©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 2, February 2013.
By Don Fisher, P.Eng., Associate Member ASHRAE; Rich Swierczyna, Associate Member ASHRAE; Angelo Karas
About the Authors
Don Fisher, P.Eng, is president/CEO of Fisher-Nickel, Inc. and manages the PG&E Food Service Technology Center (FSTC) in San Ramon, Calif. Rich Swierczyna is a senior engineer and CKV lab manager and Angelo Karas is a senior lab technician at the PG&E Food Service Technology Center.
Conditioning the outdoor air to replace air exhausted from a commercial kitchen (along with the associated fan energy) imposes a significant energy burden—typically more than half of the total HVAC load in a commercial food-service facility. Although it is known that exhaust hoods do not need to operate at full-speed all day, adoption of demand-controlled ventilation (DCV) technology has been sluggish. However, changes to ASHRAE/IES Standard 90.1-2010 recognize DCV as a key attribute in the design of energy-efficient commercial kitchen ventilation systems, and the authors believe that DCV is poised to become standard practice within the design of commercial kitchens.
A primary component of all DCV systems is the variable frequency drives (VFD) on both the exhaust and makeup air fans. Integrated with a strategy to monitor appliance activity under the hood, the DCV system will modulate the exhaust and makeup air fans in concert with appliance use. While the concept of DCV for commercial kitchen ventilation (CKV) was pioneered by one manufacturer more than two decades ago, using a combined temperature and smoke detection system, other strategies of demand control have emerged, and the technology is now offered by at least eight manufacturers. The control strategies that are (or could be) used to “sense” appliance use and level of cooking activity include:*
- Time-of-day (using an energy management system with appropriate user override);
- Appliance energy use (requires metering and algorithms capable of distinguishing standby energy use from cooking energy use);
- Sensing exhaust temperature (measured in the duct collar or in the hood reservoir) and/or temperature rise between kitchen and exhaust temperature;
- Sensing smoke or steam produced by cooking process using an infrared beam, combined with temperature sensing;
- Monitoring cooking surface temperature or activity using infrared beams, combined with temperature sensing; and
- Direct communication from cooking equipment controls to the DCV processor.
Types of DCV
Which one to choose: temperature-sensing-only DCV versus optics/cooking-activity-sensing combined with temperature-sensing DCV? This is an escalating debate within the industry as DCV systems gain traction in the design of CKV systems and as more systems are introduced. First, the authors believe that any type of DCV is better than no DCV (although there are projects where DCV is not going to be cost effective for either system).
Second, we believe that the potential turndown (average cfm reduction) for the more sophisticated DCV systems can be greater than the temperature-only systems. This is based on fan speed reduction during no-load (no cooking) periods and can be greater for optics-based DCV systems because of a quicker response time. However, if the cookline has high-heat producing, non-thermostatic appliances such as a charbroiler, conveyor oven or wok, the response time of the system may not be a significant factor and the performance of the temperature-only systems may more closely match the more complex optics-based systems.
The response time of a temperature-sensing system should be recognized as a factor in the design, particularly when thermostatic appliances such as fryers and griddles are specified. This is because the effluent produced during cooking may not be “seen” as quickly by the temperature probe in a duct collar that is providing a measure of the average exhaust temperature.
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*This article reports only performance data for the strategy that comprises both sensing temperature and sensing smoke/steam using an infrared beam. With the recent availability of various temperature-based-only systems and ongoing field monitoring, the authors anticipate a future Journal article presenting data for such DCV systems.