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issue: August 2009 APPLIANCE Magazine

Appliance Engineer - Cooking Efficiency
Improving Range-Top Efficiency with Specialized Vessels


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Greg Sorensen and David Zabrowski, Fisher-Nickel Inc.

Laboratory testing of two cooking vessel prototypes shows an increase in range-top cooking efficiency.

Figure 1. A standard 24-qt aluminum stockpot.

Range tops are one of the most widely used pieces of cooking equipment in commercial kitchens, but gas-fired models are notoriously inefficient. Typical energy efficiencies are 25–30% for standard gas range tops and 30–40% for high-efficiency gas range tops. While the range top may be used for a variety of tasks, the cooking process is always the same—the appliance provides heat to a cooking vessel from below.

There has been an enormous effort to increase the efficiency of gas range tops, but one major obstacle has always been the simplicity of the appliance itself. Unlike other cooking appliances, one cannot simply add insulation or upgrade the control technology to improve energy efficiency. Numerous designs have incorporated advanced burner types such as powered burners, sealed burners, and infrared burners, but none have achieved widespread success.

While many efforts to improve energy efficiency have focused on the appliance itself, gas-fired range-top cooking typically also involves another key element—cookware. The poor heat transfer from the flame to the cookware is mainly responsible for the inefficiency of gas cooking. The heat transfer is mainly convection heat transfer with some radiation heat transfer. The convection heat transfer coefficient is small, about an order of magnitude smaller than the thermal conduction coefficient in the cookware itself and in the typical liquid content inside the cookware. As a result, heat generated in the flame is not efficiently transferred to the pot before the flame escapes out to the side of the cookware due to buoyancy.

Possible solutions to improve the heat transfer from the flame to the cookware include the following:

  • Roughening the surface to reduce the thickness of the boundary layer, increasing the convection heat transfer coefficient.
  • Coating the surface of the cookware with an infrared-absorbing layer to improve the radiation heat transfer.
  • Increasing the cookware surface area to improve the convection heat transfer.

Eneron Inc. has developed two unique pot designs that use aluminum fins to increase the surface area exposed to the burner flame, thereby maximizing the amount of heat transferred to the bottom of the pot. While this is not the first time a company has looked at the vessel as a variable, other research efforts focused on pot size and thickness, rather than advanced pot design.



Figure 2. Prototype design 1.

Testing and Methodology

Recently, the new stockpot designs were tested at the PG&E Food Service Technology Center located in San Ramon, CA, U.S. All findings were then translated into FSTC Report 5011.08.12, Eneron Inc. Prototype Commercial Stock Pot Testing, May 2008.

The engineers at the Food Service Technology Center documented cooking-energy efficiency and production capacity of three different range tops when using two prototype 24-qt stockpot designs. These results were compared with baseline numbers obtained using a standard 24-qt aluminum stockpot. The testing followed the controlled conditions outlined in the American Society for Testing and Materials (ASTM) F1521 Standard Test Method for Performance Range Tops.

The pot used for baseline testing was a 12-in.-diameter, aluminum stockpot with a capacity of 24 quarts. It was purchased off-the-shelf from a restaurant supply house and represented a typical cooking vessel designed for use on a commercial range top.

The first prototype stockpot was the same model as the standard pot, with the bottom modified to include 1⁄16-in.-wide aluminum fins. The fins were 5⁄8-in. high and spaced 1⁄8-in. apart. These parallel fins form heat exchange channels. The second prototype pot was designed with 1⁄8-in.-wide perpendicular grooves in the fins, producing a bottom of small rectangular fins.

Three ranges were used during testing. Range A and Range B employed an open ring-shaped burner, rated at 30,000 Btu/hr per unit. Range C employed a star-shaped burner, rated at 20,000 Btu/hr per unit. A ring burner emits flame in a concentric circular pattern, and a star burner emits flame in a traditional star pattern. While both ring burners were rated at 30,000 Btu/hr, they were slightly differing in design and produced unique flame patterns.

Three test runs were performed for each stockpot on each of the three range tops. The cooking-energy efficiency and production capacity were determined by heating 20 lb of water from 70° to 200°F. The results depicted in Table I are the average of three individual test runs for each pot-range combination.

For each range, the testing results showed a substantial improvement for the prototype stockpots over the standard stockpot design.


Figure 3. Prototype design 2.

Range C was rated at 20,000 Btu/hr per burner and, therefore, was expected to have longer heat-up times than the two 30,000 Btu/hr burner ranges (Range A and Range B). However, the results from Range C were the most impressive out of the three appliances, easily showing the largest improvements over the baseline levels with the prototype stockpots. In fact, Range C exhibited quicker heat-up times with the prototype stockpots than Range A, even though Range C had 30% less horsepower than Range A.

The next test conducted was a simmer energy-rate test to determine whether there are potential energy savings when using the prototype stockpots to simmer liquid at approximately 205°F (96°C) on Range C. To conduct the test, each stockpot was filled with 20 lb of water and placed on a burner. After raising the water temperature to 205°F, the lid was removed from the pot and the burner was adjusted to hold the water temperature at 205° ± 1°F. Time and energy was monitored for 1 hour to allow calculation of the simmer energy rate.

Both of the prototype stockpots allowed the burner to operate about 3100 Btu/hr lower than the standard pot while holding a steady simmer. Unlike the energy-efficiency tests, the results from the two prototype pot designs were nearly identical, presumably because of the much lower flame level on the burner. Table II represents the results of the simmer test.

Testing found that the pots’ cooking efficiency varies based on the appliance used. This is because the flame profile plays a role in coupling the flame into the heat exchange channels; it needs to allow the flame to get in the channels (i.e., in between the fins to be able to fully utilize the increased surface area). Certain flame patterns will fit better with certain fin patterns.

Comparing the two prototype designs, the continuous fin structure pot will have better mechanical strength than the model with short fins. The continuous fin structure also provides better lateral heat transfer and improves the heating uniformity across the bottom of the cookware.

 


Pot under test.

Conclusion

The proposed stockpot designs proved to be a remarkably effective method of increasing range-top performance. Heat-up times were substantially reduced and production capacities increased. Energy performance was also significantly improved. By simply using an advanced pot design, the 25–30% energy efficiency of a standard, gas-fired range top was raised to more than 40%. When used on a range top with energy efficiency in the low 30s, the number approached 60%. 

The benefits were not limited to full-input operation, as shown by the simmer tests. Less energy was required to maintain temperature, enabling a further increase in savings. While this test was done on stockpots, the same concept can be implemented on other types of cooking vessels, such as a griddle plate, for more-efficient gas-fired range-top cooking.

This is an edited version of FSTC Report 5011.08.12, Food Service Technology Center, May 2008.

 

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