Fresh Approaches To Moving Air
By: Mike Sherer
The latest research and equipment improvements in commercial kitchen ventilation mean better performance and energy savings for your stores.
Ever been in a room with ASHRAE members on a day when commercial kitchen ventilation was the topic? You might've heard a lot of technical talk about airflow, DOA units versus MAUs, adjustable enthalpy ranges, pressure drops, extraction rates, supply plenums and so on.
Complicated topics for the rest of us, perhaps, but thanks to ASHRAE prosthat is, members of the American Society of Heating, Refrigerating and Air-Conditioning Engineersand their discussions in recent years, research in different areas of ventilation has accelerated rapidly. And research is leading to better, more energy-efficient ways to move cooking effluent.
Commercial kitchen ventilation serves a number of purposes. Proper CKV removes grease and excess heat from the kitchen and reduces the chance of a fire outbreak. A well-ventilated kitchen helps keep employees comfortable and more productive. And CKV prevents grease, smoke and odors from reaching the dining room.
CKV systems tend to be designed around the hood and how cooking effluent is going to be exhausted. Mechanical and fire codes focus on moving enough air up through the hood so that heat and effluent from cooking are exhausted and fires are prevented. Moving all that air, however, comes at a high cost to operate fans and supply replacement air.
In the past several years, however, engineers have looked at all the components of ventilation systems to see where they can make them more effective and energy efficient at the same time. Since we last looked at indoor air quality and exhaust ventilation, progress has been made on a lot of fronts. Here's what's been happening.
Test Method Leads To Improved Design
On average restaurants consume five times the energy of other retail businesses, and HVAC accounts for anywhere from 30% to 50% of a restaurant's energy bill. Of the total HVAC load, typically 75% is kitchen ventilation.
With so much design focus on how to exhaust air out of a kitchen, it's perhaps surprising that manufacturers haven't paid more attention to hood design. It's easy to build a box over the kitchen equipment, but building one that effectively traps air isn't always as easy as it looks. And after all, capture and containment are what those boxes are supposed to be all about.
Thanks to cool technology like Schlieren photography and the tireless efforts of many in the industry like the CKV Laboratory in Wood Dale, Ill., (managed and operated by Architectural Energy Corp. and the PG&E Food Service Technology Center), an ASTM standardized test method to measure hood capture and containment was developed about five years ago. (It's called "F1704-05 Standard Test Method for Capture and Containment Performance of Commercial Kitchen Exhaust Ventilation Systems.")
The test method gives manufacturers a way to see what effect hood design and airflow have on the capture and containment of cooking effluent. That led to tweaks in design as simple as adding side panels that make hoods more effective.
The way in which makeup air is introduced to the hood also affects the cooking plume, and manufacturers have worked hard to develop methods that augment rather than disrupt capture and containment.
For example, one new development is a hood with a curved top inside. Eschewing the traditional flat surface that can create effluent turbulence, this hood relies on the curved top to speed airflow to the exhaust duct at a higher rate than the updraft velocity of the cooking plume. The result is better capture and containment at a lower cfm, says the manufacturer.
Even more exciting is news that ASHRAE has funded research to design protocols for testing HVAC performance in the field. Since 75% of the cost of operating HVAC systems comes from the kitchen, research teams are starting there, looking at ways of identifying instruments and a visual method to measure hood capture and containment, supply of replacement air and other variables.
Ultimately, the ASHRAE committee's goal is to develop a field test protocol that will demonstrate to code officials a properly designed hood's ability to completely capture and contain cooking effluent at a reduced cfm. And that will ultimately help thousands of operators save money.
What About Grease Removal?
Until recently, there was no way to know how much grease a particular type of filter removed from the exhaust on the way up the stack. That all changed when the ASTM test method"F2519-05 Standard Test Method for Grease Particle Capture Efficiency of Commercial Kitchen Filters and Extractors"was developed by the PG&E Food Service Technology Center with help from researchers at the University of Minnesota. The method now helps filter makers determine with certainty how efficient filters are and for what size particles.
Most mechanical filtersbaffles, etc.remove most particles 10 microns or larger in size, and do a pretty decent job on grease particles 5 microns or larger. Cooking effluent, of course, is made up of grease and smoke particles of all sizes, and when it comes to grease extraction, you have to sweat the small stuff.
The Bay Area Air Quality Management District in San Francisco, for example, classifies particulate emissions in three categories: coarse (dust and so forth), PM 10 (particles 10 microns or smaller), and PM 2.5 (particles 2.5 microns and smaller). Commercial cooking, according to BAAQMD, accounts for about 7% of the PM 10 in San Francisco.
Grease vapor and particles in the 2.5- to 5-micron size are tougher to extract, but with the new ASTM test method, manufacturers now have a way to determine whether their filters are up to the challenge. The test method also assures that you're comparing apples to apples when manufacturers present you with filter efficiency numbers.
As a result, filters now are more competitive, and they've gotten better at removing grease. New baffle designs have made mechanical filters more effective, and when coupled with other filters or grease removal techniques, efficiency has risen significantly.
Some New Approaches
For example, a couple of manufacturers now use a mechanical filter design that spins exhaust air in a vortex. The faster the grease-laden air spins, the more centrifugal force is generated to fling grease particles out of the airstream and onto the baffles. One claims to be 95% efficient at removing grease particles 8 microns or larger. The other is 60% efficient with particles 5 microns or larger, but is 100% efficient with particles 9 microns or larger when a secondary filter is used in combination with the mechanical filter (more on those in a bit).
Another manufacturer recently released a filter using a similar extraction technique. It speeds up the exhaust and crashes the grease-laden vapor onto a horizontal surface. The collected grease and particulate matter move off the surface into a static pressure zone and flow down into the grease gutter. The maker claims efficiency of 89% down to 0.3 microns and 99% for particles between 5 and 10 microns with a very low static pressure drop.
Multi-stage filters are improving grease extraction efficiency, too. A couple of makers use a packed ceramic bead bed secondary filter to capture smaller particles. The filter combinations extract 95% of grease particles above 4 microns in size in one case, and 80% of particles 1 micron or larger in another. The new designs also don't cause as large a drop in static pressure.
A growing trend is the use of ultraviolet light in hoods to remove small particles of grease after a mechanical filter has extracted most of the big stuff. While these systems have been around for several years, they're proven technology, and relatively easy maintenance is making them a more popular choice.
In Demand: Demand Ventilation
The big news in the past few years has been the growth in demand ventilation. Twenty years ago, designing and calibrating a kitchen ventilation system was practically the only way to get the best performance at the least energy cost. Sizing the hood, fans and ductwork properly is still important, but improvements in fan motors and controls have not only made demand ventilation possible, they've become essential to offset higher energy costs.
Variable speed fans and fan controls were cutting edge technology 20 years ago. Now, several manufacturers offer hoods with infrared, heat and visual smoke sensors built in to determine if kitchen equipment is actually in use. Sophisticated computer controls then adjust exhaust fan speed accordingly. Systems can save up to 50% in energy costs to run the exhaust fan.
Even better, when you reduce the exhaust cfm, you don't need as much makeup air, so controls automatically adjust fan speed of makeup air units. You save energy costs to run the fan as well as heat or cool the makeup air.
In 2006, officials in many areas started enforcing an Int'l. Mechanical Code requirement calling for an interlock between kitchen equipment with exhaust fans. That means exhaust fans go on anytime kitchen equipment under the hood is on. If you have a demand ventilation system, in many cases you can convince inspectors that fans don't have to operate at full speed all the time, such as when equipment is first turned on or is idling. When ASHRAE's research on the field test protocol for capture and containment is complete, you'll be able to show code officials that often you can reduce the cfm of your kitchen ventilation safely even when you're cooking.
New in demand ventilation are remote communications features that let you monitor the system, verify energy savings and make adjustments. The feature shows you the temperatures at which pieces of equipment are operating, alerting you, for example, to an oven someone forgot to turn off that causes the hood exhaust fans to operate at higher speeds than necessary.
Making Up Isn't Hard To Do
Kitchen ventilation systems work on the principle that whatever goes outnamely, airmust come in. Exhaust air that goes up the hood somehow has to be replaced. Until recently, the argument went that makeup air should be delivered as close to the hood as possible, since that's where the air is going out of the restaurant.
The first corollary to that argument is that since most of the makeup air is supplied close to the hood, you don't have to temper it much. Often it's heated to prevent outside air from cooling food near the cooking line, but to save money, makeup air often isn't cooled.
The second corollary is that introducing makeup air close to the hood helps capture and contain cooking effluent.
Makeup air, however, has confounded designers and CKV installers forever, not to mention operators who have to deal with the systems once they're up and running. Too often, makeup air creates an uncomfortable environment for kitchen employees and impedes rather than improves hood capture and containment.
A few people in the industry now are looking at what some call a radical approach, yet one that's completely logicaleliminating "makeup" air entirely. Rather, they want you to think of "replacing" air that's exhausted through the hood.
Most restaurants have a DX roof-top unit that supplies heat and cooling to the building. The kitchen may or may not have a separate RTU, but in most cases it doesn't. Air exhausted through the hood is replaced in part by air from the dining room and the rest by makeup air units, or MAUs.
The kitchen environment, in other words, depends in large part on the restaurant's roof-top unit. As noted above, the MAU's primary purpose is to replace air going up the hood, not to make the kitchen environment more comfortable. So, most restaurants end up with hot and uncomfortable kitchens and CKV systems that work too hard, push too much cfm and cost too much.
Some companies, though, are looking at the kitchen more like a factory environment than a room. As a factory the kitchen has different heat gains at different times of day and places different loads on the HVAC system.
A Completely New Approach
The radical thinking goes something like this: It's more difficult to remove moisture from air with a DX unit once that moisture is in a space than it is to condition outside air before it enters the space. Since the kitchen needs to replace air exhausted by the hood, why not replace that air with not just tempered, but conditioned air? After all, when the kitchen is working at full tilt, the heat output from the equipment will make the kitchen toasty, but humidity will make it uncomfortable.
Further, the thinking goes, if air in the kitchen is being replaced with 100% outside air that's already been conditioned, there's no need for an MAU feeding air directly to the hood, which might lower the cfm requirements of the hood. Better yet, if the replacement air is being introduced into the kitchen away from the hood at low velocity, then the hood will do a better job of capturing and containing effluent by itself, reducing the cfm requirements even more.
In other words, instead of costing more to condition outdoor air for the kitchen, such a system might actually save energy by eliminating the MAU and reducing the overall cfm in the kitchen.
Working with a major HVAC manufacturer, the radical thinkers built a "direct outdoor air" system and have tested it in a number of operations. In one study of two identical stores in a chain, the DOA system with exhaust-only back-shelf and canopy hoods (with no MAU) reduced total kitchen airflow by 46%.
Even better, the DOA system created a more comfortable environmentan average of 8° F cooler and less humid than the control unitwith 15% to 26% less energy consumption.
Improvements in technology have made a sophisticated kitchen ventilation system like this not only possible but cost competitive. The system senses changes in kitchen temperature and humidity and decides what to do to make the kitchen environment more comfortable. Ten years ago, the controls would have cost as much as the DOA unit. Now, cost is competitive with traditional kitchen ventilation packages given the energy savings, and installation cost is about the same.