IGNITERS/BURNERS: Riding the Jet Stream
More than 10 years ago, the late physicist/inventor Niels Young was on a quest to create a better backpacking stove and demonstrated the principle of generating a jet of pressurized vapor from un-pressurized liquid by means of a heated, constrained wick. His son, Thomas Young, continued his father's work by developing a couple of innovative ceramic materials processes and created the first integral capillary force vaporizer (CFV).
Today, Vapore, Inc., Richmond, Calif. has developed the Vapore-Jet™, a small, ceramic device the size of a breath mint that generates a jet of pressurized vapor from un-pressurized liquid. This technology combines two common principles of nature - capillary force and phase transition - into a single device without the use of aerosol cans, pumps, moving parts or bulky mechanisms. The Vapore-Jet can be used in applications ranging from combustion appliances, including stoves and portable heaters to semiconductor manufacturing.
Nature walkTo better understand CFV, it is vital to comprehend the forces of nature that underlie the concept.
Capillary forces lift water into the crown of a tree and spread liquid through a sponge. Created from the interaction of liquids and solids, the attraction of a liquid to itself (cohesion) and to the surface it contacts (adhesion). The finer the pore size, the greater the force.
Phase transition is the energetic conversion to matter from one stable state to another. Heat is absorbed during the transition while the temperature stays the same. The phase transition from liquid to vapor entails a hundred-fold-plus expansion in volume.
Inside the Vapore-JetVapore's CFV technology is made up of three layers, bound by a peripheral glaze. The top layer is solid, with a central orifice and an undersurface consisting of alternating fins and channels. The middle layer has high porosity, with very small pores and the bottom layer consists of a more traditional, larger-pored material.
The bottom layer is in contact with a liquid, which could be a fuel such as naptha, gasoline, kerosene and alcohols. The liquid spreads through the device by wicking action.
Heat applied to the top surface conducts down the undersurface fins, including vaporization at the middle layer, with a volumetric expansion from liquid to vapor. Due to the layered porous construction and peripheral glaze seal, this expansion creates gas pressure in the undersurface channels of the top layer. All channels lead to the central orifice, where the gas escapes with force. The result is a stable equilibrium of heat flux, liquid flow, gas pressure and evolved gas.
ApplicationsThe first commercial application that uses Vapore's technology is the Cap Stove, a backpacking stove by Mountain Safety Research, Inc., Seattle, Wa. Robert Lerner, CEO of Vapore, explains the advantage of using CFV in this appliance. "The new Cap Stove combines the convenience and form factor of canister stoves with the reliability of a liquid fuel version, with the advantage of steady output until the last drop of fuel is consumed."
Lerner says that portable kerosene heaters will be the next commercial introduction. "We're also working closely with customers for medical device applications," he says.
Many medical applications involve the delivery of fluids where aerosol is preferred, ranging from humidification to pulmonary drug delivery. CFV technology is compatible with a wide range of medical materials and Lerner says it shows promise as a low-cost, high-flow solution.
There is also the possibility of using CFV technology with liquid fuels for an oil-burning furnace with the advantages being reduced emissions and efficient low-flow performance.
"The CFV makes liquid fuel behave more like natural gas," Lerner says. "CFVs perform exceptionally well at low-flow rates, in sharp contrast to traditional fuel oil burners involving mechanical systems and combustion chambers."
Researchers at Brookhaven National Laboratory in Upton, N.Y. are researching ways to achieve very low emissions of Nitrogen Oxides (NOx) from oil-fired burners in residential boilers and furnaces. C.R. Krishna, a scientist at Brookhaven says that one of the approaches that he has tested with CFVs has been used mainly with natural gas fuel and consists of burning the fuel in a porous ceramic matrix. If carried out successfully, this approach can lead to very low NOx emissions.
"In order to use the porous matrix burner approach to burning oil, it would be preferable to vaporize the oil before introducing it into the burner," Krishna explains. "One of the methods of doing this that was suggested to us was to use the capillary force vaporizer. We have been working with the system during the last few months. We have tried different burner configurations in terms of the pore sizes and thickness of the porous plates. We have been able to burn the vaporized fuel, but we have not yet optimized it for low emissions. Also, the fuel vaporizing rate is quite small. We feel we have to do quite a bit more work to meet some of our objectives like low NOx."
Still, Lerner is optimistic about the future of the capillary force vaporizer in furnaces. "CFVs will excel in energy-efficient homes, where it may be preferable for the furnace to run at low output vs. cycling off-one and for zone heating."