into it because it is not the core subject of air conditioning and refrigeration.

There are a number of points to watch with paint booths

1 - if the air velocity is too high the wet paint will start to run and disturb the finish

2 - the air must be filtered - otherwise dust will be attracted to the wet paint

3 - the exhaust will carry droplets of paint. This tends to build up on the inside of the ducts, and on the surface of the fan blades

4 - you cannot discharge paint into the atmosphere - so you have to make a plan to clean up the discharge. Often spray chambers are used to trap the paint droplets

5 - a small hood may work with a kitchen propeller fan, but for a larger volume you will need a centrifugal fan. The pressure drops involved will also dictate this. Talk to a fan Vendor for a recommendation

6 - it is important that the air distribution is as even as practical - to prevent fast moving streams developping in the painting space

If spraying anything considered hazardous by the government (almost anything besides water), there are OSHA & EPA regulations to consider.  Presumably Canadian regs. are similar to US.

OSHA regs protect the worker inside the booth; typically  require some minimum velocity of 60-100 linear feet per minute, depending on application method.

EPA regs are to collect the paint overspray on the filters, filtration efficiency depending on the hazard & volume of paint applied.  Minimum flow rates & changing filters is required when ΔP across filter reaches 1.5" water column (or similar).  So, allow a deep plenum for high capacity bag filters to minimize changing.

OSHA & Fire Code may require internal sprinklers in both booth & plenum, plus approved booth lighting, for flammables.
http://www.chemcomfg.com/overspray.htm

Chapter 9 in DOD Handbook INDUSTRIAL VENTILATION:
http://www.wbdg.org/ccb/DOD/UFC/ufc_3_410_04n.pdf

ACGIH is the Am. Conference of Gorvernmental Industrial Hygenists.

Their "Industrial Ventilation" handbook has specific paint booth requirements and design parameters.

http://www.acgih.org/home.htm

The electrical guys require explosion proof (or spark resistant) exhaust fan, what model is commonly used?

ask your new york blower rep.

Don't use an airfoil type fan.  The paint build up will quickly degrade its efficiency.  I would use an upblast axial type.  Controls and starter will have to be explosion proof.  Fire sprinklers will be required in the booth and ductwork.

Protect your fan with filters. See Viskon-aire Corporation for spraybooth design and filtration.

Also look at bifurcated fans; has the maintenance advantages of direct drive, but the motor is out of the airstream.  The fan has an integral ducting that splits around the motor and rejoins enclosing the blades.  Used also on kitchen exhausts.

Following up on lilliput1's suggestion, a common design is to erect a plenum chamber with one wall composed of paint arresters  with  the fan drawing from there. See http://www.rpezkleen.com/paintarrestors.home.cfm
B.E.