Kitchens Benefit From Demand Ventilation
October 22, 2007
Exhaust hoods in restaurants and other commercial kitchens typically operate at full speed all day long and sometimes all night long - even when cooking is not taking place. The amount of energy wasted by this continual exhaust can easily add up to thousands of dollars every year, depending on how many hoods are in use.
Given the spiraling energy costs in recent years, owners of commercial kitchens are often looking for ways to cut their utility bills. Many have found that installing demand ventilation-based energy management systems for kitchen exhaust hoods is one way to reduce energy consumption and cost. These types of controls reduce the exhaust and makeup air fan speed when little or no cooking is occurring, resulting in improved hood efficiency of up to 50 percent.
A properly implemented demand ventilation control system essentially minimizes the energy burden while maximizing the ability of the hood to capture and contain cooking effluent. Kitchens that have these controls installed usually see annual operating savings of between $1,500 and $3,000 per hood, with a typical payback period of less than four years. The controls also improve kitchen comfort, IAQ, and fire safety.
UNIVERSITY IMPLEMENTS CONTROL STRATEGYThe Clark Kerr Campus dining center at the University of California at Berkeley recently had two separate demand ventilation fan speed controllers installed. One was for the front servery cookline exhaust and supply fans and the other was for the back cooking area exhaust and supply fans.
The front line uses a single 4,800-cfm, 10-foot exhaust hood and a 6,800-cfm supply fan with hydronic heating and cooling. The rear cooking area has four separate 8-foot hoods tied into a single 7,400-cfm exhaust fan that uses a 3,740-cfm supply fan with hydronic heating and cooling.
The controllers installed at the Clark Kerr Campus were part of the Intelli-Hood® controls package from Melink Corp. The Intelli-Hood controls automatically reduce the speeds of the kitchen exhaust fans (and make-up fans or rooftop unit outside air volumes) during idle, noncooking periods in order to save fan energy and conditioned air.
This is accomplished with a temperature sensor mounted in each exhaust duct and an optic sensor mounted inside the end of each hood to measure the heat and smoke loads of the cooking appliances. These sensors send a signal to a processor, which in turn sends a signal to a variable-frequency drive (VFD) for each fan. Other components include a keypad for monitoring and control as well as an air purge device for keeping the optic sensor lenses clean.
At the Clark Kerr Campus, energy performance data for all four controlled fans was collected during March and April 2006. The estimated combined fan energy savings were about 55 percent, which equates to annual savings of 35,600 kWh and $4,095. Calculated make-up air average cfm reductions yielded an additional savings of about $2,500 per year from heating and cooling load reductions.
The cost to install an Intelli-Hood package depends on how many systems a particular kitchen needs, and there is a step up in cost for every four hoods. The total cost can be as little as $5,000 for a small restaurant to as much as $50,000 for a large hotel or institutional project, depending on the number of hoods and fans, whether the work is new or retrofit, and the complexity of the system.
The California Institute for Energy and the Environment (CIEE), a branch of the University of California Energy Institute, conducted the energy analysis at the Clark Kerr dining campus for the California Energy Commission’s Public Interest Energy Research (PIER) program. The PIER case study for the project states that demand ventilation controls are a cost-effective solution to reducing the energy load and cost associated with operating exhaust ventilation systems. The organization further noted that while a retrofit cost can be significant with respect to an operating budget, the investment will usually be returned within two to four years, and kitchen staff will have the added bonus of an improved working environment with dramatically reduced noise levels from the exhaust system.
OBSERVATION RESULTS IN CONTROLSIntelli-Hood controls have been around for about eight years, although earlier generations of a similar product by Melink have been around much longer, said Steve Melink, PE, president and CEO, Melink Corp., Cincinnati. The most current version with remote communications capability has been available for about two years.
Melink Corp. started as an HVAC commissioning service provider to the restaurant industry. About 20 years ago, company personnel observed that many kitchen exhaust fans were running at 100 percent capacity all day and sometimes all night long regardless of the actual cooking load. “Countless times we noticed on hot and humid days that these exhaust fans were throwing away cool, clean air at the same time that the rooftop unit compressors were running to keep the store cool,” said Melink.
Demand ventilation controls for commercial kitchen hoods are just now entering the mainstream, and the kitchen ventilation system is often the last place engineers and consultants look to save energy, because they do not know about the controls that are currently available.
“Energy savings is the most objective way to justify the controls on the front end, but end users enjoy two subjective benefits after the fact,” said Melink. “One is less hood noise when the fans slow down during idle, noncooking periods; and two is the improved comfort when the make-up fan is not dumping the same high volume of hot and humid replacement air into the kitchen.”
As noted previously, it can be expensive to install the Intelli-Hood controls package, and the payback will be affected by the number and size of hoods, hours of operation, variability in cooking load, climate conditions, and gas/electric rates. Melink typically performs an energy analysis for each customer to determine the fan energy, conditioned air savings, and payback period.
“If you have just one small 6-foot hood that only runs, say, eight hours a day, then this would not be a good application,” said Melink. “On the other hand, if this same application only has an electric range underneath, is rarely used, and is located in Alaska where the rates might be 30 cents/kWh, then it could be a different story.”
Any qualified mechanical or electrical contractor can install the Intelli-Hood controls in the field, although a licensed electrician is required when it comes to the VFDs. Melink typically provides on-the-job training and backs it up with additional training at its corporate office to ensure contractors are able to handle future installations on their own.
As for regular maintenance, the controls have air purge devices and an automatic recalibration algorithm, but the optic sensors may become fouled if not cleaned with a soft cloth once a month. In addition, it is recommended that the temperature sensors be wiped off once a month at the same time the optic sensors are cleaned.
Publication Date: 10/22/2007