Air Balance Fundamentals for the Modern Restaurant
The mechanical design of the modern restaurant is one of the more challenging endeavors for the HVAC engineer. Contemporary kitchens now incorporate numerous specialized cooking and heating equipment, often using large energy inputs to satisfy the proper cooking temperatures and the speed required for quality customer service.
The exhaust systems required by the modern kitchen include specialized exhaust airstreams from dishwashers and steam kettles; flue gas exhausted from gas-fired hot water heaters; nongrease-laden heat exhaust; and ultimately, the critical grease-laden airstreams from electric or gas-fired grills, impingers, and cookers. It is this unusual blend of exhaust streams in a restaurant that requires specific attention to the overall building air balance to maintain the comfort level for both the patrons and the restaurant staff.
Ventilation Standards RevisitedIndustry ventilation standards have been established by ASHRAE Standard 62, of which the latest published version is 62-2001. Recent additions have modified the standard to incorporate the latest wording and issues pertaining to IAQ. ASHRAE 62-2001 sets the industry guidelines for outside air (O/A) required by various occupancies to ensure adequate IAQ.
The HVAC designer should also ensure that proper code clearances for air intakes and exhaust are maintained for all equipment used by the restaurant. Typically this would include rooftop-mounted air conditioning units (RTU); kitchen hood exhaust fans; dishwasher exhaust fans; make-up air fans (MUA); toilet exhaust fans; combustion air intake and exhausts; ventilators; and plumbing vents.
Proper code clearance requires that OA intakes maintain a fixed distance from adjacent buildings, operable windows, building edges, exhaust airstreams, and sanitary waste vents. This is to prevent short-circuiting of foul air back into the building ventilation systems or contaminating adjacent building air intakes.
You will observe when we "crunch the numbers," that the goal in air balancing a typical restaurant is to maintain a positive overall building pressure to prevent uncontrolled infiltration from entering the building. All outside air used for A/C, ventilation, or make-up air must be processed through mechanical systems that can filter and condition the air as required for the various applications. It is not uncommon to meet the letter of the code for OA intake clearances and still have foul air being drawn into the air supply.
It is important to study the rooftop geometry (layout) to ensure that parapets, roof, screens, or equipment do not inadvertently create negative air pockets and provide short-circuiting of bad air back into the building. I was on the roof of a recently constructed major franchise restaurant during the summer that simply reeked of sewer gas since the sanitary vents were not raised above the parapet height. If it had not been for the dilution that was occurring, that store would have a major complaint that could be easily solved with a few more feet of PVC vent pipe.
ASHRAE Standard 62-2001 requires the minimum O/A as 20-cfm/person for restaurant dining and 15 cfm for the kitchens. Often, you will find in buildings designed and constructed to meet the energy standard for building envelopes, ASHRAE 90.1, that the cooling equipment selections are "driven" more by the O/A requirements than by the internal cooling loads.
In other words, based on a typical RTU system using DX coils, the O/A demands of ASHRAE 62-2001 may often exceed the actual internal loads and dictate the air conditioning tonnage required based on a manufacturer's typical recommendation of a maximum of 25% O/A for DX systems.
Modern Kitchen Hood DesignDue to the increasing variety of foods and the factors of timely customer service, the number of cooking stations in a restaurant is often increasing as the input energy is raised to accommodate faster food preparation. This directly impacts the kitchen hood sizing and exhaust cfm's required to service the cooking facilities.
Hoods are sized based on the type of cooking equipment under the hood, the size of the cooking surface, the temperature of the cooking surface, the style of hood used (island, two-sided, etc.), the height from the edge of the hood, and the type of particulate matter produced during the cooking process.
Code standards and jurisdictions have simplified the selection process by setting the cfm per area of the hood or per linear foot of exposed hood based on two conditions. One, if a hood handles smoke or grease-laden particles from deep fryers or grills, the hood and subsequent duct system are classified as a Type I hood. Two, if a hood handles moisture-laden or waste-heat-laden air, like the exhaust from a commercial dishwasher, the hood and associated duct is classified as a Type II hood.
Once the hood has been properly sized, the cfm is calculated based on code standards or UL testing. The exhaust cfm will determine the capture velocity that will adequately handle the airstream and maintain a properly functioning hood system.
Air Balance BasicsNow, with all known airstream qualities available to the designer, we now turn our attention to the most important issue of the air balance for the building.
Figure 1 is a graphical representation of the several airstreams entering or leaving a freestanding building. The typical restaurant with an integral bar will be represented by using one 15-ton, 6,000-cfm RTU for each of the dining, bar dining, and kitchen areas. Based on the seat count, the O/A requirements set by ASHRAE 62-2001 can be calculated as:
Dining RTU: 140 people x 20 cfm/person = 2,800 cfm
Bar Dining RTU: 60 people x 20 cfm/person = 1,200 cfm
Kitchen RTU: 20 people x 10 cfm/person = 200 cfm
Total = 4,200 cfm
Our ultimate goal is to meet code standards for the various airstreams and satisfy the air balance needed to maintain a positive building pressure. Toilets typically require 75 cfm/urinal or water closet per the IMC/SMC codes. The dishwashers' exhaust is per the manufacturer's design specifications, the hoods are set by UL test criteria or the local jurisdiction guidelines, and the O/A on the RTUs is based on ASHRAE 62-2001.
So it remains to determine the amount of MUA required by the building design. If the restaurant is located in a mall with common areas, the designer must take into account the mall developer's design criteria for air transfer. Some mall landlords allow air to transfer from the common areas, while some require sufficient O/A to help supply air to the common areas without any odor migration. Also, verify if a mall location has smoke evacuation ducts for smoke control in the mall.
Latent Loads at the DX CoilsBeginning with the bar area, the minimum O/A required is 1,200 cfm on the 6,000 cfm, 15-ton unit. Since the dining area requires even more O/A for the same size RTU, the bar O/A will be raised to1,550 cfm. This 1,550 cfm is 25% of the RTU capacity per manufacturer's guidelines, since the O/A is held at 25% or lower to ensure adequate latent capacity for the DX coils. (This same practice of limiting the percentage of O/A on the coil would not be required of a chilled water coil or condenser water system.)
With the guidelines required by the DX coils, we must look at the entire building as a single system to ensure that adequate O/A and hood exhaust requirements are met; since air is being transferred from the bar dining area to the dining and kitchen areas, and also from the dining area to the kitchen, we must assess the ways we can achieve IAQ and handle the large hood exhaust demands.
Ten Steps to Air Balance Calculations1. Set the bar dining area to O/A =1,550 cfm; S/A = 6,000 cfm; and R/A = 4,450 cfm, to maximize the available percent O/A through the DX coil.
2. Select the transfer cfm from the bar area to ensure a net positive pressure there. Transfer air of 1,450 cfm will allow a positive pressure of +100 cfm in the bar area.
3. The dining area is set at S/A = 6,000 cfm; O/A = 1,550 cfm; and R/A = 4,450 cfm, to maximize the percent of O/A available through the DX coil.
4. Again, since air will be transferring to the kitchen, the O/A is maximized to ensure removal of the latent load and assist in handling the kitchen loads. Since the toilet exhaust is part of this area, there is an added exhaust load of 600 cfm. Leaving a positive pressure of 200 cfm results in: 1,550 O/A - 600 cfm (toilets) - 200 cfm (pressurization) = 750 cfm available for transfer to the kitchen.
5. Combine the transfer airstreams from the bar at 1,450 cfm and the dining at 750 cfm. The combined 2,200 cfm of conditioned air is available for use in the kitchen to help with the high latent loads and the necessary exhaust requirements.
6. Calculate the pass-through velocity of the transfer airstream into the kitchen. This calculation is done to ensure a low velocity of air passing over the service counters to prevent premature cooling of the food waiting to be served. The area calculated is dependant on the kitchen layout and must take into account whether the pass doors are closed. If the pass doors are undercut or cafe style, the open area is part of the pass-through calculations.
Then observe where the hoods are located. Where does the air actually flow and how much is needed in the back of the house vs. near the service areas? Are transfer ducts used to convey air to the back of house for the exhaust hoods? These are the kinds of questions the designer must address to finalize an appropriate design.
The recommended pass-through velocity guideline is 50 fpm or less. Since 2,200 cfm is being transferred, the minimum area allowed to achieve the 50 fpm target is 2,200 cfm/50 fpm = 44 sq ft.
7. Total all the kitchen hood exhaust streams based on the manufacturer's specifications. Our typical restaurant has a dishwasher hood at 800 cfm and four separate hoods (grease and non-grease) totaling 8,200 cfm, so the total exhausted air is 9,000 cfm. Include the combustion air consumed by the gas-fired hot water heaters if the combustion air is supplied from the interior space rather than from a concentric flue or direct-ducted arrangement.
8. Again, we will maximize the percent of O/A available through the DX coil by setting the O/A to equal 1,550 cfm. This strongly exceeds the O/A criteria of 200 cfm required by the kitchen staff. Since we must look at the restaurant as a single system, the combined O/A = 4,650 cfm exceeds the ASHRAE 62-2001 requirements of 4,200 cfm for the building. Based on an occupancy load of 220 people, this provides an overall OA ventilation rate of 21 cfm/person.
The kitchen RTU is set with O/A = 1,550 cfm; S/A = 6,000 cfm; and R/A = 4,450 cfm.
9. Now, the kitchen make-up air unit is sized based on available O/A, transfer air, and total exhaust.
Total kitchen exhaust = 9,000 cfm;
Available transfer air = 2,100 cfm;
O/A intake = 1,500 cfm;
Combustion air requirements = N/A; and
Net make-up air required = 5,350 cfm.
Based on adequate pressurization of each building area: bar = + 100 cfm; dining = + 200 cfm; and kitchen = + 100 cfm and the transfer air available to partially supply the kitchen hood exhaust systems, we have determined the correct size of the make-up air unit = 5,350 cfm.
10. The final step in our air balance calculations is to provide make-up air to the correct diffuser locations and in the proper amounts to both the diffusers and hood plenums to ensure proper functioning of the entire hood and kitchen ventilation systems.
Several hood manufacturers have designed direct compensating or short-circuiting hoods which provide make-up air directly into the hood itself. Others provide a plenum along the front or sides of the hoods which is ducted for make-up air. Whether the hood is direct compensating, uses a make-up air plenum, or has no MUA features, the designer must take into account the amount of MUA allowed by the specific manufacturer(s) hood(s).
Often, the hoods will not be able to consume all the MUA, so the designer must direct additional drops through low-velocity diffusers throughout the kitchen as required by the hood layout. Using low-velocity diffusers helps prevent unwanted "drafts" affecting the hood capture effectiveness.
An important consideration for direct compensating hoods is the need for heated MUA when the air temperature becomes cool enough to prevent the effective capture of the smoke stream. This occurs due to the density of the cooler air that inadvertently forms a "barrier" to the rising hot air. The smoke and heated air can be forced outside the hood itself rendering the hood virtually useless in some atmospheric conditions. These cooler conditions can occur, although rarely, even in the more southern climates, so it is best to provide a method for heating the make-up air on direct compensating hoods during the winter season.
Separated Smoking Bar DesignsFigure 2 represents a typical restaurant featuring a bar in which smoking is allowed. Some jurisdictions require separation by physical walls with complete separation of the airstreams using separate RTUs. Other jurisdictions require a negative pressure to be maintained in the smoking section to prevent secondhand smoke from entering nonsmoking areas. Still others may allow an open architecture but require a negative pressurization and HEPA-style filters to remove particulate matter from the return air. The designer should verify these issues with the local authority having jurisdiction (AHJ) before proceeding with the design.
Figure 2 reflects this unique application of a separated bar by adding exhaust fans in the bar and allowing air to transfer from the dining area to the bar to provide a neutral building pressure in the bar to prevent infiltration, but also a negative pressure differential between the bar and the dining area to ensure that no smoke migration occurs towards the dining space.
The two conditions mandated by a separated smoking bar requirement stand out:
1. With the same RTUs and O/A requirements, the designer will set the O/A based on occupancy to minimize the O/A load in the bar.
At 60 people, this amounts to 1,200 cfm. So, instead of the 1,550 cfm used in the earlier example, the O/A is minimized, since it is basically exhausted without assisting the kitchen hood exhaust demands.
To provide for effective pressurization, the design will yield:
O/A = 1,200 cfm;
Transfer air = 100 cfm (from dining area); and
Bar exhaust (s) = 1,300 cfm.
2. The remaining dining and kitchen space are now analyzed as a composite system. The designer will quickly recognize that the resulting design will demand an increase in the make-up air by the amount of air exhausted in the bar. This 1,300 cfm is not available for transfer, so the MUA must be sized to accommodate this additional demand.
This additional 1,300 cfm adds 25% more to the MUA demand of the Figure 1 example of 5,350 cfm. This 6,650 cfm MUA load requires the designer to adjust the MUA selection. Since this load is usually not cooled due to capital and operating costs, the designer must recognize it presents additional concerns that may impact the modern kitchen. Calculating the sensible load of this MUA as sensible load = 1.08 (cfm) delta T. This reflects a typical load of 147,960 Btuh for delta T = 20 degrees F.
This represents just over 12 tons of air conditioning load imposed on the kitchen. The latent load represented by the cfm depends on the location's rh, so the designer may have to take steps to lessen the impact of this large load. It is obvious that the typical kitchen temperature will tend to be higher than the dining spaces. The designer may need to enlarge the kitchen RTU to allow additional O/A to be conditioned and reducing the noncooled MUA.
It is not uncommon to see moisture on the S/A diffusers in the kitchens due to higher rh conditions condensing water vapor from the large MUA load. Often you will see several "dry" diffusers in the same kitchen. This is because those particular "dry" diffusers are MUA drops and no surfaces are reaching the dewpoint condition to precipitate condensation.
A Recipe for SuccessAir balance worksheets are provided for the designer to document the calculations necessary to effectively design restaurants and similar challenging building designs.
The contemporary kitchen design continues to provide design challenges for the HVAC engineer to meet ventilation standards, IAQ, and airstream separation. Consistent use of design protocols and sound engineering principles will ensure that your building design meets all jurisdictional authorities, code standards, and good engineering practices.
The unique challenges that are posed by the modern restaurant kitchen are leading to new technological advances to ensure IAQ and control the demanding humidity loads. Future articles will be devoted to the application of new technology and design criteria of kitchen hood developments.ES
EDITOR'S NOTE: Some of the images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.