Patent Application
20110207060
1. Field of the Invention
This invention relates to a fluidized bed furnace with a fluidizing chamber containing a coarse layer of unfluidizable material located beneath a fine layer of fluidizable material and no distributor to receive a premixed air/gas mixture. Preferably, the fluidized bed is a heat treating furnace.
2. Description of the Prior Art
Fluidized beds have been used for various purposes for many years for many uses including chemical processing and treating waste. Fluidized beds have been used for heat treating wires using a mixture of air and natural gas which is ignited in the bed to provide heat. Previous designs use two types distributor. One type of distributor is a porous tile located on top of an air plenum to allow air to enter the bed and a perforated pipe sitting above the distributor feeds natural gas premixed with some air into the fluidized bed, the total air being sufficient to burn the natural gas. The perforated pipe is usually buried in a layer of coarse sand to protect the perforated pipe and the porous tile from heat generated in the hot fluidized sand above the coarse layer. It is also known to use perforated pipes or perforated metal plates as the distributor.
The porous ceramic tiles are cemented in a steel frame work using a high temperature cement, which is fairly brittle. If the tiles and the steel frame work get too hot, the cement will crack and cooling air must be circulated through the support steel. In addition, an emergency blower that will keep air flowing in case of power failure must be installed. Both of these requirements are expensive. When the natural gas is introduced above the bed through perforated pipes, many of the advantages of the porous tile distributor are lost. Lateral mixing of air and gas in a fluidized bed is fairly poor, so this design does suffer from non-uniform combustions resulting in high emissions and low combustion efficiency. The porous tiles are expensive as is their installation. When perforated pipes are used as the distributor (with no porous tiles), a large number of perforated pipes are required and the pipes are made of stainless steel and are very costly and labour-intensive to produce. Small holes are drilled in the tubing at approximately one inch spacing.
Drawn steel wires can also be heat treated using a bath of molten lead to heat the wires to the annealing or stress-relieving temperature of around six hundred to one thousand degrees Celsius (600-1000° C.). The wires are submerged in the molten lead using a cast ceramic sinker while the wires are pulled through it at a fixed speed that varies with wire diameter. While this system is simple and effective, many jurisdictions have restricted or banned the use of lead due to its toxicity and environmental concerns.
It is an object of the present invention to provide a fluidized bed that does not have a distributor and passes a fluidizing gas directly through a coarse layer of unfluidizable material located beneath a fine layer of fluidizable material.
It is an object of the present invention to provide a fluidized bed heat treating furnace that does not have a distributor and passes an air/gas mixture directly through a coarse layer of unfluidizable material located beneath a fine layer of fluidizable material.
A fluidized bed comprising:
Preferably, the fluidized bed is a fluidized bed heat treating furnace and the fluidizing gas is a premixed air/gas mixture, the mixture being combustible, the fluidized bed having an igniter to ignite the air/gas mixture.
A method of operating a fluidized bed, said bed having a fluidizing chamber with a fluidizing gas inlet and a bottom thereof, the method comprising:
Preferably, the method of operating a fluidized bed comprises operating a fluidized bed heat treating furnace to heat objects and connecting the premixed air/gas inlet to a supply of combustible gas into a source of air, premixing the air and gas to form a premixed air/gas mixture that is combustible, locating objects to be heat treated within the chamber to be within the fluidizing zone when the bed is operating. The method comprises igniting the air/gas mixture as the mixture exits from the coarse layer and operating the fluidized bed at a temperature, flow rating duration to heat treat the objects.
In
A fine layer 16 of refractory particles of sufficiently small size so as to be fluidized when the bed 2 is operating is located immediately above the coarse layer 14. A plurality of wires 18 to be heat treated are mounted within the fluidizing chamber 4 at a height that is above the fine layer 16 when the bed is not operating, but is within a fluidized zone and is contacted by the particles of the fine layer 16 when the bed is fluidized. An interior surface of the side walls 10 is coated with a heat resistant material 20. The top 12 has vents to exhaust gases from a fuel/air mixture (not shown in
In
Depending on the size of the fluidized bed heat treating furnace, the number of premixed air/gas inlets can range from one to several inlets to insure that the premixed air/gas mixture is evenly distributed when it exits the coarse layer 14. The furnace 2 does not have a distributor and therefore has several advantages over previous fluidized bed heat treating furnaces. In the furnace of the present invention, no cooling air is required and there is no distributor or perforated pipes or tubes. Since no cooling air is required and the pressure drop across the two layers 14, 16 is much lower than previous furnaces, a smaller blower size can be used for the same furnace area of the present invention than for previous furnaces. This results in lower energy consumption, a higher combustion efficiency and fewer contaminants being exhausted through the vents 34 as more complete burning is achieved by the furnace of the present invention than is achieved by previous furnaces. Also, the explosion hazards that can be present with previous furnaces are eliminated by the furnace of the present invention. Further, the furnace of the present invention is much less expensive to construct and operate than previous furnaces. Maintenance costs of the furnaces of the present invention is much less than the maintenance costs of previous fluidized bed heat treating furnaces because no distributor is present.
The size of the particles in the coarse layer is chosen to be less than the quenching distance of the fuel. When the fuel is natural gas and air, the quenching distance is three millimetres (3 mm). The spacing between the particles of the coarse layer is therefore less than three millimetres (3 mm) so that no combustion will occur within the coarse layer. Combustion occurs in the fine layer, which is fluidized. Preferably, the coarse layer and fine layer are particles of sand. The coarse layer has sand particles that are sufficiently large not to be fluidized at the flow rates used to fluidized the finer sand located above the coarse layer. The air/gas mixture entering the coarse sand distributes uniformly in the lateral direction due to the low flow resistance of the coarse grit sand coupled with the increasing flow area available as radial distance from the inlet increases. By the time the air/gas mixture reaches the fine fluidizing sand, it is uniformly distributed and at an even pressure across the entire coarse grit sand layer.
Preferably, the coarse grit sand of the coarse layer is the same material as the fine fluidizing sand of the fine layer so that the coarse layer can withstand the same temperatures as the fine layer and does not need to be protected by heat in any way. While the coarse layer and fine layer preferably consist of the same material, the particles of the coarse layer, of necessity, are larger than the particles of the fine layer. It is not essential that the coarse grit sand be the same material as the fine fluidizing sand. For example, the coarse grit layer can be gravel. Experiments have shown that even with the fine fluidized sand at a thousand degrees Celsius (1000° C.), the coarse grit sand one inch below the coarse/fine sand interface is below one hundred degrees Celsius (100° C.) and cannot support combustion.
The heat treating furnace of the present invention can accommodate a number of wires to be heat treated (for example twenty to fifty) running in parallel approximately twenty-five to forty millimetres (25-40 mm) apart. The fluidized bed of the present invention can be operated between six hundred degrees Celsius (600° C.) and one thousand degrees Celsius (1000° C.) and range in length from around fifteen (15) feet long to over eighty (80) feet long depending on wire speed. The bed is typically around three (3) feet wide but the width varies with the number of wires and the space in between the wires. The wires are preferably drawn through the fluidizing zone and extend through suitable openings (not shown) in end walls 36 of the chamber 4 on a continuous basis using pulleys located outside the chamber. The wires are pulled through the fluidizing chamber at a height to be within the fluidized portion of the fine layer particles when the bed is operating. Preferably, the heat resistant material 20 is a steel alloy.
| Number | Date | Country | |
|---|---|---|---|
| 61305821 | Feb 2010 | US |
