Revac Systems83, PadmaNagar Phase II ChintalHyderabad 500054

Mobile No +91- 9246 377 652

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Kitchen Ventilation

Hotels Restaurants Hospitals Canteens Retail Malls Residences Jails

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Buildings with Kitchens

Air Conditioning Ventilation Fire Safety Building Pressurization Refrigeration Air Distribution Food Service Equipment

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System Design for Kitchens

Basic Purpose of Kitchen Ventilation

To provide a comfortable environment in the kitchen To ensure the safety of the people working in the

kitchen and other building occupants by:

-Effective Removal of Effluents which may

include gaseous, liquid and solid contaminants produced by the cooking process and products of fuel & food combustion.

Effluents can be life threatening and flammable

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Effluent Generation in Kitchens

Heat is a primary ingredient of kitchen effluents

50% to 90% of the appliance energy input is released in the form of a rising thermal plume (convective). Balance is released into surrounding space through radiation.

This plume also contains most of the food and fuel generated effluents.

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Primary objective of kitchen ventilation is to capture and remove the air and effluents that constitute the plume through an effective exhaust system.

Heat radiated into space form the appliance must be addressed by the space air-conditioning system.

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A Commercial Kitchen

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Typical Kitchen Exhaust System

HOOD (8'x4'6") with Baffle FilterVelocity Across Hood - 150 FPM

VCD VCD

600x400 600x400

600x6005600 CFM1500 FPM

TRION Air CleanerModel T2002

Velocity Across Air Cleaner 450 FPM

Kitchen Exhaust SystemSchematic Diagram

ESP

ESP

Centrifugal Fan (Exhaust)6000 CFM 60mm SP 5hp motor

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Grease Emissions

Amount of grease in vapour phase varies from 30 to 90% by mass. This is an important factor in designing the grease removal system.

CO, CO2 and NOx emissions are present in gas and not electrical appliances.

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Grease Emissions by Cooking Appliances & Food Products

0 10 20 30 40 50 60

Gas

Electric

Gas

Electric

Gas

Electric

Gas

Electric

Gas

Electric

Gas

ElectricG

riddl

eB

roile

rB

roile

rF

ryer

Ove

nR

ange

Ham

burg

erH

ambu

rger

Chi

cken

Pot

atoe

sP

izza

(Sau

sage

)S

pagh

etti

Sau

ce

Grease (kg per ton of Food)

VapourParticulates

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In the absence of cross-drafts, a heated plume rises vertically, entraining the ambient air which enlarges the plume, cools it and slows it down.

An Exhaust Hood, generally located above the appliance, guides this plume into a ducted exhaust system.

The exhaust system flow rate must be slightly higher than the plume volume rate. Extra exhaust capacity may be required to resist cross-drafts.

The concept of capture velocity is not applicable to kitchen exhaust if a hood of sufficient size is placed at the correct height.

If the appliance and hood are placed against a backwall, the plume is drawn towards the wall by the “Coanda effect”.

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Behavior of Hot Effluent Plume

Effluent Plume Flow Rates

0 200 400 600 800 1000 1200 1400

Gas

Electric

Gas

Electric

Gas

Electric

Gas

Electric

Gas

Electric

Gas

ElectricG

riddl

eB

roile

rB

roile

rF

ryer

Ove

nR

ange

Ham

burg

erH

ambu

rger

Chi

cken

Pot

atoe

sP

izza

(Sau

sage

)S

pagh

etti

Sau

ce

CFM

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An Exhaust Hood

The Centre Piece of any Kitchen Ventilation System (KVS)

Not just a sheet metal box but a critical component of a well engineered system

Much greater awareness today about the need for an effective KVS

Issues are health, safety, efficiency and energy savings

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Hot Air Plume

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Exhaust Hoods

Type I – for removing grease & smoke : used for cooking appliances

Type II – for steam, vapour, heat & odour when grease is not present : used for dishwashers, steam tables, etc.

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Basic Styles of Type I Hoods

Wall Mounted Canopy Single Island Double Island Back Shelf or Proximity Pass Over Eyebrow

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Hood Styles

Wall Mounted Canopy Hood(With Baffle Filter)

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Schematic of an Exhaust Hood

Hanging Brackets

EXHAUST

GREASETROUGH

HOOD LIGHT

BAFFLEFILTER

EXHAUSTCHAMBER

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Wall Canopy Hood

Cooking appliance placed against a wall

Minimum air flow requirement 3” built in gap between hood & wall Three finished sides Less susceptible to cross drafts Minimum overhang requirement:

6” on sides and 6”-12” in front for full capture

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Single Island Hood

Cooking appliance/s (row) placed in the middle (not against a wall)

Four finished sides More susceptible to cross

drafts Recommended overhang: 12”

on all 4 sides Baffle filters in ‘V’ configuration

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Double Island Hood

Double row of appliances placed back to back

Two wall canopy hoods placed back to back

Recommended overhang : 6”-12” on all 4 sides

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Basic Styles of Type II hoods

Oven Hoods

Condensate Hoods

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Type II Hood – OVEN Hood

Removal of Heat & Odour Canopy hood with no filters Hood size determined with oven door open

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Type II Hood – Condensate Hood

Removal of heat, odour & moisture Condensate baffle and gutter to condense moist air & drain

the water Used over dishwashers

Removable Condensate

Baffles

Condensate Gutter

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Hood Overhang

Hood Type End Front RearWall Mounted Canopy 150 150 -

Single Island 150 150 150Double Island (per side) 150 150 -

Eyebrow - 150 -Back Shelf 150 250* -Pass Over 150 250* -

* Maximum Setback

Minimum Hood Overhang Requirements(mm)

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Exhaust Flow Rate

Calculation of proper exhaust flow rate very critical for effective operation of a KVS

The upward velocity of the effluent thermal currents is mainly a function of the temperature of the cooking surface and varies from 16 FPM over steam equipment to 160 FPM over charcoal broilers.

Appliances categorized in 4 groups by cooking duty

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Appliance Duty Category

Light Medium Heavy Extra Heavy

200o 200o 315o 370o

- Ovens - Hot Top/Element - Open Burner - Appliance using

- Steamers Ranges Gas Ranges solid fuels e.g.

- Cheese - Griddles - Broilers wood, charcoal,

Melters - Fryers - Wok Ranges briquettes

- Pasta Cookers

- Conveyor Ovens

(Pizza)

- Rotisseries

50 FPM 85 FPM 150 FPM 185 FPM

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Equipment Duty => Light Medium Heavy Extra HeavyWall Mounted Canopy 660 990 1315 1800

Single Island 1315 1645 1975 2300Double Island (per side) 830 990 1315 1800

Eyebrow 830 830 - -Back Shelf 990 990 1315 -Pass Over 990 990 1315 -

Minimum Exhaust Flow Rates by Equipment CategoryCFM per Linear Meter of Hood

For a combination of appliances in a row under a single hood, the flow rate is based on the heaviest duty appliance unless the hood design permits different rates over different sections of the hood.

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Schlieren Optical Imaging

Shows an optical image of the heat and flow pattern

Very useful tool in kitchen ventilation research

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Exhaust Flow Rate - 2750 CFM (Spillage)

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Exhaust Flow Rates- 3600 CFM (Full Capture)

Baffle Filter

The primary device for grease removal Series of vertical baffles designed to capture

and drain the grease into a container Made of stainless steel or aluminium Air stream is made to rotate and centrifugal

force throws the heavier grease particles out of the exhaust air stream.

Filters are normally removable and are cleaned by hot water.

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A Baffle Filter - Schematic

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Hood Pressure Drop

Exhaust Flow Rate per Linear Meter Ps(CFM) (mm)

530 - 850 6 - 12850 - 1150 12 - 19

1150 - 1500 19 - 26>1500 >26

Pressure Drop across a Hood with Baffle FiltersA General Guide

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More Effective Grease Removal

Clean air considerations have led to the need for higher efficiency (HE) grease extraction systems.

Mechanical filters (e.g. baffle) are not effective in removing small grease particles or grease in vapour form.

More effective devices reduce grease buildup downstream of the hood, lowering duct cleaning frequency and improving fire safety.

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Electrostatic Precipitators (ESPs)

Used in the exhaust stream downstream of the hood and the most common HE device.

By high voltage ionization, particles are collected on flat electrostatic plates.

Efficiency may drop as the ionizer section becomes dirty and effective surface area is reduced.

Under heavy loading condition, unit may shut down because of voltage drop.

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High Efficiency ESP System

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A Dirty Electronic Filter

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Cleaning of a Dirty Filter

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A Cleaned Filter

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Ultra-Violet (UV) Destruction

UV light chemically converts grease into an inert substance.

Adequate exposure time for chemical reaction is required.

High intensity UV lamps must be shielded from eyes.

Some forms of UV may generate ozone . It is imperative to have the exhaust on while UV lights are on.

UV lamps need to be replaced periodically.

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Water Mist Scrubber and Water Bath

Passing the effluent through water entraps particulates and condenses grease vapour

Static pressure drops are high Grease laden water tends to clog drains. Water hardness a consideration in spray

nozzle system

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Exhaust Duct Design

Kitchen exhaust ductwork carry hot grease laden air.The following general guidelines should be followed in their design: Ducts can be Round or Rectangular Must be grease tight : should be free of traps that can hold

grease. Min. sheet gauge should be 16g steel or 18g ss. Horizontal dust runs should pitch towards the hood for

continuous drainage. The slope should be 2% for runs under 75’. For higher runs, refer to local codes.

NFPA Standard F96 (US) sets minimum duct velocity of 7.5 m/s (1500 FPM) for exhaust ducts.

Maximum velocities are limited by pressure drop & noise and normally do not exceed 12.5 m/s (2500 FPM).

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Duct Design Considerations ASHRAE research finds no evidence to support the NFPA

minimum of 7.5 m/s, but recommends it be reduced to 2.5 m/s (500 FPM). This allows flexibility in design of variable speed exhaust systems and retrofitting.

For new single speed systems, a design velocity of 7.5 m/s (1500 FPM) is appropriate. Duct velocities above 2000 FPM cause noise and at less than 1000 FPM, ducts become large and expensive.

Straight GI ducts, at duct velocity of 1500 FPM, will have a pressure loss of about 0.15 mm per meter length. This pressure loss is proportional to square of duct velocity.

The total pressure loss is the sum of the loss across the hood, Y/T connections, bends, straight duct length & convergent/divergent sections

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Fans for Kitchen Exhaust

Kitchen exhaust consists of hot, grease laden air with some solid particulate matter also.

Fan must be capable of handling this air. Motor must be kept outside the air-stream. The recommended kitchen exhaust fan is a

SISW centrifugal fan with backward incline wheel.

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Condition of a FC Impeller used in Kitchen Exhaust

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Advantages of Backward Inclined Fans

Self cleaning properties Higher efficiency Limit load characteristics Can work without scroll housing

(Plug/Plenum Fan)

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Types of Kitchen Exhaust Fans

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Plug Type Kitchen Exhaust Fan

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SISW BI Impeller

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Plug Type Exhaust Fan – Suction End

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Plug Type Exhaust Fan – Drive End

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Roof Top Centrifugal Fan with Vertical Discharge

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Terminations of Kitchen Exhaust Systems

Roof Top

Outside Wall

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Roof Top Terminations

Preferred choice Fan at the end of termination Accessibility Discharge directed away from building Precautions Discharge direction such to minimize re-entry into

fresh air intake. Knowledge of prevailing winds. Grease to be collected and drained to a closed

container : a fire safety precaution Rainwater to be kept out of the grease container

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Outside Wall Termination

Fan may or may not be at the terminal Precautions - Discharge direction such to minimize re- entry into fresh air intake. - Away from combustibles - Horizontal duct sections to pitch towards the hood for grease collection - Discharge not to be directed downward or towards pedestrian areas.

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Makeup Air Considerations

Exhaust air volume must be replaced with clean outside air.

Negative pressure in the kitchen to prevent odours migrating to outside. Not to exceed 5 Pa as per NFPA Standard 96. Excessive negative pressure prevents proper drafting of direct vent appliances.

Proper design of make up air system ensures hoods operate as per design.

IAQ and thermal comfort are also important considerations in designing a make up air system.

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IAQ : ASHRAE Standard 62

Outside Air per Person

(CFM)

1. Restaurant Dining Area 21 (max. 75 persons/100 sq.m) 2. Cafeteria/Fast Food Dining Area 21 (max. 108 persons/100 sq.m)3. Bars/ Cocktail Lounges 30 (Max.108 persons/100 sq.m)4. Kitchens 15 (Max. 22 persons/100 sq.m)

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Replacement Air Categories

Makeup air for kitchen ventilation must integrate into the total building fresh air system design

There are three sources of makeup air:Supply Air : Outside air brought in by the HVAC system dedicated to comfort conditioning of kitchen space.Make up Air : Outside air brought in to provide replacement air specifically for the hood. May or may not be conditioned & is typically delivered close to the hood. Transfer Air :Outside air brought into the kitchen but introduced in the building by the HVAC system dedicated to the space adjacent to the kitchen.

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Replacement Air Distribution

Design to eliminate high velocities, eddies, swirls & cross-drafts that can interfere with the natural vertical rising of the effluent plume.

Deliver replacement air to the hood

- At proper velocity

- Uniformly from all directions For conditioned air, non-directional perforated ceiling diffusers

are the best option. They can distribute large mount of air throughout the kitchen at low discharge velocities.

Four-way ceiling diffusers near the hood are not recommended.

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Compensating Hoods

Hoods with built in (integral) replacement air supply

Air Curtain (Down Discharge) Back Wall Discharge Front Face Discharge External Supply Plenum Combination of above

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Compensating Hoods

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Air Curtain (Down Discharge)

Used for spot cooling of cooking staff Along the perimeter of the hood (front and/or side) Uses conditioned air Can be used to keep un-conditioned air close to the hood at

the cost of comfort. Replacement air supply – 10% to 50% Discharge velocity a critical factor affecting hood

performance – at too low a value, air enters the hood directly & at too high a value, it will entrain the exhaust plume and spill it in the room.

Recommended design supply rate is 65 cfm per foot. Max. can be 125 cfm/ft. under ideal conditions.

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Back Wall Discharge

Makeup air plenum between back wall and hood (about 6”

deep). Extends about 6” below cooking surface. Runs along the entire length of the hood. Very effective way of supplying un-conditioned make-up air

near the hood. Recommended design supply rate @ 150 cfm/ft, although

max. can be up to 250 cfm/ft.

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Front Face Discharge

To throw supply air across the room. Used to supply conditioned air in a conditioned kitchen or

conversely for non-conditioned air into a non-conditioned kitchen.

Replacement air supply – 40% to 80%. Not recommended for conditioned hot air in cold climates. Face velocity at discharge should be less than 150 fpm. Recommended design rate @150 cfm/ft. Max. can be 250

cfm/ft.

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External Supply Plenum

Plenum mounted at the ceiling or the top front edge of the hood along its full length.

Supplies air close to the hood. Can be used for conditioned or

un-conditioned air. Advantages over air curtain

hood – permits higher discharge velocity.

Max. supply rate can be 180 cfm/ft. Recommended rate 110 cfm/ft.

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Effect of Four Way Diffuser on Hood Capture

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Exhaust Flow Rate 2750 CFM – 1800 CFM Makeup Air (Spillage)

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Exhaust Flow Rate 3600 CFM – No Makeup Air (Full Capture)

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Makeup Air – Conditioned or Unconditioned?

If the objective is comfort, use conditioned air. If the objective is low cost, use un-

conditioned air. Both can be used, provided their distribution

is designed to minimize cost, maximize comfort and optimize hood performance.

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Makeup Air Distribution – Conditioned Air

For conditioned air, select the system that will distribute the air throughout the kitchen to improve comfort - (listed in decreasing preference):

1. Perforated ceiling 2. Front face discharge 3. Back wall discharge 4. External supply plenum 5. 4-way diffuser 6. Air curtain

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Makeup Air Distribution – Unconditioned Air

For un-conditioned air, the supply system should deliver the air as close to the hood as possible without adversely impacting hood performance and without mixing with room air – (Listed in decreasing preference):

1. Back wall discharge

2. External supply plenum

3. Front face discharge

4. Perforated ceiling

5. Air curtain

6. 4-way diffuser

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Integration and Balance

Supply air system must integrate & balance with the exhaust system

Balancing using controls and flow adjustments optimizes system performance

Every kitchen should be slightly negatively pressurized compared to surrounding area to contain the grease vapors, odors and hotter kitchen air within the kitchen

In a stand alone restaurant, the overall building should be at slightly positive pressure compared to outside to prevent infiltration of heat, dirt, dust and insects.

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Thank You