Fcsi Quiz - Halton Hood Load[1]

7/27/2019 Fcsi Quiz - Halton Hood Load[1]

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Heat Load Based Design -Radiant, Convective and Conductive HEAT™

• Types of Heat Sources and Effect on Kitchen Environment

• Exhaust Air Flow Rates (UL vs. Heat Load based Design)

• Hood Comparison Studies

Since the inception of the ASHRAE technical committee on commercial kitchen ventilation

(TC 5.10) back in the early 1990’s the commercial kitchen ventilation industry has moved

toward more resolute definitions of capture and containment and also calculation ofexhaust air flow rates based on the actual cooking equipment.

In the past, building codes called for capture CFM based on the square foot area of the

hood system, and these rates changed depending upon whether the hood system was

a wall or island arrangement (100 CFM wall 150 CFM island per square foot of hood).

Today, the International Mechanical Code calls for CFM per linear foot of hood based

upon equipment operation for the following equipment categories: extra-heavy-duty,

heavy-duty, medium-duty and light duty. These equipment category designations are

outlined in the ASHRAE Applications Handbook chapter on Kitchen Ventilation.

Picture 61Correct exhaust air flow rates

should be based on heat generatedfrom the cooking equipment



If the hood system is UL Listed, the hood does not have to meet the exhaust air flow

requirements of the International Mechanical Code and may fall back on the rating

obtained during the UL 710 testing for 400°, 600° or 700° F surface temperatures.

The most accurate method to calculate the hood exhaust air flow is a heat load based

design. This method is based on detailed information of the cooking appliances installed

under the hood including type of appliance, equipment dimensions, height of the cooking

surface, source of energy and nameplate input power. All of this data allows calculating

how the particular appliance emits energy into the kitchen. Part of this energy is emitted

to the space in the form of the convective plume - hot air rising from the cooking surface

The other component is heat in the form of radiation. This radiation component does not

affect the sensible space temperature until it heats up a surface which, in turn, releases

heat to the space in the form of convection. The majority of the radiant heat load cannot

be affected or captured by the hood, unless the hood system is single wall constructionin which case over an extended period of time the surfaces of the hood can absorb the

radiant heat and then convect it into the kitchen itself.

Picture 62Double wall hood construction canlimit radiant transfer to convectiveload



The amount of air carried in a convective plume over a cooking appliance at a certain

height can be calculated using the following equation

qp = k (z + 1.7Dh) 5/3 • Qconv 1/3 • Kr

Where:

qp - airflow in convective plume, CFM

z - height above cooking surface, inches

Qconv - cooking appliance convective heat output, Btu/h

k - empirical coefficient, k = 61 for a generic hood

Kr - reduction factor, taking into account installation of cooking equipment (free, near

wall or in corner)

Dh - hydraulic diameter, inches

Dh = 2L • W---------------

L + W

L, W - length and width of cooking surface, inches

Picture 63Hydraulic Diameter



Kitchen hoods are designed to capture the convective portion of heat emitted by cooking

appliances, thus the hood exhaust air flow should be equal or higher than the air flow in

the convective plume generated by the appliance. The total of this exhaust depends on

hood efficiency, and the air distribution system..

qex = qp • Khoodeff • Kads

Where:

Khoodeff - kitchen hood efficiency

Kads - spillage coefficient taking into account the effect of the air distribution system on

convective plume spillage from under the hood. The recommended values for Kads are

listed in Picture 74.

The efficiency of capture and containment for various models and styles of hoods canbe validated by comparing the C&C flow rates for two hoods that have been tested using

the same cooking process.

Table 7.Spillage coefficients as a function of the air distribution system



The heat load based design gives an accurate method of calculating hood exhaust air

flow as a function of cooking appliance shape, installation and input power, and it also

takes into account the hood efficiency. The only disadvantage of this method is that it is

cumbersome and time consuming if manual calculations are used.

Halton’s Hood Engineering Layout Program (HELP™) is specifically designed for

commercial kitchen ventilation and turns the cumbersome calculation of the heat load

based design into a quick and simple process. It contains the up-dateable database of

cooking appliances as well as Halton Capture J et®hoods with their higher efficiencies.

It is still common practice to estimate exhaust airflow rate based on rough methods. The

characteristic feature of these methods is that the actual heat gain of the kitchen appliance

is neglected. Thus the exhaust air flow rate is the same whether the appliances under

the hood are a heavy load like a wok or a light load like a pressure cooker. These rough

methods are listed for information, but should only be used for preliminary purposes and

not for the final air flow calculation. They will not provide an accurate result.

• floor area

• air change rate

• cooking surface area

• face velocity method (0.3-0.5 m/s)

• portions served simultaneously

Neither of these rules takes into account the type of cooking equipment under the hood

and typically results in excessive exhaust air flow and hence oversized air handling units

coupled with high energy consumption rates.

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Heat Load Based Design -

Types of Heat Sources and Effect on

Kitchen Environment

Click here to return to the Heat Load Based Design page

Kitchens operate year round and cooling is almost always required, even during the colder

winter months. This is due to the heat loads generated in a commercial kitchen from the cooking

processes.

The modes of heat gain to a space may include:

solar radiation through transparent surfaces

heat conduction through exterior wall and roofs

heat conduction through interior partitions, ceilings, and floors

heat generated within the space by occupants, lights, and appliances

energy transfer as a results of ventilation and infiltration of outdoor air

miscellaneous heat gains

However, in commercial kitchens, cooking processes contribute the majority of heat gains to

the space.

Sensible heat (or dry heat) is directly added to the conditioned space by conduction, convection

and radiation. Latent heat gain occurs when moisture is added to the space (e.g., from vapor

emitted by occupants, equipment and processes). Space heat gain by radiation is not immediateRadiant energy must first be absorbed by the surfaces enclosing the space (walls, floor, and

ceiling) and by the objects in the space (furniture, people, etc.). As soon as these surfaces and

objects become warmer than the space air, some of the heat is transferred to the air in the

space by convection.



To calculate a space cooling load, detailed building design information and weather data a

selected design conditions are required. Generally, the following information is required:

building characteristics

configuration (e.g., building location)

outdoor design conditions

indoor design conditions

operating schedules

date and time of day

Cooking can be described as a process that adds heat to raw or precooked food. As heat is

applied to the food, effluent is released into the surrounding environment. This effluent release

includes water vapor, organic material released from the food itself, and heat that was not

absorbed by the food being cooked. Some forms of reheating, such as rethermalizing limit theeffluent released to the space but still emit water vapor to the surrounding space.

The hot cooking surface (or fluid, such as oil) and product vapors create thermal air currents

(typically called a thermal plume) that are received or captured by the hood and then exhausted

If this thermal plume is not captured and contained by the hood, they become a heat load to

the space. The velocity of these thermal plumes depends largely on the surface temperature

of the cooking equipment, and varies from 25 feet per minute over some steam equipment, to

200 feet per minute over some char-broilers. Because of this variation, cooking equipment is

typically classified in four categories: light duty (such as ovens, steamers, and small kettles up

to 400°F), medium duty (such as large kettles, ranges, griddles, and fryers up to 400°F), heavy

duty (such as broilers, char-broilers, and woks up to 600°F) and extra heavy duty (such as

solid-fuel-burning equipment up to 700°F). By far, in a commercial kitchen, cooking contributes

to a majority of the heat loads in the space.





Exhaust Air Flow Rates


It is still a common practice to estimate exhaust air flows of hoods based on very rough “rules-

of-thumb.” One of these rules for wall-mounted canopy hoods is to exhaust 100 cfm per square

foot of hood face area (national code requirement). If all four sides of the hood are open (island

hood), the rule has been to use 150 cfm/ft² of hood face area. Neither of these rules takes

into account the type of cooking equipment under the hood and typically results in excessive

exhaust airflow and hence oversized air handling units and high energy consumption.

The most accurate method to calculate the hood exhaust airflow is heat load based design. This

method is based on detailed information of the cooking appliances installed under the hood

including type of appliance, its dimensions, height of the cooking surface, source of energy

and nameplate input power. All this data allows calculating how the particular appliance emits

energy into the kitchen. Part of this energy is emitted to the space in the form of the convective

plume – hot air rising from the cooking surface. The other part is ejected into the space by

radiation warming up the kitchen surfaces and eventually the air in the kitchen.

The Amount of air carried in a convective plume over a cooking appliance at a certain height

can be calculated using Equation 1.

( ) r convh p K Q D zk q ⋅⋅+⋅= 3

1

3

5

7.1

(1)



Where

qp – airflow in convective plume, cfm

z – height above cooking surface, in

Qconv – cooking appliance convective heat output, Btu/h

k – empirical coefficient, k = 61 for a generic hood

Kr – reduction factor, taking into account installation of cooking appliance (free,

near wall or in the corner)

Dh – hydraulic diameter, in

L,W – length and width of cooking surface accordingly, in

Kitchen hoods are designed to capture the convective portion of heat emitted by cooking

appliances, thus the hood exhaust airflow should be equal or higher than the airflow in theconvective plume generated by the appliance. The total of this exhaust depends on the hood

efficiency.

adshoodeff pex K K qq ⋅⋅= (2)

Where

K hoodeff

– kitchen hood efficiency.

K ads

– spillage coefficient taking into account the effect of the air distribution

system on convective plume spillage from under the hood. The recommended

values for K ads

are listed in the table below.

The kitchen hood efficiency shown in equation 2 can be determined by comparing the

minimum required C&C flow rates for two hoods that have been tested using the same

cooking process. Table below presents recommended values for spillage coefficient as

function of air distribution system



Table Spillage Coefficients As A Function of the Air Distribution System

Type of air distribution system Kads

Mixing ventilation

Supply from wall mounted grills 1.25Supply from the ceiling multicone diffusers 1.2

Displacement ventilation

Supply from ceiling low velocity diffusers 1.1

Supply from low velocity diffusers located in the work area 1.05

For the short-cycle hoods equation 2 will change into (see Ref 5)

intqK K qq adshoodeff pex +⋅⋅=

(2.1)

Where

qint

– internal discharge airflow, cfm

The Heat Load based design gives an accurate method of calculating hood exhaust airflow as

a function of cooking appliance shape, installation and input power, and it also allows to take

into account the hood efficiency. The only disadvantage of this method is that it is cumbersome

and time-consuming if manual calculations are used.

Standard UL 710

The primary emphasis for UL 710 test protocol is the establishment of a uniform standard tes

for commercial ventilation systems. This test encompasses airflow testing (of visible smoke)

and verification of compliance to NFPA-96 construction standards. In addition, it quantifies

clearances and overhang requirements of tested hood systems based on results.



The intention of the UL airflow testing is not to establish design exhaust rates, rather to establish

minimum exhaust rates for purposes of fire safety and ventilation. Since the testing is based on

the removal of visible cooking vapors, it falls well short of quantifying the efficiency or ability of

a hood system to remove the convective heat from a given group of appliances.

• Measures minimum exhaust air flow rate based on 400F

• 30% fat burgers are placed on a 2.5 sq. ft. griddle

• The Burgers are cooked at the prescribed surface temperature and flipped

• The U.L. inspector observes the capture and containment of visible cooking

vapors

• Test was not established to measure hood efficiency or heat gain to the space

Commercial Kitchen Ventilation Systems

The commercial kitchen is a unique space where many different HVAC applications are takingplace within a single environment. Exhaust, supply, transfer, refrigeration, building pressurization

and air conditioning all must be considered in the design of most commercial kitchens.

It is obvious that the main activity in the commercial kitchen is food preparation. This activity

generates heat and effluent that must be captured and exhausted from the space in order to

control odor and thermal comfort. The amount of supply air into a space, both tempered and

un-tempered coupled with transfer air from surrounding spaces to maintain the pressurization

and balance of a building are all dependent upon the amount of exhaust air removed from the

space. So, if the systems design all stems from the amount of air to be exhausted, it stands

to reason that the exhaust air quantity is critical to good system operation, providing therma

comfort and productivity of workers.

Let’s explore this concept further.

What is the reason for exhaust levels in the kitchen? As previously mentioned the answer is

simply to remove the heat and cooking effluent generated during the cooking process. With this

in mind, what is the most accurate method of determining the exhaust quantity required in the

kitchen? It should begin with the heat load generated by the cooking process. Any other method

of determination (such as cfm/foot, cfm /ft2, etc.) will not provide an accurate result. Only by

basing HVAC operation on the total loads (conduction, convection and radiation loads) present

in the space can an accurate determination be made. And only by an accurate determination

can a good operating system be designed.



Many manufactures of commercial kitchen ventilation equipment offer design methods for

determination of exhaust based on cooking appliances. Any method used is better than no

quantification at all.

The method of determining exhaust levels based on the heat generated by the coking process

is referred to as heat load based design. It is the foundation of accurate and correct design

fundamentals in a commercial kitchen environment.





Hood Comparison Studies


In this section a variety of techniques and research findings are presented that demonstrate

the methods utilized to measure hood efficiency such as ASTM 1704 (Capture & Containment

using thermal imaging) and CFD (Computational Fluid Dynamics). ASTM 1704 allows for the

comparison of different hood systems under the same load condition to determine their relative

efficiency.

Wall Canopy Case Study

Halton is using state-of-the-art techniques to validate hood performance. These include

modeling of systems using CFD, Schlieren imaging systems and smoke visualization. All of the

Picture 25KVE with Capture J et®off



Halton’s standard canopy hood (model KVE) utilizes Capture Jet®technology to enhance hood

performance and consequently hood efficiency.

In this case study, the KVE hood has been modeled using CFD software. Two cases are

modeled for this analysis; one with the jets turned off, in effect, this simulates a generic exhaust

only canopy hood and a second model with the jets turned on. As can be seen from the following

picture results, at the same exhaust air flow rate, the hood is spilling when the jets are turned

off and capturing when the jets are turned on.

The same studies were repeated in a third party independent laboratory. The Schlieren Therma

Imaging system was used to visualize the plume and effect of Capture J et®technology. As can

be seen below, the CFD results were in agreement with the visual tests.

Picture 27Schlieren KVE Capture J et®off



Picture 28KVE with Capture J et®on

Picture 28Schlieren KVE Capture J et®on



Comparison to Exhaust Only

Utilizing the tools of CFD and Schlieren in conjunction with the ASTM 1704 standard, one

can validate the Capture & Containment threshold and determine the effectiveness of

dynamic features such as Capture J et® in hood performance. Suction only devices with static

modifications to the geometric design along the edges can produce improvements on capture

and containment. These systems are more significantly more susceptible to cross drafts and

changes in the velocity of the thermal plume.

With the availability of ASTM 1704 one can compare the relative claims of manufacturers

regarding performance levels of one hood compared to another under the same load conditions

The following graphics show testing of Capture J et® style hoods compared to exhaust only

canopies with static modifications.

Picture 29.

‘Triangle’ style exhaust only hood versus a Capture J et®

View the Hood Comparison Video

Download the video in MPEG 1 Format 1.8mb

Download the video in Windows Media format 0.4 mb

http://dasp2.sulatto.fi/halton/usa/cms.nsf/files/8BA6032681BA2D65C22574390048CFE9/$file/captiveair.m1v


http://dasp2.sulatto.fi/halton/usa/cms.nsf/files/51514DC9CA541A14C22574390048BF44/$file/captiveair.wmv






Picture 30

‘Lip’ style exhaust only hood versus a Capture J et®

As can be seen from the visual results below, in order for either one of these style exhaust

only hoods to capture and contain the same effluent release, it required 70 CFM more

exhaust per foot of hood for this under-fired gas broiler that was tested on all three models.

This equates to a 71% increase in airflow requirements to do the same function as the

Capture J et®system.

Backshelf Hood Case Study

Independent research has been performed to evaluate the capture efficiency of Halton’s

backshelf style (Model KVL) hood.

The first set of results for the KVL hood demonstrate the capture efficiency using a Schlieren

thermal imaging system. Pictures 31 and 32 show the results of the KVL hood with the jets

turned off and on at the same exhaust air flow. The off position would simulate an exhaustonly type system. Once again, it becomes readily apparent that the Capture J et®technology

significantly improves capture efficiency. The KVL hood is spilling when the jets are turned off

and capturing when the jets are on.



Picture 31KVL Capture J et®Off

Picture 33CFD KVL Capture J et®Off

Another study conducted in-house was to model these two cases using CFD in order to see

if the model could predict what was observed in a real world test. Pictures 33 and 34 present

the results of the CFD models for the jets off and jets on, respectively. Note that the jets in

the KVL hood are directed downwards, where they were directed inwards on the canopy KVE

discussed earlier.

When you compare the CFD results to those taken in the independent imaging tests, you will

notice that they produce extremely similar results. This demonstrates than not only can CFD

models be used to model kitchen hoods but they can also augment laboratory testing efforts.



Picture 31KVL Capture J et®On

Picture 33CFD KVL Capture J et®On


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Fcsi Quiz - Halton Hood Load[1]

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