Kitchen Hood Analysis.pdf

7/27/2019 Kitchen Hood Analysis.pdf

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COMMERCIAL/INSTITUTIONAL KITCHEN EXHAUST AND MAKE-UP AIR

ANALYSIS, DESIGN AND SELECTION

APPLICATION GUIDE

Preface

The procedure of ventilation equipment selection should follow an analysis based on thecooking line-up, menu, operating hours and quantities of cooked food. Predeterminedselections based on initial costs are very short sighted and unrealistic.

Kitchen ventilation is subject to many constraints and forces which must be examined andevaluated together. Cooking as a process within a building must be treated as such. When

any consideration is understated operating costs will increase and sanitation will suffer.Climatic conditions vary greatly, therefore each application must be related to its local

characteristics, It is unrealistic to design a system the same for northern Alaska as forSouthern Texas. This simply is not possible.

With energy costs having dramatically risen to a point where the kitchen exhaust hood

system is the biggest energy user in the kitchen, it has unleashed those who would preyon the uninformed. They speak of free air or no cost to you exhaust/make-up systemswhen, in reality, the costs are greater than if a system had been analyzed an designed for

its intended purpose.

New building and fire code requirements have recently been enacted which have caused

significant changes in engineering requirements. These code requirements are in aconstant state of flux and must be monitored constantly to assure compliance of the hoods

and all related equipment. Indoor air quality, air pollution control methods and heatrecovery systems are also becoming integral parts of the exhaust hood system.


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PART 1

Contemporary Exhaust Control Devices

Kitchen Exhaust Ventilation

Currently there are two common types of exhaust hoods.

Those with internal baffles which are called grease extractor hoods. These may

include an internal wash system or may be manufactured as cartridges for manual

washing.

Those which use removable filters are called grease filter hoods. These must be

removed and manually washed.

Both are subject to the same standards when evaluated by an independent testing agency.

Grease Extractors

By design Grease Extractors should remove in excess of 95%, by weight, of the greaseproduced by the cooking process. This very efficient method of grease and particleremoval from the air stream is especially important when the exhausted air must be

cleaned to satisfy ecological requirements or where very high grease loads will causedamage when deposited on the roof or sides of abuilding.

Most extractors utilize centrifugal force as the grease extraction method. The ventilatorcauses the exhaust air to be pulled through a series of tight turns at high velocity causing

the heavier grease particles to agglomerate, separate from the air stream and collect onthe interior surfaces.

When an automatic wash system is a part of the Grease Extractor, automatic controlsshould include a hot water and detergent wash down system to remove the debriscollected during the cooking operation. The exhaust fan normally does not operate during

this sequence. The wash system should be automatically activated in the event of highexhaust gas temperatures above 325 F. This will serve as a secondary back up to the fireo

suppression system. Activation of the system maybe usedtoshut off the exhaust fan and

close a fire damper located either at the chamber inlet or outlet.

The Grease Extractor has the highest grease removal efficiency and is suited for

applications where grease loading is high. Grease Extractorsmay utilize continuous coldwater sprays to congeal grease particles in the air stream as a secondary method ofextraction. Such systems offer an advantage over centrifugal type extractors when


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extremely high (140 F or greater) exhaust air temperatures are encountered. The coolingo

water spray will cause vaporized grease to congeal, yielding an extraction efficiency higherthan the straight centrifugal unit. Furthermore, these systems serve to regulate exhaust

gas temperatures, there by stabilizing or reducing exhaust requirements between idle andload use of the cooking equipment. High operating and maintenance costs are associated

with continuous water sprays.

Grease Extractors are also available with removable cartridges in lieu of the wash system.This allows for the cleaning of the cartridges in a dishwasher and a significant additional

initial cost saving. When labor costs are high or when little shut down time is available forhood cleaning, the addition of a wash system is advantageous.

It should be noted here that products claiming to be grease extractors are beingmanufactured which are intended for use in filter hoods. These products have a horizontalslot opening. However the grease extracting performance is much lower and not suitable

for use as a Grease Extractor. (See filter hoods)

Filter Hoods

The filter hoods use metal filters to remove already agglomerated grease droplets from the

air stream prior to entering the exhaust duct. Filter hoods are referred to as Bulletin 96canopies, a reference to NFPA Bulletin 96 which governs construction requirements forcommercial kitchen applications. Filter hoods are available as Listed if they have been

successfully tested.

Underwriters Laboratories establishes the standards for the filters which are known as UL

Classified Filters. The primary purpose of these filters is to remove liquid grease droplets

from the air stream, drain the grease away and serve as a fire stop. All classified filterscause grease removal by centrifugal force and the collected grease drains from the

surface into a grease gutter. The filters are installed in a vertical position to no less than45 degrees off horizontal plane. The efficiency of these filters is approximately 50 to 60%,by weight. It should be noted that these filters do not reduce duct grease buildup as

efficiently as grease extractors do.

Pre-code filters were typically wire mesh that removed grease by impingement. Such filters

are no longer permitted due to their flammability and the rapid increase in pressure dropassociated with grease collecting and congealing in the filters.

High Velocity Hoods

High Velocity (HV) hoods combine a high velocity type slot with a filter hood. Their designincorporates the UL classified filter as the primary grease removal means. The openingis limited to only a few inches in order to accelerate and direct the inlet of gases, thereby


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improving smoke capture. Air volume selections are the same as for the greaseextractor, when applying rules of directional air flow. They are applied in moderate greaseload applications. Initial cost is slightly above the standard filter hood and below that of the

grease extractor.

UL Hood and Damper Assemblies

Exhaust hoods may be constructed as UL Hood and Damper Assemblies through theaddition of an automatic fire damper located in the exhaust duct collar. The automatic

damper prevents fire from reaching the duct work and contains the fire. The use of Hoodand Damper assemblies is determined solely by local fire codes.

UL 710 (as revised in 1992)

The UL 710 testing procedure is the nationally accepted method of determining thesuitability of a manufacturers hood for use over cooking equipment.

The results of this test procedure places restrictions on how a specific hood may be used.

The main restrictions include:

The maximum cooking surface temperature over which the hood may be placed.The maximum vertical distance between the front lower edge of the ventilator and

the cooking equipment.The minimum allowable exhaust air volumeThe maximum allowable internal make up air volume for each type of internal make-

up air delivery methodThe maximum overall length of the hood per exhaust collar locationSpecial methods of construction

This information is available from the independent testing agency which performed thetests.


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Devices - Principles of Operation

Figure 1Bulletin 96 DesignUL Listed Hood

Figure 2Grease ExtractorUL Listed Hood

The most frequently used hood arrangement containsfilters (Figure 1).The exhaust fan draws room air into

the canopy and through the filter. The pan on top of therange is the source of contaminants and heat that are

working against the air curtain. The air curtain isdenoted by the arrowed lines from the front edge of theequipment. When the air curtain is not strong enough

to contain the contaminants, it is very easy for thesecontaminants, at the velocity emanating from the range,to escape into the room. This why it is important to

maintain a 20 to 60 feet per minute velocity at the front

edge, depending upon the type of equipment beingventilated, Note that these velocities are obtainedwithout any internal make-up air. The heat of thecooking equipment creates an upward thermal current

that will naturally enter the canopy if not disturbed byexternal air currents.

The edge velocity is in part related to the speed ofexhaust gases entering the filters but diminished by thedistance away from the same filter. The use of very

large filters will cause low pressure drop and very low

edge velocity. Frequently the area nearest the ductconnection works well but spillage (leakage) occurs at

the ends.

The Grease Extractor (Figure 2)draws exhaust airthrough a lineal slot and creates velocities at the front

edge of the cooking equipment of 20 to 60 feet perminute, depending on the manufacturers design and theequipment being ventilated. Because a grease extractor

uses higher extraction velocities than a filter hood it can

create higher capture velocities at the front edge of thecooking equipment (or lower CFM/lineal foot). The

rising thermal currents are accelerated into the highvelocity inlet slot. As the capture speed increases airrushes in to take its place causing a draft and thus

capture.


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Figure 3 (Figure 3) The high velocity filter hood utilizes aHigh Velocity 96 Design moderately high slot speed to establish capture. The

slot may vary from 3" to 5" of opening to accommodate200 to 350 CFM/lineal feet of hood in the same manneras the more familiar grease extractor. Consequently

required static pressure for the velocity increase mustbe in addition to the usual requirements for aconventional filter. The major advantages are capture

with less cfm/lineal foot and reduced system initial costover grease extractors.


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TABLE 1: TYPICAL EXHAUST HOOD DATA

Hood Style Filter Hood Grease Extractor High Velocity96" Wash Hood HV96"Figure 1 Figure 2 Figure 3

Per Lineal Foot Greatest ModerateCFM 250 - 400 150 - 350

Lowest +

150 - 350

Static Least High ModeratePressure .4 - .9" W.G. 1.3 - 1.7" W.G. .7 - 1.3" W.G.

HP Same Same Same

Initial Cost ** Lowest Highest Moderate

Operational Cost * Highest Least Moderate

Grease Removal Fair Best Moderate

40 -60% 90 - 95% 50 -60%

* Includes cost of energy exhaust, routine daily and periodic maintenance.

** Lower figures are for use with light duty cooking equipment.

+ Use in conjunction with higher volume island and sectionalized hood for combined

rating.

Contemporary Make-Up Air Methods

Evolution has been from no make-up air, to opening doors and windows, through health

and sanitation codes and finally the evolution of high energy costs. Engineers now mustmake exact value judgements on the selection of how to introduce make-up air. Thereason for make-up air is profound. The exhaust system is the most expensive consumer

heating and air conditioning source in the building.


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Figure 4Front Grill, Style F

Figure 5Down Discharge, Style D

The first filter canopies with make-up air were grills

(Figure 4) in the front panels that introduced air in the

a manner which diffused the air into the kitchen,allowing it to drop and be pulled into the hood, similar toa hood without make-up air. The results were the same

capture effect as a standard filter canopy without make-up air. Grills are ideal for dry climates where evaporativecooling is used. However, if the kitchen is air

conditioned a draw back results in the temperature ofthe make-up air in the summer time being higher than

that of the conditioned air in the kitchen. The make-upair is mixed into the return air of the HVAC system of thekitchen causing an increased air conditioning load. The

system is ideal for desert climates such as Arizona.

The first attempts at air curtain make-up air delivery

were the use of down discharge grills (Figure 5), slots

or holes. Unfortunately, excess velocity caused many

problems which included loss of inbound edge velocity.This permits the contaminants produced by the cookingequipment to move into the kitchen. Another problem is

that air delivered from the perimeter slot does not tendto flow in a straight line - it feathers. Any smoke caughtin this situation has a tendency to work its way to the

outside and, because of the feathering, some of the

smoke rolls into the kitchen atmosphere. The make-upair should be heated to 65 degrees to make it

comfortable for the chef. Another objection to thissystem is that the air velocity striking the cooking staff isso strong that it is very annoying. It is common to find in

practice that the chef has turned off or reduced themake-up air supply to such a low velocity that it defeatsthe initial purpose.


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Figure 6Air Curtain/ Low Velocity,Style EZ

In the final evolution of curtain air delivery two factorsemerged which have proved vital for good air delivery toenhance capture; low velocity and downward orientation

delivered from a front supply point. The EZ Air system

(Figure 6) is a low velocity air curtain that creates a

make-up supply area directly in front of the hood. Theair supply, working in conjunction with the lowerpressure developed under the hood from the exhaust,

causes a controlled air movement through the areabetween the bottom of the hood and the cooking

surface. Using low velocity, the system can utilize make-up air temperatures of 45 to 50 degrees F when allowedby code. In most localities this will reduce the energy

consumption of the hood by 50% to 75% over theutilization of room air at 65 degrees or above.

Short Circuit Make-up Air

Essentially, there are only two types of make-up air chamber distribution methods. Theseare air curtain as described above, and internally injected make-up air known as shortcircuit or internal space compensating.

Short circuit hoods were designed initially to circumvent initial equipment mandated by

codes which set dilution air (room air) requirements for fire safety. The code official gavelittle consideration to the initial operating cost of systems, The requirements were simplyto reduce the exhaust temperature. Thus, an unwitting discussion has opened the door

for many ideas: mostly short sighted.

Short circuit hoods have several common failings. The air introduced under the hood

reduces the air curtain effect. As the percentage of air introduced inside the hood airincreases, the edge velocity reduces. At the equivalent of 50% of exhaust as make-air, theedge velocity is 0 to 15 FPM. From that point on the designer must realize that cross

current form other air systems and employee movements will cause spillage. Additionalsudden flare-ups such as charbroiling, frying and opening of compartment/oven doors willemit grease vapor to the room. Use of short circuit hoods beyond low input cooking line-

ups such as for nursing homes, soup line-up and grills will emit grease vapor to the

atmosphere and floor.


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The following three designs have seen their trials and failed:

Figure 7 Figure 8 Figure 9

High Velocity Short Slow Velocity Short Multi-point Short

Circuit Make-up Air Circuit Make-up Air Circuit Make-Up Air

Figure 7

While high velocity make-up air injected as shown in (Figure 7)can induce room air near

the aspirating point the effect is nearly zero FPM at the surface edge. As the percentage

of make-up air increased above 50%, it negates the room air or drives out thecontaminated gas from under the hood.

The design velocity at the make-up air slot is usually 1,000 to 2,000 FPM. The filtervelocity is aroundis around 300 to 400 FPM. The make-up air rebounds off of the filter because it cannot

accept the velocity. Imagine a water hose with a quarter inch orifice and a full flow ofwater. Try to force the water through a 1 inch hole 3 feet away. The same effect is true withthe air in a canopy.

When the outside temperatures fall below freezing, the air that normally flows straightacross the canopy from the front to the filter has a tendency to drop because it is heavier

or more dense. When the temperature drops in to the teens, the air will prevent the

contaminants from rising. Cold air does not mix well with hot air being produced at thesurface of the cooking equipment. A good analogy of this is a weather front. When a cold

front from the north meets warm air form the south, a storm develops. The same thing istrue of a short circuit hood. As the Cold air enters and falls, it will force the contaminated

hot air into the room or down to the floor.


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Canopies designed on the principle that hot air will eventually rise utilize larger overhangs,such as 12 to 24 inches plus 30 inch deep hoods to help capture lighter smoke. However,the heavier contaminated particles will be lost below the hood.

Figure 8

Slow velocity short-circuit hoods, as shown in use a series of large linear grills. The

purpose of the grills is to bring the air in at a lower velocity to avoid causing the air tobounce off the filters as discussed above. Because it is a low velocity type the problem is

the thermocline effect commonly known as inversion. Hot, polluted vapors cannotpenetrate the colder, denser barrier above the cooking equipment. The grease and watervapors condense and fall to the floor or onto the cooking personnel.

Figure 9

The difference between Figure 8 and Figure 9 is that the manufacturer is introducing

make-up at the rear as well. As outside temperatures fall below freezing, the raw air willexpand and push out the contaminants and therefore negate capture in the attempt toincrease the percentage of make-up air by introducing air behind the cooking equipment.

If the amount of this air is too great a large amount of grease may be deposited behind andbelow the cooking equipment as the cold outside air comes in direct contact with the hothighly contaminated fumes of the cooking equipment. This causes a severe sanitation

problem if not properly designed. Furthermore, this air may cause a problem with gas pilotlights and could cool the cooking surface thus interfering with cooking operations and

greatly increasing cooking equipment energy usage.

Short circuit designs such as Figures 7, 8, and 9 are very limited by physical conditions.

Many manufacturers depend on the cook to change damper settings for winter or summerto compensate for changing air volume due to changes of air density of outside air. It is

likely that the cook will eventually close the outside air because it interferes with his workor blows on him. When the make-up air supply is eliminated the more expensive air fromthe building is used in its place.


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A New Design

In light of the well established short comings of present designs, the Tri-Air hood has beendeveloped to minimize the physical and thermal impact of raw air injected inside the hood.

Figure 10Style Tri-Air

The Tri-Air is a truer design of the short-circuit systemthan ever offered before. Approximately 50% of the

exhaust gas volume A , is injected at the top of the

hood. It is slip streamed across an air foil surface and

into the area between the filter and the HV baffle. Theventuri action induces the rising current carrying vaporsand particulate to enter the filters. The

temperature/thermal expansion impact is very minimalsince the exposure area is limited to only a few inchesexternally. The mixing will occur at or beyond the filters.

Two additional make-up air volumes of approximately15% of exhaust each are injected under the hood. The

volume B moves gently across the interior of the hood

and contains the effluent toward the HV baffle andfilters.

The additional volume, C , is introduced as curtain air

and cools the cooking personnel. The cold outside airexpands when mixed with the internal source of heatedair, thus increasing the actual percentage of volume of

short - circuit air. In all short -circuit hoods, the volumeof make-up air supplied during the cold season must beless than the volume during the warm season. Two

speed makeup air fans will manage the volume bymoving full volumes of warm air and 2/3 volume of coldair. More on this matter in Part III.


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TABLE 2: TYPICAL MAKE-UP AIR DATA

Distri- A/C Temper- from Make-up Air And/0r Room Personal Comments

bution Kit- ature Radiant Recommended Air Loss Comfort

For Discharge Relief Max. % Of Volume

chen Required Heat Summer-Winter Summer Winter

Expansion

Figure 4

Front GrillSTYLE F

Room No 70 F Poor 85% 85% 15% 15% None Evap. Cooledo

kitchens only.

Figure 5

DownDischargeSTYLE D

Chefs Yes 70 F Fair 60% 60% 40% 40% Poor Discomfort causedWork by velocity supplyArea air.

o

Figure 6

E-Z AIR

Curtain Style

Chefs Yes 40-50 F Good 85% 85% 15% 15% Good - Ideal for cold climateWork Excellent with high radiant he

Area Ideal for A/C kitchen

o

Figure 7High VelocityShort Circuit

Inside Yes 35 F None 50% 35% 50% 65% None Requires excessiveHood amount of room air.

o

Figure 8Low VelocityShort CircuitSTYLE IM

Inside Yes 35 F None 60% 40% 40% 60% None Requires excessiveHood amount of room air.

o

Figure 9

Multi PointShort Circuit

Inside Yes 35 F None 50% 35% 50% 65% None Requires greater Hood overhangs and

o

excess room air.

Figure 10Tri-Air

Inside Yes Any Fair 80% 54% 20% 46% Excellent Ideal for mild to hotHood humid climates or A

kitchens. For light tomedium duty line-up


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PART II

DESIGN ANALYSIS

Exhaust

Why Use A Hood?

1. To control an environment for capture of effluent.

2. To induce air flow.

3. To protect the region from flame, heat and particulate matter.

What is Important?

1. An exhaust hood should shape the air flow characteristics, be easily cleanable, beconstructed of non-combustible materials and not interfere with the process which

it is ventilating.

2. Hood designs relate directly to process characteristics: the energy load beneathit, the contaminant load, the process and the characteristics of the building in whichit is contained. The process energy is the cooking line input which expands

gases; i.e., products of combustion, water vapor, grease, hydrocarbons. Anexhaust fan creates an air flow that moves the gases in a specific direction.

So The Primary Considerations Are:

1. Exhaust volume is determined by heat energy input of the cooking line.

2. The exhaust air volume must be greater than the heat expansion from the cookingsurface plus any internally injected make-up air.

3. Capture can only be maintained if air movement between the lower edge of thehood and the cooking surface has an inward velocity of not less than:

a. 15 feet per minute for light duty

b. 25 feet per minute for medium dutyc. Exceptional heat loads may require up to 60 feet per minuted. Particulate loads should determine the efficiency requirement of the

grease extraction unit.


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How Does It Work?

Capture is the controlled movement of gases in a specific direction which combines thosegases into one mixed gas temperature. The mixed gas condition of a kitchen exhaustsystem is comprised of room air, make-up air at another temperature and the expanded

effluent produced by the cooking line. The hot effluent rising from the heated surfaces willcreate a slight pressure differential which induces room air replacement. It is this volumeof rising gas which must be exhausted since it contains grease as liquid and vapor, water

vapor, various odors and noxious gases. To achieve capture, a mechanical draftestablishes the direction of flow through the physical structure of the hood. The capture

characteristic is constant and must be great enough to overcome the movement of gasesemanating from the cooking line and other air turbulence from outside sources includingmake-up air and room air.

The capture effect of a hood can be improved by overhangs which restrict the outwardexpansion of the gases. Overhang is the space between the edge of the cooking

equipment and the edge of the hood in a vertical plane.

The exhaust volume requirement for capture is determined by the process and the

distance of the hood from the process. Normally, a shelf hood requires less draft than awall canopy for the same process.

Make-up Air

Why Use Make-Up air?

1. To replace the excess air needed to control capture and dilute gas temperatures.2. To manage energy expenses.3. To manage building air pressure.

What Is Important?

Air Density

Hoods function at ranges of conditions 70 to 110 F. degrees and approximately 14.7 PSIA.

The operating conditions of the exhaust gas as measured at the exhaust collar of the hood

will range between 90 and 140 degrees F. depending upon the cooking load and make-upair temperature.

Most cooking lines have a fairly constant energy input controlled by thermostats. The

kitchen room temperature varies and the make-up air can vary greatly. Since the exhaustfan is a constant volume device, it is critical that the make-up air variable be controlled.


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Variations in the make-up air load are related directly to the air density. Example: St. Paul - winterdesign -10 degrees F and a summer roof-top temperature of 110 degrees F yields a 26% to 37%variation (depending on relative humidity) which must be compensated for during the course of theyear in order to maintain uniform conditions. Raw air cannot be brought in without seasonaladjustment.

Make-up air tempered to 50 degrees F or evaporatively cooled to 84 degrees F has a densitycontrolled within +4.8% to -2.5%.

Table 3: TEMPERATURE CORRECTION FACTORS

Outside Air Density Correction Dry Air (@29.921" Moist Air (@29.921'Temp. Degree F Factor Hg.) Lbs/CFT HG.) Lbs/CFT

-40 1.26 .095 .095

-30 1.23 .092 .092

-20 1.20 .090 .090

-10 1.18 .088 .088

0 1.15 .086 .086

10 1.13 .085 .085

20 1.10 .083 .083

30 1.08 .081 .081

40 1.06 .079 .079

50 1.04 .078 .077

60 1.02 .076 .075

70 1.00 .075 .073

80 .98 .074 .071

90 .96 .072 .069

100 .95 .071 .066

110 .93 .070 .064

120 .92 .068 .061

130 .91 .067 .057

140 .89 .066 .05

150 .87 .065 .049


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This table has been designed as a convenient method to determine the actual exhaust gasvolumes being applied in an exhaust/make-up system. To determine the actual air volumefor other than standard air at 70 degrees F and 29.92" barometer, multiply the air volume

(CFM) by the LBS/CFT dry air at the initial temperature divided by the LBS/CFT at the newtemperature to equal the ACFM. Since the conditions include substantial water vapor,

then use the Moist Air values.

CFM X Initial LBS/CFT / New LBS/CFT = ACFM @ 29.921" Hg.

Primary considerations Are:

1. The kitchen area must be negative in pressure in relation to surrounding public

areas, but may remain positive in relation to atmosphere. Exception: Health carefacilities.

2. Make-up air injected internally to the hood does not improve capture but only serves

to reduce gas temperatures under the hood.

3. Internal make-up air reduces room air changes.

4. Make-up air volume is determined by exhaust air volume. Make-up air distributionlocation is determined by room air change requirements. The type of make-up airdistribution is determined by radiant heat load.

5. Make-up air design temperature should be based on rooftop temperature conditionsrather than climatic temperature conditions.

Room Air

Sufficient supply air should be introduced to compensate for air being exhausted throughthe ventilator, but not being made up for by the hood system. This can be accomplishedthrough HVAC supply ceiling diffusers located a minimum of six feet from the ventilator.

The volume required must be calculated by the HVAC designer/contractor taking intoaccount:

1. All exhaust and supply sources in the kitchen area (code may require no morethan .02 inches water column negative pressure in the kitchen areas).

2. Air flow patterns in the kitchen area (making sure no drafts interfere with smokecapture).3. HVAC heat/cooling loads


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PART III

SIZING AND APPLICATION DETAILS

Quantifying Exhaust Gas Volumes

By Prevailing Code

Prevailing local codes specify required minimum exhaust standards. The coderequirement may exceed traditional rates per lineal foot or square foot rating establishedby the Uniform Mechanical Code.

By Experience

Through experiences based on trial and error an art has emerged which factors numerous

values into a judgement call. The designer will judge the line-up for energy input, flash

load, radiant and contaminant loads, overhangs and other design characteristics in orderto establish an exhaust volume.

Third Method

Through comparative analysis an engineer can determine the exhaust requirements fora hood section. He will assign a value of 1 to 5 for each of the four (4) characteristics

relating to each appliance in the cooking line-up. Each value is the squared and addedtogether. The square root of the total is the average rating per foot of the appliance.

Table 4 offers some typical examples of cooking equipment.

Second step: the average rating per foot is multiplied by the length in feet of the appliance.

The total ratings are added and then divided by then length of the line-up to establish anaggregate rating for the hood section. Overhangs are added at the same rating whenrequired.

Gas Versus Electric Input

The energy input of a device causes some expansion of surface gas without any frying,

broiling or boiling actually occurring.

The HVAC industry requires 80% to 90% combustion efficiency in boilers and furnaces.Typically up to 65% efficiency is usual with kitchen cooking equipment. Gas fired cookinglines require greater exhaust air flow. Electric cooking inputs are a precise measure sinceflue gases have to be handled by the exhaust system with greater input per foot.

Radiant Value


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The radiant value is a device which is a strong indicator of its input value. A slow walkpast a cooking line-up will teach a physical lesson showing radiant heat discharge to theroom can be as much as 40% of the input of a unit.

Flash Load

Flash loads are indicative of high input, direct fired equipment such as deep fryer, ovens,char-broilers, roasters.

Steady load is found in equipment which maintains food at low temperatures such as is

found in convalescent homes. Kettles, compartment steamers and other indirect firedappliances are typical steady load equipment.

Contaminant Loads

Char-broiling produces the greatest concentration of grease and hydrocarbon particulate.

Second in the line of priority is deep fat frying followed by griddling. Ovens, convectionovens and steam holding are minimal contaminant loads. The use of food additives (suchas marinated meats) greatly increases the contaminant load. Chinese cooking on woklines is also an exceptionally heavy grease load. Frozen potatoes produce more airborne

grease than fresh or refrigerated.

Procedure to Establish Exhaust volume: Third Method

1. List cooking equipment; width of item in feet and rating average values for eachcooking appliance from Table 4. Multiply and total.

2. Divide total in #1 by length (in feet) of line-up.

3. Refer to equivalent capture chart (Table 5) and select the CFM/FT under thespecific hood to be selected. Round off to next higher whole number, i.e. 6.7 select#7 rating.

4. Multiply the CFM/FT. x the length of the cooking line. Overhangs, if required, must

be added to establish length of hood.

Example:

Cooking line contains:


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Table 4

1 each - Griddle - 4' - 4 x 6.6 = 26.4

2 each - High Output Fryers - (2x1-1/2)= 3' - 3 x 9.1 = 27.31 each - Dump Station 1' - 1 x 0.0 = 00.0

Total Line 8'0" Total Rating 53.7 / 8Aggregate Rating = 6.7

Refer to Table 5, Equivalent Capture Chart.For 6.7 aggregate rating, use 7 rating.

Therefore: Filter Hood 400 CFM/FT. x 8 ft. = 3200 CFMWash or HV 96 300 CFM/FT. x 8 ft. = 2400 CFM

Note: The above air volume estimate is based on minimum practical requirements

for typical conditions. Local codes or special considerations may require the use

of high volumes.


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Table 4: TYPICAL OF COOKING EQUIPMENT RATINGS

TYPE TYPICAL TYPICAL MBA LOAD RATING VALUE

MBH MBH LINEAL

INPUT FT FT2

Low 1 High 5 normal range Extra High 6 Extremely High 7

INPUT | RADIANT | FLASH | CONTAMINANT

*UNIT

RATING

AVG/FT

CHAIN BROILERS 120 15 7 1 5 6 10.5

CHARBROILERS (4') 120 13.3 30 5 5 5 5 10.0

FRYER High output 140 39.4 105 5 4 4 5 9.1

Standard 110 31 82.5 4 3 3 4 7.1

GRIDDLE (4') 95 10 24 3 3 3 4 6.6

RANGE Heavy DutyHot Top With Oven 132 16.5 49.5 3 5 1 2 6.3

FRYERCounter Type 34 8.6 17 2 3 3 4 6.2

BROILER (Upright) 80 9.2 26.7 2 4 3 1 5.4

BAKING OVENUpright 90 6.5 20.5 3 3 3 1 5.3

RESTAURANTRANGE 175 13.2 35.2 2 3 2 3 5.0Open Burner, Griddle,Oven

RANGEHeavy Duty, OpenBurner w/ Oven 125 15.2 45.7 3 2 2 2 4.6

TILTING SKILLET(4') 120 10 30 2 2 2 3 4.6

CONVECTION OVEN( 1 Deck ) 110 11 33 3 2 3 1 4.1

STEAM KETTLES40 Gallon 75 15 33.3 1 2 1 1 2.6

2 Gallon 7 7.5 6.3 1 1 1 1 2.0

Compartment

Steamer 17 4.3 8.5 1 1 1 1 2.0

WORK TABLES/DUMP STATIONS 0 0 0 0 0 0 0 0

*IF OTHER EQUIPMENT IS USED ABOVE THE LINE-UP SUCH AS CHEESE MELTER OR SALAMANDERTHEN ADD 1.0 TO THE AVERAGE / FOOT FACTOR


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TABLE 5 EQUIVALENT CAPTURE CHART

CFM / LINEAL FOOT

TYPE OF HOOD

AGGREGATE FILTER GREASE HIGH VELOCITY BACKRATING C96 EXTRACTOR HV96 SHELF

10 700 500 500 *

9 600 400 400 *

8 500 350 350 *

7 400 300 300 350

6 350 275 275 300

5 300 250 250 250

4 250 200 225 225

3 200 150 200 200

2 200 150 200 200

Notes:* Use overhead canopy style hood

SECTIONAL HOOD DESIGNS

Dual Volume Ratings

Each application should be viewed by the designer to minimize exhaust volumerequirements without jeopardizing capture. The engineer has a number of tools to work

with including zoning of the line-up. One half of the cooking line might be high intensitywhereas the other would be medium. Two different ventilation rates can be utilized on asingle cooking line. Perhaps the left side might be 350 CFM per foot over a char-boiler,

deep fat fryer hot top range side of a cooking line where the right side with ovens, steamkettles and urns could be ventilated at only 200 CFM per foot. The average volume perfoot meets U.L. listing and code requirements and the total air volume is less. A single

hood could be used with two duct collars, different filter sizes and an internal divider would

separate the hood functionally. Example: Within a 16 foot hood the left hand 8 foot sideincludes a 4 foot char-broiler and a 4 foot hot top range while the right side is urn,

convection ovens and steamers. The left side aggregate value is 8.1 and requires 350CFM per foot X 8 feet. The right side aggregate value is 3.0 which requires only 200 CFMper foot, for a total of 4400 CFM or 275 CFM per foot average.


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The conventional methods would require the selection to be rated on requirements of thechar-broiler at 350 CFM per foot or more. 350 CFM X 16 feet equals 5600 CFM exhaust.The sectionalized approach offers a reduction of 1200 CFM. However, it will be necessary

to provide two duct collars, both requiring the same static pressure if a single exhaust fanis used.

Table 6: Exhaust Volume / Static Pressure Relationships

At The Exhaust Collar

Filter Hoods Grease Extractor High Velocity

CFM/ Static Slot Static Slot Filter FT Pressure Width Pressure Width Size

200 Filter hoods are balanced N/A 3" 1.25 3.5" 10"

250 by Pressure drop of the filters. 1.3 3" 1.25 4" 12"

300 Multiply the filter pressure 1.7 3" 1.25 5" 16"

350 drop X 1.15 this equals 1.7 4" 1.5 5" 16"

400 the total pressure 1.3 (2) 3" N/A N/A N/A

500 drop of the hood 1.3 (2) 3" N/A N/A N/A

Overhangs

Canopy hoods enclose a large area between the canopy and the line-up which must

overcome any expansion. Overhangs are employed to restrict the outward thrust of the

gasses. The height of the hood is determined by the process, the personnel activities,overhangs and application, but not over 84 inches above the finished floor.

Back shelf hoods do not usually require overhangs.

Overhang minimums are frequently set by codes. Consider the following table as minimum

for good design.

Canopy Only Walled No Wall

Sides 0" 6"Back 0" 12" *

Front 12" 12"

* The use of a single inlet exhaust plenum as an island ventilator should locate the inlet

vertically over the rear edge of the equipment.

Use of the aggregate rating is for the full length of the line-up if enclosed on three sides.


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If not walled, then add the overhang length to the cooking line-up and use the aggregaterating for the full length of the hood.

Make-up Air Selection Guide

1. The kitchen should always be negative pressure in relation to adjacent public areas.

2. Is the kitchen area air conditioned?A. Yes - Use Tri-Air, PLV or EZ Air.

B. No - Use grills facing out (F) or air through ceiling grills for maximumventilation effect.

3. Kitchen air changes:A. Air conditioned kitchen - 6 to 12 air changes per hour (5 to 10 minutes each

change)

B. Non air conditioned kitchen - 20 to 30 air changes per hour (2 to 3 minuteseach change)

4. Use E/Z Air over high radiant equipment.

5. Use E/Z Air in areas where heating and air conditioning costs are a prime factor.

6. Use Tri-Air for light duty line ups, to 78% make-up air. Use of E/Z Air for mediumand heavy duty line-ups is good for up to 85% make-up air.

Make-up Air Roof Top Units

Make-up air units are selected as a percentage of the exhaust air volume, typically, 80 to85% make-up air depending upon the method of distribution. However, other exhaustpoints such as condensate hoods, dishwashers, and storage room exhaust may be added

into the total to be supplied.

The delivery air temperature is dependant upon the method of introduction. In air curtain

distribution systems a temperature averaging 50 to 55 F air should be satisfactory. As lowo

velocity air is introduced as an air curtain, it may operate at 45 F or lower outleto

temperature. When grills distribute the air through the room 55 to 60 F air should beo

utilized, In short circuit applications, a minimum air temperature of 35 F should beo

maintained or reduce the volume of make-up air to allow for the thermal expansion.


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Some codes require make-up air heating capacity be sized to provide air tempered to noless than 10 F lower than design room air temperature.o

Generally codes stipulate that 10 feet horizontal or 3 feet vertical clearance exist betweenthe make-up air intake and the exhaust outlet. Check local code requirements.

The solar heat gain on the roof top machinery is greater than the heat loss. Insulation isbeneficial for exposed duct runs above the roof for both summer and winter operation.

Exhaust Fans

The following criteria should be used for the selection of exhaust fans operating for

commercial kitchen hoods:

1. Wheel design should be the non-overloading type, i.e., backward inclined or

backward curved.

2. Select adjustable belt driven fans which protect the motor from grease buildup

and exhaust air heat. Use of direct drive fans prohibits accurate system balancingsince volume dampers are not permitted ( see dampers).

3. Fans should discharge away from building surfaces, normally vertical in direction.Most codes require that the fan discharge be located a minimum or 40 above theroof surface, at the same time the requirement for maintaining the horizontal duct

run at least 18" above the roof surface is satisfied.

4. Fans should be able to be cleaned of any grease accumulation. Drain provisionsto a catch pan or drain should be used.

5. The fan mounting should allow access to the adjacent ductwork.

6. Fans should be sized with the largest diameter wheel turning at the slowest RPM

permitting the most laminar air flow.

Exhaust Duct

Exhaust ducts are designed by a constant velocity method. A minimum of 1500 andmaximum of 2200 to 2500 feet per minute (depending upon various codes) is mandated.

A fully welded 16 gauge black iron or 18 gauge stainless steel is required by NFPA.

Square or rectangular ducts are most common to meet the fully welded constructionrequirement.


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The routing of exhaust duct is controlled by codes which require that the duct leading tothe exterior of the building be the most direct possible route. Horizontal exhaust ductshould be pitched either toward the hood or toward a clean out sump located at the lowest

point. Usually a sump is located where the horizontal duct changes to the vertical. Properlydesigned clean out ports must be provided at specified distances and at each change of

direction in the duct run. Cleanout access panels should be located on the sides of theduct with the bottom edge of the opening not less than 1" from the bottom of the duct. Theplates should be gasketed and fastened with bolts for a liquid tight seal. Exhaust ductsshould not be installed closer than 18" to combustible surfaces. The clearance

requirements may be reduced by shielding the combustible materials with fire ratedmaterial. NFPA 96 allows joints in ductwork to be made with companion flanges and hightemperature seals. However, some counties require all joints to be fully welded.

Codes require that exhaust ducts penetrating the roof extend at least 18" above the roofsurface. When a duct runs horizontal to the roof the bottom of the duct must be 18" above

the roof surface. The clearance from combustible requirements apply above the roof as

well as below the roof and also include the roof curb. Steel pre-fabricated and insulatedroof curbs are recommended. Local codes may exceed these requirements and should be

consulted.

Dampers

1. Exhaust

Building codes state that no damper may be used in a kitchen exhaust ductunless that damper is a integral part of a listed grease extractor or hood anddamper assembly. The requirement prohibits the use of back draft, volume,

smoke, fire or balancing dampers. Exact sizing must assure the design of theductwork along with pressure drop calculating.

2. Make-up Air Dampers

NFPA Bulletin 91 mandates a U.L. fire damper as part of supply air plenum whichintroduces make-up air inside the hood cavity, i.e., short circuit. The use of back

draft dampers in cold climates is highly recommended to prevent cold air fromentering the building in off hours. Dampers in make-up air ducts for back draft,volume, smoke and/or fire control are governed by NFPA Bulletin 91 sections

referring to air ducts.

Fire Protection Systems

National codes require automatic fixed type fire protection systems to protect ducts,


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plenums, and cooking surfaces whenever the cooking process releases grease vapors.

The respective systems and the general code and standards which covers the application

are as follows:

Wet chemical system - NFPA 17AWater sprinkler system - NFPA 13NFPA 10 also encompasses requirements for portable hand type extinguishers in additionto fixed systems.

National codes require that activation of the automatic fire protection system alsodisconnect the supply of fuel from the cooking line protected by the system. Fuel shut-off

is by means of shunt trip beakers, contactors, solenoid activated or mechanically activatedgas valves through a spring and cable system linkage. The application of these systemsis a specialty. Codes require the system be designed and installed by certified personnel.

Some systems and regional codes require that exhaust and/or make-up air systems beinterlocked with the automatic fire system. Generally, wet chemical, and sprinkler type

systems require the exhaust fan remain operational during system activation. It isdesirable, and in most localities mandatory, for activation of the fire protection system toshut off the make-up air supply which will cause the space in which the hood is located to

become greater in negative pressure to prevent the migration of smoke to other areas.

Stop Station and Control Interlock

A simple push button or switch control should start exhaust and make-up air systems foruse by unskilled kitchen personnel. Under normal operation, the exhaust fan should start

and operate an interlock with the make-up air system. Since the make-up air system mayrequire provisions for heat, cooling or vent air, the function should be provided by aselection switch or outside air thermostat. A wash hood must incorporate the mandatory

wash cycle at the end of each operation through a control cabinet supplied by the hoodmanufacturer.

Lighting

The requirement for minimum levels of lighting measured at the working surface is defined

by the local health codes. Some form of lighting device is required in all canopy typehoods. The application of these lights is specifically covered under NFPA Bulletin 70, theNational Electrical Code.


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All lighting fixtures used in a commercial cooking hood must be listed for the application.U.L. has specified that the fixtures must be installed a minimum of 48" above the cookingsurface which effectively prohibits the use of lights on shelf type hoods. If the hood being

used is not listed under U.L. 710 for a lower distance.

Three types of lights bear the necessary listing for the application: surface mountedincandescent, recessed mounted incandescent and recessed mounted fluorescentfixtures.

The National Electrical Code considers the inside portion of a hood to be contaminated airduct and prohibits the mounting of wiring or conduits inside the hood. It also requires thathigh temperature insulation types be used. The hood shell may be penetrated for lighting

and fire equipment fixtures utilizing approved grease penetration fittings.

Both OSHA and NSF (National Sanitation Foundation) require that the globe be protected

from breakage by either a removable (thus cleanable) metal guard or be a non-shattering

type globe. Shatterproof glass and plastic coated globes are commonly used. The glassmust be of a type that is resistant to the high temperatures.

National And Local Codes

Principle codes which apply to commercial kitchen hood equipment are: NFPA, Bulletin 96;

Uniform Mechanical Code, BOCA Mechanical Code, Southern Building Congress Code,and miscellaneous state and local codes. Additional attention is required in some areasas they have city and/or state codes that differ greatly from the national codes. E.G. State

of Michigan, State of Maryland, City of Chicago, Denver, New York City, Los Angeles and

others.

The installation and design of commercial kitchen hoods are governed under themechanical code; however, many areas allow the products to be covered under theDepartment of Public Health. Hoods may be covered under the jurisdiction of the

mechanical and health inspectors and therefore must meet both codes. Some areas tonote with regard to conflicting health and mechanical codes are Denver, Miami, State ofMaryland, New York and Chicago.

Ecological Codes

Many areas, especially communities with high population density and communities with

air quality problems, as determined by the EPA, may require that the air be cleaned ofgrease, particulate and odor before releasing the exhausted air into the atmosphere.These must be inquired about and if required designed into the job on an individual basis.


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Test and Balance

Test and balance is the final step in assuring that a project meets the design andengineering criteria. It is the customers assurance that the maximum effectiveness of the

system is achieved.

It is most often the responsibility of the contractor which furnished the exhaust fans andthe make-up air unit to do the test and balance.

After the entire building HVAC system and hood system meet the design and engineeringcriteria, final adjustment can be made to achieve optimum building comfort and cost

efficiency.

Air balance should be checked on an annual basis to ensure the system is performing at

peak efficiency. In order to accomplish this, it is important to know the proper method used

to determine the air volume required.

On all ventilators the air volumes required should be listed on a label under the canopy oneach section of hood. Air volume is commonly read as cubic feet per minute CFM.

Once the volume requirements are known, the velocity at the exhaust can be determinedby dividing the stated volume by the area of the opening expressed in square feet.

Example: Length of exhaust slot equals 120 inches at 4 inch width.4" X 120" = 480 square inches, divided by 144 = 3.333 square feet of slot

opening.

Therefor, if the design CFM is 3500 CFM divided by 3.333 square feet, the

velocity at the slot will average 1050 FPM.

By using a vane type air meter with a 3" head and taking three reading per section,

average actual face velocity can be determined. At this point the blower wheel speedshould be increased or decreased depending on readings taken in order to coincide withdesign requirements.

Because there are several types of air meters available, a thorough knowledge of their use

is important. Check the instrument manufacturers instruction to ensure measurements areaccurately recorded.

Documents

Kitchen Hood Analysis.pdf