Demand Control Kitchen Ventilation

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What is DCKV?

Demand control kitchen ventilation (DCKV) is a building controls approach of slowing down kitchen exhaust fans and subsequent supply air in response to the actual cooking loads in a commercial kitchen. Traditional commercial kitchen ventilation systems operate at 100% fan speed independent of the volume of cooking activity and DCKV technology changes that to provide significant fan energy and conditioned air savings. DCKV was invented in 1990 by Stephen K. Melink, US Patent #4903685, founder of Melink Corporation in Milford, OH. By deploying smart sensing technology, both the exhaust and supply fans can be controlled to capitalize on the Law of Affinity for motor energy savings, reduce makeup air heating and cooling energy, increasing safety and reducing ambient kitchen noise levels. 

Why is it important to control kitchen ventilation?

According to the United States Energy Information Administration, commercial kitchens consume over three times the energy of the average commercial building per square foot and present impactful savings for kitchen operators. The kitchen ventilation system comprised of the exhaust and supply fans consume nearly 30% of this energy.

The energy savings realized by DCKV systems are the sum of the motor operating savings (energy used to physically spin the fan motors) and conditioned air savings (energy saved by not needing to condition outside air).  While conditioned air savings have a linear, roughly 1:1, relationship with fan speed reductions, the motor operating savings benefits from speed to power requirements per the Law of Affinity.  For example, reducing fan speed by 20% reduces the energy consumed by that fan motor by nearly 50%.  Reducing fan speed by 40% results in the motor consuming 75% less energy than when it is running at 100%. This compounding energy savings in the fan motor along with the conditioned air savings make for a powerful energy saving combination for the kitchen operator.  

In addition to energy savings, DCKV has been recognized for improving kitchen comfort, indoor air quality, and improving fire safety in the commercial kitchen space.  By modulating both exhaust and supply air equipment, the ideal building pressure can be achieved in the ventilation system leading to improvements in overall building pressure and health.  Advanced DCKV system can also mitigate the fire risk in commercial kitchens by providing a high-temperature alarm that will notify occupants of a dangerous condition prior to fire suppression system activation. Systems can be further interfaced with electronic gas solenoid valves or shunt-trip breakers to deactivate the cooking fuel source in the event of exhaust fan failure.  CO, CO², and VOC sensors can also communicate to the DCKV system to provide notification of a dangerous condition in the kitchen and command the fans to an appropriate exhaust rate to alleviate the danger to occupants.

Technology Development & Code Adoption

The system is comprised of two primary sensor sources to detect cooking, temperature probes in the exhaust duct collars or hood canopy and optical sensors to detect the presence of smoke or effluent as a result of cooking in the canopy.  As the temperature probes sense a rise in exhaust air temperatures, the signals are sent via the system controller to Variable Frequency Drives (VFDs) to increase the fan speeds proportionately from a predetermined minimum speed, typically 30%.  The optic sensors visually monitor the hood canopy and are activated by the presence of smoke or steam produced by the cooking process. When the optical sensors are sufficiently obstructed by the effluent the fans will speed up to 100% to maintain capture until the effluent is sufficiently removed from the canopy.

Significant barriers to commercial adoption of the DCKV were identified and overcome beginning in 2000 with the NFPA 96 Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations.   Previously section 8.2.1.1 cited minimum duct velocities of 457.2m/min (1,500ft/min), however following an ASHRAE research project, RP-1033, Effects of Air Velocity on Grease Deposition in Exhaust Ductwork, the code was changed to read: “The air velocity through any duct shall be not less than 152.4m/min (500ft/min)”. 

Following this change was the recognition of the DCKV as a best practice for design in the ASHRAE Handbook under Standard 154 – Ventilation for Commercial Cooking Operations in 2003. The committee members responsible for the handbook change also successfully included the design parameters into the 2003 edition of the International Mechanical Code.  The Uniform Mechanical Code also adopted provision for multi-speed kitchen ventilation systems in the 2004 edition.

Another catalyst for market adaptation happened as a result of changes to ASHRAE/USGBC/IES Standard 90.1 – 2010, as DCKV was again recognized as a key attribute when designing an energy efficient kitchen. The 2012 edition of the International Green Construction Code references the same standards as detailed in 90.1-2010 and IES Standard 189.1-2011 along with the requirement in LEED v4 EA Credit 1. The United States Environmental Protection Agency (EPA) identified DCKV technology for the 2015 ENERGY STAR Emerging Technology Award as catalyst for further commercial acceptance. Further, in 2014 California’s Building Energy Efficiency Standard (Title 24) adopted similar language for compliance and the US Department of Energy has set a compliance mandate by September 27, 2016, across the United States.

Energy Savings Performance and Case Studies

Several independent research groups including U.S. Federal and State government agencies, utility providers and industry trade associations have conducted analysis of the technology.  One such group, Fisher-Nickel Inc., has developed numerous case studies while operating the Food Service Technology Center (FSTC) in San Ramon, CA supported by the California Public Utilities Commission.

Case Study: Demand Ventilation in Commercial Kitchens, An Emerging Technology Case Study: Melink Intelli-Hood® Controls Super Market Application (FSTC Report 5011.06.13)

Summary Findings:

• ·        Average daily fan consumption reduced by 74%, from 118 kWh/day to 31 kWh/day
• ·        Annual energy reduction of 31,370 kWh
• ·        Reduced airflow by approximately 40%

Case Study: Demand Control Ventilation for Commercial Kitchen Hoods (Report ET 07.10), by Design & Engineering Services Customer Service Business Unit, Southern California Edison, 2009.

Summary Findings:

• ·        Average kW reduction of 54% across five separate locations and markets

Implementation of Demand Control Kitchen Ventilation Systems

DCKV systems can be implemented into commercial kitchens either during the new construction process or via retro-fit into existing systems.  

New Construction:

Due to the complicated nature of DCKV systems, it’s recommended that the primary specification be carried in the Construction Specifications Institute's MasterFormat, Division 23 – Mechanical, further coordinated and referenced with automation systems in Division 25. It’s also imperative to list the controls package in Division 11-400 with a reference to the previously mentioned sections so that the project team is property coordinating the same system across the scope of work for all trades. The scope of work for the controls will typically encompass the electrical and mechanical trades for the installation and powering of the controls devices. 

For the mechanical designer it’s important the control strategy being proposed is coordinated with the food service designer based on the appliance types, facility usage and local codes.  It is typically more cost-effective on a construction and operating basis, as well as less risky from a liability standpoint, to minimize the length of high-temperature grease ducts. For example, most hotel, hospital, and other large commercial kitchens are designed as part of a single-floor building and only connect to a multi-story building to minimize this impact. This reduces the first cost of installing grease ducts on multiple floors that waste valuable space in a high-rise, and the risk of extending a potential fire hazard any further than necessary. And it facilitates a dedicated fan per hood design without the need for dampers, of which there are many benefits.

But even in a high-rise building, purposely designing obstructions such as dampers in a long high-temperature grease duct which is designed to convey heat, smoke, and grease vapors and away from the building is problematic for several reasons. First, is the liability concern of having long high- temperature grease ducts distributing grease into areas of the building beyond the kitchen. Since grease is a combustible substance, this poses a potential risk. Codes require regular cleaning of kitchen hood, ducts, and fans. High-rise buildings with long ducts and obstructions are inherently more exposed from a liability standpoint than single-story buildings with short ducts and no obstructions.

Once a DCKV system is deployed on a project, one of the most critical and often overlooked pieces is the commissioning process.  Ensure your controls provider has a robust and document commissioning plan, ideally submitted for review during the selection and specification process, so you can confidently have a system that will save energy. The commissioning document should include functional testing, VFD parameters settings, airflow measurements, temperature sensor calibration and balancing results.

Retro-Fit Applications:

The first step is to obtain detailed site information pertaining to the kitchen ventilation system, kitchen hoods, appliances, electrical systems and any other pertinent systems associated with the mechanical systems.  A thorough assessment of the existing ventilation systems performance and ability to capture and contain cooking effluent is critical prior to applying a DCKV system.  This information and observance should be made by a qualified contractor or system supplier with in depth knowledge of HVAC balancing and commercial cooking environments. 

A critical step in the initial assessment, and a key cost driver, is the existing electrical system configuration and fan motor ratings and performance.  The scope of work for implementation includes the mounting and powering of the VFDs to take the place of the traditional motor starters for fan control.  It’s important to note that motor load wiring needs to be run in separate conduits for multiple fan motors as to not cause electrical interference between motors being commanded to run at separate frequencies by the VFDs.  Likewise, the use of fan motors capable of being controlled by VFDs and being in compliance with NEMA MG31 Part 1.4.4.2 standards. 

Following the site assessment, the installing contractor / manufacturer will design and engineer the kitchen DCKV system in accordance with the findings for optimal performance and energy savings.  It’s also critical to ensure that the installed product is UL 710 listed for usage in a commercial kitchen hood.  In some cases a DCKV system that is non-UL compliant may void the manufacturer’s warranty on the original kitchen hood. Some systems may be UL 510 listed, however it’s important to note that this certification is only for the electrical component enclosure and not the sensors used to modify the exhaust hoods or duct work. 

Installation of sensors and powering of the system components does not constitute a functional DCKV system.  To ensure optimal operation and safety standards it’s critical to have the contractor conduct a comprehensive commissioning of the installed system. This commissioning should be inclusive of ensuring correct Fire Mode response, calibration of the sensors to the cooking appliances, properly programmed VFDs, supply air integration and verified capture and containment of cooking effluent.  The kitchen operations and building maintenance personnel should also be property trained in the operation and maintenance of the system. A properly maintained DCKV system will ensure continued energy savings and safety for the building occupants.  

References

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