State-of-the-art kitchen ventilation

September 12, 2000
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Only 10 years ago, it seemed the commercial kitchen industry was looking to its past for answers to common ventilation problems. Manufacturers were still largely promoting the same systems sold in the 1970s — principally short-circuit hoods and belt-drive fans with manual on-off controls.

But now the industry is embracing its future armed with research and proven technologies borrowed from related fields such as fume hoods, vav systems, and analog-digital controls.

As a result, today’s kitchen ventilation systems are no longer just about heat and smoke removal, fire protection, and lowest first cost. Owners and operators of food service establishments are more sophisticated with the passing of the 1990s and the deployment of so many new technologies.

They want smarter systems that are more energy efficient and less maintenance prone. They also want their kitchens to be more comfortable, and at the same time in compliance with indoor air quality standards. Finally, they want reduced noise levels and enhanced fire safety capabilities.

The impact on the hvac industry is far reaching. Everyone from specifying engineers and consultants, to hood and fan manufacturers, to mechanical reps and contractors, is having to raise the bar in order to successfully compete in this new, technology-driven marketplace.

Fortunately, many have learned that electronic controls are available to provide such wide-ranging capabilities.

Intelligent controls

Until recently, controls for kitchen ventilation mainly consisted of a manual on-off switch and a magnetic motor starter or relay. Exhaust and makeup air fans either operated at 100% or not at all. This all-or-nothing approach was adequate in the old days, but is certainly not in synch with the digital age in which we now live.

Today, the state of the art is microprocessor-based controls with sensors that automatically optimize fan speed based on a range of parameters, such as cooking activity (heat and smoke/vapor load), kitchen comfort (indoor vs. outdoor temperature comparison), and indoor air quality (CO2).

Intelligence of this kind is long overdue, considering the prevalent nature of electronic controls in every facet of our lives and the tremendous advantages they provide. Electronic controls have already been widely used in food service establishments for commercial ovens and fryers, lighting systems, and heating and air conditioning systems.

End-users are increasingly aware of this and are beginning to demand such ventilation solutions as a way to become more competitive.

From a hardware standpoint, a microprocessor with sensors is a relatively simple accessory to a kitchen hood. A temperature sensor mounted in the exhaust duct collar and an optic sensor (for smoke/vapor detection) mounted inside the hood monitor the cooking load and send a signal to the processor which, in turn, controls fan speed via a variable-frequency drive (vfd).

An air purge mounted on top of the hood (Figure 1) keeps the optic sensor clean. For applications with a makeup air fan, a second drive operates as a slave to the master. This slave drive is interlocked with the fire suppression system (like a typical motor starter) for shutdown if necessary.

Finally, a well-engineered system will include plug-and-play cables so that connections are no more difficult than, say, plugging a mouse into your computer.

This relatively simple control setup can yield significant fan energy and conditioned air savings during non-cooking conditions. Anyone who has been on the roof of a restaurant on a hot day and stood next to an exhaust fan discharging cool 72ÞF air while the air conditioning system is running its heart out can immediately understand why.

The installed cost per hood for these types of controls is typically $2,000 to $4,000, depending on the number and size of hoods and fans involved. The expected energy savings can be $1,000 to $4,000 per hood depending on operating hours, climatic conditions, variability in cooking load, and gas-electric rates.

Thus, the bottom line is usually a one- to two-year payback, with a positive cash flow for the end-user in subsequent years.

No yelling required

Reducing the fan speeds also helps reduce noise, which makes the kitchen environment far more tolerable as employees can hear one another without shouting over the infamous roar of the kitchen hoods.

And reducing the amount of hot, humid makeup air being supplied into the kitchen during the peak outside load hours of 1 to 5 p.m., when there is not much cooking activity in many food service operations, can make a dramatic difference in kitchen comfort.

The idea is, you do not want the peak ventilation load to coincide with the peak outside load, because the air conditioning system is rarely sized large enough to handle both and keep the kitchen comfortable.

Expanding the intelligence of the kitchen ventilation system beyond what is described above is fairly easy. One way is to incorporate a temperature sensor in the supply duct collar and another in the kitchen space, to provide economizer-like capabilities.

When the outside temperature is cool and the inside temperature is warm, the processor will want to take advantage of “free cooling” by speeding up the makeup and exhaust fans rather than slowing them down during idle periods.

There are two advantages over conventional economizers:

1. No modulating dampers and associated maintenance headaches; and

2. No building pressure problems, since both the makeup and exhaust air are controlled.

Similarly, the intelligence of the system can be expanded by including a CO2 sensor in the dining room, to provide demand-based, fresh air ventilation for occupants. When the CO2 level rises above a threshold of, say 1,000 ppm, the processor will speed up the makeup air and exhaust fans until the indoor air quality dictates the fans can slow down again, depending on the cooking load and kitchen comfort level.

Under this scenario, some of the makeup air would ideally be ducted to the dining room rooftop units so that fresh air is delivered where it’s most needed. Again, the advantages over conventional control strategies are no moving parts (like dampers) and no positive building pressure problems.

Many options

The controls can be customized for virtually any application.

Some hoods may only have ovens underneath, which would obviate the need for an optic sensor (there would be no smoke/vapor generation).

Also, some food service operations may have a large moisture load that is not covered by a hood; thus, a humidity sensor in the kitchen would augment system intelligence and regulate hood fan speed accordingly.

Even fire safety can be enhanced by continuously monitoring the exhaust air temperature. The idea is to sound an alarm or shut down the cooking appliances before the temperature reaches the rated melting point of the fusible link, not after.

The problem with conventional fire suppression systems is that they are designed to put out a fire rather than prevent one. And once activated, these conventional systems impose a tremendous burden on the operator because of clean-up costs, idle labor, lost business, and recharging costs.

Cooking revs up fan

Now let’s review a typical sequence of operations:

1. The cook/chef turns the hood fan switch on, and the fan speed goes to some preset minimum of, say 25%.

2. He/she then turns on the cooking appliances and the fan speed increases in proportion to the exhaust air temperature and eventually reaches a steady state of, say, 75%.

3. Once the chef starts cooking (as evidenced by the production of any smoke/vapors), the fan speed immediately increases to the maximum of 100%. Fan speed is also overridden to 100% if additional sensors are incorporated for kitchen comfort and indoor air quality purposes and the conditions are right.

4. Finally, a manual bypass switch is always present in case the operator wants manual vs. automatic control.

Minimum speed settings for systems with untempered supply fans, and steam and hot water heat, can go all the way down to 0% speed.

The minimum speed for makeup air fans with direct-fired gas heat is typically 10% to 50%, depending on the type of burner being used.

Some engineers do not realize that there are burners specifically designed for vav applications. And the minimum speed for indirect-fired gas heat is typically 40% to 60%.

Foreign concept?

The concept of variable-speed kitchen ventilation systems is still foreign to some people, even though such controls have been around for more than 10 years.

One of the misconceptions is that you cannot vary the speed of the exhaust fan because this goes against code, which requires a minimum duct air velocity of 1,500 fpm.

However, the fact of the matter is that this code’s intent was only to ensure a sufficient transport velocity of particulates in the exhaust airstream. If no cooking is taking place (as determined by an optic sensor), there are no particulates to worry about, so it is acceptable to reduce the fan speeds under this condition.

Even NFPA 96 provides an exception that allows lower exhaust air volumes during non-cooking conditions. And official interpretations from BOCA, UMC, SBCCI, as well as numerous states and cities, also agree with this view.

Finally, it should be noted that other industry organizations and companies, including ASHRAE, McDonald’s, Fisher Consultants, UL, and several prominent utilities and hood manufacturers, have brought support and credibility to this movement.

Benefits abound

A side benefit of going to variable-speed controls is that the vfd eliminates the conventional on-off magnetic motor starter, which affords the opportunity of going to direct-drive fans.

For those familiar with the food service industry, it is well known that the belt is the “weak link” in the overall kitchen ventilation system. No other single component is so prone to failure and constant maintenance.

The primary reason we have used a belt-drive fan up until recently is that the motor operates at a constant speed, so we had to rely on adjustable pulleys to balance the system to the proper air volumes. By going to direct-drive fans, you also save on reduced equipment costs and eliminate the drive losses associated with belts.

Finally, by using vfd’s, you eliminate the stresses and high-current inrushes of instantaneous starting through gradual soft-starting, and provide precision air balancing capability.

Like computers and other electronics, vfd’s have come down dramatically in price over the last decade, to where a fractional-hp drive does not cost much more than a magnetic motor starter.

In fact, the marginal cost of the vfd is easily justified by the first cost and operating cost savings of going to direct drive, without even weighing in the earlier-mentioned benefits.

In conclusion, a true systems approach to kitchen ventilation involves more than just a hood, duct, and fan. It also involves controls.

The goal is to match equipment performance with the process load in order to maximize energy efficiency, as well as ensure optimal kitchen comfort, indoor air quality, and fire-safety.

“Smart” kitchen hoods will become the standard rather than the exception in the near future. A more sophisticated marketplace is pulling the technology forward, and the more progressive manufacturers are pushing it into reality.

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