demand controlled ventilation.pdf

7/27/2019 demand controlled ventilation.pdf

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3 6 A S H R A E J o u r n a l a s h r a e . o r g N o v e m b e r 2 0 1 2

Food service facilities have high energy consumption with equipment

and commercial kitchen ventilation (CKV) being the primary energy

consumers in a restaurant. Exhaust hood airow drives HVAC energy

consumption for CKV, so the rst step in reducing this exhaust airow is

designing high efciency hoods with low capture and containment (C&C)

airow rates. The next step is using demand control ventilation (DCV) to

further reduce exhaust airow when cooking is not taking place under

the hood, but when appliances are hot and ready for food preparation.

Airow reduction is not the sole ob-jective o a DCV system; it also mustensure exhaust airow and the corre-sponding supply airows are increasedto C&C levels as soon as cooking starts(to avoid spillage o convective heat andcooking euent into the kitchen space).

The current NFPA-96 Standard1 and In-

ternational Mechanical Code2

require

that a hood operate at ull design air-ows whenever ull-load cooking activ-ity occurs underneath an exhaust hood.

DCV has evolved rom simple two-speed an control systems to proportion-al control with variable requency drives(VFDs) based on exhaust temperature.

This improvement allowed or varying

airows throughout the day. Then, an

optical sensor was added to the temper-ature-based control to detect cookingactivity taking place under the exhausthood to urther enhance perormance.The latest system introduced to the

market added measurement o exhaustairow and automated balancing o mul-tiple exhaust hoods connected to a singlean (or a dedicated an) and modulationo replacement air or the space. Futuresystems need to be designed to consider

the entire kitchen status to maximize en-ergy savings.Laboratory testing was conducted or

common appliances in the commercialkitchen to evaluate system perormancewhen equipped with various DCV al-gorithms: operating at a fxed exhaust

About the Authors

Derek Schrock is U.S. research director and Jimmy

Sandusky is research engineer at Halton Company

in Scottsville, Ky. Andrey Livchak, Ph.D. is direc-

tor of global research and development at Halton

Group Americas in Bowling Green, Ky.

By Derek Schrock, Member ASHRAE; Jimmy Sandusky, Associate Member ASHRAE; Andrey Livchak, Ph.D., Member ASHRAE

Demand-Controlled Ventilation

For Commercial KitchensT

etraImages/Corbis


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N o v e m b e r 2 0 1 2 A S H R A E J o u r n a l 3 7

setpoint temperature, operating on a temperature curve to in-crease exhaust airow proportional to the temperature dier-ence between exhaust and space temperature and operating ona temperature curve in combination with a cooking activitysensor (CAS) to drive the system to design when cooking is

detected. Additionally, an evaluation was done to determineenergy savings or a DCV system with balancing dampers in-stalled on a our exhaust hood, island confguration.

Test Setup

The objective o the frst round o tests was to compare per-ormance o DCV systems that use temperature sensors onlyto those that incorporate cooking activity and temperature sen-sors. Currently, only two manuacturers oer the latter. Onedesign uses optical opacity sensors to detect the presence ocooking euent in a hood cavity. Another design uses inrared(IR) temperature sensors to monitor the surace temperatureo cooking appliances. Data rom these IR sensors along withspace temperature and hood exhaust temperature sensors areanalyzed to interpret the status o cooking appliances (idle,cooking or o) and adjust hood exhaust airow accordingly.

A 72 in. (1.8 m) long wall canopy exhaust hood was confg-ured to simulate various DCV control algorithms available onthe market: exhaust temperature-based system that operates ata fxed setpoint, exhaust temperature-based system that oper-ates on a curve and exhaust temperature coupled with a cook-ing activity sensor system (includes IR sensors).

Fixed setpoint exhaust temperatures o 90F, 100F and130F (32C, 38C and 54 C) were evaluated. For these con-fgurations, the minimum exhaust airow rate was 80% o de-

sign airow rate (a common value or temperature only basedsystems due to the limited ability to detect when cookingstarts and ramp-up o exhaust airow). Exhaust airow wasvaried by a VFD in an attempt to maintain the tested tempera-ture setpoint.

For exhaust temperature systems that operated on a curve,the minimum exhaust airow rate was again 80% o design.When using the curve, exhaust airow was incrementally in-

the timer expiration, i no new cooking activity is detected, thesystem returns to the curve control algorithm.The exhaust hood was installed at 80 in. (2 m) above fn-

ished oor with a temperature sensor mounted in the exhaustcollar. The inrared sensors were positioned in the ront, in-terior ace o the canopy to sense the cooking surace. Thetemperature sensor was installed so that it was centered in thehood collar.The test protocol included a range o appliances that all

demonstrated similar trends, but due to space limitations onlydata or appliances most commonly seen in kitchens is pre-sented: a charbroiler, griddle and open-vat ryer.

Airows inTable 1represent hood C&C airow, and when-ever the hood operates below this value when cooking occurs,the hood is spilling. Details o appliance uel source and prod-uct cooked are shown as well.

During testing, each combination was evaluated at the idle

and cooking states. Exhaust airow rate and temperature wereplotted versus time. The onset o the cooking process was not-ed to determine system response time.

Results and Discussions

Charbroiler

Figure 1summarizes testing conducted with the charbroiler,which exhibited the highest exhaust temperatures o all tested

Table 1: Cooking appliance and associated food product.

ApplianceFuel

SourceLoading

Design/Capture

And Containment

Airfow

600F

Charbroiler

Natural

Gas

Frozen

Hamburger Patties1,800 cm

400F Griddle,

Thermostatically

Controlled

Natural

Gas

Frozen

Hamburger Patties1,000 cm

350F Open-Vat

Fryer, Single Vat

Natural

Gas

Frozen

French Fries1,000 cm

2,000

1,900

1,800

1,700

1,600

1,500

1,400

150

145

140

135130

125

120

115

110

105

100

Airfow(cm)

Temperature(F

)

10:04:00a

10:07:22a

10:10:34a

10:13:16a

10:16:41a

10:19:25a

10:22:04a

10:24:42a

10:27:17a

10:29:50a

10:32:24a

10:34:59a

10:37:35a

10:40:11a

10:42:50a

10:46:25a

10:48:58a

10:51:33a

10:54:11a

10:56:51a

10:59:29a

11:02:06a

11:04:47a

11:07:25a

11:10:03a

11:12:43a

11:15:20a

11:17:58a

11:20:37a

11:23:16a

11:25:55a

11:28:35a

11:31:17a

11:34:01a

Temperature+ CAS

TemperatureCurve

Patties On

ConstantTemperature,

SP = 90F

ConstantTemperature,SP = 100F


Airfow Duct TemperatureDesign Airfow

Figure 1: Charbroiler testing.

creased as the temperature dierencebetween exhaust and kitchen space in-creased. This algorithm ensures C&Co convective heat rom appliances in-

stalled under the hood.Minimum exhaust airow or thesystem with cooking activity sensorsinstalled was limited to 40% o the de-sign rate to ensure that the exhaust answere operated in their recommendedrange. This system used the curvetemperature control as described previ-ously when appliances are in idle modeand transitioned to design exhaust air-ow or an adjustable period (which wasset to seven minutes or this test) upon

detection o cooking activity. Following

This article was published in ASHRAE Journal, November 2012. Copyright 2012 ASHRAE. Reprinted here by permission rom ASHRAE at www.haltoncompany.com.This article may not be copied nor distributed in either paper or digital orm by other parties without ASHRAEs permission. For more inormation about ASHRAE,visit www.ashrae.org.


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3 8 A S H R A E J o u r n a l N o v e m b e r 2 0 1 2

appliances. With the exhaust temperature and cooking activitysensor (CAS) algorithm, there was a small dierence (approxi-mately 5%) in idle and cooking exhaust airow rates due to theelevated exhaust temperature. The cooking activity sensor wasable to detect placement o patties on the cooking surace and

orce the system to design exhaust airow, albeit a small change.Following expiration o the cooking timer at approximately10:13 a.m., the airow rate dropped, but was orced back todesign value as the exhaust temperature rose. With the testedalgorithm, when the exhaust temperature exceeded 120F(49C), the system went to design exhaust airow (regardlesso the cooking activity sensor values signal) as this was theupper limit o the temperature curve.

Comparable results were seen with the temperature onlysystem operating on a curve and constant temperature systemswith a setpoint o 90F and 100F (32C and 38C). Due to anelevated exhaust temperature, the exhaust airow remained ator near the design airow level or the entirety o the test dueto the temperature dierence threshold o the operating curvebeing exceeded. When operated on the temperature curve,there was a small decrease in exhaust airow at approximately10:30 a.m. due to a decrease in exhaust temperature.

When the setpoint or the constant temperature system waschanged to 130F (54C) the exhaust airow dropped to 80%

o design and remained there or the duration o the test asexhaust temperature did not exceed the setpoint. There wasone exception where the exhaust temperature peaked at 134F(57C). Being 4F (2C) above the threshold, this value waswithin the deadband o the system. Had the temperature con-

tinued to rise, the exhaust airow would have increased pro-portionally in an attempt to maintain a constant exhaust tem-perature.

It should be noted that with the charbroiler, the exhaust tem-perature rose steadily throughout the test as buildup on thecooking surace occurred. I the test had lasted longer, the ex-haust airow would have likely increased to design airowsor all tested systems.

Griddle

The results o the griddle testing are summarized in Fig-ure 2. The exhaust temperature with the CAS algorithm oper-ated at approximately 70% o design airow at idle. The ex-haust airow was greater than the minimum setpoint o 40%to maintain C&C o heat generated by the appliance. Whenhamburger patties were placed on the surace, a decrease insurace temperature was noted as a cooking signal, and airowwas driven to design almost immediately, and ell back to idleater the cooking timer expired.

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This test showed that the constant temperature setpoints o90F and 130F (32C and 54C) were not suitable or the ap-plication. With a 90F (32C) setpoint the system remained atdesign airow at all times due to the setpoint being lower thanthe exhaust temperature, even at idle conditions. A site confg-ured to operate in this manner was no dierent than having astandard canopy hood without DCV. The opposite was true o

the 130F (54C) setpoint. The system remained at idle airowas the exhaust temperature never exceeded the setpoint, result-ing in both heat and cooking euent spilling to the space.

Open-Vat Fryer

Figure 3shows data taken or the open-vat ryer. Whentesting with the CAS, the idle airow rate uctuated between

For an exhaust temperature-based sys-tem operating on a curve, exhaust airowincreased to the minimum idle rate o80% o design. Ater patties were placedon the griddle, the airow increased grad-

ually with the exhaust temperature andthen ell as the temperature decreasednear the end o the cooking cycle. Notethat airow was not at design or the en-tirety o the cooking process.

When operating with a constant tem-perature setpoint o 100F (38C), thesystem was able to reach and maintaindesign airow ollowing placing pattieson the cooking surace. However, theinitial response was not as ast as ob-tained with the CAS.

Figure 2: Griddle testing.

1,200

1,100

1,000

900

800

700

600

140

130

120

110

100

90

80

70

60

Airfow(cm)

Temperature(F)

1:02:00p

1:06:55p

1:11:52p

1:17:04p

1:22:18p

1:27:34p

1:32:55p

1:38:21p

1:43:53p

1:49:22p

1:54:50p

2:00:13p

2:05:36p

2:10:49p

2:15:49p

2:20:44p

2:25:33p

2:30:12p

2:34:45p

2:39:16p

2:43:50p

2:48:18p

2:52:48p

2:57:19p

3:01:50p

3:06:20p

Temperature+ CAS

TemperatureCurve

Patties On


SP = 90F




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40% and 50% o design as the appli-ance fred to maintain oil temperaturein the vat. Upon dropping the basketso ries into the oil, the drop in temper-ature detected by the sensor indicated

cooking was occurring and drove theexhaust an to design airow and thenreturned to idle ater the cooking timerexpired.

With the algorithm operating on atemperature curve, the system idled atthe minimum prescribed 80% o designairow and increased proportionally asthe exhaust temperature rose. The tem-perature dierence between exhaust andspace did not exceed the upper limit togo to design airow.

With the constant temperature setpoint confgurations, allsystems idle at 80% o design. When the setpoint was 90F(32C), the system did reach design airow ater the initia-tion o the cooking process, but a signifcant time lag betweenonset o cooking and meeting design airow was noted. For100F and 130F (38 C and 54C) confgurations, the systemremained idle at all times due to exhaust temperature not ex-

ceeding the setpoint. This was indicative o the requirementto be very amiliar with the given cooking process when con-fguring temperature-based DCV systems. For example, theopen-vat ryer potentially could perorm better with a lowertemperature setpoint value, but common practice dictates90F (32C) and above setpoints to achieve any measurableairow reductions.

Figure 3: Open-vat fryer testing.

1,200

1,100

1,000

900

800

700600

500

400

300

200

120

110

100

90

80

70

60

Airfow

(cm)

Tempera

ture(F)

2:58:00p

3:01:23p

3:04:37p

3:07:57p

3:11:21p

3:14:48p

3:18:11p

3:21:37p

3:25:01p

3:28:25p

3:31:45p

3:35:04p

3:38:19p

3:41:36p

3:44:52p

3:48:01p

3:51:12p

3:54:25p

3:57:39p

4:00:51p

4:04:06p

4:07:22p

4:10:39p

4:13:55p

4:17:16p

4:20:36p

4:23:58p

4:27:22p

4:30:46p

4:34:12p

4:37:36p

Baskets Dropped

Temperature

Curve


SP = 90F



Temperature

+ CAS




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Response Time

In addition to evaluatingthe exhaust airow achievedwith dierent types o DCVsystems, the response times

o the systems were comparedas shown inTable 2. An entryo N/A indicates that thesystem either was not able tomeet or remained at designairow during the test. Note

or tested appliances. When using only the temperature sensor,system response is more dependent on the correct selection set-points and is always delayed. I improperly confgured, temper-ature-based systems can operate at either idle or design airowat all times, resulting in loss o C&C or no energy savings.

Each appliance shown has dierent exhaust temperaturesthat represent the transition rom idle to cooking states. Rare-ly, appliances are confgured so that each has a dedicated ex-haust hood; the mixed lineup under a long hood is typical.Appliance lineups will vary rom site to site, making a generictemperature curve or setpoint nearly impossible to obtain.

Some confgurations can perorm well without the CAS.For example, convection or conveyor ovens produce little to

the response time o the systems with the CAS was substan-tially aster than the temperature-based systems. This is be-cause the sensor actively monitors what is occurring at the ap-pliance level, and can indicate when cooking occurs as soon asthe product is placed in/on the appliance and drive the exhaustan to design airow or the entirety o the process; whereas,a temperature-based system can only react to a by-product othe cooking process: a change in temperature that takes longerto observe.

Discussion

The inclusion o the cooking activity sensor ensures the sys-tem goes to design airow at the onset o the cooking process

Table 2: Response time comparison.

Time From Start o Cooking (Seconds) When Design Airfow Reached

Appliance Temperature

+ Cooking

Activity Sensor

Temperature

Only Curve

Constant

Temperature,

SP=90F

Constant

Temperature,

SP=100F

Constant

Temperature,

SP=130F

Charbroiler 23 N/A N/A N/A N/A

Griddle 35 174 N/A 181 N/A

Open-Vat Fryer 23 N/A 297 N/A N/A



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no smoke during the cooking process, which would be di-fcult or an optical opacity or inrared sensor to interpret. Insome installations, a combination o temperature-based andtemperature/CAS-based systems may be necessary or thesystem to perorm at an optimal level.

Space temperature will vary during cooling and heating pe-riods, aecting exhaust temperature as well. As a result the ex-haust temperature setpoint must be adjusted rom one seasonto another or DCV systems with exhaust temperature sen-sors only. A space temperature sensor installed in the area toconstantly evaluate the dierence between exhaust and roomtemperature maximizes energy savings potential o the DCVsystem.

A an speed profle or each confguration was generatedto compare energy savings, rather than using average percentreduction alone. These profles were used in conjunction withan outdoor air load calculator to determine savings associatedwith makeup air cooling and heating, as well as exhaust andsupply an energy. Table 3compares the annual energy sav-ings associated with both confgurations.

Although both confgurations save energy, the installation o

balancing dampers maximizes these savings by allowing thehoods to operate independently. Without dampers, when onehood is in the cooking state, all are orced to design airowregardless o state. The value o the balancing damper lies inthe ability to lower the airows or hoods that are not cookingin single exhaust an, multiple exhaust hood confgurations.

In this particular case with our hoods connected to a singleexhaust an, the DCV system with balancing dampers savesnearly twice as much energy when compared to a similar sys-tem without balancing dampers. This is a conservative esti-mate because additional savings when all hoods are in idlemode are not accounted or. Indeed, when the DCV system

is in idle mode (appliances are hot, but no cooking occurs),

Case Study: DCV With and Without Balanc-

ing Dampers

For installations where the exhaust hoods eachhave a dedicated exhaust an, balancing dampersare not needed since the airows can be modu-lated by changing the an speed. However, whenmultiple exhaust hoods are connected to a singleexhaust an, balancing dampers listed in accor-dance with the UL 710 standard can be installedon each exhaust hood section to maximize theenergy savings o a DCV system. This is becauseeach hood needs to have the ability to indepen-dently regulate airow based on its state (o,idle or cooking). To illustrate the energy savingsthat can be achieved with dampers installed, asite is evaluated with both confgurations.

The examined site is in Seattle, and is a 24/7operation. The only time the kitchen exhausthoods are shut down is or a daily water-washoperation (approximately 15 minutes). The ex-

Figure 5: Case study with balancing dampers installed.

120

100

80

60

40

20

0

ExhaustFanSpeed(%)

Average Airfow = 73% o Design

5/13/12 5/14 5/15 5/16 5/17 5/18 5/19 5/20 5/21 5/22

Exhaust Fan VFD Speed, %

Figure 4: Examined site exhaust hood conguration.

haust hoods are installed as back-to-back island style canopyhoods and are connected to a single exhaust an (Figure 4).Each hood is ftted with a balancing damper at the exhaust col-lar. The DCV system operates with cooking activity sensorsinstalled on all hoods. The design airow or the site is 11,290cm (5328 L/s). Figure 4shows monitored data or exhaustan speed. On average, the exhaust airow rate was 73% odesign. Figure 5shows the system rarely operated close to

design airows because the our hoods did not have cookingoccurring at the same time.Figure 6shows the exhaust an speed or the same DCV sys-

tem and time period without the dampers installed. To modelthe system without dampers installed, hood status was alsomonitored with the an speed and exhaust airow data. Theseags are generated by the control algorithm based on the inputsrom the cooking activity, space and duct temperature sensor.

I one o the our hoods was in cooking state, that an speedwould increase to 100% to reach design airow or the par-ticular exhaust hood. I status was o or idle or all hoods,an speed was assumed to be equal to that o the system with

balancing dampers.


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and the exhaust airow is controlled based on ahoods exhaust temperature (more accurately thetemperature dierence between hood exhaustand space temperature) a dilemma is revealed:which exhaust temperature (or hood) should be

used as a control signal or DCV without damp-ers?The hood with the highest exhaust temperature

would be the saest bet, but this would requirea more sophisticated control algorithm (not thecase or many DCV suppliers) and will still endup with a higher total exhaust airow comparedto DCV with dampers. In some cases, a fxedleading hood is assigned, and its exhaust tem-perature is used to control exhaust airow or thewhole system in DCV systems without dampers.

Future of DCV Systems

As evidence shows, the cooking activity sensoris an important component o an efcient DCVsystem. However, this is not the most eectiveway to identiy appliance status. Taking a signaldirectly rom the cooking appliance is a more e-ective way to detect appliance status (cooking,idle or o). Most modern cooking appliances areequipped with programmable logic controllers

Compared to DCV systems with cooking activity sensors,systems that use only temperature sensors can have signifcantlags in response time; more than two minutes in the evaluatedcases o the open-vat ryer and griddle. Not detecting cooking

in a timely manner results in loss o C&C, which allows heatand cooking euent to spill to the kitchen space. Any sav-ings associated with an energy can quickly be oset by anincreased load on cooling and heating equipment.

When temperature only systems are used, setpoints mustbe calibrated or a given application (appliance combination).

This is key to ensuring the system operates as intended. In-appropriate setpoints can result in hoods that run at designairow constantly (an expensive exhaust-only hood) or idlecontinuously (allowing spillage to occur). The setpoints alsoshould be reset or winter and summer to account or variationin kitchen space temperature, unless a space temperature sen-

sor is used or automatic reset.Using automatic balancing dampers listed per UL Standard710 or DCV systems with multiple hoods connected to a singleexhaust an signifcantly improves system energy efciency. Theenergy savings or a our-hood system can be double when com-pared to an identical DCV system without balancing dampers.

References

1. NFPA. 2011. NFPA Standard 96-2011,Standard for Ventilati onControl and Fire Protection of Commercial Cooking Operations.

National Fire Protection Association.

2. ICC. 2012. 2012 Internati onal Mechanical Code. InternationalCode Council.

Figure 6: Case study without balancing dampers installed.

120

100

80

60

40

20

0ExhaustFanSpeed(%)

Average Airfow = 86% o Design

5/13/12 5/14 5/15 5/16 5/17 5/18 5/19 5/20 5/21 5/22

Exhaust Fan VFD Speed Without Dampers

Table 3: Energy savings comparison with and without balancing dampers.

Estimated Savings

SystemHeating

(Therms)

Cooling

(kWh)

Exhaust Fan

(kWh)

Supply Fan

(kWh)

DCV With

Dampers1,133 6,435 32,554 10,851

DCV Without

Dampers623 3,539 15,697 5,232

Dierence 510 2,896 16,857 5,619

(PLCs) that already know appliance status. Establishing com-munication between the appliance and DCV controller is allthat is needed.

As noted previously, cooking equipment and CKV are a

kitchens primary energy consumers. The term demand con-trol ventilation implies that hood exhaust is modulated basedon demand by cooking appliances under the hood. Cookingappliances defne overall kitchen energy consumption becauseCKV energy consumption is, to a large extent, driven by ap-pliances being used, and their status defning DCV exhaustairow. However, DCV doesnt optimize the energy consump-tion o the source: the cooking equipment.The next step in the development o an energy-efcient

kitchen is implementing a demand-controlled kitchen (DCK)strategy, where appliances are controlled based on cookingdemand and communicate their status to DCV to minimize

CKV energy consumption. Indeed, how many times have youseen a range with all burners on and no pots on it or a triple-stack conveyer oven with all stacks on and just one conveyerbeing used? When we implement a DCK strategy with energy-efcient cooking appliances that are integrated with a DCVsystem (controlled based on cooking schedule and demand),we will have a truly energy-efcient kitchen.

Conclusions

Commercial kitchen DCV systems can oer great energysavings to the end user when properly implemented. Careshould be taken to ensure the proper DCV system and sensor

type are selected or a given appliance lineup.

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