Vent hoods: A Simple Explanation
Commercial vent hoods are required in restaurants and kitchens for one reason: to remove the smoke, grease, and steam vapors that are expelled by the various pieces of kitchen equipment. This is achieved by removing the air with an exhaust fan. If the equipment expels grease vapor, then grease filters are installed inside the vent hood to capture the excess. Grease hoods are referred to as Type I. If the vent hood is purely for removing steam vapors and heat such as those caused by high temperature dishwashers, then the vent hood is referred to as Type II. As simple as this may seem, it is a science that takes careful calculation and analysis. Here are three primary questions to consider before deciding on a commercial vent hood: How large will the vent hood be? How much air must the vent hood remove? How will this affect my building over all?
Vent Hood Size:
For our purposes we will be using a wall canopy style vent hood. Although there are a variety of vent hood styles, this is one of the most commonly used. The size required on commercial vent hoods are determined by building codes. The International Mechanical Code states:
The inside lower edge of canopy-type Type I and II commercial hoods shall overhang or extend a horizontal distance of not less than 6 inches (152 mm) beyond the edge of the top horizontal surface of the appliance on all open sides. The vertical distance between the front lower lip of the hood and such surface shall not exceed 4 feet (1219 mm). - IMC 2006 507.12
This means the your vent hood must extend past the boundaries of your restaurant equipment by at least 6".
Real world example:
A kitchen plans on cooking its menu on 3 pieces of equipment; a 36" flat grill, a 24" char-broiler and a 36" range top. Assuming the equipment is lined up side by side you would get an overall measurement of 96" or 8 feet. Add 6" to each end of the equipment and you have an overall of 108" or 9 feet. The length of the vent hood you need will be 9 feet. Keep in mind that this measurement is the entire linear length of your equipment line up including items that are between each piece cooking equipment. If you added a 36" stainless steel prep table between the grill and the broiler, then then the size of the vent hood would grow larger accordingly. Please see picture below:
Type I vent hood with common equipment
Type I vent hood with s/s table inbetween equipment
Calculating Air Flow of the Vent Hood
Air flow is measure by volume over time. The most common unit of measurement is Cubic Feet per Minute or CFM. If you have a room that is 1800 cubic feet (15'x15'x8') and apply an exhaust fan rated at 900 CFM, then it will take approximately 2 minutes to completely remove the air from that room (2min x 900cubic feet/minute= 1800 cubic feet).The amount of air a vent hood will remove is determined by a set amount of CFM per linear foot dependant upon the type of cooking equipment being exhausted. Common recommendations are 200 CFM per foot for medium heat equipment like flat griddles, ranges, ovens and 300 CFM for high heat equipment such as char broilers, wok ranges, etc. Lets assume we require a vent hood 10' long and that the cooking equipment is a medium heat application. We will then multiply 200 CFM per foot by 10 ft which is 2000 CFM.
How Will the Vent Hood Effect My Building?
Your building contains a myriad of components that must work together in order to keep the structure safe and efficient. Installing a commercial vent hood introduces a new and unique variable into this harmony.This conformity is achieved with a 'balanced' vent hood. Balancing a vent hood means that the amount of air being removed is made up by forcing air back into the building. Its a tightly knit integration between your HVAC system and vent hood. Type I vent hoods will typically be installed with a fresh air or make-up air device for this reason. A secondary make-up air fan system will be installed using a separate duct and connect to the fresh air device on your hood. Typically this is a called a fresh air plenum and is installed in the front of the hood. It will often appear as a shallower box and have perforations along the surface. A common mistake people make is to assume the make up air return acts as a wind screen to contain the smoke. This is not the case, as your vent hood exhaust is designed to remove the smoke, grease and vapors independent of a make-up air system. If it were not for the make up air, your HVAC would have to fight against the constant siphoning of air out of your building through the hood's exhaust fan. An HVAC which works harder uses more energy and using more energy means paying more for electricity. The amount of air being returned into your building should generally be about 80% of the exhaust CFM. 2000CFM of exhaust will translate into 1600CFM of make-up air. The remaining 20% of air should be made up through your HVAC fresh air intakes. Its is important to note that local building codes vary, and the amount of make up air required may be different where you live. As a general rule for any examples given here, always check with your local building codes to make sure you are in compliance.
The points made here only cover a few of the basics regarding commercial vent hoods. There are many more elements that integrate into the ventilation system including the fire suppression system, duct work, electrical shunt trip wiring, pull stations, and much more. We encourage you to continue your study of restaurant vent hoods and kitchen exhaust system in our vent hood section. There you will find a wealth of information about the various elements not covered in this section. Call us at 1-866-618-4999 to get consulting on any commercial vent hood system.
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