INDUSTRIAL FURNACES

STEP GRATE TYPE STOKER FURNACE

Outline

The sewage sludge is supplied to the furnace after having been dried by the steam type drier. The combustion exhaust gas generated within the furnace is supplied to the waste heat boiler arranged at the upper part of the furnace, and is recovered as steam. It is intended to save the energy by utilizing the recovered steam for a drying heat source.

Capacity

-Self-retained combustion limit : Calorific heat of dehydrated cake 320 kcal/
kg og over
-Furnace outlet gas temperature : 900 ~1,100 deg. C
-Exhaust gas property (outlet of furnace) :
Dust (0.5 ~ 3.0g/ m3N) NOx 100 ppm or less
-Power consumption : 50~ 70 kWh/ t x cake
-Combustion capacity : 70 ~500t/ d (based on dehydrated)

Features

The waste heat during combustion is recovered by the boiler and is use for a drying heat source, etc. Therefore, it can be an energy-saving system. The stoker furnace of this type can be compatible with the high-calorie sludge simply by changing the waste heat boiler construction.
Since the incineration ash is in 0.5 ~ 50 mm semi-melting slag state, it is accompanied by little elusion of heavy metals, and can be disposed of without pollution, or utilized effectively.
The operation of the incineration furnace is fully automated within the range from sludge laying and ignition start-up to stopping of banked fire.

Structure

The step grate type stoker furnace is made up of step grate type hearth as shown in Fig. 1, in which the movable stage and fixed stage are arranged alternately. The sludge dried up to the moisture content of approx. 40% is supplied in the furnace from the hopper, and further supplied to the fixed stage downward by the operation of the movable stage. And it is moved downward sequentially within the furnace. The combustion air is supplied from the lower side of the fire bed, by which the air makes efficient contact with the sludge, to dry, ignite and burn the sludge. The sludge is subjected to self-sustained burning at the sludge combustion unit at the temperature of 1,000 ~ 1,200 deg. C and is discharged as half-melting slag-state incineration ash.

The steam recovered by the waste heat boiler is utilized for a drying heat source. When the sludge contains limited calorific heat, the steam is compensated by burning oil within the boiler, however, almost all sludges will not require any aux., fuel (oil).

Furnace Types Defined

-central warm-air furnace
-steam or hot-water system
-heat pump
-floor, wall, or pipeless furnace
-built-in electric units
-heating stove (which burns wood, coal or coke)
-room heater (which burns gas, oil or kerosene)
-fireplace
-portable heater.

Central Warm-Air Furnace: A type of space-heating equipment in which a central combustor or resistance unit--generally using gas, fuel oil, or electricity--provides warm air that circulates through ducts leading to the various rooms. Heat pumps are not included in this category. A forced-air furnace is one in which a fan is used to force the air through the ducts. In a gravity furnace, air is circulated by gravity, relying on the natural flow of warm air up and cold air down; the warm air rises through ducts and the cold air falls through ducts that return it to the furnace to be reheated, thus completing the circulation cycle.

Steam or Hot-Water System: Either of two types of a central space-heating system that supplies steam or hot water to radiators, convectors, or pipes. The more common type supplies either steam or hot water to conventional radiators, baseboard radiators, convectors, heating pipes embedded in the walls or ceilings, or heating coils or equipment that are part of a combined heating/ventilating or heating/air-conditioning system. The other type supplies radiant heat through pipes that carry hot water and are inlaid in a concrete slab floor.

Heat Pump (Reverse-Cycle System): A year-round heating and air-conditioning system in which refrigeration equipment supplies both heating and cooling through ducts leading to individual rooms. A heat pump generally consists of a compressor, both indoor and outdoor coils, and a thermostat.

Floor, Wall, or Pipeless Furnace: Space-heating equipment consisting of a ductless combustor or resistance unit, having an enclosed chamber where fuel is burned or where electrical-resistance heat is generated to warm the rooms of a building. A floor furnace is located below the floor and delivers heated air to the room immediately above or (if under a partition) to the room on each side. A wall furnace is installed in a partition or in an outside wall and delivers heated air to the rooms on one or both sides of the wall. A pipeless furnace is installed in a basement and delivers heated air through a large register in the floor of the room or hallway immediately above.

Furnaces

Furnaces are indispensable tools for solid state chemists. A variety of different kinds of furnaces exist, each with their own capabilities and limitations. This document discusses the two most common types found in a solid state or Inorganic chemistry laboratory.

Tube Furnaces

A tube furnace is designed to heat a tube that is usually 50 to 100 cm in length and from 25 to 100 mm in diameter. Samples are placed inside the tube in ceramic or metal boats using a long push rod. The tube is surrounded by heating elements which may also incorporate a thermocouple (a thermocouple can also be inserted down the tube if desired).

Pictured below are common types of tube furnaces. The first is rather large example of a typical "clamshell" design found on many Lindberg-Blue M furnaces. The two halves of the heating elements are encased in a hinged cover. This permits easy removal of the tube and also permits one to put samples into the furnace without using a tube. The second furnace is a Linn High Therm tube furnace with an attached controller. Notice that this furnace does not open like the clamshell one. The tubes are not shown in these pictures:

While these are shown horizontal, tube furnaces (particularly those that are not of clam-shell design) are often used in a vertical orientation. This permits the tube, furnace or a suspended sample to be slowly moved during the experiment, a technique often used for slow cooling or crystal growth. Tube furnaces are the furnaces of choice for crystal growth given their small volume and precision with which the temperature can be controlled.

Tube furnaces also have a significant advantage over other types of furnaces. The ends of the furnace tubes (which usually protrude 10 or more centimeters from each end of the furnace) do not get very hot and so a variety of different adapters may be placed on the ends. This permits one to perform a reaction under a controlled atmosphere (pure oxygen, nitrogen and CO/CO2 are typical examples). In a typical setup, gas flows in one end of the tube and then the gas (and any evolved gases) exits through a bubbler.

The Lindberg furnace shown above is a single zone furnace, meaning that it has only one set of heating elements and controller. For techniques such as vapor transport synthesis or chemical vapor deposition, a two or three zone furnace is often used. The Lindberg three zone furnace shown below and the Linn furnace shown above can be set to heat three different segments of the tube to three different temperatures simultaneously.

Furnace tubes can be made out of a variety of materials. Quartz is commonly used for temperatures below 1,200 degrees C, and alumina or yttria-stabilized zirconia (among others) can be used for higher temperatures.

It should be recognized that the temperature inside a tube furnace drops off rather quickly as one moves away from the center of the heating zone. If maintaining an exact temperature is important, the sample(s) should be placed as close as possible to the center of the zone (or thermocouple). One can also insert a calibrated thermocouple down the tube to generate a temperature vs. distance profile if exact temperature control is critical.

Box furnaces

Box furnaces are convenient furnaces to use. As the name implies, the furnace has a box shape and a box-shaped interior. The interior dimensions (10 to 50 cm per side) are much larger than a tube furnace, but the unit is often compact enough to sit on a benchtop. A door on the side permits quick and easy access to the interior. Several examples of box furnaces and their controllers are shown below.

Box furnaces are very handy when large or many samples need to be prepared. It is much easier to add or remove samples from a hot box furnace than a hot tube furnace.

Box furnaces are not often used for crystal growth because the convective currents can cause the temperature inside to fluctuate slightly. In addition, maintaining a controlled atmosphere in a box furnace is usually not feasible.

Box or tube?

No matter what kind of furnace one uses, it is important to keep materials from spilling or depositing on the walls of the furnace. In tube furnaces, this is not a problem unless a tube is not used (usually a bad idea). If material spills on the furnace refractory material or heating elements, these can burn out prematurely and are expensive to replace. In addition, certain compounds and elements may undergo vapor transport reactions between the walls of the furnace and the samples (often with disastrous consequences).

Furnace controllers

Box and tube furnaces are available starting at about $1,000 for a small unit and non-programmable controller. Prices can go much higher as one adds multiple zones or accessories. Programmable controllers are very desirable, especially for tube furnaces. The one shown here from Eurotherm has dimensions of approximately 10 x 10 x 20 cm and has many useful features such as temperature ramp programming, alarms, two digital inputs and accuracy of 0.25%. A good controller can cost $1,000.

A hypothetical temperature ramp program for single crystal growth in a tube furnace might include ramping to 1000 degrees C at 25 degrees per minute, holding isothermal for several hours, ramping down at 0.2 degrees per hour to 975 degrees C and then ramping down at 0.5 degrees per hour to 500. There are many variations, of course.

TYPES OF FURNACES

Standard Natural Gas Furnaces - are the most commonly used units in houses today. Their main disadvantage is a low seasonal efficiency range of 55 to 65%, due to the constantly burning pilot flame and warm air loss through the chimney during non-operating cycles. Because of this, standard furnaces are no longer manufactured or available in Canada.

Medium Efficiency Natural Gas Furnaces - are similar to the standard units, but have electronic ignition, a built-in flue damper or induced draft venting. Seasonal efficiencies range from 76 to 85%. Electronic ignition improves efficiency by eliminating the constantly burning pilot flame.

-The built-in flue damper reduces heat loss up the chimney by closing the chimney flue when the furnace is not operating.

-The induced draft fan eliminates the draft hood, thereby reducing the amount of warm air lost. With induced draft venting, a fan forces the products of combustion through a vent pipe and up a small size chimney, or in some cases through the wall, eliminating the need for a conventional chimney.

High Efficiency Natural Gas Furnaces - use improved combustion techniques and extra heat exchangers to achieve seasonal efficiency levels of 90% or better. These are the most efficient furnaces available, and therefore usually cost more than other units.

-Heat exchangers lower the exhaust gas temperature to a point where the water vapor condenses and releases a large amount to latent heat. The cooled exhaust gases are removed through a 5 cm plastic vent in the wall.

-Most high efficiency furnaces also have induced draft venting, eliminating the need for a chimney. The liquid condensate produced is simply piped to a floor drain.

Reference :
Website
http://nett21.gec.jp/CTT_DATA/WATER/WATER_4/html/Water-189.html
http://www.furnacecompare.com/index.html
http://www.ilpi.com/inorganic/glassware/furnace.html
http://www.saskenergy.com/appliances/products/furnace.htm
http://www.surplusrecord.com/srg/007780.htm

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