Commercial Range-Oven Change-out.

An anemometer will measure air flow.

1 - As previously mentioned, a vane or hot-wire anemometer can be used to get velocity readings in the duct, from which volume can be determined (VxA). You will need a small hole in the duct to insert the anemometer and should take several traverse readings to get an good average velocity. Anemometers can range widely in price & quality - check omega.com for a decent, low-cost one. Air balancers also use a flow hood that fits over a supply grille (usually 2'x2') and provides velocity readings, although they are fairly expensive.

2 - Per fan affinity laws, Q₁/Q₂ = n₁/n₂ and the differential pressure dp₁/dp₂ = (n₁/n₂)². The increase in volume (50%) for such a small increase in speed (14%) seems incorrect - perhaps they have reported this performance at the same dp as for operation at 1314 rpm, but the dp will actually be higher due to higher air velocity. The actual flow should be pretty close to that predicited by affinity laws, (1500/1314) x 800 = 913 cfm. Ask the mfr for fan curves at each speed and overlay with your system curve to find out what you can really get out of the fan.

3 - Get the mfr & model off the nameplate and get an IOM manual from the mfr.

4 - This is not the way I would design it, but I don't think there is a code issue with it. If the MUA register is properly adjusted, the throw should prevent short circuiting, but it also might blow some of the fumes/smoke into the space.

5 - Higher velocity = higher friction/pressure loss, resulting in more power required. Per affinity laws, P₁/P₂ = (n₁/n₂)³ so even small increases in speed can result in large increases in power requirements. Given the size and velocity you posted, the pressure drop is about 1.5" WC/100 ft of duct (get a Ductulator or similar from HVAC mfr) which is alot higher than I would size at, but is probably OK as long as you dont overload the fan motor (confirm with fan curves). The other issue that you may run into by increasing velocity is that it will get noisier, which might cause complaints depending on current noise levels.

Good morning dac1267, 1. Thx for the lead to the anemometers. I have requested the flow quantities on the existing ducts.

2. Based on the affinity laws, I have calculated the new 1000 cfm flow rate based on 1642 rpm. However, in looking at the Performance Table for the installed fan, for 0.375"wc ESP and 1350 rpm, the flow rate would be 1546 cfm. I estimate the friction loss at 0.33"wc based on your 1.5"wc/100'. So, based on the Table, the flow rate may already be at or over the required 1000 cfm.

Surprisingly, at 800 cfm and 0.33"wc ESP, the required rpm falls far below the minimum fan speed of 1150 rpm. At the new flow rate of 1000 cfm and an estimated new ESP of 0.5"wc, the new required rpm is still below the minimum rpm.

3. The contractor is to supply more detailed manufacturer data on the MUA unit.

4. MUA Discharge Grille location inside hood. The contractor chose that location based on a recommendation by the exhaust duct fabricator, who does this type of work in the area. The City Fire Inspector did not like this location, but approved it anyway.

5. Based on an existing equivalent exhaust duct length of 22', I estimate the friction loss at 0.33"wc.

Thx to you and the others who offered opinions.

Its been a while, however, I believe that the code you need to apply for the exhaust duct is NFPA-96, for commercial exhaust from a range hood. Depending on the local building code, you will need to verify which version (meaning year as it changes frequently) they require you use. The minimum exhaust velocity will be given there, although from memory I believe it is 1500 fpm. You will have to confirm with local authorities.

You may need to revise the duct to meet the code as well depending on when it was built. Also, there may be other issues. These things can turn out to be much larger than initially anticipated. You will have to do your homework on this one, because these issues tend to be up to the AHJ.

Cheers,

Duane