Mmmmmm, roast pork. Mmmmmmmmmm, apple pie. No longer will you have to eat out or spend hours in the kitchen for a great meal. Engineers at Enersyst Development Center (Dallas, TX), in collaboration with those from appliance manufacturer Thermador (Huntington Beach, CA), are bringing fast, high-quality, cooking into your kitchen.
Look at these cooking times of the new JetDirect(TM) oven compared with a conventional convection oven:
Pork & potatoes 30 mins. 60 mins.
Vegetable medley 7:30 25
Bundt cake 20 40
The JetDirect combines the best of two worlds--the speed of a microwave with the browning capability of Enersyst's air impingement oven. This technology is not new. Restaurants, such as Pizza Hut, Dominos and Red Lobster have been using Enersyst's impingement ovens for the last 20 years. But the combination of impingement and microwave has not been available for home use.
To help tailor their technology for residential use, Enersyst partnered with Thermador in 1997. "The JetDirect oven for us is a continuation of something we introduced to the market 25 years ago--combination cooking," says Alan Leukhardt, vice president of Thermador.
Engineers from both companies worked together to overcome challenges such as:
- Air flow
- Size constraints
- Heat Transfer
- Power limitations
- Combining technologies.
Impingement System: The principle of cooking lies in the heat transfer rate, the "h" factor--or the speed that heat moves from one point to another. The air impingement process heats the surface of food through extremely hot, directed jets of air. The 450F degree blasts speed the rate of surface heat transfer by 60% over that of a conventional oven. Air impingement sweeps away the cooling stagnant layers of air created as water evaporates from the food.
Food cooks evenly in this type of oven when moved through the "jet stream" on either a turntable or conveyor belt. "If you take a piece of bread, place it in an impingement oven and keep it stationary, you would see toasted spots in the bread where the jets of air hit," says Michael Dobie, vice president of engineering for Enersyst. But because of size constraints, designers can't put conveyor belts in residential units. And market surveys show that people don't like turntables in their "conventional" ovens. "They want to be able to use their standard cooking utensils such as rectangular, metal cooking pans for their cookies and cakes," he points out.
Since they couldn't move the food, engineers moved the air. Jay Dougherty, project engineer, designed a "stirrer" that literally mixes both the microwave field and the jet air stream.
"We tried several iterations before we got the design right," says Dobie. Because of space constraints, Enersyst engineers had to design one stirrer that moved both the microwave field as well as the hot air. "If a stirrer affected the microwaves, it did nothing for the hot air and visa versa."
Finally, Dougherty, designed a patented rotating, tapered stirrer that met performance objectives. Dougherty drew on his prior knowledge of air vectoring and mathematical modeling and developed his 3D model using Vellum Solids from Ashlar (Santa Clara, CA).
Another requirement for home use was that no moving parts could be exposed. "When I thought about this for home use, I knew I couldn't have any exposed blades, motors, and blowers, so a child could stick his or her hands inside and not get hurt," Dougherty says.
But moving columns of air without seeing the parts became a materials problem: the stirrer, motor, and blowers had to be hidden, while the microwaves and the hot air generated had to reach the food at specified angles. Dougherty and team decided on a ceramic material to form the jets of air. Not only can ceramics handle high temperatures, but this particular ceramic allows microwaves to pass through to the food.
Size considerations. To accommodate home cooking needs, individual jets had to be re-engineered for a 30-inch cavity. The Enersyst restaurant ovens had a cooking chamber of no more than 8 inches in height. "Ninety percent of the foods we eat are cooked within a 4- to 8-inch range," says Dobie. "However, for the occasional Thanksgiving turkey, people like to have the option of having more room."
Expansion of the oven cavity meant redistributing the columns of air and microwaves without affecting performance, says Thermador's Leukhardt. "We had to project air streams further to reach the food," he continued. Because impingement ovens have air jets above and below the oven rack, engineers found adjusting the spacing and the size of the top air holes did the trick. The bottom jets, on the other hand, weren't a problem. Everything at home is typically cooked in a pan, which automatically distributes the heat. Bottom jets could therefore be less powerful than the top, with smaller air holes, and a tighter pattern.
While the cooking chamber had to be larger than commercial units, the oven's outside dimensions had to be downsized. "Squeezing all the equipment from a commercial model in to a compact residential model was tough," says Dobie.
This required rearranging components and cutting the size of the blower. Instead of one large device, Enersyst used two smaller blowers stacked on top of each other. These steps produced the added benefit of reducing duct work.
A smaller size required better insulating materials as well. "Because we didn't have as much room for insulation, we had to use a better quality insulator," says Dobie. Their choice: ceramics.
Self Cleaning/Materials. Enersyst's original proof-of-concept oven did not have a prototype self-cleaning mode. Thermador required this feature because of the high demand in the consumer market. "This was probably our biggest challenge," says Dobie. 'We relied on Thermador for its self-cleaning and mass production know-how.'
The first change was in the material used to line the oven cavity. Enersyst's commercial models are made of stainless steel. But stainless steel darkens after repeated self-cleaning cycles. The new models are made from low-carbon-cold rolled steel with porcelain coating.
Fixing one problem led to another challenge. The cavity walls of Enersyst's commercial ovens have sharp edges. In order to use porcelain coating, "we couldn't have any sharp corners," says Dobie. "Essentially, you are melting glass directly on the cavity and we couldn't have it warp." So they used curved corners instead.
Nor could they use any low temperature bearings. Traditionally, Teflon was used for restaurant commercial ovens where temperatures only reach 550F. Now materials had to withstand temperatures of 900F for a two- to four-hour period. They used high temperature grease between ball bearings and interior driven parts.
Certain components had to be respecified to withstand the higher temperatures. For example, the blower wheel would deform at high speeds, so a stronger blower was used, although the blower motor was acceptable, says Leukhardt.
The fan in the cooling system also had to be redesigned. This promotes flow of air through the door. Hot-air exhaust with cool air is forced through the bottom of the unit. Engineers used a reverse-flow cooling system, says Leukhardt. "This keeps warm air out of users' faces and away from services that people would touch."
The self-cleaning requirement created a problem for the microwave as well. A traditional microwave is manufactured to tight tolerances, he says, and those tolerances must be maintained before and after self-cleaning.
The temperature requirement limited the materials that could be used. Forget low-temperature plastics and Teflon. Instead, engineers used high-temperature plastics and resins. They also replaced metal surfaces with dielectric materials.
Controls. The electronics controls developed by Thermador in conjunction with Digital Appliance Controls (Elgin, IL) specified the user interface.
The panel had to reflect how the end user would interact with the oven in terms of audio and visual tactile performance, as well as manage the power control for the oven. Physical requirements of control include such items as dimensions, color, layout, graphic design, mechanical size, and power relay boards that relay heat to the oven.
These had to be easy for consumers to use and understand, "which was about as tough as making it cook quickly,' jokes Dobie.
The oven cooks food so quickly that engineers needed a control that was user-friendly, says Leukhardt. Thermador engineers developed a controller, Cook-Smart(TM), with pre-programmed modes to optimize performance for the user. It defines various food groups that have common cooking temperatures and microwave power levels. The controller converts radiant cooking time to the equivalent time for a jet oven.
For example, the user selects the Cook-Smart mode for cookies and inputs the normal cooking time of 12 minutes. The new oven automatically converts to JetDirect times.
Two at once. The JetDirect will be part of a two-oven unit, with a standard convection oven as the lower half. As Enersyst took the lead on the JetDirect, Thermador simultaneously developed its convection platform. This meant an integration of both ovens from a mechanical and structural standpoint. "We needed to fasten parts together. We need to integrate the two cooling systems, the insulation systems, and the electronic controls including wiring," says Leukhardt.
This required constant communication between the two companies. In addition to weekly conference calls to discuss plans, and informal calls in between, Thermador employed a new EDM/PDM/CAD system: Pro/ENGINEER from Parametric Technologies (Waltham, MA). "We set Enersyst up with the same system," says Leukhardt, "and provided them with a design station of Pro/ENGINEER. Through electronic transfer of files, we sent them our basic design information and they sent us back integration data. We had daily updates."
"It's amazing what you can do with Pro/ENGINEER," says Dobie. "If it fits on the screen, it'll fit in real life."
Look for the JetDirect ovens expected on the market by January 1999.
For more information
To speak with a company representative, call 1-800-828-6344, x 011 and key in the specific Product Code below
Pro/ENGINEER from PTC (C): Product Code 4439
Sens-A-Touch(TM) from TouchSensor Technology (E): Product Code 4440
Digital Appliance Controls (E): Product Code 4441
Pilkington Seraphic (E): Product Code 4442
January 1995: Decided to develop a microwave/air impingement oven for the home
March 1995: Began designing a 24-inch prototype
February 1997: Entered into agreement with Thermador
February 1997 to August 1997: Tested proof-of-concept oven
June 1997: Developed specifications for JetDirect oven
June 97 to August 97: Initial design
July 1997: Began design in Pro/ENGINEER.
September 1997: Began construction on prototype
December 1997: Intermediate design review, discussed hardware
January 1998: First prototype; began tooling for long lead time items
February 1998: Tested UL needed laboratory approval
March 1998: Prototypes #2,3,4 were built. These are presently being used for agency approval.
April 1998: Debuted at the Kitchen and Bath show (Chicago)
December 1998: Target production startup
Partners in design
- TouchSensor Technologies, LLC Component worked on: patented, proprietary, Sens-A-Touch(TM) electronic glass touch system.
- Digital Appliance Controls Component worked on: electronic clock and oven control system including display head, power relay control boards and microwave triac power control board.
- Pilkington Seraphic Component worked on: curved, tempered glass control panels and door panels.
- Food behavior
- Impingement system:
--No moving parts
--Heat transfer management
- Size Constraints
--High temperature allowances
- Smart energy utilization
- Power management