The control and removal of grease from hoods by means of grease filters, baffles and similar devices is a subject of sufficient concern to warrant a separate chapter in NFPA 96. The standard specifies the conditions of installation and minimum distances between the cooking surface and the grease removal device.
Grease Extraction Rates
The National Bureau of Standards is not equipped to test ventilators.
Manufacturers test their own units according to NBS test procedure IR74-505.
This is therefore the procedure VM uses to test its Cyclo-Wash 3, Cyclo-Maze and Cyclo-Vent units.
Although VM tested its own units to the specified NBS test procedure, it used the services of the Institute for Storm Research to verify the tests. Dr. J.
C. Freeman, President of the Institute, has expertise in air movement, temperature, point and factors related to condensation extraction.
The grease extraction rates of VM units, expressed as a percentage by weight of grease removed, are on the basis of averages of a series of tests.
Filters (stainless steel baffle type GF Series)
Of the various devices available to collect and remove grease from hoods, steel filters were for many years the most common means. The main requirements for a filter are that it must be:
C of steel construction, tight fitting and firmly held in position;
C easily accessible and easy to remove for cleaning;
C installed at an angle not less than 45o from the horizontal;
C equipped with a drip tray pitched to drain into a container having a minimum capacity of 0.13 gallon (0.5L).
Figure 23 - Air flow in
Some devices for removing grease also serve other functions, such as removal of combustion gases, heat and cooking odors. VM has pioneered a number of combination units, which this Section explains. For installation, operating and maintenance procedures, refer to the relevant sections and the appendices.
Filter Grease extractor chambers (Cyclo Maze series)
There are two types of Cyclo Maze ventilators. One is a dry type; the other is a water mist unit that uses a cold water mist to extract grease and airborne solids, and a hot water wash for cleaning. The principle of operation for both types is the same.
Baffles cause grease-laden air entering the plenum to move in a spinning motion. With cold water mist nozzles, the air passes through a curtain of cold water mist, which causes grease particles in suspension to solidify and fall into a drainage trough or to spin off and collect on the baffle surfaces.
The high velocity flow of the air stream through a further set of baffles removes the residual grease in suspension under the effect of centrifugal force.
The larger volume of the upper section of the plenum greatly reduces the air velocity, causing even more grease to fall out of the air stream before it flows through the head exhaust collar into the exhaust duct, as shown in Figure 23.
The unit operates on the principle of high capture velocities and low exhaust volumes. These two features combine to provide economical grease extraction. Baffles cause air entering the plenum from the cooking surface to move in cyclonic motion. The centrifugal action of the air causes heavier particles of airborne matter to collect on the baffle surfaces and drop to the bottom surface of the plenum. Stages in the air flow through the Cyclo Maze unit (refer to the items shown in Figure 23 and Figure 24) are:
1 The inclined face plate directs the air into the lower baffle chamber.
2 Air entering the chamber is spun in cyclonic motion.
3 The air then splits into two separate streams.
4 Each stream encounters separate intercepting baffles.
Figure 24 - Air flow in Cyclo Maze cold water mist unit
5 Manually adjustable baffles allow for air balancing along the length of the hood when more than one type of cooking appliance is installed.
6 The upper chamber reduces the air velocity before it exits the pod to further drop out grease particulates.
7 The air stream passes the exhaust control dampers used to adjust air volumes and balance different hood sections.
8 In the final stage, the cleansed air column passes through the listed fire damper assembly.
9 The cold water mist grease extractors are fitted with cold water nozzles to supply a continuous stream of mist to the extractor chamber (see Figure 24). This reduces the temperature in the chamber and increases the extraction of grease (CCM series).
Note
Three factors affect the removal of grease in exhaust hoods:
C Air velocity
C Change of direction and spinning thin air C Lower exhaust air temperatures
An efficient design uses a combination of one or more of these factors throughout the path of travel of the exhaust air in the grease removal chamber.
A good design in an extraction chamber requires a low static pressure to operate efficiently; i.e., low internal chamber resistance allows for lower horsepower and energy consumption to achieve maximum grease removal efficiency.
Grease Extraction Unit as an Engine
If we recognize an engine as the energy source which makes an installation or equipment function, we realize that the engine of a restaurant is its kitchen. In turn, the cooking bank is the engine of the kitchen and the exhaust hood or ventilation system keeps the cooking bank engine running smoothly.
Exhaust hoods come in various shapes, capacities and styles. The grease extractor is the engine of the hood and the main feature on which the efficiency of the hood depends. This report is a summary of the characteristics and specifications of hoods and their grease extraction engines.
What makes a hood engine efficient?
C Changing the direction of air to spin out grease.
C Changing the air velocity to drop out grease.
C Changing the air temperature with cold water mist to scrub and condense out grease.
What types of engines are there?
Automobiles with which engines are most frequently associated are equipped with four, six and eight cylinder engines, together with many features that make them different. The same is true of exhaust hood engines.
The power of the engine lies in its capacity to remove grease. The 4 cylinder engine (2” deep) is the smallest in the range and removes the least amount of grease. The 6 cylinder engine is larger (12” deep) and can be manually cleaned or self cleaning using a hot water wash. The 8 cylinder engine has a cold water mist scrubber added and, using the third grease removal principle, removes a lot more grease. The 12 cylinder unit is the largest engine (18” deep). It has 200% more cold water.
4 Cylinder
areaThe exhaust hood body is the same as a car body.
Vent Master hoods accommodate all engines
Figure 25 - Types of hood engines
Ducts
As part of an overall ventilation system, the ducting serves two important functions. One is to exhaust the kitchen air to the outside environment; the second, when replacement air is supplied through the ducting system, is to deliver outside air to the kitchen space.
This chapter discusses the use of ducting in any ventilation system and considerations designers, suppliers and owners need to take into account.
Here, in a general way, it is worth noting that ducting is to the system what piping is to the water supply in that it directs the air flow from and, in some designs, to the kitchen space. The shorter the duct run, the less power is needed to move the air stream. In single-story structures where air is exhausted through the roof, the size of the ducting is not critical. The run is short, the resistance to flow negligible. In multi-story structures where the duct run length is considerable, the designer must calculate the size of the run with accuracy.
Ducting resistance to air flow and duct length also determines the size of the exhaust and inlet fans.
Space is the important criterion. A long, straight duct run with minimum turns to smooth the air flow are basic to good design. There are, however, other important factors to consider. Many codes and standards specify that ducts must not pass through fire walls or partitions; kitchen ventilation ducts must not be interconnected with any other ventilation ducting or exhaust system; and duct runs must not have dips or traps in which cooking residues can collect. Openings for access are also required, the openings clearly marked to prevent the placing of obstructions to access.
These are some of the major considerations in the design and use of
ventilation ducts, which make the ducting an important part of any system. The applicable codes (see the Codes and Equipment Specifications Section), provide specific detail and consultation of those considerations that apply. Here are other important topics worth noting.
Ducts & ventilation rates
To give some idea of the cfm capacity requirements of exhaust systems, Table 3 lists cfm values used for various typical kiosk food court installations and the hood sizes installed for food court applications.
Using the values tabulated in Table 3 for the eight kiosks, there is an average cfm value of 17,800/8 = 2,225. It is therefore safe to work on an average value of 2,500 cfm per kiosk, but allow for individual kiosk sizing for cfm. (See also the Auxiliary Equipment Section.)
Openings
Access openings are required either at the sides or on the top of duct runs at every change in direction. Hoods with dampers in the exhaust or supply collar require an access opening for cleaning and inspection purposes. Similarly, cleaning and inspection openings are needed for exhaust fans having duct work connected to both sides, the opening or openings to be within 3 ft. of the fan.
Duct cleaning is such an important part of maintenance that openings must allow for cleaning the duct work along its entire length. This, in turn, means that the system designer must make sure inspection and cleaning openings satisfy this cleaning requirement. Kitchen ventilation is not the same as ordinary building air conditioning ducting. Venting odors and grease vapor material can pose fire hazards about which local and jurisdictional authority inspectors can be and are particular.
Ducting installation
The emphasis placed on fire safety and protection by all authorities with regard to ducting is reflected in the installation requirements of ducting seams, penetrations and connections.
Many codes specify that duct-to-hood collar connections must have a liquid-tight continuous external weld. NFPA 96 from which
Figure 27 is taken is specific in its requirements for connections at the hood collar that are not continuously welded.
Duct installation clearances
A strong emphasis of NFPA 96 Standard concerns the hazardous fire
10' 0"
Not less than 1-hr fire resistance for
Roof Access panel
building less than 4 stories in height Not less than 2-hr fire resistance for
building 4 stories or more in height Opening in enclosure
Second Sealed around the duct at this point since fire-rated floor
Exhaust hood
Figure 28 - Typical section view for building with two stories or more with non-fire-rated ceiling and fire-rated floor
For SI units: 1in = 25.4 mm;
1 ft = 0.305 m
Source: NFPA 96, 1994 Edition
potential of cooking operations. Figure 28, taken from NFPA 96 Appendix A, shows detail to cover various types of installation requirements for a typical commercial cooking exhaust system. Figure 28 is followed by Table 4 (also courtesy NFPA 96 Standard) giving examples of types of construction assemblies containing noncombustible, limited combustible and combustible materials.
Exterior installation
Many codes, and NFPA 96 in particular, recommend that duct work be installed vertically and adequately secured to the building exterior. The fasteners - bolts, screws or rivets - used to secure the duct must not penetrate the duct walls.
Interior installation
As specified in NFPA 96, in buildings having more than one floor, but also in single-story buildings with a specified fire-rated roof-ceiling assembly, ducts have to be enclosed in a continuous enclosure to maintain the integrity of fire separations required by the applicable building codes. The enclosure must extend from the lowest fire-rated ceiling or floor above the hood through
any concealed spaces to or through the roof.
Further, if the building is less than four stories, the enclosure wall has to have a fire rating of not less than one hour. In buildings of more than four stories, the enclosure fire rating has to be two hours or more.
If fire does occur in a ducting system, inspection by a qualified inspector is necessary before further use to determine if the structural integrity of the duct and enclosure still meets requirements for fire protection purposes.
The whole point of duct design and installation for ventilation systems is the need to consult, and comply with, applicable code requirements.
Exhaust terminations
Codes are specific for exhaust system terminations for both rooftops and walls.
For example, for rooftop terminations, NFPA 96 specifies minimum clearances between the exhaust outlet and property lines, adjacent buildings and air intakes as well as minimum height levels of the outlet above adjacent air intake devices.
The requirements for wall terminations are equally specific, particularly with respect to clearances from the outlet to adjacent buildings, power lines, air intakes, doors and windows, etc. At no time should grease from a commercial kitchen cooking exhaust be allowed to run down the wall of a building.
Note: Exhaust termination requirements are shown in codes as minimum requirements. Consideration must still be given to make sure that grease build up will not occur through roof top air intakes or in surrounding structures.