Kitchen Exhaust Fans, Fire Protection And Exhaust Make Up Air
4. Kitchen Exhaust Fans
Kitchens Exhaust fans (AKA blowers or blower packages) are the last link in the total exhaust package. The fan draws the air into the hood, through the grease duct, through the air purifier (if present), and discharges the air out of the building to the outdoors. Kitchen Exhaust is always discharged outdoors; it is never recycled back into the building.
In properly configured Kitchen Exhaust applications the exhaust fan is always placed downstream from the hood, precipitator, and the odor module. Configuring the system this way creates negative pressure inside the purification equipment and prevents polluted air from escaping through cabinet penetrations (access doors etc.). It also prevents the fan from loading with particulate. In other words you want to pull the air through the system, not push it.
Fans for Kitchen Exhaust need to be rated for handling grease laden air. Appropriate fans will have a UL 762 approval and listing. The main determining factors in fan selection are CFM need, total system static pressure, noise volume and discharge preference. There are two main types of fans used in Kitchen Exhaust applications.
- Utility Set – A fan typically designed with a single-inlet, scroll housing, and a backward-inclined or an airfoil centrifugal impeller. It can provide a higher static efficiency capability than a typical power roof ventilator. Air enters the impeller axially and leaves it in a substantially radial direction. These can be mounted indoors or outdoors in-line having additional duct between the fan outlet and the point of discharge. Utility sets offer a lower price point than mixed flow fans.
- Mixed Flow – Mixed Flow Fans combine the benefits of axial flow and centrifugal flow fans. They have the advantage of compact design, straight-through airflow, as well as the preferred acoustic characteristics and high pressure capability. Mixed Flow fans offer superior air and sound performance.
5. Commercial Kitchen Fire Protection & Suppression
Grease in a Commercial Kitchen Exhaust duct is highly flammable. Because of this danger commercial kitchens are required to have automatic fire suppression systems. In the USA Fire Suppression systems are governed by the codes under the National Fire Protection Agency (NFPA). Fire Suppression Systems use a combination of dry chemicals and/or wet agents to suppress fires. Suppression systems help control damage and loss to equipment and buildings. Common means of detection are through heat sensors, wiring, or manual detection (depending on system selection).
The hoods, ducts and air purification equipment are equipped with piped nozzles that carry fire suppressant to the various components of Kitchen Exhaust system to extinguish fires should they occur. These systems work in conjunction with the exhaust fan and when the systems are designed and installed this is an important consideration. In general when the system detects a fire the precipitator should shut down but the exhaust fan should still run. This is to allow the system to evacuate the heat and smoke from the fire as the chemical agents take care of the flames without interference from the precipitators.
All kitchen fire suppression systems manufactured after 1994 must comply with the UL300 standard to account for increased commercial cooking times and temperatures. Dry chemical kitchen systems have been replaced with wet systems to accommodate hotter and longer cooking times.
Some manufacturers pre-pipe their filtration units with nozzles so that during installation the fire protection company need simply hook up to these with their lines carrying the suppression agents. Air Quality Engineering does not pre-pipe for fire suppression. Local codes and ordinances along with NFPA rules are subject to change. Our preference is to have local fire suppression experts install the nozzles where local codes require them.
6. Kitchen Exhaust Make-Up Air
There needs to be a system in place to replace the large amount of air evacuated by the average Kitchen Exhaust duct system. That is where the aptly named Make-Up air system comes in. This system brings in outdoor air to replace the air that is lost up and out of the building through the exhaust system.
If this air is not replaced at the same rate that it is expelled, the building will experience a negative pressure situation (the air pressure outside the building is greater than the air pressure inside the building). Some of the most common indicators of a negative pressure problem include the suction pressure that makes exterior doors difficult to open, exhaust fans that don’t work properly, drafts around doors and windows, poor indoor air quality, and infiltration of outdoor air. These problems are more pronounced during the cold season when windows are kept closed, reducing the natural pressure-balancing effect.
A make-up air system maintains the proper balance between inside and outside air pressures while also providing many other benefits:
- supplies conditioned replacement air that can be heated, cooled, humidified or dehumidified
- greatly reduces infiltration of cold outdoor air and drafts through opened doors
- promotes proper operation of exhaust systems
- improves indoor air quality
- eliminates suction pressure at exterior doors
- decreases load on existing HVAC system
- lowers energy costs by as much as 35%
- maintains a more comfortable and productive working environment
The make-up air unit consists of a blower (fan) that takes in fresh air and passes it through a bank of filters to remove dust and other contaminants. The amount of fresh air brought in is regulated by the dampers in the damper assembly. An actuator opens the dampers to varying degrees depending on how much fresh air is needed at the time. In a heating application the passing air is then efficiently heated by a direct-fired gas burner. The heated air is then either blown directly into a building space or it can be ducted to target locations inside a building via the air distribution system.