Hi snow man:
Replying to sizing a vent hood:
Sizing the hood is the simplest part of the ventilation equation.
The hood must extend 6 inches over all sides of the equipment under it.
Exception: If against a wall only a 3 in air brake is required between the hood and the wall, no extention over the equipment. Most competent hood manufactures automatically build in a 3 inch air brake on hoods that are against a wall.
In addition curtains are allowed on the ends of a hood if they do not interfere with the operation of the equipment. The size of the hood can be reduced to where the curtains are adjacent to the equipment.
The above is only the start to designing a hood - ventilation system. The amount of air to be extracted is determined by the various units of equipment under the hood. That can very from 300 CFM per foot hood over ovens to 525 CFM per foot of hood over a broiler.
What ever the total CFM of air extracted, an equal amount of air, no less then 10 degrees cooler then acceptable room temperature, must be returned to the building. That is known as make up air.
Considering that the cost of warming make up air averages about $1.50 per CFM per year that can become a significant cost.
Make up air units are not, as of now, required by code to cool the warm weather air they bring into a building. The business operator has the option of, adding a rather expensive cooling system to his make up air unit, suffering the summer heat the unit brings in or installing a lot of additional A/C .
It should be apparent to all that for considerations of operating cost and work area comfort a properly designed ventilation system can cut both operating costs and improve work area comfort.
Example a pizzeria hood over a high volume conveyor oven that is used in most high volume shops. Assuming an 111 in long by 60 in deep oven it requires a 123 in long by 66 in deep hood.
Assuming an un certified hood, ( a certified hood has been tested by UL or ETL and the lowest CFM at which they can operated has been certified) uncertified hoods require more air to be exhausted, That hood will require 4700 CFM of exhaust and 4700 CFM of make up air.
At about $1.50 per CFM of make up air per year that is about a $6000.00 cost, per year, for gas and electricity. That figure can be substantially more or less depending on the geographical location. Additional costs are incurred for additional A/C to combat hot summer air brought into the building if the make up air unit does not include A/C
Note also that the above calculations apply to ventilation systems recently installed or to be installed now under current codes.
Without going further into details let me state that we are now installing ventilation systems,
over conveyor ovens, such as described above, that only extract 800 CFM of air from a building.
George Mills