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of 23 Rev 0316 Airepure Australia 2016

Kitchen Exhaust Treatment

Technical Paper

Written by Jonathan Bunge (M.Eng -Chemical) for Airepure Australia 2016.

of 23 Ph. 1300 886 353 www.airepure.com.au Rev 0316 Airepure Australia 2016

Contents 1. Kitchen Exhaust Background ............................................................................................................................ 3

2. Why Treat Kitchen Exhaust? .............................................................................................................................. 4

3. Hood Filters ........................................................................................................................................................... 4

4. UV Lamps ............................................................................................................................................................. 5

5. Ozone ................................................................................................................................................................... 6

6. Electrostatic Precipitator ................................................................................................................................... 7

6.1. Velocity of Air

6.2. Basis of Specification

6.3. Temperature of the Air

6.4. Cell Passes

6.5. Servicing Mechanism

6.5.1. Manual Clean Duct Mounted ESP

6.5.2. Automatic Self-Cleaning Plantroom ESP

7. Carbon ............................................................................................................................................................... 13

7.1. Carbon Implementation

8. Multi-Staged Filter Packs .................................................................................................................................. 15

8.1. Filter Pack Variations

9. Wet Scrubbers ................................................................................................................................................... 16

10. Dilution/Dispersion ............................................................................................................................................. 16

10.1. Discharge Location

10.2. Dispersion / Dilution

10.3. Treatment Prior to Fan

11. Do’s, Don’ts, Myths and Misconceptions ...................................................................................................... 18

11.1. Using the Australian Standards as a Minimum

11.2. Fire Risks

11.3. Always Consider Servicing

11.4. How Removal Efficiencies are Specified

11.5. Velocity of Air through ESP's

11.6. Flow Distributions after Duct Bends

12. Recommended Treatment and Application Pairings ................................................................................ 20

12.1. Horizontal Exhaust

12.2. Vertical Exhaust

Appendix 1: Cooking Styles ..................................................................................................................................... 22

References .................................................................................................................................................................. 23


1. Kitchen Exhaust Background

Every commercial kitchen is different, and depending on the type of cuisine produced there are varying amounts of moisture, grease, smoke and odour being emitted. Food products rapidly oxidise/vaporise at cooking temperatures and form organic compounds, which are driven off in particulate and gaseous form. This is often accentuated by the food water content vaporising and the entrainment of solid particles. The result is a combination of solid particles, liquid droplets, and vapour/gaseous phase contaminants. While there remains some debate on the characterization of the particulate size distribution and its variance with temperature and cooking style, it is established that the general range is about 0.1 to 10.0 µm.

Figure 1: Particle size distribution at 260°C1. Note: the distribution varies significantly with different cooking styles.

When designing a kitchen exhaust treatment system it is important to not only know the type of cooking being undertaken, but to also establish the necessary end result that you are trying to achieve. Australian standard AS1668.2 (2012)2 provides calculations for required kitchen exhaust flow rates of 7 different types of cooking (Appendix 1: Cooking Styles) with various hood configurations. This flow rate should be approached as a design minimum. The flow rate specified by AS1668.2 (2012)2 is to provide enough dilution air to reduce the concentration of contaminants and reduce the

temperature of the kitchen exhaust air, both critical parameters for a successful treatment system.

As part of this you need to install the appropriate equipment in the right order. The order of treatment systems to remove various contaminants has an optimum sequence to it – like a set of sieves that are correctly sized and ordered – to take out large bits first – then sequentially removing ever smaller components. This prevents any given stage fouling earlier than expected due to excessive and inappropriate load. There are a number of prevalent ways to treat kitchen exhausts all of which can be applied individually or in series with each other. The success of any given system is defined by how appropriately the particular kitchen can be paired up with a cost effective, high performing combination of treatment systems.

These systems are:

• Hood Filters (Particle) • UV Lights (Particle & Odour) • Ozone (Particle & Odour) • Filter Packs (Particle) • Activated Carbon (Odour) • Electrostatic Precipitator (Particle) • Wet Scrubbers (Particle & Odour) • Dilution/Dispersion (Odour)

Note: removing particles also reduces odour generated by particles but not the odour in the gaseous phase.

A successful kitchen exhaust system must also be paired up with a cost effective fan. The fan needs to be able to handle the dynamic static pressure that kitchen exhausts encounter whilst maintaining flow. The type of kitchen exhaust treatment system implemented will have an impact on the capital cost and the ongoing energy costs of the associated fan. This should always be taken into account when designing a kitchen exhaust treatment system.


2. Why Treat Kitchen Exhaust?

The Building Code of Australia (BCA) and therefore the National Construction Code (NCC) refers to Australian standard AS1668.2 (2012)2 The use of ventilation and airconditioning in buildings. Part 2: Mechanical ventilation in buildings. Therefore by law AS1668.2 (2012)2 must be adhered to when treating kitchen exhaust.

According to AS1668.2 (2012)2 untreated kitchen exhausts must be discharged vertically at over 5m/s or horizontally at less than 1000L/s, be at least the distance in Table 1 from a property boundary, any boundary to a public street and any outdoor air intake opening or any natural ventilation device or openingi and be treated to reduce the concentration of contaminants where necessaryii.

Table 1: Minimum Separation Distances from Discharges to Intakes, Boundary or Natural Ventilation Device1

Airflow rate within the minimum distance (L/s)

Minimum Distance (m)

<200 1 <400 2 <600 3 <800 4 <1000 5 ≥1000 6 If these conditions cannot be met then there are concessions that can be made, all 3 of the following conditions must be met concurrently1:

1. The flow rate used in Table 1 can be reduced via particle and odour filtration using the following formula:

𝐷𝐷𝐷𝐷𝐷𝐷 𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝐷 = �̇� − �𝜂 × �̇��

i Consider the location of future air intakes ii Council action generated by complaints can make treatment necessary despite meeting other conditions

Where:

�̇� = 𝐴𝐴𝑟𝐴𝑟𝑓 𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝐷 𝜂 = 𝑇𝑟𝐷𝑟𝑟𝐷𝐷𝑇𝑟 𝐷𝑓𝑓𝑒𝐷𝑇𝐴𝑒

2. The treatment system must not result in the undue discharge of other contaminants (e.g. ozone).

3. The treatment system should involve mandatory testing and maintenance to ensure ongoing, satisfactory contaminant removal.

These concessions allow the use of horizontal exhaust if the treatment option is able to lower the deemed flow rate to below 1000L/s. These guidelines when followed are almost certain to prevent council action against a kitchen generated by community complaints; however authorities can still enforce additional action.

3. Hood Filters

Hood filters are often undervalued with respect to their importance in a successful kitchen exhaust treatment system. A well designed hood filter can significantly reduce the amount of grease that enters the duct. This significantly prolongs the service life of treatment systems downstream. Challenges of the hood filter are cost, ease of cleaning, fire regulations, static, noise and the effectiveness.

A traditional commonly used baffle filter will remove 30% of the grease and lower the load on the duct. However a more proactive solution is to use an improved hood filter in the form of a honeycomb, coil or multi-stage porous ceramic filter etc. The efficiency of hood filters scale up with cost and some claim to remove up to 98% of grease particles; however the basis of the efficiency figure must be investigated as outlined in section 11.4: How Removal Efficiencies are Specified.


An efficient hood filter is the most underutilized tool in kitchen exhausts. The savings in maintenance costs is significant and the reduction in fire risk is worth the initial investment. If this stage is done properly all the downstream elements can work in their optimum range giving the most cost effective long term results.

4. UV Lamps

Using UV lamps to break down organic and bio-organic material is a proven sterilisation technology used in multiple industries including water treatment and hospitals. The correct application of UV lamps can also be very useful in reducing particle and odour concentration in kitchen exhaust if applied correctly.

There are three types of UV rays, UV-A, UV-B and UV-C. UV-C has the shortest wavelength (~185 nm) and is capable of simultaneously producing ozone whilst undergoing photolysis with the grease particles. The additional benefit of ozone production is why UVC is the preferred wavelength of ultra-violet rays for kitchen exhaust treatment.

The photolysis reaction involves the energy from UV rays breaking down predominately grease particles and vapours which consist of long chains of carbon and hydrogen. These smaller chains can continue to be broken down by photolysis or be oxidised by ozone present in the air to form H2O and CO2. Some white ash is also generated due to the formation calcium, potassium and sodium salts etc.

A certain residence time is required to allow enough time for a sufficient amount of photolysis of the grease particles to occur. A velocity of 1m/s to 2m/s is generally used in industry for standard sized lamps. The variance in speed corresponds to the contaminant loading with a longer residence time required for

highly contaminated air streams. There is limited testing of the actual performance of UV with respect to residence time and loading; this introduces an element of risk associated with specified systems underperforming.

The ozone generated also needs residence time to oxidise odours; however of higher concern is residual ozone existing at the outside exhaust location. There are significant health concerns associated with ozone exposure. Please read section 5: Ozone.

The placement, arrangement, thermal and vibration isolation, safety interlocks and lamp monitoring are other critical design factors to allow successful use of UV lights. Subsequently, the installation and maintenance of these devices is governed by obtaining good quality lamps of the correct type from a verified vendor, changing them on a regular cycle, and finally avoiding surface contamination by excessive water, grease or oils from the cooking process.

One of the most significant challenges with these systems is staying below the contaminant loading point for the lamps. Below this level, the lamps are essentially self-cleaning due to the ratio of photolysis/oxidation to contaminant loading. Above that level, the photolysis is not fast enough and the lamps build up surface contamination and progressively drop to lower and lower efficiencies.

UV lamps placed in the hood can be effective at removing grease close to the source to lower fire risk and duct cleaning, but they run into performance problems due to higher temperatures. For temperatures below 40°C standard quartz UV lamps are sufficient. For higher temperatures it is essential to use Mercury Amalgam lamps, as these are more capable of maintaining performance at higher temperatures3.


The performance of the ozone component of UV lights is reduced at higher temperatures as the half-life of the molecule decreases exponentially as a function of temperature4,5.

When correctly applied, high quality UV lamps can be an effective way to reduce particles and odours in kitchen exhaust. However for assured performance it is recommended not to use UV on its own; instead UV should be treated as a load reducing mechanism to lower maintenance costs on subsequent treatment systems.

5. Ozone

Ozone when introduced to a kitchen exhaust stream will oxidize grease and odours to form predominately H2O and CO2. This process is known as ozonolysis. There are four commonly used methods to generate ozone:

• Corona discharge • Ultraviolet (UV) Radiation • Electrolysis • Radiochemical

Essentially all methods use energy to break the bonds of oxygen molecules (O2) and allow the oxygen atoms to re-form as ozone (O3). Corona discharge generation and UV radiation are the only methods applicable to kitchen exhaust.

Both methods involve drawing air through a modular unit attached onto the side of a kitchen exhaust duct that is under negative pressure. The negative pressure in the duct creates a flow of outside air through a unit containing high voltage plates (corona discharge) or 185nm UV lamps (ultraviolet radiation) generating ozone. The amount of ozone can be controlled via a damper between the unit and the duct. The amount of air an ozone generator can clean is dependent on the contaminant loading and the residence

time available. A unit is designed to treat a volumetric amount of air, generally a minimum of 2 seconds residence time is required to allow the ozone to clean the specified volume of air with a moderate contaminant loading before exhausting outside.

One major advantage of ozone generators over in duct/hood UV lights is the location of the unit; grease particles don’t pass through the unit preventing the need to clean the unit. However the UV lights inside units that rely on UV still need to be periodically replaced, this is due to the natural degradation of UV lights over time.

There is a major concern relating to residual amounts of ozone that are exhausted to occupied spaces. Ozone exposure in humans has been associated with dizziness, insomnia, coughing, chest pain, reduction in lung function and irreversible obstructive airway diseases6. In particular ozone exposure is damaging to the un-developed lungs of children5.

There are three ways to ensure that no residual ozone is present in the final exhaust; this is also a requirement of AS 1668.2 (2012)2 that must be adhered to.

The first option is to install an ozone detector just prior to the building kitchen exhaust point; the ozone level detected here can control the damper position on the outlet of the ozone generator. As the cooking load increases and decreases the amount of ozone consumed varies and therefore the amount required to be generated also changes.

The second option is to install a modest carbon trap to capture any long lived organics and contact them with remaining ozone in the stream, destroying remaining organics; refreshing the carbon surface and containing the residual ozone. Note that whilst the carbon life is extended


through cleaning with ozone it will still require periodic replacement.

Provided with a reliable control system the first option ensures that no ozone is ever released, this is not the case with the second option as ozone will begin to pass through if the carbon bed becomes saturated before being replaced.

The second option ensures a higher efficiency of odour removal as additional residence time is effectively created by having both ozone and organics react while adsorbed onto the surface of the carbon.

Thirdly it is recommended to ensure ozone generators are operated under negative pressure, this is to ensure that any leakages will be into the duct and system, eliminating concerns of ozone leaking into populated areas.

In any case ozone is not recommended as the sole mechanism to remove both particles and odour, generally the residence time available will only be long enough to completely break down the smaller gaseous phase organics (odour). It is highly recommended that another high efficiency particle removal system is used in conjunction with ozone for complete particle removal.

The control measures required and inability to operate as a sole treatment system make ozone and therefore by extension UV a difficult treatment option to implement safely with a high level of success. Therefore for assured success it is recommended not to use ozone on its own; instead ozone should be treated as a load reducing mechanism to lower maintenance costs on subsequent treatment systems.

6. Electrostatic Precipitator

Electrostatic precipitators (ESP’s) use high voltages to initially charge particles before utilising the charge on the particle to capture it via attraction to metal plates that are of the opposite charge or grounded. There are many different types of ESP’s tailored for applications in a vast array of industries, however it is commonly accepted that the two stage, low voltage ESP is the applicable type for the treatment of kitchen exhaust.

ESP’s are highly effective at treating large amounts of air with relatively low operating costs due to the maintenance regime consisting of cleaning instead of replacement. No other kitchen exhaust system can effectively treat large, dirty volumes of air like a correctly designed ESP can.

ESP’s also have a relatively low pressure drop that does not rise over time like conventional filters; this saves a considerable amount of money through power savings and the initial investment in the fan. Despite ESP’s operating around 12.5kVDC the power consumption for a correctly specified ESP is around 50W per 600mmx600mm facial area in the direction of airflow.

The ESP collecting element, also known as a cell, contains two stages; charging and collecting. In the first stage, a uniform corona field is created between charged and grounded electrodes. The charged electrodes are either tungsten wires, stainless steel plates or stainless steel spiked plates. The stainless steel spiked plates offer the greatest mix of strength and performance. Tungsten wires are prone to breaking and plain stainless steel plates are not as efficient compared to the spiked plate design. The initial charge can be either positive or negative. All particles pass through this charging stage and each one takes on a static charge.


In the second stage, alternating parallel plates are charged and grounded. As the charged particles move into the second stage, they are repelled by the charged plates if the polarity is the same as the initial charge and attracted to and collected on the grounded plates. If the polarity of the secondary charged plates is the opposite of the initial charge then the particles are attracted to the charged plates. The particles are accumulated on the plates until the cells must be periodically cleaned, either automatically or manually depending on the ESP system.

Figure 2: Particle capturing mechanism of an ESP

Ionisers generate a small amount of ozone with negative polarity and the wire type ionizer generates the most7. This will only be a health issue if the ESP does not have carbon behind it; however ESP’s almost always use carbon for odour control which will also clean up the residual ozone.

Important design parameters are the size and number of collection plates, the voltage gradient across the ionisers and collectors and the air composition, temperature, velocity and flow rate.

The physical construction of most ESP cells is the same as most use the most efficient combination of voltage, amps, cell size and layout.

It’s commonly thought that a higher voltage differential across the plates, regardless of the gap between plates will result in higher collection efficiency; this is not exactly the case. However a higher voltage gradient will result in higher collection efficiency. The voltage gradient is the difference in voltage between the

charged plates and the grounded plates divided by the gap in-between. For example:

6.25𝑘𝑉𝐷𝑘 − 0𝑘𝑉𝐷𝑘7.4𝐷𝐷

= 0.84𝑘𝑉𝐷𝑘/𝐷𝐷

The maximum voltage gradient that can be used is called the breakdown voltage (also known as dielectric constant); beyond this voltage gradient consistent arcing will occur. This will result in 0% efficiency for the ESP. 1-3kVDC/mm is recognised by physicists as the maximum voltage gradient for pure air before arcing occurs8. However due variances with particle loading reducing gap size, electrical resistance of particles, humidity, temperature, air pressure, electrode size, shape, orientation, finish and the materials used the design limit for ESP’s for kitchen exhaust is generally 0.9-1 kVDC/mm.

It is recommended that ESP’s are designed slightly lower than this maximum and/or have automatic voltage control. It is also not recommended to use ESP’s for solid fuel applications as ash particles have a tendency to quickly reduce the gap size of the collection plates causing arcing.

Due to most ESP’s available on the market operating close to the maximum possible voltage gradient, the remaining main difference’s seen in industry are the velocity of air through the cells, the basis on which specifications are listed, temperature of air, the amount of cell passes and the servicing mechanisms.


6.1. Velocity of Air

To achieve minimum 95% removal of particles of size 0.3 µm for a standard cell depth the velocity of the air cannot exceed 1.7m/s. If an ESP has two cell passes the velocity allowable rises to 2.1-2.3m/s. Independent testing confirms this result:

Figure 3: Collection Efficiency Vs Face velocity (1 Pass and 2 Pass)

Commonly ESP’s are specified with a throughput velocity of around 3.5m/s; the laws of physics prove this is impossible. The reason the velocity is sometimes specified at 3.5m/s instead of 1.7m/s is because the size of the ESP required halves and therefore the capital cost of the unit halves.

The collection efficiency at any given air flowrate is dictated by the particle migration velocity. The particle migration velocity is the velocity in which the particles will move perpendicular to the direction of airflow due to its electrical charge and the charge of the field it is in. This is shown in Figure 4. This particle migration velocity is predominately a function of the particle size, field voltage gradient and the resistivity of the particle. A number of standard parameters including temperature, pressure and humidity also apply.

Figure 4: Theoretical Particle Migration Velocity (Nominal Conditions) 6.

Figure 4 shows a theoretical migration velocity under nominal conditions, the effective migration velocity under kitchen exhaust conditions will vary but the trends relating to particle size and voltages gradients from Figure 4 remain the same.

For example a particle entering the middle of a 7.4mm gap needs to travel 3.7mm in a perpendicular direction before passing through the ESP cell to ensure collection. At a migration velocity of 0.05m/s and air velocity of 1.7m/s the plate depth must be at least 126mm for there to be a probable chance of collection.

𝐷𝑒𝐷𝑟𝑟𝑇𝐴𝐷𝑃𝑟𝑟𝑟𝑒𝐴𝑓𝐷 𝑀𝑒𝑀 𝑉𝐷𝑓

× 𝐴𝑒𝑟 𝑉𝐷𝑓 = 𝑃𝑓𝑟𝑟𝐷 𝑓𝐷𝑇𝑀𝑟ℎ 𝑅𝐷𝑅

3.7𝐷𝐷0.05𝐷/𝐷

× 1.7𝐷/𝐷 = 126𝐷𝐷

The actual efficiency of collection is a function of probabilities and the particle migration velocity known as the Deutsch Anderson equation. For example the particles won’t always start halfway between the collecting plates. It seems obvious that doubling the velocity of air through an ESP will double the required length of the collection plates for the same result; however it is actually worse than this.


Figure 5: Effect of gas velocity on ESP performance for differing particle sizes of 3.5, 11 and 51µm diameter6.

Figure 5 shows the migration velocity affected by a parameter called particle mobility. At low velocities the effective optimum migration velocity is reduced due to the large amount of turbulence that is generated by an energized field with low velocity. At high velocities the effective optimum migration velocity is reduced due to re-entrainment and scouring. Lower velocities whilst an issue are not as detrimental as higher velocities as there is additional residence time in the ESP to make up for the loss in migration velocity.

Coming back to the previous example let us assume conservatively from Figure 5 that the particle migration velocity is 2.5 times lower at 3.5m/s (0.02m/s) than at 1.7m/s (0.05m/s). Therefore the plate length required is:

=𝐷𝑒𝐷𝑟𝑟𝑇𝐴𝐷

𝑃𝑟𝑟𝑟𝑒𝐴𝑓𝐷 𝑀𝑒𝑀 𝑉𝐷𝑓× 𝐴𝑒𝑟 𝑉𝐷𝑓

=3.7𝐷𝐷

0.02𝐷/𝐷× 3.5𝐷/𝐷 = 647.5𝐷𝐷

Probability states that a standard cell depth of 380mm would collect the particle at 1.7m/s requiring 126mm of plate length almost always, whilst it would be very rare for the particle at 3.5m/s requiring 647.5mm of plate length to be collected on a 380mm plate. This highlights the rapid decrease in efficiency of ESP’s when excessive flow rates are specified. Figure 6 visually explains the example.

Figure 6: Calculated horizontal distance travelled by particle at air flow velocity of 1.7m/s and 3.5m/s

6.2. Basis of Specification

The basis of specification of an ESP can also vary; 95% efficiency at 0.01 µm is different to 95% at 0.3 µm or 95% at 5 µm. Conventionally, the efficiency of particle filtration systems decreases with particle size; however this is not the case with ESP’s.

Figure 7: Nominal efficiency of ESP's as a function of particle size9.

Figure 7 shows how the efficiency of an ESP varies with particle size with a distinct minimum between 0.3 µm and 0.7 µm. This aligns as expected with the region for the lowest particle migration velocity as shown in Figure 4.

The 2 peaks in efficiency are due to the different predominant particle charging mechanisms present. Field / impact charging mechanisms are prevalent for particles greater than 1 µm whilst particles under 0.2 µm are strongly charged by diffusion charging also known as Brownian motion6. The particle sizes in-between are charged by a mixture of poor impact and diffusion charging. The reduction in charge efficiency leads to a minimum in particle migration velocity and therefore collection efficiency6.


Importantly the size of the particles in which ESP’s are least effective (0.3 µm to 0.7 µm) poses the most significant risk to human health. Research has indicated that there is a significant rise in the probability of lung cancer when subjected to increasing levels of particle matter under 2.5 µm10,11.

Always be wary of how specifying ESP’s in different ways can lead to false confidence. For example an ESP with a specification of 95% removal of 0.01 µm particles is a far inferior performing ESP compared to an ESP with a specification for 95% removal of 0.3 µm particles. Figures 4 and 7 show a similar particle migration velocity and therefore efficiency at 0.01 µm and 5-10 µm for any given system. This shows how deceiving a specification with efficiencies listed for 0.01 µm particles can be. Always ask for test reports proving the removal efficiency of 0.3 µm particles to increase the likelihood of a like for like comparison.

6.3. Temperature of the Air

The temperature of the air along with the humidity of the air has a large impact on the resistivity of the air. In short if the air/particle mixture has more resistance, the charge efficiency decreases and therefore the particle migration velocity and collection efficiency decreases. Conversely if the resistance decreases the opposite applies, however if the resistance decreases too much arcing may occur as the dielectric constant (breakdown voltage) of the mixture of air and particles has been reduced to below the ESP design voltage gradient.

Figure 8 proves that as temperature rises from ambient temperatures above the recommended temperature limit of 50˚C for kitchen exhausts, there is an increase in the resistance and therefore a decrease in efficiency. However temperature will rarely be an issue if the kitchen hood has the

correct amount of dilution air as specified by AS1668.2 (2012).

Figure 8: Resistivity of air/particle mixture as a function of temperature and moisture12.

6.4. Cell Passes

A double pass unit allows a higher velocity (2.2m/s) and therefore higher volumetric flowrate through the ESP than a single pass (1.7m/s) ESP while maintaining efficiency with the same facial dimensions. This is due to the efficiency retention associated with an increase in length of the collector cells that the air is in contact with.

Alternatively for treatment of the same volumetric air flow an effective double pass unit has:

• A longer overall unit length. • A smaller facial area (usually via a

reduction in width, sometimes in height).

• A smaller amount of carbon behind the ESP’s and a corresponding increase in frequency of change out.

There are essentially 3 reasons you would go for a double pass, despite the added cost and length:


• Space requirements dictate you have no more width to work with so you can instead increase the length.

• 12 m3/s is the limit for the amount of air that can be treated by a single pass; the cells facial area becomes too wide after this to sustain sufficient power across all of the cells and an increase in height would require a mezzanine platform for servicing. So a double pass becomes the only viable option for increasing treatment capacity.

• You want an increase in the capture efficiency of 0.3 µm particles and are therefore willing to operate a double pass unit at the same velocity as an equivalently sized single pass unit (no volumetric airflow increase in this case).

Note: a double pass is approximately 30% more expensive than a single pass for an equivalent volumetric air flow

6.5. Servicing Mechanism

The grease collected must be cleaned off the ESP cells periodically for continued efficient performance of the ESP. There are two ways of achieving this; manually cleaning the ESP cells or using an automatic in-built washing system.

6.5.1. Manual Clean Duct Mounted ESP

Manual clean ESP’s are cheaper, smaller and have no hydraulic requirements compared to automatic wash ESP’s. However over time the expense and inconvenience of repeatedly manually cleaning cells far outweighs the lower capital costs. If there are no on-site facilities for cleaning, a cell swap program is a viable solution provided that the ESP retailer can offer this. The constant handling of ESP cells will conceivably lead to physical damage to the mechanical

structure of the cells over time, incurring additional costs for replacements.

Manual clean ESP’s often come with only in-built pre filters. It’s highly recommended to also place safety filters and carbon behind them to complete the treatment. The cost of the additional housing and filters must be taken into account when comparing manual clean ESP’s to automatic wash ESP’s which commonly feature complete treatment systems.

A potentially significant issue with manual clean ESP’s is the difficulty of servicing, especially when placed in the roof space as seen recently at multiple restaurants in Sydney. The 2ft cells can weigh up to 20kg whilst the 3ft cells can weigh 30kg. When placed in hard to access areas the difficulty of maintenance is huge. Compounding this is the need to perform maintenance weekly for your average kitchen exhaust; furthermore extreme contaminant levels can reduce the service period to daily.

Cleaning cannot be ignored as the build-up of grease shortens the gap between plates causing arcing (Figure 9). Consistent arcing means zero efficiency and if the thickness of the collection plates is small, permanent damage of cells can occur through bending of the cell plates under a high voltage gradient.

Figure 9: Grease connecting charged and grounded plates causing arcing due to irregular cleaning


6.5.2. Automatic Self-Cleaning Plantroom ESP

ESP’s can also be manufactured with an automatic wash system. The frequency of the automatic wash is highly dependent on the contaminant load; similar to the frequency of manual cleans on the manual ESP’s. Depending on the quality of the wash system present and the contaminant loading; the cells in an automatic wash ESP can last from 3 months to 5 years before a manual deep clean is required.

Cell washes can be scheduled overnight when the cells are not in use and there is maximum hydraulic capacity available.

The method by which the ESP is washed is an important differential in the quality of the product. Hot wash ESP’s with a detergent/water mixture are the most effective and will require the least amount of manual cleaning. For example a hot wash ESP would typically require a manual clean every 12 months; whereas a cold wash ESP would typically require a manual clean every 3 months.

The most common reason for the lack of availability of hot water is the size of the hot water system required. An ESP that utilizes spray washing instead of bath washing will significantly reduce the hydraulic requirements. This will generate manageable space requirements and capital and operating costs.

An automatically cleaning plantroom ESP is undeniably the most effective long term solution for the treatment system for kitchen exhaust.

7. Carbon

Kitchen exhaust consists of a number of different volatile organic compounds (VOC’s); the composition and quantity vary depending on the type of cooking. The bulk of cooking fumes consist of n-alkanes and fatty acids with n-alkanols, aldehydes, ketones , sterols, polycyclic aromatic hydrocarbons (PAHs) and aromatic amines (AA’s) amongst many others also present13,14,15.

Plain activated carbon is effective at removing n-alkanes and fatty acids which are the bulk constituents of kitchen exhaust. In most cases this is sufficient to return the exhaust to contaminant concentrations that are below the odour threshold.

However on the rare occasions that initial concentrations of other contaminants are high enough then odours will still be present in the form of PAHs, formaldehyde and AAs etc. Only a suitably impregnated media, preferably of the permanganate type will be effective at removing these types of contaminants. Therefore plain carbon mixed with a permanganate impregnated media is seen as a more expensive complete solution for odour removal.

Figure 10: Activated carbon

7.1. Carbon Implementation

There are many different ways to use carbon in a kitchen exhaust system including bonded panels, disposable and refillable modules, bonded pleats and extruded monolithic blocks. The effective contact residence time, the quality of the


carbon and the pressure drop are the three critical components of an effective system.

Figure 11: Contact efficiency as a function of residence time.

At a fixed volumetric flow rate through the same size duct the contact residence time is directly proportional to the mass of carbon that the air comes in contact with. The velocity through the carbon may vary due to the arrangement of the carbon, for example V-bank arrangements increase the cross sectional area for flow. In short relative to airflow a higher mass equals a higher residence time which equals higher contact efficiency (Figure 11) and therefore removal efficiency.

To bring odours under the odour threshold with reference to AS1668.2 (2012) (Appendix 1: Cooking Styles): cooking styles 1 and 7 can use bonded pleats, style 2 can use monolithic blocks and styles 3, 4, 5 and 6 are strongly recommended to use bonded panels, disposable modules or refillable modules.

Economies of scale is typified by carbon filters, therefore a greater initial outlay on a larger mass of carbon will generally result in greater value for money; both in dollars per unit mass of carbon and reduced maintenance.

The relationship between the contact efficiency (Figure 11) and the removal efficiency is a function of the quality of the carbon. The quality of the carbon is largely dictated by a surface area/volume ratio,

a smaller pore size and/or a greater pore density increases this ratio and therefore the quality of the carbon. There are a number of test methods to confirm quality such as the CTC adsorption, the iodine value and BET testing. Other parameters such as moisture content, ash content and hardness are also important.

Different configurations of carbon also exhibit different pressure drops, this has an effect on the long term operating costs of your treatment system. Naturally a trade-off between quantity of carbon and pressure drop is often required.

Figure 12: 1. Monolithic blocks) 2. Bonded pleats 3. Bonded v-bank pleats 4. Disposable modules 5. Flat bonded panels


8. Multi-Staged Filter Packs

Multi-staged filter packs utilize conventional filters to remove grease particles in progressive stages. These filter packs generically are comprised of 4 sets of filters: A pre-filter, an intermediate bag filter, a 0.3 µm 95% DOP mini-pleat filter/HEPA and carbon for odour treatment.

The major benefits of a multi-stage filter pack are the cheap modular construction, one piece field assembly and ease of maintenance, even in the roof space.

The recommended pre-filter is G4 rated (30% efficient, ASHRAE 52.1). These are cheap filters that act as a sacrificial filter to eliminate large particles and prolong the life of the more expensive filters that follow. It is recommended to use a filter with a metal or plastic frame as cardboard tends to expand with grease and moisture and can be hard to remove.

The recommended intermediate filter is a F6-F8 bag filter (70-90% efficient, ASHRAE 52.1). These filters act as the workhorse and have a large holding capacity relative to the other filters. The rating of F6, F7 or F8 is a trade-off between performance and the proportional additional energy cost associated with a higher grade filter.

The recommended final filter is a rigid H11 mini-pleat filter (0.3 µm 95% DOP efficient, ASHRAE 52.2). This filter will clean up almost all of the particles remaining providing both a clean exhaust and protection for the subsequent carbon stage. A H14 HEPA filter can also be used for higher capture efficiency, this also brings a higher capital and energy cost. This level of treatment is rarely required.

The fourth stage is a carbon stage to remove odours, the type and quantity of carbon recommended is discussed in 7.1: Carbon Implementation.

The servicing period can vary anywhere from monthly to yearly depending on usage hours, contaminant levels and installation/design quality. It is not recommended to use multi-staged filter packs with intense cooking processes as the frequent servicing intervals will generate a huge operating cost that won’t be offset by the lower capital investment.

Normally when conventional filters are filtering dust a flow rate of 2.54m/s is acceptable. However there are two key reasons why it is important that the multi-stage filter packs are not run at higher than 1.9m/s.

The pressure drop of 4 stages of filtering becomes very inefficient energy wise; the static is 55% higher at 2.54m/s compared to the recommended velocity of 1.8m/s.

Conventional filters are also designed for dust, not grease; a lower velocity is required to ensure the filters can perform at the levels prescribed by ASHRAE 52.1 and 52.2 with grease instead of dust. Figure shows the damaging effect that a flow rate of 2.5m/s can cause to conventional filters under grease loading. Additionally this system also had insufficient air dilution at the hood causing significant additional damage with higher air temperatures and contaminant concentrations.

Figure 13: Bag filters from an undersized multi-staged kitchen filter pack with a velocity of 2.5m/s


Like all kitchen exhaust treatment systems it is important that the kitchen hood complies with AS1668.2 (2012)2, this is to ensure adequate dilution air to reduce temperature and contaminant concentration.

The multi-staged filter packs are extremely effective when paired with high efficiency hood filters. The lives of the filters are significantly increased with protection by high quality hood filters and the servicing of both components of the treatment system is simple.

9. Wet Scrubbers

Wet scrubbers operate by forcing smoke, grease and odour through water; this removes particles via entrainment in the water and can condensate odours through a cooling effect. AS1668.2 (2012)2 specifies that charcoal and solid fuel exhaust must be treated separately to general kitchen exhaust; wet scrubbers offer the perfect solution.

Figure 14: Smoki ® Wet Scrubber

They are specifically designed to treat solid fuel applications including wood fire pizza ovens. Unlike all previous kitchen exhaust treatment systems listed, a wet scrubber can handle high temperatures and large embers.

Traditional kitchen exhaust systems can only treat solid fuel kitchen exhaust with the introduction of dilution air to lower the

air temperature for treatment. This procedure is typically performed by a dilution fan that is manually powered on by kitchen staff prior to the oven fire being lit. There is no sure way within these solid fuel applications to interlock the operation of a dilution fan when a fire is lit; which allows human error (by forgetting to operate dilution fans) to overheat treatment systems.

Figure 15: Melted filter after failure to operate dilution system on a solid fuel pizza oven exhaust.

Wet scrubbers are severely limited by the amount of air that can be treated with the larger systems only capable of 700L/s. This restricts the application of wet scrubbers to isolated solid fuel applications. Most general kitchen exhaust exceeds 700L/s and it is not economic to use numerous wet scrubbing systems for large quantities of air.

For very large solid fuel exhausts it may be feasible to use dilution air with alarmed temperature gauges in the ductwork instead of multiple wet scrubbers.


10. Dilution/Dispersion

A powerful fan tailored for kitchen exhaust can serve two purposes: to ensure the discharge point is further than the required distance as specified by AS1668.2 (2012)2 and dilution/dispersion to lower the odour concentration below the odour threshold for any applicable sensitive locations (receptors). A tailored fan can be used instead of or in conjunction with existing treatment systems.

10.1. Discharge location

For any vertically discharge kitchen exhaust above 1000L/s it must be discharged at over 5m/s and more than 6 meters from a property boundary, any boundary to a public street and any outdoor air intake opening or any natural ventilation device or opening.

A fan can extend the effective discharge location through the use of a stack to make the exhaust system compliant with AS1668.2 (2012)2. The discharge velocity only needs to be 5m/s however an investment in a more powerful fan for dispersion and dilution may also solve an odour problem.

10.2. Dispersion/Dilution

Even if a vertical exhaust complies with the required distances and velocities there is a clause in AS1668.2 (2012)2 that states the air must be “treated to reduce the concentration of contaminants when necessary”. This is to ensure appropriate treatment of the air is enforced when contaminants are still present at sensitive locations despite the rest of the standard being adhered to. Instead of an expensive treatment system which is not required for a vertical exhaust, dispersion and dilution of the air can be employed. Through an appropriately designed fan the odourous concentration of contaminates at sensitive locations can be eliminated.

Figure 16: Dispersion and dilution of kitchen exhaust, jet plume fan vs traditional stack fan (equivalent exit velocity, double static on RHS).

Both fans in Figure 16 generate a stack height that is likely to remove the odour from the building envelope and disperse the odours to a level below the odour threshold before it reaches sensitive locations. Often the stack height is large enough to prevent the air reaching any sensitive locations. A lower profile fan is more aesthetically pleasing and requires less static due no physical stack requirement.

Dilution is also important; a fan that can draw in outside air through dampers on the plenum and/or entrain and mix air as the exhaust passes through the nozzle will significantly reduce the concentration of odours.

A fan is always required for kitchen exhaust, so sizing a fan than will prevent the need for other expensive odour removal systems is often well worth the investment.

10.3. Treatment Prior to Fan

The fan if correctly designed can stand solely as a method for odour abatement. In the case that odour remains an issue with sensitive receptors after dilution and dispersion then additional odour control prior to the fan needs to be assessed.

It is preferable to have particle arrestance before exhaustion to reduce the cleaning and equipment maintenance costs. In addition to this it is desired to have a fan


that has an inbuilt drip tray in the plenum to capture excess grease.

If there is no efficient particle filtration before the fan, the fan must be serviceable, able to withstand certain levels of grease and the fan must be able to have downtime (Example: 11.3 Always Consider Servicing).

11. Do’s, Don’ts, Myths and Misconceptions

11.1. Using the Australian Standards as a Minimum

It is crucial that the flow rate of exhaust air specified by the Australian standard is treated as absolute minimum. The specifications are in place for a reason and that reason is to ensure there is adequate dilution of the exhaust air with respect to temperature and contaminant concentration. It’s one of the easiest places to take a shortcut as lowering the amount of air to be treated will lower the cost of the entire treatment system including the ductwork and fan.

If responsible for signing off on kitchen exhaust designs, ensure you have always double checked the minimum flow rate. Retrofitting a bigger system after an undersized installation is expensive and sometimes not possible.

It must also be noted that the values specified should be used as a minimum, and if possible allow an additional design margin, especially when dealing with cooking types 5 and 6 (Appendix 1: Cooking Styles).

11.2. Fire Risks

Fires in mechanical exhausts caused by kitchens can spread rapidly within grease and oil laden duct work. Fire dampers do not work in kitchen exhaust as grease on the downstream side of the damper will ignite before, and irrespective of damper

closure. Therefore a number of fire prevention measures as outlined in AS1668.1 (2015)16 and AS1668.2 (2012)1 must be undertaken, these include:

• 300mm separation between ducts and combustible material

• Separate shafts for kitchen exhausts from different fire compartments.

• Flame barriers in accordance with UL 1046 when ductwork is longer than 10 metres and exposed flame or embers may be present as part of the cooking process.

• Minimum distance from the cooking surface to the hood filter of 1350mm for solid fuel, 1050 for gas cooking, 600mm for electric cooking and 200mm for kebab cookers.

• Hood filter must be installed not less than 30˚ from vertical.

• Ductwork structurally independent of dedicated fire resistant shafts/enclosures must be minimum1.2mm galvanised steel or 0.9mm stainless steel.

• Drainage points with grease tight plug must be installed.

• Access panels for cleaning are required for every turn in the duct or every 3m length of horizontal ductwork.

• Access panels and flexible connections must be air tight, grease tight and unaffected by water, grease or cleaning solvents.

• Horizontal ducts must be graded upwards (at least 1:200) in the direction of the airflow to allow grease to run back towards the hood.

To complement all of these measures it is also highly recommended to install high efficiency hood filters to reduce duct cleaning and reduce fire risk.


11.3. Always Consider Servicing

When designing or signing off on a kitchen exhaust you should always consider the ease and cost of servicing. Consider the access available, the downtime available and the frequency in which servicing will be required. The type of system initially chosen and the quality of the design has a large impact on servicing costs.

Figure 17: Example of a poorly installed dilution system.

Figure 17 shows a fan that is difficult to service while following OH&S regulations. This system does have particle treatment to protect the fan in the form of manual wash ESP’s. However in this particular application the ESP did not prevent grease build-up on the fan due to a number of possible reasons:

• The velocity of air through the ESP is too high due to an undersized ESP.

• There is not enough straight ductwork either side of the ESP to allow even airflow over the ESP.

• There are no safety filters after the ESP.

• The ESP’s may not have been washed regularly due to access issues or poor management.

This leaves the fan needing regular servicing with poor access, something that should also be avoided.

Other common poor service examples include:

• Manual wash ESP’s placed in hard to access roof spaces, the cells can weigh up to 30kg ensuring expensive labour and logistic costs.

• Not leaving enough room to slide in replacement filters and cells.

• Large distance to vehicle access points for a frequent service system.

11.4. How removal efficiencies are specified

The way in which removal efficiencies are specified can vary greatly over the industry and there are significant differences to be aware of.

Figure 18 shows the difference between a 95% efficiency that is specified based on the total number of particles captured and the total mass of particles captured. This is because larger, easier to capture particles contain more mass, this makes the overall system sound more effective than it is. In Figure 18 when 95% particle efficiency is listed, effectively 97.3% of the mass is captured, likewise when 95% mass efficiency is stated the particle capture efficiency is effectively 90%.

Figure 18: Particle capture on a particle basis and mass basis, based on particle size distribution in Figure 1.

Likewise a system specified 99% efficient on a mass basis at 1.5 µm may well be 50% efficient at 0.5 µm. Products that have test reports proving the efficiency for a standard and equivalent particle size are the easiest to compare. A good example


is products that have passed the ASHRAE 95% 0.3 µm DOP particle testing method.

11.5. Velocity of Air through ESP’s

Single pass ESP’s cannot remove 95% of 0.3 µm particles at velocities greater than 2m/s. Be aware that there is a removal efficiency minimum between 0.3 µm and 0.7 µm, the removal efficiency of 0.01 µm particles by an ESP is the same as the removal efficiency of 5-10 µm particles for any given system. This fact is often used to make ESP’s look better than they are on specifications. Refer to Section 6.1: Velocity of Air to understand why this is the case.

11.6. Flow Distributions after Duct Bends

Figure 19: Underutilised filters are coloured lighter, this is due to being positioned immediately after a duct turn

Figure 19 shows that when a treatment system is placed directly after or before a sharp turn in the duct work the treatment system becomes underutilized. This is because the majority of the air will flow around the outside of the bend leaving the duct work on the near side of the bend underused.

This reduces the value for money of the treatment system as the effective surface area is decreased. This is a likely cause of inefficiency previously highlighted in Figure 18.

12. Recommended Treatment and Application Pairings

12.1. Horizontal Exhaust

There are three applications regions where there are obvious best choice treatment systems with respect to cost (capital + operating), service intervals and performance.

When cooking styles 1, 2, 3 or 7 are being employed with flow rates less than 5000L/s it is recommended to use a multi-stage filter pack. The capital cost is small and the contaminant loading will result in acceptable operating costs. As the flow rate exceeds 5000L/s the energy cost of the higher static system become accentuated and a lower static option with a higher capital cost like an ESP becomes a competitive choice.

Figure 19: Recommended systems for applications

When cooking styles 3, 4, 5 and 6 are being employed with flow rates greater than 5000L/s than an automatically cleaning plant room ESP is the recommended option. The operating costs of any other system to effectively treat air of this quantity and contamination are prohibitive. When the air flow rate is under 5000L/s contaminant reducing mechanisms such as coil filters, UV lamps and ozone prior to a multi-stage filter pack can be economical.

When solid fuel is being used the potential ember load demands the use a wet


scrubber, no other system treats high temperature exhaust as effectively or safely as a wet scrubber. For very large solid fuel exhausts it may be feasible to use dilution air with alarmed temperature gauges in the ductwork instead of multiple wet scrubbers.

For all the flow rate and contamination level combinations not mentioned there are multiple effective solutions and an expert in the field should be consulted.

Additionally there are sometimes special requirements that require a more detailed assessment of best practice. For example there may space limitations, a level of aesthetics required, or sound pollution restrictions.

12.2. Vertical Exhaust

On the occasion that vertical exhaust requires treatment the same principles from horizontal exhaust apply, however instead of in duct odour treatment systems, a fan, preferably a jet plume fan can be used to dilute and disperse of the odours.


Appendix 1: Cooking Styles

Cooking styles according to AS1668.2 (2012)

Process Type 1 Non-grease producing equipment and void spaces under the hood, which serve to ventilate other cooking equipment.

Process Type 2 Low-grease, medium-heat producing equipment such as griddles, ranges, conventional fryers, tilting skillets, steam kettles and gas ovens.

Process Type 3 High-grease, low-heat producing equipment such as electric deep-fat fryers, grooved griddles, hot tops and hot top ranges.

Process Type 4 High-grease, medium-heat producing equipment such as countertop barbecues and gas-fired deep fat fryers.

Process Type 5 High-grease, high-heat producing equipment such as woks, salamanders, and open flame charcoal equipment utilising solid fuel.

Process Type 6 Oriental cooking tables and/or woks.

Process Type 7 Bread ovens and steam-producing combination ovens.


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