Natural Draft Boilers
The term "natural draft boiler" is used to refer to a boiler with a specific type of burner assembly. As one might expect, a natural draft boiler drafts it's combustion air naturally, or without the utilization of a fan. When air is heated, the molecules move more quickly, causing them to spread further apart. Because the air molecules spread further apart when heated, a certain volume of air that is heated will have less air molecules in it, and thus less mass (or weight) than the same volume of air which is not heated. For example: one cubit foot of air at 70 degrees F weighs 0.07492 pounds while one cubic foot of air at 300 degrees F weighs 0.05224 pounds (source: http://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html). When the burners are on and there is a flame, the air being used to provide oxygen to the flame (which is typically the air in the room) mixes with the fuel (typically natural gas) at or near the point of combustion. At the point of combustion the oxygen in the air and the fuel undergo a molecular change in which heat is produced and the carbon in the fuel bonds with the oxygen in the air to form CO2. The "air" produced during combustion. is called "combustion air", by tradesmen and boiler makers. Because the molecules in the combustion. air are of greater temperature than that of the air being provided to the equipment (ambient air), the combustion. air is less dense than the ambient air, which causes it to rise. The movement of air upward through the boiler is called draft, and that is where the term "natural draft" comes from.
Natural draft boilers have been around for thousands of years, however, they were not widely used for space heating until the industrial revolution. Natural draft boilers may utilize solid fuel, liquid fuel, or vapor fuel. Because natural draft boilers rely on the hot air to draft naturally, which is in an upward direction, a common trait of every natural draft boiler is that the burners are near the bottom of the system, and that the combustion air exits the system at an elevated position, relative to where the ambient air and fuel enter the system. Because the combustion must occur at a lower elevation than that of where the combustion air exits the system, it is quite common for natural draft boilers to have the burners located directly under the boiler's heat exchanger. The most common type of natural draft boiler in the United States of America is a natural gas, cast iron sectional, natural draft boiler (as shown in the image to the left).
Although the natural draft boilers are the oldest and simplest type of boiler design, their construction is still quite sophisticated and interesting. Today's modern natural draft boilers are the result of a work in progress that has been on-going since before the time of the Roman Empire, until today. Locomotive engines in the trains used to win the "Wild West" utilized natural draft boilers, and today natural draft boilers are used for industry and to heat many buildings throughout the United States.
Although the natural draft boiler is an "old invention", it is still very important that a qualified and experienced boiler maker install your natural draft boiler. As may be evident in the subject mater, natural draft boilers need to be vented properly in order to function well. The fuel and air mixtures need to be proportioned, for the boilers to burn well, and air dampers on the burners should be adjusted during every new natural draft boiler installation. The improper venting of a natural draft boiler could not only cause boiler failures and efficiency loss, but could also endanger the safety of those in the structure utilizing the boiler system. The sizing and construction of the chimney (technically called "Stack") need to be correct or the boiler may vent (technically called "draft") too quickly, too slowly, or may even cause the combustion air to "spill" into the structure (which may lead to carbon monoxide poisoning). When a natural draft boiler vents it's combustion air too quickly, the stack temperature is increased and the boiler losses efficientcy. The Ideal situation with a natural draft boiler is to obtain a consistent and reliable flow of combustion air going out of the stack, but to not vent the combustion air more quickly than is necessary. If combustion air vents too quickly out of the boiler system, the combustion air does not come into contact with the boiler's heat exchanger long enough to transfer as much heat to the liquid in the boiler, and the boiler is not as efficient as it could be. If a boiler drafts too slowly, it is because the boiler is under firing (not producing enough heat), the boiler's stack is improperly designed, the building is under negative pressure, or some combination of the three. Boilers that draft too slowly are dangerous because that condition is indicative of a problem in the boiler system. The danger of a slow drafting boiler is that the combustion air or even flames may be pushed into the room that the boiler is located in. Obviously, the flames "rolling back" into the boiler room needs little explanation as to the danger that that condition poses, but the combustion gasses "rolling back" into the room may need a little explanation. Combustion gasses often contain some level of carbon monoxide. Typically most boilers operate with levels of carbon monoxide production that will only cause illness to the occupants of a structure that has a gas burning appliance that spills combustion air into the structure, but some produce enough carbon monoxide to be deadly. Regardless of the concentration of carbon monoxide in the combustion air, an improperly vented natural draft boiler is always dangerous because any amount of combustion air being inhaled is not good.
There are various methods and designs of boiler stacks that are utilized by natural draft boilers. The simplest type of stack is quite simple and is just a straight pipe going to the outdoors from the boiler. The air enters the boiler at or near the point of combustion, and exits the system via a pipe exhausted to the outdoors. This type of boiler stack or vent is not common because it makes no provision for controlling the draft of the boiler. Moreover, this type of stack design is not safe or efficient, but it is worth noting because some people install boilers with straight pipe stacks that do not make provision for draft control, all be it rare as it is. Most natural draft boilers utilize some form of "draft diverter". A draft diverter, as may be seen in the image above, simply allows the air in the boiler room to enter the chimney or stack, while also allowing the air, from a strong gust of wind, to enter the boiler room instead of the boiler, in the event that air is pushed downward through the stack. Draft diverters also serve the purpose of allowing the air in the boiler room to enter the stack above the draft diverter. Having air enter the stack at the draft diverter is important because it cools the rest of the stack, above the draft diverter, and keeps the negative pressure in the stack above the draft diverter from sucking the combustion air out of the boiler's heat exchanger, as it would if the stack was a strait pipe. The draft diverter acts as a sort of natural stack or chimney thermostat. As the chimney or stack becomes hotter, the pressure in the stack drops, causing more air to draft through the chimney or stack. As the pressure in the chimney or stack drops, more boiler room air is sucked into the chimney or stack, thereby lowering the stack temperature. Eventually an equilibrium is met, and with out a draft diverter, a much larger amount of air would travel through the boiler's heat exchanger area, thereby having the same cooling effect noted, only on an undesirable part of the system, the heat exchanger.
Some natural draft boilers are equipped with something called a stack damper. Boiler makers in Michigan commonly call stack dampers by the name of a popular stack damper manufacturer, Effikal, because Effikal dampers seem to be the stack damper of choice in North America. Because natural draft boilers will tend to draft as long as the air in the boiler system is hotter than the ambient air, and because the direction of heat transfer of a boiler's heat exchanger can go both ways, stack dampers may be used to stop the draft of hot air out of the boiler system when the boiler's burners are not energized or "firing". Stack dampers trap the air in the boiler's heat exchanger by closing the stack via a metal damper controlled by an actuator motor. The stack dampers have safety end switches built into the boiler's control circuit that stop the boiler's burners from energizing or firing in the event that the damper fails to open fully on a call for heat. Typically, stack dampers are spring loaded so that when power is applied to the damper's actuator, the damper opens, but when the power going to the damper ceases, the damper actuator's spring closes the damper. The energy saved by the utilization of stack dampers has caused the use of such devices in commercial, industrial, and residential boiler applications.
The burners on a natural gas or propane, natural draft boiler tend to be pretty consistent among the most widely used boiler designs. A typical natural gas or propane, natural draft boiler will utilize a gas pressure regulator, a gas valve, a manifold, nozzles, air baffles, and burners. some natural gas boilers may use an uncommon central burner chamber, wherein the gas is shot out of one burner into a large chamber, but most natural draft boilers utilize a manifold, nozzle, air baffles, and burners system. Being practical about the subject matter, we will not focus on the uncommon type of natural gas or propane, natural draft boiler types. In a natural gas, natural draft boiler, the components from the gas regulator to the nozzles is called the "gas train". The pressure regulator is responsible for maintaining a consistent gas pressure which is somewhat less dependant on line pressure. Because each building may have a gas pressure slightly different from the next, it is necessary that provision be made for ensuring that a gas pressure be supplied to the burners which is within the manufactures design criteria, and that is why gas pressure regulators are used on the gas trains of boilers.
In a typical natural gas, natural draft boiler, upon a call for heat, the control circuit is energized. When the control circuit is energized, electricity is applied to the safeties which are wired in series with one another. If one safety has failed, the boiler will not fire. If all safeties are closed (or OK), the power goes to the ignition control, which in turn either checks for a pilot proof (on a standing pilot boiler) or opens the pilot gas valve, and lights the pilot using a hot surface ignition device (similar to how a light bulb heats up), or using high voltage electricity to created a spark. Once the ignition control has fired or verified the pilot flame, it can open the main gas valve. When the main gas valve is opened, the gas travels through the gas pressure regulator, through the gas valve, through the manifold, out of each of the nozzles, down the center of the air baffles, and then into the burners. The natural gas flow is determined by measuring the gas pressure in the manifold, which is a large piece of steel pipe that runs along the base of the boiler, at the level of the burners, for the width of the boiler. The nozzles are the outlet's of the manifold, letting the gas out of the manifold into the burners. The nozzles are of a specific orfice size determined by the boiler manufacture to supply just the right amount of gas to the burners. The nozzles create enough back pressure to keep the manifold pressurized while the gas valve is in the open position. After the gas leaves the nozzle it enters the air baffle portion of the burner. The air baffle portion of the burner allows for air (most importantly O2) to mix with the fuel (natural gas) prior to combustion. Burner design varies based on manufacturer and era of boiler, but typically the burners either are cast iron burner which have a tube that connects to the center of the burner to the nozzle by going down and then out, or the burners are straight steel tubes with stamped slots called flame ribbons.
Regardless of what particular design a boiler is, cleanliness and regular maintenance are very important. Soot, iron, and fire block debris cause all boilers to not only operate poorly, but can also be dangerous. At Johnson Heating and Cooling, LLC, we don't take short-cuts when doing boiler service work, boiler CSD-1 reports, boiler installation, or boiler repairs; everything that we do is by the book. We have great respect for your equipment, and moreover, we just enjoy doing boiler work. When the safety of the building occupants is in our hands our customers don't need to worry about the work that we do, because it is done thoroughly, with skill, and done right. If you live in the State of Michigan and need boiler repair, boiler installation, boiler service, or just a good boiler repair company, give us a call. We have two decades of experience working on boilers and hydronic heating systems, our rates are competitive, and we always treat our customers the way that we would like our families to be treated if they were in your shoes. If you would like to learn more about some of the work that we have done, and look into some of our references that are local to you, just navigate to our contact page and click on the link to your township or city. We have listed a few references from several areas in Macomb and Oakland Counties that are organized under their respective township or city.
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