Kitchen ventilation systems: Saving energy without sacrificing performance
New options for owners looking for ways to reduce operating costs.
Commercial kitchens consume a significant amount of energy in restaurants and other establishments that have food service operations. The typical constant-volume kitchen exhaust system generates 20 to 30 air exchanges per hour. If that air is being heated or cooled, money is being spent not only on operating costs, but to condition the air as well.
To help promote energy efficiency, manufacturers have been asked to apply their products in a way that reduces the exhaust rate, which in turn reduces the required supply rate and the number of air exchanges per hour. While testing has provided the industry with information on improved performance at lower exhaust rates with greater overhangs, end skirts, appliance positioning, variances in hood design, etc., a point of diminishing returns exists where some lose sight of the kitchen exhaust system's purpose.
The ventilation system has to capture the heat and other effluent generated by the cooking equipment, oftentimes in imperfect conditions. For example, the simple act of walking by a cooking battery generates enough draft to interrupt the capture and containment if there is not enough capture velocity on the hood edges. Even if the hood appears to be capturing the smoke, it may not be capturing all the invisible heat. This heat gain to the space decreases comfort and will ultimately find a return vent, then end up being cooled by the HVAC system, requiring even more energy. Other examples of situations where unwanted air currents may be created include pass-through windows, doorways, supply air distribution configurations, and drive-through windows.
It's not that using what we've learned from testing is a bad approach to saving energy—but it is limited. Excessive constant-volume exhaust rate reductions may sacrifice performance and could lead to additional energy spending. Alternative products and technology can be used to generate greater savings with sustained performance and comfort, some of which are described below.
Proximity hoods (see Figure 1), sometimes referred to as backshelf hoods, can effectively reduce the net airflow through the space while maintaining performance. These hoods are designed to be hung in close proximity to the cooking appliance when compared to standard canopy hoods. The height from the cooking surface will vary depending on the manufacturer but, as an example, might range from 17 to 36 in. Due to the closeness to the cooking surface, these hoods have less vulnerability to cross drafts and can generally exhaust at lower exhaust rates. Furthermore, proximity hoods take up less space because in most cases the equipment can extend beyond the front edge of the hood (underhang) as determined by the manufacturer's UL 710 Listing . The best applications for these types of hoods are light- and medium-duty applications such as fryers, griddles, and ranges. The use of proximity hoods with char-broilers is not recommended, due to their excessive heat loads. Another limiting factor may be appliance size, so other technologies should be considered to complement other ventilation products.
Demand ventilation, also known as variable volume, is capable of generating operating savings and a quick payback. This technology has been available in the market for several years and is now gaining traction. And with several new variations of products available from different manufacturers, these systems are more affordable than ever.
Demand ventilation systems are based on the premise that cooking loads vary throughout the day. Some operations, such as supermarkets, may have batch-style cooking only a few times throughout the day, while restaurants might have breakfast, lunch, and dinner rushes with idle periods in between. Regardless, most operations are not cooking at full capacity the entire time the kitchen staff is in the building and the ventilation system is operating.
Demand ventilation systems monitor the cooking operation and adjust the exhaust, supply, and rooftop unit fans so that when the cooking load is reduced, the fans operate at a reduced level and save energy, especially considering the heating and cooling loads mentioned earlier. These systems typically use a temperature sensor to monitor heat load from the cooking equipment and send a signal back to the control system, which then provides a signal to a variable frequency drive (VFD). The VFD then adjusts the fan speed. Other systems may have additional sensing technologies, but all are intended to monitor the current state of the system and adjust it accordingly.
Figures 2 and 3 illustrate how these systems reduce airflow. Note that in keeping with our goal discussed above, airflow rates are reduced only to a point that maintains proper performance (capture and containment) and meets minimum code requirements such as minimum duct velocities and UL-listed hood limits. Reductions of airflow up to 50% during idle cooking periods are achievable. Furthermore, using AMCA-certified fans will improve performance of demand ventilation systems by assuring that the fans operate as specified. Additional benefits include reduced sound levels in the kitchen and soft-starting motors with VFDs that decrease wear and tear on fan belts and bearings.
If a facility is considering or attempting U.S. Green Building Council LEED certification, the use of demand ventilation may help qualify for up to two points depending on the facility and application. Use of a demand ventilation system alone cannot assure LEED points; however, when used with other products in the same category it is likely to contribute positively to the overall performance required to earn points. The areas where these products typically apply are:
Innovation and Design Process: ID Credit 1—Innovation in Design
Energy and Atmosphere: EA Credit 1—Optimize Energy Performance.
In addition, the payback period can be decreased by offsetting some of the upfront costs through rebates and credits. These, too, are not guaranteed or available in all areas, but many state and local governments have rebate programs for which demand ventilation systems will qualify.
ELECTRICALLY COMMUTATING MOTORS
Electrically commutating motors use their internal circuitry to convert 115 V single-phase power to a controllable DC voltage. The result is increased efficiency and full-speed control. These motors can be used on direct-drive fans that are UL 762 Listed for cooking applications and applied to kitchen systems, and are easily adjusted during test and balance for optimum performance. These motors have up to 80% usable turndown, significant power use reductions, no belt or pulley losses, no belt maintenance, and no requirement for a VFD to adjust the speed of the fan when used in conjunction with a demand ventilation system. These motors are soft-starting, which increases motor and bearing life, and run much cooler than typical motors and permanent-split capacitor motors. Again, because of the significant energy savings realized by these motors, they too may contribute to acquiring LEED points. The limiting factor to using these motors in some cases is size, but manufacturers are continuing to develop larger horsepower motors that deliver more value to the industry.