Hoods Go Green

George Zawacki, Guest Author -- Foodservice Equipment & Supplies, 4/1/2008

George Zawacki, DFS, CFSP Founder and Publisher Up Your Stack Arlington Heights, Ill.

Those of us who looked at energy-saving devices 5-to-10 years ago were basically ignored. The only relevant factor was cost. Customers always wanted to know how much would it cost them? Owners, architects and general contractors only smiled, then deleted energy-saving items that cost more or quickly asked for a cheaper alternate.

Now, it is 2008 and we operate in a different world. Suddenly, being green or the idea of energy efficiency and saving energy is beginning to catch on in all corners of the industry. In fact, it is much more than catching on; if you are not leading with energy efficiency, you are being left behind.

But what about the unsung workhorse of the kitchen: the stainless-steel box we call an exhaust hood? How can that box join the revolution?

Surprisingly to many, a few forward-looking individuals began working to improve the efficiency of exhaust hoods more than a decade ago. These people did not subscribe to the vagaries and myths that have surrounded commercial kitchen exhaust hoods.

The backbone of the questioning rested in a new and little known ASHRAE technical committee known as TC5.10 Kitchen Ventilation. The acronym stands for the American Society of Heating, Refrigeration and Heating Engineers. The committee was made up of a few dedicated engineers from companies that manufacture and sell commercial kitchen hoods and ventilators, plus a few operators who understood energy costs to their companies. They began to challenge a lot of the old rules and began asking “why?”

Their line of questioning spanned a vast subject area including: Why are hoods designed the way they are? Why do we need 1800 FPM per minute being sent up the stack and out of the building hour after hour only to be replaced with more expensive heated and air-conditioned air? Does the placement of equipment under the exhaust hood make a difference? Are there ways to cut down on the amount of air it takes to effectively remove the grease, heat, odor and by-products of cooking? Why does the exhaust fan always have to run at full speed all day long? Why do the various codes conflict?

Out of this willingness to question the status quo, progress was made. Out of it all has arisen a respect for the work being done and a gradual recognition by code bodies that there are better ways to design a commercial kitchen and the exhaust systems that enable them to work more efficiently.

For the past 50 years, the gold standard of ventilation stated that in order to keep grease moving inside the ductwork and away from the kitchen, air had to move at no less than 1500 feet-per-minute (FPM) to a maximum of 2100 FPM. Anything less was thought to create significant grease deposits in the ductwork that in turn became a serious fire hazard. This was gospel and this was what all of us who designed kitchens subscribed to as the way to figure the FPM load for the mechanical engineers.

Myth No. 1: 1500 FPM Is the Minimum
After an ASHRAE grant, a contract was released to the University of Minnesota to challenge that myth. The testing proved without a doubt that the minimum air movement of 1500 FPM was indeed a myth. The testing set a new threshold of 500 FPM for continuous movement of grease-laden air within the ductwork. Now, the new minimum FPM is 500, recognized by the International Code Council (ICC) and the National Fire Protection Association as NFPA96.

Systems are still designed today using a range of between 1500 to 1800 FPM and generally stay away from the higher limit of 2100 FPM . So, your question would probably be what good is a lower limit at 500 FPM? In fact, it is an excellent question. See Myth No. 2 for the answer.

Myth No. 2: Variable-Speed Fans Are Not for Commercial Kitchens
Engineers normally designed exhaust fans to operate at full speed. Some systems included a dual fan speed that would save a little energy when or if used. This would be the case if the cook ever gave it a thought. And indeed most cooks who came into their kitchen early in the morning, would turn on all their cooking appliances so as to be hot no matter if they were needed at 5 a.m. or 3 p.m.

Then, an engineer came along who challenged the idea of the exhaust fan running full speed all day long. He reasoned that, no matter how large or small the operation, a kitchen’s cooking loads vary significantly during the day. His idea was to develop a system that did not depend on the cook to flip the switch but to automatically vary the speed of the fan by reading the effluent and heat radiating off the cooking equipment.

Variable fan speed studies done testing existing kitchen operations were very informative. Some of the studies, especially with large 24-hour hotel kitchens, proved that seldom, if ever, did the variable-speed fans operate at full speed. More often than not, the peak fan speeds came in very short cycles while the fans tended to operate in the 50-percent to 75-percent range most of the time. Those studies began to demonstrate that we designers were probably oversizing many hood systems and ultimately contributing to a waste of energy.

The answer relating to Myth No. 1 is that the lower FPM allowed for the variable-speed fan to be employed in the design. Once it was proved that commercial kitchen exhaust systems could successfully operate at lower speeds than 1500 FPM , it gave engineers an answer to a perplexing problem involving existing exhaust duct installations. That being the knowledge that if the ducts in a kitchen exhaust system proved too small to serve a larger-sized hood, by being able to calculate FPM needs using the lower velocities available, it became a possibility to expand an operation without needing to tear out the old duct system. That is usually next to impossible when the duct shaft passes up through multiple spaces in floors above the kitchen area.

This possibility may not always be practical, but it can now be explored.

Myth No. 3: The Hood Can’t Be Improved
Wrong! Busting myths one and two, gives engineers the opportunity to design exhaust hoods to operate much more efficiently than ever thought possible in the past.

Myth No. 4: More Is Better
The rule of thumb was to design using the highest CFM rating for the type of cooking equipment being used, then for good measure add 10 percent to ensure the exhaust hood system would properly exhaust all the heat and noxious fumes. Again, the ASHRAE TC5.10 committee went to work and challenged our long-held assumptions. They questioned all kinds of things including placement of equipment under the hood. Would the location of devices make a difference in how much CFM was needed?

All these practices and ideas and more have been gradually introduced into the mainstream of design, codes and standards over the last decade. They are making a difference. That stainless-steel hood is looking a little green if you look at it from the right angle.