3. Filtration of Kitchen Exhaust
Filtration of the Kitchen Exhaust air can be desired for a variety of reasons: Local code requirements, complaints from neighbors, concern about the environment, concern about grease build up on the roof, protecting the integrity of the fan, etc. and so on. In the trade these systems are known as and referred to by a few different names that are all essentially the same thing.
- Pollution Control Unit (PCU)
- Electrostatic Precipitator (ESP)
- Air Scrubber
When it is desirable to avoid dumping the contaminated air from Commercial Kitchen Exhaust into the outdoor environment these are the main areas of concern
- Particulate Filtration – particulate is tiny pieces of solid or liquid matter in the Kitchen Exhaust airstream. This pollution is typically filtered either using media filtration or electrostatic precipitation. An electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally impede the flow of gases through the device (low static pressure loss), and can easily remove fine particulate matter such as dust, smoke and grease from the air stream.A plate precipitator is a Penney Type (two-stage) system that contains a row of thin vertical wires, and followed by a stack of large flat metal plates oriented vertically. The air or gas stream flows horizontally through the spaces between the wires, and then passes through the stack of plates. A high voltage of several thousand volts is applied between wire and plate. If the applied voltage is high enough an electric (corona) discharge ionizes the gas around the electrodes.Ions flow to the plates and charge the gas-flow particles. The ionized particles created by the power supply, move to the grounded plates. Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to electrostatic pressure (given from layer resistivity, electric field, and current flowing in the collected layer)Air Quality Engineering manufactures two systems that filter particulate in the Kitchen Exhaust airstream. Both of these series units are Penny Type (two-stage) plate electrostatic precipitators.
- F-72 Series – A manual clean Penney Type Precipitator system. The F-72 series is preferred when installation size is limited, the particulate stream is not considered heavy, or when automatic cleaning (maintenance) is not desired or necessary. This series may also be preferred when the primary concern is budgetary. Key benefits of the F-72 Series systems:
- Very small footprint; allows unit to be placed in areas where competing models will not fit.
- Available in modules or complete:You only need purchase the components that you need if assembling on-site; however turnkey systems are available.
- Lower cost of ownership due to the washable filters (cells) versus the expensive replacement filters found in media type filtration systems.
- Green product: doesn’t fill landfills with disposable filters like media type systems.
- Entry level type system, very affordable
- AutoClean Series – An automatically (self) cleaning Penney Type Plate Precipitator system. The AutoClean Series is preferred when the particulate stream is heavy, or when automatic maintenance is preferred. Key benefits of the AutoClean Series Systems:
- Efficiency of the wash system: AutoClean units have a wash arm that rotates and traverses for excellent coverage of the cells and the pre and post filters compared to much less cleaning efficiency on competing models.
- Lower cost of ownership due to the washable filters (cells), versus the expensive replacement filters found in media type filtration systems.
- Available in modules or complete: You only need purchase the components that you need if assembling on-site; however turnkey systems are available.
- Green product: Uses less wash water than competing auto wash systems and doesn’t fill landfills with disposable filters like media type systems.
- The self-washing feature reduces or eliminates the necessity for expensive cell cleaning services. Based on this cost savings versus manual clean units the AutoClean upgrade can pay for itself over time.
- VOC and Odor Removal – Volatile Organic Compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, VOCs like odors are gaseous and cannot be captured by ordinary filtration means (media or electrostatic precipitation). VOCs and odors must be trapped by adsorption of the gases into an adsorbent media such as activated carbon or Potassium Permanganate. Air Quality engineering utilizes large odor control modules that contain panels filled with activated carbon or an activated carbon/potassium permanganate blend for the removal of VOCs and odor in the kitchen exhaust airstream. These panels are bulk refillable and the sorbent media must be replaced periodically when it becomes saturated and unable to adsorb further. Saturated sorbent media will off-gas and can be an odor source when saturated. Refill frequency will vary depending on the level of contaminant in the airstream. Activated carbon is a form of carbon made from many different raw materials including coal, coconut, wood, etc. The most common raw materials are coal and coconut. The activation process takes place at elevated temperatures in the presence of steam or gas to create a large quantity of molecular size pores throughout the carbon. Through the activation process, the carbon attains a very high surface area to mass ratio. This ratio is on the order of 500 – 1400 m2/g. Activated carbon adsorption capacity and efficiency relies on the surface area, pore size distribution, adsorbate molecule size, temperature, pressure, moisture content, and concentration of adsorbate. Typical removal efficiencies for activated carbon are 95 to 99 wt%.Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO−4 ions. Formerly known as permanganate of potash or Condy’s crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the evaporation of which leaves prismatic purplish-black glistening crystals.Odor modules in the Kitchen Exhaust airstream are intentionally placed after the precipitator or other particulate filter. Because sorbent media has a large open pored structure it is susceptible to “face loading”. In other words if there is particulate in the air stream it can clog the front panels of the carbon (similar to a prefilter clogging) and keep the panels from performing their intended function. Similarly if the precipitator fails for any reason the carbon panels can be compromised. For this reason it is not uncommon to see some sort of media filtration between the precipitator and the odor module. This is typically referred to as a “safety filter”.
- Size, Type, and Location of the Filtration System – Filtration systems are located nearly anywhere along the grease duct run as long as they are physically located somewhere between the hood and the fan. That leaves a wide range of options for size, type and location of the systems. Choosing and locating the proper Kitchen Exhaust filtration system depends on a wide range of variables.
- Static Pressure (SP) – Static pressure is resistance to airflow. It can be due to twists and turns of the ductwork, the ductwork itself, and the impedance from filtration. It is measured in inches of water gauge (wg). For Kitchen Exhaust applications the static pressure is typically only a factor in fan selection. There are three typical designations of static pressure.
- External Static Pressure – The static pressure of the ductwork and other factors that are external to the air purification system.
- Internal Static Pressure – The static pressure created by the air purification system only.
- Total Static Pressure – The sum of the external and the internal static pressures.
- Kitchen Exhaust System CFM – This is the Cubic Feet per Minute volume of air that is traveling through the hood and will enter grease duct and ultimately the air purification system (if so equipped). This is not to be confused with airflow velocity. Velocity is used to calculate CFM but they are not the same. Velocity is not a constant as different hoods have different recommended velocities. (CFM = Hood Area Sq. Ft. x Velocity)Air purification systems perform at different efficiencies depending on the CFM through the particular air purification unit. At Air Quality Engineering we strive for a minimum of 95% efficiency (per ASHRAE 52.1 test standard, efficiencies to various other test standards available upon request) and size thusly. For liability reasons Air Quality Engineering does not calculate or suggest airflow requirements. The CFM figure is typically supplied from the engineer or manufacturer that designed the hood system for the particular application.
- Level of Contaminant – The level of contaminant is a factor in determining the type of equipment and the number of passes to be utilized. In light duty applications (pizza ovens, bread ovens, kitchens without charbroilers for example) the AutoClean feature is not as critical because the maintenance won’t be as frequent and the F-72 series will perform well.Similarly, in heavy duty applications (heavy charbroiling of chicken, hamburger, or steaks for example) the AutoClean is preferred as washing may be necessary weekly or even daily. The F-72 series can be used in heavy applications but it should be noted that frequency of maintenance can be a concern.In heavy duty applications where the utmost in efficiency is desired a double-pass may be utilized. In these instances there are two passes of precipitators used to capture particulate. The first pass gets the bulk of the particulate (>95%) and the second pass is intended to capture (>95%) of the remaining particulate (the particulate missed by the inefficiency of the first pass). It’s not quite as simple as this in practice, but this illustrates the double-pass philosophy.
- Physical Space Limitations – The actual space available for installation is often a consideration in the type and/or location of the air purification unit. The F-72 series has a smaller footprint than the AutoClean series so it can more easily fit into tight spaces such as above the ceiling tile. There are two main locations where these units are typically located. Each has different and very specific needs and concerns.
- Indoor installations – Indoor installations have inherent benefits for the customer and the equipment. Indoor units are typically less expensive and have a longer lifespan as they are not exposed to the elements nor do they need to be prepped for such exposure. Indoor units are often placed above drop ceilings so access for maintenance can be a concern. Another concern that engineers/architects need to address for indoor installation is making sure the ceiling that is supporting the unit can handle the additional weight. Physical space limitations are a key concern of indoor installations. There needs to be space for the equipment and there also needs to be space to open the doors and access the cells and the odor module panels for maintenance. Indoor installation may also be preferred for high-rise buildings so that the exhaust can exit the side of the building. Running grease duct to the roof of a high-rise building is typically cost prohibitive.
- Outdoor Installations – Most outdoor installations are rooftop jobs however there are instances where a mezzanine or courtyard can be utilized for unit placement. The model AutoClean or F-72 series units need to have an outdoor enclosure (AKA Doghouse) when being installed outdoors. The electronics and cells need to be protected from the weather. The AutoClean water supply, detergent line, and drain may also need to be heat traced in cold weather locales. When the air purifier is to be located on the roof there are some roof preparations that need to be considered as well (curb, weight restrictions, and the rigging and/or actual hoisting of the unit into place). For all of the above reasons outdoor installations are more expensive than indoor installations.
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