O c t o b e r 2 0 0 1 A S H R A E J o u r n a l 3 7

ASHRAE Journal

A

Kitchen Exhaust:Issues and Solutions

About the Author

By John A. Clark, P.E.Fellow/Life Member ASHRAE

John A. Clark, P.E., is an associatevice president in the Mechanical Engi-neering Group of Hammel, Green andAbrahamson, Inc. in Minneapolis.

kitchen exhaust system is more than a hood in the kitchen area

to capture, contain, and remove vapor, smoke and grease from

cooking operations. Hood manufacturers have designed cano-

pies that capture and contain. Filter banks also are part of the package. It

is important to understand how the exhaust fan, stack discharge, and

replacement air contribute to a successful kitchen exhaust system.

Temperature rise is measured.d. Fan failure test with 50% of mini-

mum cfm, 1 pint (473 mL) of grease ig-nited. Temperature rise is measured.

e. Fire test with minimum cfm and 3pints (1.4 L) of grease ignited. The hoodshould not be damaged.

f. Burnout test with minimum cfm. Thehead is coated with 0.3 lbs of grease persquare foot (0.01 kg/m2) and 1 pint (473mL) of grease ignited. Temperature riseis measured.

The measured temperature rise is com-pared to an acceptable tabular value.

The four categories of cooking equip-ment tested are listed according to thecooking surface temperature:

• Light duty such as ovens, steamersand small kettles up to 400°F (200°C).

• Medium duty such as large kettles,ranges, griddles and fryers up to 400°F(200°C).

• Heavy duty such as upright boil-ers, charbroilers and woks up to 600°F(315°C).

• Extra heavy duty such as solid-fuelequipment up to 700°F (370°C).

The design exhaust rate is the desiredairflow rate in cfm/ft times the length ofthe hood canopy. Table 1 is reproducedfrom the 1999 ASHRAE Handbook—Ap-plications.

As a reminder, the primary purpose ofthe grease filter in the hood is to preventflame penetration into the duct. The ther-mal plume will rise into the canopy andbe contained if there are no disturbingcross currents of air.

FansMost systems place the exhaust fan

at or near the outlet of the duct system.Placing the fan at the outlet end keepsthe duct under negative pressure soleakage tends to be into the duct. If theduct was pressurized by a fan near thehood, grease would tend to leak out. Aroof-mounted fan often is used forease of service and cleaning. The pur-poses of the fan, other than to move theair, are:

• Keep the contaminants off the roofsurfaces,

• Discharge hot, grease-laden air awayfrom air intakes, and

• In the case of fire, direct the heat orflames away from the roof or nearby sur-faces.

The up-blast power roof ventilator(PRV) is the fan most often selected forthis duty. This type of fan is a compactunit and, with the proper curb, dis-charges the exhaust at least 40 in. (100cm) above the roof surface to satisfy therequirement in Section 4-8.2.1 of NFPA96 designed to keep the grease off theroof surface.

The up-blast PRV has some shortcom-ings. Its static pressure is normally lim-ited to 2.0 to 2.5-in. w.c. (500 to 600 Pa).This limitation becomes a problem when

Basic System ConsiderationsA hood canopy is used to contain the

rising thermal plume produced by hotcooking processes. The hood includesa filter bank to remove grease and toprevent flame penetration into theductwork. Air movement transports theheat and contaminants through the fil-ters, up the ductwork, and out of thebuilding. A fan provides the air move-ment.

The design airflow for a UL-listed hoodis based on the minimum amount of airrequired to capture and contain the ther-mal plume. The newer codes will relatecfm per linear foot of hood to the cook-ing surface temperature hood tests asconducted per UL 710 Standard. This ULcfm is a minimum established under testconditions. Consider using a value of10% to 15% more cfm to deal with fieldconditions.

The UL710 rating is based on the fol-lowing tests:

a. Temperature test with minimumcfm and cooking equipment on. Tem-perature rise is measured.

b. Cooking test with minimum cfmand cooking 70% lean meat patties. Nosmoke spillage allowed.

c. Abnormal flare-up with minimumcfm, 1 pint (473 mL) of grease is ignited.

3 8 A S H R A E J o u r n a l w w w. a s h r a e j o u r n a l . o r g O c t o b e r 2 0 0 1

ASHRAE Journal

there is a long exhaust duct and/or whenthe hood contains cartridge-type filtersthat have greater resistance than baffle-type filters. For example, a cartridge-typehood designed for charbroilers at 300 cfmper linear foot (43 L/s per linear meter)has a static pressure loss of 1.5 in. w.c.(370 Pa). This does not leave much staticpressure capacity for the ductwork andstack outlet losses. If the fan cannot de-velop the necessary static pressure, it willnot move its design exhaust volume. Theresult will be grease discharged out ofthe fan outlet and onto the roof and/ornearby surfaces.

When large amounts of grease appearon the fan or roof, often the cfm is belowdesign values. Once the fan capacity isreturned to its proper cfm, the grease dis-charge drops significantly.

The condition of grease discharge atlow exhaust cfms is a result of not enoughair at the filter surface to cool vaporizedgrease and moisture. The cooling at thispoint is needed for the filters to removegrease. The grease and water vapor areseparated from the airstream at the filter,if condensing can occur at the filter sur-face. The larger particles are then sepa-rated as the air changes direction as itpasses through the filter bank. If the pro-cess produces a large amount of vapor,water mist can be used to cool the vaporsat the hood. The ASHRAE-sponsored re-search project to measure what is contained in the kitchenexhaust confirmed this cooling and condensing phenomenon.

If the up-blast PRV is not appropriate, what other types offans can be used? A utility fan set with backward inclinedblades is a good choice for systems with higher static pres-sure requirements. The fan set should have the UL 762 ratingfor high temperature service. The fan arrangement can behorizontal if the discharge is at least 40 in. (100 cm) off theroof, or it can be upwards. An in-line fan with the motor lo-cated out of the airstream and belt/pulley protection fromgrease is another choice. A horizontal or vertical discharge isacceptable with the proper bearing and support. Consider-ation should be given to drain the rain and moisture that mayreach the fan scroll. Drain outlets are available in the lowerarea of the fan scroll. Exhaust stack configurations may beone of the types as shown in Figure 1. Some fan manufactur-ers lay the centrifugal fan wheel in a horizontal arrangementand angle the outlet upwards. This is a specialized arrange-ment that can be considered.

Stack DischargeFan discharge stack design often is neglected. Most designs

simply satisfy the height requirement above the roof surface

and the direction of discharge. The discharge plume will tendto remain at the elevation where the discharge vector is over-come by the wind vector. The result is that nearby intakes re-entrain the exhaust contaminants. The average dischargevelocity from PRV units is 1,000 fpm (5 m/s).

Therefore, with a 15 mph (24 km/h) crosswind, the dischargewill stay at the 40-in. (100 cm) discharge above the roof. Also,with the fan’s low profile, the stack outlet is vulnerable to windbackwash from adjacent structures. To make matters worse, theexhaust fan and the replacement air unit often are surroundedby a visual screen or wall enclosure. In these cases, the dis-charge effluent often stays within the enclosure and is re-en-trained into the replacement air system.

Figure 2 illustrates a proven design that has been success-ful in industrial and laboratory exhaust systems. The designoutlet velocity of these stacks should be in the 2,500 to 3,000fpm (13 to 15 m/s) range. The discharge plume will extendanother 5 to 10 ft (1.5 to 3 m) above the stack outlet beforethe wind vector turns the plume horizontal. This height sepa-ration keeps the exhaust out of intakes that are below theplane of the plume discharge. Induced air from the drain open-ing also dilutes the odor and smoke particulate in the ex-haust air. If the exhaust is unusually odorous or smoky, air

Figure 1: Typical exhaust outlets.

Figure 2: High velocity vertical discharge.

Minimum40 in.

DischargeHeight

PRV

Vented BaseDrain

HorizontalCentrifugal Fan Utility Set

Roof

ExtendedBase

Drain Lip

Drain Roof

D+14D

6 in. Min.D

From Indoor Fan

D+14D

D6 in.Min.

doohfotoofraenilrepmfc,etaRwolFtsuahxEmuminiM

dooHfoepyT ytuDthgiL ytuDmuideM ytuDyvaeH ytuDyvaeHartxEyponaCdetnuoM-llaW 002ot051 003ot002 004ot002 +053

dnalsIelgniS 003ot052 004ot003 006ot003 +055)edisrep(dnalsIelbuoD 002ot051 003ot002 004ot052 +005

worbeyE 002ot051 052ot051 — —revOssaP/flehSkcaB 002ot001 003ot002 004ot003 dednemmoceRtoN

Table 1: Typical minimum exhaust flow rates for listed Type 1 hoods for cookingequipment type.

O c t o b e r 2 0 0 1 A S H R A E J o u r n a l 3 9

Kitchen Exhaust

can be introduced into the exhaust air-stream before the fan (Figure 3).

Outlet stack velocity is governed onlyby the consideration for fan static pres-sure/horsepower and noise. The stackheight is usually about 10 ft (3 m) abovethe roof surface. At velocities above3,000 fpm (15 m/s), the velocity pres-sure loss of duct fittings increases, re-quiring more fan horsepower, and thevelocity noise rises above NC 40 levels,which is objectionable. The stack canbe round and made of stainless steel, alu-minum, or painted galvanized steel,to minimize visual objection to thestack.

An exhaust stack for a system that shuts down at night in acold climate causes problems. The problems are heat loss fromthe building up the stack; start-up problems moving cold denseair up a multistory stack; and freezing of sprinkler heads at theroofline near the stack outlet. One solution to these problemsis to provide a closure damper at or near the system outlet. Theauthority having jurisdiction often allows a closure damper atthe horizontal discharge of the centrifugal fan or in the inlet tothe fan at the roof curb, if the damper is accessible at the roofline(Figure 4).

Replacement Air ConceptReplacement air for the air exhausted should come mainly

from the kitchen area. Ten to 15% of the outdoor air can betransferred from the dining area adjacent to the kitchen. Thisis also advantageous to control the migration of cookingodors into the dining area. Be careful that the transfer airpath does not occur mainly at the pass-through windowbetween the kitchen and the dining room. The air velocitytends to cool off the food resting on the shelf in the pass-through window.

The supply air to the kitchen should be through outlets thatdo not disturb the rising thermal plume off the cooking sur-face. Avoid using four-way diffusers at or near the hood canopy.The goal is to avoid blowing toward the hood so the airflow

Figure 4: Typical closure damper arrangement.

Butterfly Damper

Roof

1/2 × 1/2Mesh

Screen

does not impinge on the hood face and drop down, whichdisturbs the thermal plume. Typical supply air outlets are per-forated panels or grilles in the front face of the hood, slot dif-fusers blowing away from the hood face, low velocity massflow down discharge diffusers, backwall discharge ducts be-low the cooking surface, and vaned diffusers located awayfrom the hood face. Hood side curtains control disturbing crossdrafts at the hood. The replacement air quantity must comefrom outdoor air intakes of supply units to the space. Theseoutdoor air intakes cannot be closed after occupancy in anattempt to save energy.

ConclusionTo achieve more predictable and reliable performance from

a kitchen exhaust system, select UL-listed hoods appropriatefor the cooking equipment and select a fan that will producethe tested minimum cfm at the system static pressure. Selectfan types that will not re-entrain the exhaust back into theoutdoor air intakes. Provide an appropriate exhaust stack thatdirects the discharge away from the same level as outdoor airintakes. Provide replacement air supply devices that do notdisturb the thermal plume from cooking surfaces. Balance theoutdoor inlet cfm to the exhaust cfm. Lack of attention to oneor more of the components listed previously is likely to causefailed or substandard kitchen exhaust systems.

Figure 3: Exhaust stack dilution inlet.