There are several types of furnaces as follows:
Household Furnaces - A household furnace is a major appliance that is permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water. The most common fuel source for modern furnaces in the United States is natural gas, other common fuel sources include LPG (liquefied petroleum gas), fuel oil, coal or wood. In some cases electrical resistance heating is used as the source of heat, especially where the cost of electricity is low.
Combustion furnaces always need to be vented to the outside. Traditionally, this was through a chimney, which tend to expel heat along with the exhaust. Modern high-efficiency furnaces can be 98% efficient and operate without a chimney. The small amount of waste gas and heat are mechanically ventilated through a small tube through the side or roof of the house.
Modern household furnaces are classified as condensing or non-condensing based on their efficiency in extracting heat from the exhaust gases. Furnaces with efficiencies greater than approximately 89% extract so much heat from the exhaust that water vapor in the exhaust condenses. Such furnaces must be designed to avoid the corrosion that this highly acidic condensate might cause and may need to include a condensate pump to remove the accumulated water. Condensing furnaces can typically deliver heating savings of 20%-35% assuming the old furnace was in the 60% Annual Fuel Utilization Efficiency (AFUE) range.
Heat Distribution - The heat is transferred from the furnace through an intermediary distribution system. If the distribution is through hot water (or other fluid) or through steam, then the furnace is more commonly termed a boiler.
A condensing furnace - Most modern furnace installations in the United States used forced-air heat, where ductwork carries air through the heat exchanger of the furnace, whence it is blown throughout the building. One major advantage of this type of system is that it enables easy installation of central air conditioning.
In such an air (convection) distribution system, a cold air return feeds the cooler incoming air into the heating chamber, where it passes into a plenum, or chamber, from which it goes into the ductwork to various parts of the building.
Air convection heating systems have been in use for over a century, but the older systems relied on a passive air circulation system where the greater density of cooler air caused it to sink into the furnace, and the lesser density of the warmed air caused it to rise in the ductwork, the two forces acting together to drive air circulation in a system termed "gravity-feed".
The following rare but difficult-to-diagnose failure can occur. Typically the warm air is moistened by re-directing part of the heated air from the air supply duct, through the humidifier. If the furnace was installed incorrectly, the diameter of this tube may be too large, and cause an excessive amount of hot air to feed back into the furnace. If the temperature inside the furnace exceeds a maximum threshold, a safety mechanism with a thermostat will shut the furnace down. A symptom of this failure is that the furnace consistently shuts down, even though the temperature inside the house is cold. The solution is to reduce the diameter of the cross-feed tube, or install a baffle that reduces the volume of re-fed air.
Metallurgical furnaces - In metallurgy, several specialized furnaces are used mostly for smelting of metals.
Industrial furnaces - A furnace or direct fired heater, is an equipment used to provide heat for a process or can serve as reactor which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, all furnaces have some common features.
Fuel flows into the burner and is burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. Burners can also be floor mounted as in the picture, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section. In the chamber where combustion takes place, known as the firebox, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The heating fluid passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as flue gas. After the flue gas leaves the firebox, most furnace designs include a convection section where more heat is recovered before venting to the atmosphere through the smokestack.
Parts of a furnace are as follows:
Radiant section - The radiant section is where the tubes receive almost all its heat by radiation from the flame. In a vertical, cylindrical furnace, the tubes are vertical. Tubes can be vertical or horizontal, placed along the refractory wall, in the middle, etc., or arranged in cells. Studs are used to hold the insulation together and on the wall of the furnace. They are placed about 1 ft (300 mm) apart in this picture of the inside of a furnace. The tubes, which are reddish brown from corrosion, are carbon steel tubes and run the height of the radiant section. The tubes are a distance away from the insulation so radiation can be reflected to the back of the tubes to maintain a rather uniform tube wall temperature. Tube guides at the top, middle and bottom hold the tubes in place.
Convection section - The convection section is located above the radiant section where it is cooler to recover additional heat. Heat transfer takes place by convection here and the tubes are finned to increase heat transfer. The first two tube rows as seen in the picture below are in the bottom of the convection section and at the top of the radiant section. This area of bare tubes (without fins) are known as the shield section, so named because they are still exposed to plenty of radiation from the firebox and shield the convection section tubes, which are normally of less resistant material from the high temperatures in the firebox. These tubes may be of the same material as the radiant coil tubes. The area of the radiant section just before flue gas enters the shield section and into the convection section called the bridge zone. Crossover is the term used to describe the tube that connects from the convection section outlet to the radiant section inlet. The crossover piping is normally located outside so that the temperature can be monitored and the efficiency of the convection section can be calculated. The sight glass at the top allows personnel to see the flame shape and pattern from above and visually inspect if flame impingement is occurring. Flame impingement happens when the flame touches the tubes and causes small isolated spots of very high temperature.
Burner - The burner in a vertical, cylindrical furnace as above, is located in the floor and fires upward. The burner tile is made of high temperature refractory and is where the flame is contained in. Air registers are devices with movable flaps or vanes that control the shape and pattern of the flame, whether it spreads out or even swirls around. Flames should not spread out too much, as this will cause flame impingement. Air registers can be classified as primary, secondary and if applicable, tertiary, depending on when their air is introduced. The primary air register supplies primary air, which is the first to be introduced in the burner. Secondary air is added to supplement primary air. Burners may include a premixer to mix the air and fuel for better combustion before introducing into the burner. Notice that in the picture of the floor of the furnace, it is a different material than the wall. It is made of hard castable refractory known as kastolite so the floor can be walked on during maintenance. The brown dust on the floor is soot from the flame and rust from the tube.
The pilot flame here is lit by an ignition transformer. The pilot flame in turn lights up the main flame. The pilot flame uses natural gas while the main flame can use both diesel and natural gas.
Sootblower - Sootblowers utilize flowing media such as water, air or steam to remove deposits from boiler tubes. There are several different types of sootblowers used. Wall blowers are used for furnace walls and have a very short lance with a nozzle at the tip. The lance has holes drilled into it at intervals so that when it is turned on, it rotates and cleans the deposits from the wall in a circular pattern. It after it has turned a predetermined number of rounds, the sootblowing is completed and stops. Below is a convection section sootblower utilizing medium pressure (10-12bar) steam.
Stack - The stack is a cylindrical structure at the top of all the heat transfer chambers. The breeching directly below it collects the flue gas and brings it up high into the atmosphere where it will not endanger personnel.
The stack damper contained within works like a butterfly valve and regulates draft in the furnace, which is what pulls the flue gas through the convection section. The stack damper also regulates the heat lost through the stack. As the damper closes, the amount of heat escaping the furnace through the stack decreases, but the pressure or draft in the furnace increases which poses risks to those working around it if there are air leakages and the flames can then escape out of the firebox.
Insulation - Insulation is an important part of the furnace because it prevents excessive heat loss. Refractory materials such as firebrick, castable refractories and ceramic fiber, are used for insulation. The floor of the furnace is normally castable since it has to be hard enough to walk on during maintenance. Ceramic fiber is commonly used for the roof and wall of the furnace and is graded by its density and then its maximum temperature rating. For eg: 8# 2,300°F means 8 lb/ft3 density with a maximum temperature rating of 2,300°F. An example of a castable is kastolite.