5 Minutes to Understand Kitchen Grease Filter

Baffle grease filters are designed to remove grease particles from the exhaust hoods in a kitchen air stream. They are the first layer of protection in any restaurant's grease containment and exhaust ventilation system. Baffle hood filters capture airborne grease, smoke, steam, heat, odors, and other substances that are produced during cooking under the hood. They are necessary and required for any commercial restaurant kitchen to operate smoothly, safely, and efficiently. 

Why need a kitchen grease filter?

Kitchen baffle filters are one of the first lines of defense for restaurant safety. Without the proper baffle hood filters, your kitchen will be filled with airborne particles you don’t want your staff breathing! Exhaust systems that have broken, missing, or undersize air filters are prone to collect accumulations of grease deposits throughout the entire duct system. Kitchen grease is highly flammable, and because of the chimney effect in vertical ductwork,  a very intense rapidly spreading flash fire can engulf the entire system. The vent mesh grease filters block flames and flammable debris from entering your ductwork system, preventing a major fire hazard. Therefore, replacing and maintaining restaurant hood filters is an important step in reducing the risk of kitchen grease fires.  

Types of kitchen grease filter

Equipment available ranges from simple mesh or baffle grease filters to highly complex, specialized and expensive multi-stage filtrationand odor treatment systems.

Grease

The build up of grease on kitchen equipment and in associated exhaust ventilation systems needs to be prevented primarily to reduce fire hazard and for general food hygiene. Typical grease loads for different types of appliance are compared in the table below. Grease will be in the form of particles and vapor.

Grease and moisture load

Cleaning is essential for hygiene and for fire safety and may need to be as frequent as a week where there is a high level of grease generated (e.g. frying). Grease trays should be checked daily and cleaned by dishwasher or steam clean. The efficiency of various types of grease filters is compared in the table below. 

Restaurant hood filters performance compared 

Generally higher efficiency is accompanied by a corresponding higher pressure drop.

Mesh grease filters

Mesh type grease filters are formed from layers of stainless steel or galvanized steel mesh on which grease is deposited by impingement. They are not effective for higher grease loads and should not be installed above high grease-producing cooking appliances (e.g. deep fat fryers, chargrills, griddles, etc). The performance of mesh grease filters is affected by temperature and they should also be mounted not less than 45° from horizontal. Pressure drop is low when they are clean but increases with use between washes. Mesh filters and require frequent washing (e.g. twice weekly) and are cleaned by soaking in very hot water with detergent which eventually degrades them so they need replacement. They also increase the risk of fire compared with alternatives and are not normally suitable for commercial applications. Many insurers will not permit their use because of this.

 

Performance factors

Baffle grease filters

Baffle type grease filters are more efficient than mesh filters and are suitable for general cooking with moderate grease loads. Baffle grease filters are formed from a series of interlocking stainless steel vanes creating two rapid changes in the velocity of the exhaust air stream. Grease is deposited on the vertical vanes and is collected in removable drawer troughs. The interlocking vanes also help to prevent the direct spread of flame. They have a non-overloading characteristic. Baffle grease filters should be mounted not less than 45° from horizontal. They can normally be cleaned in a commercial dishwasher. Some baffle type filters are arranged to induce a swirl into the upward moving airstream to improveextraction efficiency.

Performance factors

Slot cartridge filters  

Cartridge filters are more efficient than baffle filters and are suitable for moderate to heavy grease load applications. They are not to be confused with disposable cartridge filters. Cartridge filters comprise high-velocity slots that open onto a series of baffles causing four changes of direction. This results in higher efficiency levels but also higher pressure drops. They are mounted horizontally fitted over the full length of the extract plenum with grease drawers positioned below. Like baffle grease filters, they have a non-overloading characteristic.

 

Performance factors

Water wash grease filters are normally slot cartridge type filters fitted with an integrated automatic hot water and detergent wash cleaning system. The wash cycle is usually operated at the end of each working day. Water wash grease filters are easier to maintain but expensive and have a high-pressure drop. A hot water supply and drain are also required.  

 

Water mist grease extractors  

Water mist grease extractors are one of the most effective ways of extracting grease from kitchen exhaust air with efficiencies of 90-98% possible under ideal conditions. They operate by spraying a continuous mist of cold water into the exhaust air stream that emulsifies grease causing it to drop into collection troughs. Automatic cleaning is provided for the water wash system and so they are easier to maintain than non-automatic systems. However, they are expensive and have a very high-pressure drop. A hot and cold water supply and drains are also required. 

Odor control treatment of kitchen grease filter

Odor control 

 

Cooking odor is mainly gaseous and therefore not removed by particulate filtration. Perception of odor is very subjective and much research into how to measure and categorize odor has been carried out. Some of the attributes used to assess odor which influence its potential for annoyance are listed below. 

Odor assessment  

The response to a particular odor varies between individuals. The level of odor control required depends on a number of factors such as those listed in the table opposite.

 

● Size of facility

● Type of food cooked

● Type of appliances used

● Location of premises

 

Level of odor control  

Typical odor and grease production characteristics associated with different types of catering establishments are compared below.

 

Odor characteristics of catering establishments 

Odor treatment  

New premises or premises subject to planning restrictions may require special odor control systems to prevent harm to amenity. Best practical means should be taken for existing premises. Odor control is achieved by odor treatment and stack dispersion. The performance of stack dispersion depends on a number of factors (e.g. local wind pattern) which are often not easy to assess accurately. Rules of thumb must therefore be treated with a degree of caution and it is always best to discuss proposals with the local environmental health officer as early as possible in the design process.

 

A recent DEFRA guide suggests exhaust air should be discharged not less than 1m above the roof ridge of any building within 20m and if this cannot be complied with (e.g. for planning reasons) the discharge point should be 1m above the roof eaves and appropriate odor control measures provided as required. If these measures are not possible an exceptionally high level of odor control will be required. Various types of odor control systems available are listed in the table opposite.

Odor control systems require very good levels of pre-filtration (e.g. grease extraction, coarse and fine filtration prior to carbon filtration). Stack dispersion should always form part of the overall odor control treatment unless there are exceptional circumstances that prevent it. Refer to the DEFRA guide for more information and guidance on risk assessment. Neutralizing systems are available but their effect is subjective and should not be relied on as the first line of attack. They can be used to ‘polish’ in highly sensitive situations. Dosing levels may lead to other odor problems.

 

Activated carbon filters have an upper temperature limit of 40°C and so should be located as far away as possible from the heat source.  Higher temperature air streams should be cooled by fresh air bleed or a cooling coil.  The relative humidity should be maintained below 75-80% as efficiency is significantly reduced above these levels as water is preferentially adsorbed.     Residual particles reduce efficiency and increase pressure drop significantly.  Therefore effective grease filtration and pre-filtration for particulates is required.  The grade of pre-filter used should be as recommended by the carbon filter manufacturer and not less than F5 to BS EN 779.  Where smoke (a particulate) is generated by cooking processes a higher grade of pre-filter is required (e.g. F8 up to various classes of HEPA filter) to maintain reasonable carbon filter life.        The service life of activated carbon filters depends on the original mass of carbon compared with the weight of gas adsorbed.    The capital and running cost of carbon filtration with HEPA pre-filtration is high.

 

Activated carbon filters  

Activated carbon filters work by adsorption where gaseous molecules impinge on the surface and remain there for a period of time before desorbing into the gaseous phase again. The adsorption bed requires replacement or regeneration after all the active sites have been used. Activated carbon filters can be very effective at removing odors under optimum conditions.

 

Activated carbon filters have an upper-temperature limit of 40°C and so should be located as far away as possible from the heat source.  Higher temperature air streams should be cooled by fresh air bleed or a cooling coil. The relative humidity should be maintained below 75-80% as efficiency is significantly reduced above these levels as water is preferentially adsorbed. Residual particles reduce efficiency and increase pressure drop significantly.  Therefore effective grease filtration and pre-filtration for particulates are required. The grade of pre-filter used should be as recommended by the carbon filter manufacturer and not less than F5 to BS EN 779. Where smoke (a particulate) is generated by cooking processes a higher grade of pre-filter is required (e.g. F8 up to various classes of HEPA filter) to maintain reasonable carbon filter life. The service life of activated carbon filters depends on the original mass of carbon compared with the weight of gas adsorbed. The capital and running cost of carbon filtration with HEPA pre-filtration is high.

 

Typical minimum dwell times for various applications are shown in the table below. 

Electrostatic precipitators 

 

Electrostatic precipitators are sometimes used after a mechanical grease filter for additional grease removal before odour treatment.  Particles are removed by being ionised by a high voltage source and collected on flat plates.  Collector plates need to be kept clean to maintain efficiency and to prevent restriction of air flow.   Electrostatic precipitators are expensive.    

 

UV Treatment  

Ultraviolet light treatment can be included as part of the treatment regime to convert grease into inert substances.  UV systems work best on small particulates and vapour but  require adequate exposure time for effective operation.  High intensity UV light sources should be screened from view and interlocked with fans to remove ozone which is produced by some forms of UV.    

 

Cost  

The relative costs for various types of grease filter are compared in the table below.

 

Grease filter cost indicators

The relative costs for various types of odour control system are compared in the table below.

 

Odor control systems cost indicators 

Precaution of kitchen grease filter

● Mesh grease filters are not suitable for commercial kitchen applications.  Insurers may not permit their use. 

● Regular cleaning is essential both for hygiene and for fire safety.  Essential cleaning may be as frequent as weekly where high level of frying.  

● Grease trays should be checked daily and cleaned by dishwasher or steam clean.      

● There is an increasing expectation for improved odour control.  Dispersion techniques may not be adequate on their own in high density urban environments.  

● Assessment of minimum stack height required is complex and rules of thumb are not always adequate. 

● See DEFRA Guide for Risk Assessment methodology.

● Problems are often caused by poor maintenance.