Make up Air Help
Not being and expert in make up air I have a question. Hibachi steak house which only uses the kitchen cooking equipment on occasion.
I have a hood that is 17’ X 4’ Long. The ventilator is a FX24BFT. The hood has 4 rectangular openings(close to 7”X10”) inside the front which lead up to two round take offs(9”) which lead up to a 30” X 30” fresh air opening. When the ventilator kicks on it creates a negative pressure condition in the kitchen pulling open the doors. Can I add a make up air unit to this hood? If so, how do I size it? How do I know the right ventilator for this hood was selected?
IF the FAI intake is mounted on the roof. It's probably best to transition the duct into a MAU. Or any supply air fan to interlock with the kitchen exhaust fan. CFM of the fan should be equal to the KEF CFM. Maybe a little bit over.
If the hood was manufactured by a company that makes specialty equipment the intake collars were probably sized correctly. Is it a long duct run to the outside?
Each 9” take off runs with flex about 4 feet, ties into a transitional trunk then turns up to the 30 X 30. The take offs are welded which leads me to belief they were designed with the hood however the fresh air was only figured to have an elbow with bird screen on the roof. The exhaust fan is not the same size as the curb which was cut into the roof originally which gives me the feeling it was replaced at some time and perhaps oversized which is why the fresh air with out a fan unit is insufficient This exhaust fan can move between 2500-10500 CFM so my concern is, how much CFM should the MAU be rated for? If I select a unit that with a 15” wheel which can move between 2000-8000 CFM(found at http://www.empirehood.com/RTP/rtp_performance_data.htm), how do I know the hood is designed properly to handle the air change?
Thanks for the help,
If I recall this correctley an island hood open on all four sides requires 150 cfm per sq.ft.
A hood against a wall require 100 cfm per sq.ft.( open on three sides ).
Using your size : 17 x 4 = 68 sq. ft. x 100 cfm = 6800 cfm.
The exhaust velocity needs to be 1500 fpm.
Make-Up Air (supply) needs to be approx. 80% at the hood.
Using your size hood, 6800 cfm x 80% = 5440 cfm
The buiding (kitchen) pressure needs to be neutral or slightly positive.
With your hood, that means 6800 cfm - 5440 cfm = 1360 cfm of fresh air is still required to be supplied to building. Generally advised to be supplied thru economizers from Roof Top Gas Pak or A/C.
This should help you pick the correct sized Make-Up Air Ventilator. Then you must size the duct based on the static. The Mfg. of the ventilator will be able to help you size this up.
If i remember right , i had that problem after a couple of
power hood exhausts were added, and so i compensated with the yacs or rtu's
the only time i had a prob was when it came to winter, so i then compensated and left the doors pefectly closed the units closer to the doors i compenstated for this and kept the kitchen in a .04 neg for the odors
and the fresh air was always there, the gas on the hot water demand for the chafing dishes etc , was a relighting prob, until i compensated with the gas pressure and pilot light adjustment,,,
ashrae use to have kitchens set for 20/15/8 thats occupancy per person/cfm per person/ L/S -person telaire systems inc., IAQ
it all depends on how your air is set up and where on your ac systems or pacs or rtu's too, and how much they can move, i had four 80 tonners, they didnt have enough space for the air i could move, or enough heat load to give me a run for the money!
I usually had to readjust the air , because they had guys that had passwords to make adjustments to make everyone miserable ,and themselves dangerous!
Changed a 40 hp copeland or two because of that same reasoning.. that was fun
Those face velocities sound about right for non listed hoods. The listed ones will have lower airflow usually.
Originally Posted by mardoman
A good reference on kitchen ventilation is
The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.