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August 2003

By Anthony J. Spata, PE

When it comes to getting your airflow right all around, even new ventilation systems pose a challenge. And unlike wine, they don’t get better with age. 

No doubt you’ve read plenty about indoor air quality lately. The volume of information published recently on the topic certainly qualifies as information overload.

But what you really need to think about—and what most stories overlook—is how the introduction of new technologies and modifications can detrimentally affect existing HVAC systems, if you’re not careful. With the numerous air-moving components present in a restaurant, the potential for good systems to go bad is high.

To get the results you want, first you need to see all restaurants as a collection of multiple, dynamic ventilation systems. Kitchen exhaust hoods, makeup air units, dining-area HVAC equipment and restroom exhaust fans, in general, may operate separately, but their performance is inexorably linked.

So front-of-house and back-of-house in reality are fundamentally tied to each other. “The view that they are separate will lead the operator, designer or builder to big problems and dissatisfied customers,” says Steve Grover, v.p. of health and safety regulatory affairs for the National Restaurant Association. “Time after time I hear of new buildings that need to have their HVAC redesigned because [somebody] forgot it’s one big system.”

And in the big picture, governing every part of the building and the building overall, is the first rule of ventilation: “Air out equals air in.”

Sounds basic, but sometimes we lose sight of the basics. Air exhausted from a space ultimately will be replaced with an equal amount of air from another source. It’s identifying, specifying and controlling that other source that’s the key to preserving IAQ standards.

No two concepts are exactly alike, and designing these ventilation systems to work in harmony means your engineer has to understand the unique factors in a restaurant that influence IAQ and HVAC operation. Is the facility a QSR or fine-dining unit? Is the hood a Type II (heat and odor) over a pizza oven, or is it a 20’ Type I (grease exhaust) canopy hood with integral makeup air over an under-fired broiler, deep fryer and six-burner range? Seating for 50 or 500? Freestanding or on the ground floor of a 30-story office building? All these things make a difference for your mechanical design.

And when all goes right, the “trained arrows” on the plan point where they’re supposed to. (The “Indoor Air Quality Handbook” available from Options-Philip Morris USA provides an excellent guide to creation of an “airflow footprint.”) Positive and negative air pressure areas are neatly labeled. The arrows move in unison, clearly defining the direction of each and every CFM of air. Problem solved. Air balance achieved, no odors to worry about. Right? Wrong. You’ll quickly find the arrows suffer from Attention Deficit Disorder, and are easily influenced and led astray from their intended path.

Trained Arrows
Even if everything starts out right—the engineering design properly identifies the interaction of the supply and exhaust systems, the mechanical contractor follows the drawings during installation, and the test-and-balance company achieves the stated air quantities—everything is fine only for a short period after commissioning. Then, degradation begins.

Unlike wine, ventilation systems never get better with age. You need to keep up with ongoing maintenance to ensure the arrows continue to move as desired. In addition, you have to account for operational changes that affect airflow in all systems that are impacted.

IAQ literally starts at the very top—that is, up on the roof. You need to locate exhaust fans and plumbing vents to prevent the reintroduction of cooking emissions and odors into outside air intakes. Being in compliance with the separation distances mandated by applicable codes is not enough. You or your engineer must consider influences such as the prevailing wind direction and speed, roof shape, and any adjacent structures.

John Clark, PE, of HG&A Inc. in Minneapolis, has performed significant research in this area. Along the way, he’s taken a high-velocity upblast exhaust stack design typically used with laboratory fume hoods and modified it for restaurant application. The design introduces additional outside air from above the roof into the kitchen fan flow, enhancing dilution and dispersion of the exhaust, greatly reducing recirculation of cooking odors into HVAC fresh air intakes.

Because everybody can see that a hood’s exhaust and makeup air are related, those systems usually are interlocked. But that’s not the end of the system, obviously. To prevent cooking odor migration into adjacent areas, the makeup air system normally accounts for only a fraction of what’s needed to offset the exhaust. The remainder needed to balance our “air out equals air in” equation is in kitchen supply air and in air transferred from adjacent spaces, often the dining area.

Without enough of this transfer air, you can wind up with a stronger than normal negative pressure zone in the kitchen. Too much negative pressure can disrupt complete capture of cooking vapors. In extreme cases, gas-fueled appliances may “back-flue” (i.e., combustion by-products are drawn into the space rather than being vented to outside), drawing carbon monoxide into the room rather than up the flue.

So you need to be sure you have adequate transfer air coming in, not only for comfort, but for safety. If HVAC systems serving adjacent areas are designated to provide transfer air, you’ll want them running any time the exhaust fan is running. (Note, too, that many state codes now limit how much negative pressure a kitchen can run.)

Apart from insufficient transfer air, another common cause of errant arrows is operating the nonkitchen HVAC units in intermittent supply-fan mode. In this mode, the supply fan operates only when heating or cooling of the space is required. When the fan cycles on and off, outside/transfer air volumes swing, and airflow patterns within the restaurant change dramatically, complicating temperature control.

Broken Arrows, Bent Arrows
Even with adequate makeup air volume, how and where it’s supplied can cause problems. All exhaust hoods, regardless of type, can have performance disrupted depending on how air is supplied in close proximity to them.

Canopy hoods, typically found in restaurants other than QSRs, can be exhaust-only, or have some type of makeup air system integrated with them. With exhaust-only hoods, any air movement external to them that disrupts the natural convection of, and induced draw on, cooking vapors will result in incomplete capture. The use of four-way supply diffusers is highly discouraged in the kitchen area because of their primarily horizontal, high-velocity discharge pattern. “Perforated face” diffusers, with a gentle, vertical discharge air pattern, not only minimize interference with exhaust hood performance, but they can also be positioned to provide optimum “spot cooling” of the kitchen staff.

When makeup air is integrated into a canopy exhaust hood, then both sources of air supplied in proximity to the hood must be carefully regulated to affect complete capture of vapors. Properly applied, integrated makeup air systems that use air curtain, face-discharge, perforated perimeter or rear discharge supply will yield acceptable results.

Each of these makeup air methods shares the same critical design parameters: The quantity and temperature of air supplied, its discharge velocity and the direction of discharge. The Commercial Kitchen Ventilation Lab located in Wood Dale, Ill., operated by Architectural Energy Corporation of Boulder, Colo., has done extensive research into air movement patterns for each of the above hood styles. Through use of a Schlieren Visualization System, the CKV Lab has been able to examine the effect of each makeup air method, and provide guidance for correct usage.

One of the easiest and most cost-effective fixes this research has validated is the use of side panels on canopy hoods to improve their performance. By minimizing the effects of drafts on the exhaust plume, side panels help capture cooking vapors even while reducing the total exhaust rate.

Backshelf hoods (predominant in QSRs, as typically each hood is designed to serve a specific type of cooking equipment) have the benefit of being closer to the source of vapor generation. However, performance is still susceptible to most of the factors detrimental to canopy hood performance. In addition, a backshelf unit’s ability to capture cooking vapors is more easily influenced by the movement of staff in front of it because backshelf designs only partially project over the equipment.

A “walk by” test, simulating the frantic pace of QSR employees in the kitchen, is a common method used to determine their real-world minimum exhaust quantity. As with canopies, studies show that the placement of side panels at the ends of a backshelf hood dramatically improves performance.

An excellent reference for engineers involved in commercial kitchen design is the American Society of Heating, Refrigerating and Air-Conditioning Engineers “HVAC Applications Handbook.” Chapter 30 in the current 1999 edition (with a significantly expanded chapter in the soon-to-be issued 2003 edition) provides in-depth detail on exhaust hood types, their applications and associated makeup air strategies.

Also, the California Energy Commission has published a “Design Guide: Improving Commercial Kitchen Ventilation System Performance,” co-produced by Architectural Energy Corp. and Fisher-Nickel Inc. Updated in February of this year, it provides numerous illustrations of the various exhaust systems in use, and examples of how the Schlieren system is used to evaluate their performance.

And soon to be available from the National Restaurant Association, “Indoor Air Quality—Tips for the Independent Restaurant Operator” by Stephen K. Melink, PE, will provide information targeted to that significant segment of the industry.

Seminars, too, are great sources of information, and IAQ seminars put on by industry organizations and suppliers alike are on the rise. Coinciding with the National Restaurant Show held annually in Chicago, for example, Greenheck, of Schofield, Wis., partners with the University of Wisconsin in a three-day seminar devoted to all aspects of commercial kitchen ventilation. Many of the most active R&D companies regularly participate in such events, with course content ranging from providing a basic understanding of exhaust systems to the latest developments in ventilation technology.

If you have a smoking section in your dining area, it too can be susceptible to the operation of the kitchen’s ventilation systems. Unless that smoking section is 100% enclosed and isolated from the rest of the facility, a highly negative kitchen can cause migration of smoke from it into other areas. Supplemental, recirculating air-cleaning devices (such as an electrostatic precipitator) are rendered useless in these situations.

Front House, Back House, Same House
Going back to the aforementioned airflow footprint, you should create (either by yourself or with the help of an HVAC professional) alternative diagrams that represent situations of one or more dining-area HVAC units not functioning, and assess the results. In addition, you may want to determine the tolerance level built into the design of your air balance. If a reduction of as little as 5% in design airflow quantities will result in your arrows changing direction, you’re going to need to follow a comprehensive, regimented maintenance program to stay at your specified volume and balance.

Any discussion of IAQ must include the operation of the restroom exhaust fan, too. Even if you have single-occupancy facilities, the exhaust fan should operate continuously and not be switched locally with the room’s lights. Why invite problems? The amount of air exhausted from the restroom is typically minor compared to the total amount of kitchen exhaust. However, the effect of odor migration from this area is much more noticeable, and is significantly more offensive. A restroom’s design exhaust quantity should always be above the mandated minimum to allow a safety factor.

What’s That, Mold?
Remember “Sick Building Syndrome”? During the energy crisis of the 1980s, architects and engineers thought a good way to reduce the expense related to HVAC operation was to build “tighter” buildings (i.e. have less air infiltration) and reduce the amount of outside air brought into them. Combined with the predominant use of synthetic materials, the unforeseen result was conditions that facilitated the growth of molds and a general feeling of “stale” indoor conditions. Numerous lawsuits resulted for illnesses real or imagined, with the resultant cost of related building remediation incalculable.

While ventilation standards for all buildings have been substantially improved since then, the problem of mold growth continues today. In fact, increased ventilation can actually contribute to the problem, especially in areas with predominantly high outdoor humidity.

“I am not ready to put mold on a par with asbestos, but it is a growing concern,” says Grover at the NRA. “The scientific link between illnesses and mold is much less clear than the link between illnesses and asbestos. However, we are seeing growing litigation concerns.”

Rick Bagwell, president of Halton Co., similarly shares the feeling. “Mold growth is a true problem; however, it seems widespread only in geographic regions that have high levels of outdoor relative humidity,” he says. “The engineering community needs to be educated that unconditioned makeup air does not ‘magically’ enter the space and then exit through the hood system. It does exit, but only after mixing with room air, increasing its humidity.”

To help prevent mold growth, it is imperative you regularly maintain the HVAC system, especially its filters, cooling section, condensate pan and drain. Any accumulation of water and nutrients in the dark, warm environment of the HVAC system can result in the rapid growth of undesirable organisms.

Tricks With Economizers
In an about-face on the tight-building theory, industry standards now mandate more fresh air per occupancy. To accommodate the requirement, and still keep a lid on heating and cooling costs, the HVAC industry refined the concept of using “economizer” systems to provide both ventilation and, when weather conditions permitted, “free” cooling.

An economizer consists of a motorized damper assembly that regulates the amount of outside air admitted into an HVAC unit, in response to controls that determine when environmental factors are favorable to provide cooling without the use of any additional refrigeration (i.e. compressor) devices. The often-used analogy is that of opening the windows of your house on a sunny but cool-and-breezy day. With an economizer, the total amount of air supplied to the building remains constant, while the outside and return air quantities vary in inverse proportion to each other. (Meaning, as the amount of outside air brought through an HVAC unit increases, the quantity of return air decreases.)

Economizer controls can simply monitor outside temperature, or be as sophisticated as comparing the temperature and humidity of outside air to that of the return air from the space. However, with the effect excessive humidity has on propagating mold growth, most HVAC manufacturers have adopted the use of controls that take humidity into account for you.

In general, the economizers in use today are reliable and require no more maintenance than a typical motorized damper and temperature control system. Kitchens, with their large internal heat gains, are an ideal application for economizers. With a kitchen’s need for cooling far into winter months, economizers quickly yield savings that amortize their added cost. When applied to HVAC equipment serving dining areas, the payback period is still attractive, but will take somewhat longer to reach the break-even point.

Alone, however, they’re not a panacea. Economizers vary their outside air component by temperature and perhaps humidity, don’t forget, not by exhaust or ventilation demand. So it is very easy to mislead our friends, the arrows, from their intended direction. This is especially true when economizers are present on HVAC systems serving a kitchen.

To properly apply an economizer to a restaurant, an additional exhaust fan, which operates in conjunction with the economizer, is required to maintain consistent building pressurization. A contact switch that simply indicates a certain opening of the economizer has occurred typically controls this “power exhaust,” turning it on whenever the switch is making contact.

While many HVAC equipment manufacturers use this basic control system, it may not be sufficiently sensitive to maintain the designed air balance. Typically, with only a few hundred CFM separating the positive and negative areas in a restaurant, you might need a more sensitive “differential pressure” switch to control operation of the power exhaust. With this control, the actual air pressure inside the space served by the HVAC unit is monitored, and operation of the power exhaust is regulated by sensors to maintain the desired air balance.

New Belts, No Belts And Heavy Breathers
So while we’re on the topic of smaller things, like switches, have you ever noticed that very often it’s one of the least expensive components and can be the most critical to proper operation of a complicated system?

Which brings us to…the fan-drive belt! Be it smooth or notched, single or double, the drive belt is a perfect example of how the small can wield great influence.

Who hasn’t had an exhaust fan belt go out at the worst time? While belt life is greatly improved these days, lots of external factors still conspire to shorten it. Pulley misalignment, improper tension, etc., all take a toll and shorten belt life. Even under ideal conditions, you should check your drive belts at least every six months and replace them annually. Keeping spare belts either inside the fan box where they will be used, or in a specific parts box at the restaurant, allows for quick replacement when they do break.

Even before a belt breaks, it can stretch, and that too affects the restaurant’s air balance. During the startup/ balance phase, you should record final rpm settings of all fan shafts (not motor speeds) on a summary form retained at the restaurant. In addition, the final rpm settings should be written (in permanent marker) inside the fan.

Dodging the belt issue completely, direct-drive fans with electronic speed controls today are finding wider acceptance within the restaurant segment. As with anything new, there are pros and cons.

On the upside—no more drive belt. On the downside—the motor is typically unique to the fan, and obtaining a replacement could take longer then finding a replacement belt. On the upside—with electronic speed control, it really is “set it and forget it.” On the downside—many fans with electronic speed control require three-phase power, while a typical fractional-horsepower belt-drive fan uses only single-phase power.

With the refinement and wider use of electronic speed control, kitchen exhaust systems are evolving to the next level—variable volume exhaust systems that respond to the intensity of cooking activity.

Variable-Speed Fan, Variable-Speed Makeup?
For many years, national codes and recognized standards had mandated that a fixed exhaust volume, determined by hood size/type, appliance type and products being prepared, be maintained at all times, even when no cooking was actually occurring. Within the last few years, exhaust systems that automatically respond to the amount of smoke and heat being generated during cooking have gained wider acceptance. Typically, exhaust quantities can be reduced anywhere from 33% to 50% during “no-load” cooking times.

“There is no doubt that tomorrow’s kitchens will depend on variable-speed exhaust,” says Steen Hagensen, president of Exhausto in Atlanta. “Variable-speed fans are more efficient, use less energy and provide for a variety of control and monitoring possibilities. Improved cooking consistency can be achieved by controlling the fan speed based on temperature and other parameters.”

But once again, a potential detour for the airflow arrows lurks inside the benefits. If the variable-speed exhaust rate is reduced, makeup/outside airflow also has to be reduced simultaneously, or else the kitchen designed to be negative will go positive, and cooking odor migration will result. Thus, you’ll controls to regulate both ends of that equation.

Depending on the equipment and ventilation strategy used, either a speed/volume reduction in the supply air fan must occur, or outside air dampers must be repositioned to reduce flow. It is important that the outside air adjustment occur with equipment directly serving the kitchen. This approach will maintain the same relative transfer of air from adjoining areas at all times, and avoid the creation of intermittent drafts as the exhaust fan goes from full- to partial-speed and back.

Analogous to variable-speed kitchen exhaust/makeup systems, which control incoming outside air based on smoke/heat, demand control ventilation regulates the amount of incoming outside air for the dining areas based on occupancy, or more precisely, carbon dioxide levels within the space. The system regulates the amount of outside air introduced to keep CO2 below a predetermined maximum level.

Basically, a demand-control ventilation system works much like an economizer, except that it responds to CO2 levels instead of temperature/humidity. Engelhard Sensor Technologies manufactures a wide range of products that can be factory installed or retrofit into HVAC units. In many cases, they are compatible with existing economizer components.

This concept can be beneficial in restaurants with large dining areas and/or private rooms that, depending on time and situation, may have 10 or 100 customers present. Instead of always bringing in (and conditioning) outside air based on the maximum expected occupancy of an area, the system lets you regulate ventilation based on actual needs. During hot or cold weather periods, utility savings will be substantial.

As with the variable kitchen exhaust concept, proper application is critical. If you depend on dining area HVAC equipment to provide any outside air to be transferred to the kitchen, the system must minimally provide that quantity at all times. Also, during those periods when the dining-area units are introducing large quantities of outside air, it may be necessary to operate a supplemental exhaust fan to relieve the excessively positive condition created.

Making A Symphony From The Notes
As you can see, you can’t deal with IAQ on a component-by-component basis. The pieces comprise a unified system that requires the same attention to detail as do your valued customers. While you’ll need trained HVAC professionals to preserve good IAQ, there’s a lot you can do to assist. Watch for signs that things are off, and take the appropriate corrective action.

And keep your eyes on the arrows.

1) You replace outside air filters in HVAC equipment with a more efficient (higher-resistance) type that actually reduces air volume brought into the building, causing a pronounced negative pressure. Conversely, you install higher-efficiency filters in the exhaust hood, and they slow the exhaust, resulting in incomplete capture of cooking vapors. (Good intentions require a COMPLETED thought process to be beneficial. Changing filter types might change the whole equation.)

2) You substitute a different fan-drive belt or pulley in an emergency to get back in operation, and then you forget to go back and install the properly sized component. Always write down original fan RPM values for future reference.

3) You clean the HVAC ductwork but allow water to collect in low points or soak into interior insulation. And then you wonder where the mold is coming from. Always vacuum ductwork with interior insulation.

4) You install programmable thermostats to automate HVAC equipment that provides make-up and transfer air to the kitchen, but:
a) The thermostats do not have the correct time.
b) The thermostats are not programmed to start HVAC equipment before exhaust fans are turned on, and they continue to operate the HVAC units until after the last fan is turned off.
c) The thermostats are not adjusted for Daylight Savings Time changes.
All of the above will result in highly negative buildings.

5) You clean the exhaust filters in the kitchen hood periodically during the day to keep them efficient, but you allow cooking to occur while no filters are installed in the hood. And you wonder why there is so much grease leaking from your exhaust fan.—AS

Anthony J. Spata, PE, has more than 25 years of experience in HVAC design and energy conservation research for restaurants and commercial buildings, serving the last several years as corporate engineer for McDonald’s Corp. A registered engineer in 47 states, he is also a senior member of the Association of Energy Engineers and serves on ASHRAE’s Standards Committee for Commercial Kitchen Ventilation. He’s a frequent speaker at restaurant and utility association meetings, typically relating his experience as a technical professional who represents end users.

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