General Boiler Information
Total Page：16
File Type：pdf, Size：1020Kb
General Boiler Information 1 boiler horse power is about 42,000 BTUs of INPUT 1 pound of steam is about 1,200 BTUs of INPUT fuel, and about 1,000 BTUs at the point of use, depending on the pressure of the steam Low pressure steam is considered to be up to 15 psi; high is generally 100 psi and higher. Superheat is a term that refers to higher temperature steam, as a result of a second special steam heat exchanger in the boiler that allows steam pressure to increase, thereby taking on more BTUs (in excess of 500 psi is typical of superheat). Superheated steam is very dry steam. Smaller boilers are generally rated in horse power; larger are generally rated in thousands of pounds of steam (500 hp and under will typically be rated in hp) Typical boiler efficiency will be in the 75 - 85% range; new highest efficiency boilers may be near 90%; newer quick heat up types of boilers with copper heat exchangers can be more efficient, especially at startup and part load than older, heavy mass cast iron boilers. Source: DOE 'Improving Steam System Performance - a Sourcebook for Industry' Oct.2004 Click on image for larger view Fire Tube Boilers In firetube boilers, the combustion gases pass inside boiler tubes, and heat is transferred to water on the shell side. Scotch marine boilers are the most common type of industrial firetube boiler. The Scotch marine boiler is an industry workhorse due to low initial cost, and advantages in efficiency and durability. Scotch marine boilers are typically cylindrical shells with horizontal tubes configured such that the exhaust gases pass through these tubes, transferring energy to boiler water on the shell side. Scotch marine boilers contain relatively large amounts of water, which enables them to respond to load changes with relatively little change in pressure. However, since the boiler typically holds a large water mass, it requires more time to initiate steaming and more time to accommodate changes in steam pressure. Also, Scotch marine boilers generate steam on the shell side, which has a large surface area, limiting the amount of pressure they can generate. In general, Scotch marine boilers are not used where pressures above 300 psig are required. Today, the biggest firetube boilers are over 1,500 boiler horsepower (about 50,000 lbs/hr). Firetube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water. Each pass sends the flue gases through the tubes in the opposite direction. To make another pass, the gases turn 180 degrees and pass back through the shell. The turnaround zones can be either dryback or water-back. In dryback designs, the turnaround area is refractory lined. In water-back designs, this turnaround zone is water-cooled, eliminating the need for the refractory lining. Water Tube Boilers In watertube boilers, boiler water passes through the tubes while the exhaust gases remain in the shell side, passing over the tube surfaces. Since tubes can typically withstand higher internal pressure than the large chamber shell in a firetube, watertube boilers are used where high steam pressures (as high as 3,000 psi) are required. Watertube boilers are also capable of high efficiencies and can generate saturated or superheated steam. The ability of watertube boilers to generate superheated steam makes these boilers particularly attractive in applications that require dry, high- pressure, high-energy steam, including steam turbine power generation. The performance characteristics of watertube boilers make them highly favorable in process industries, including chemical manufacturing, pulp and paper manufacturing, and refining. Although firetube boilers account for the majority of boiler sales in terms of units, water-tube boilers account for the majority of boiler capacity. For more info on boiler type terminology, see Boiler Basic Types Steam Generators Steam generators are like boilers in that they are fired by gas and produce steam, but they are unlike boilers in that they do not have large pressure vessels and are made of light-weight materials. The fact that they do not have pressure vessels means that in most locations they do NOT require a boiler operator (always confirm with local codes). This can be a substantial savings when there is no other reason to have an operator other than the local code requires it for a large pressure vessel. The fact that they are made out of light weight materials means they perform well at part loads and respond quickly to changes in loads. This greatly increases part load operating efficiency. For more information see Steam Generators Compact and Modular Boilers Modern materials, controls and the pursuit of ever higher energy efficiency and reduced emissions is leading to boilers that are smaller in physical size, have cleaner emissions and produce dryer steam. Materials are critical because old cast iron boilers relied on mass to prevent them from thermal shocks that could split the boiler apart. New metals reduce mass which improves thermal transfer and can handle the thermal stress of going from cold water to steam in seconds. Space is money, especially in new construction. Boilers of similar output capacity made smaller to reduce their space requirements can result in overall lower first cost of equipment plus space. Boilers have a certain efficiency curve that tends to result in the boiler having the highest efficiency at full-fire. As the boiler is 'turned-down' to run at less than 100% capacity, efficiency typically drops. To counter this situation, a modular boiler bank of 3 or more boilers with a programmed controller can improve operational efficiency over a single boiler operating in a turn- down mode, and provides a certain amount of redundancy for back-up. Tubeless and Condensing Boilers High energy prices along with improved material and combustion technology is resulting in a new generation of high efficiency gas boilers. Traditional boilers are designed to PREVENT condensation because it is corrosive to boiler components and the wide variations in temperatures cause problems with thermal shock. However, without condensation, boilers can not be higher than about 85% efficient. Boilers that are designed for condensation and use advanced controls to squeeze every possible BTU from the combustion process are able to achieve efficiencies in the high 90's. There is a first-cost premium, but when energy prices are high, paybacks are more acceptable. "Tubeless" Boilers use tubing coils instead of rigid tubes. "Direct Contact" water heaters have no tubes, tubing or coils; they have heat transfer media such as spheres or cylinders and allow flue gases to come in direct contact with the water. Steam Information Steam is an invisible gas that's generated by heating water to a temperature that brings it to the boiling point. When this happens, water changes its physical state and vaporizes, turning from a liquid into a gas. Conversely, when heat energy is removed from steam, it loses its ability to retain a gaseous state and condenses back into a liquid. The resulting liquid is called condensate. The temperature at which condensation takes place is known as the dew point. When water is heated at atmospheric pressure, its temperature rises until it reaches 212°F (100°C), the highest temperature at which water can exist at this pressure. Additional heat does not raise the temperature, but converts the water to steam. One pound of water takes 1 BTU per Degree of Temperature rise up to 212°F; to form steam, an additional 970 BTUs is required for the "Latent Heat of Vaporization". Therefore, steam has (970 + (212 - Condensate Temperature)) BTUs per pound. EXAMPLE: If the condensate temperature is 160°F, (970 + (212 - 160)) = 1,022 BTUs per pound. This clearly shows why steam has more energy content than hot water. NOTE: These are BTUs delivered to the water; efficiency must also be factored in to determine INPUT BTU requirements. For more information about Steam For specific information about Steam Trap Audits Superheat Superheat refers to the process of increasing the temperature of steam above about 400°F and 100 psi to produce a very "dry" steam with absolutely no water vapor. This feature is most common in very large power plant boilers of watertube construction. An additional heat exchanger capable of the high temperatures and pressures is required. At least one company promotes a "direct fired" superheater, that could have some advantages for facilities with smaller sized boilers that need higher temperatures and pressures, but do not want to invest in a new boiler or use a Thermal Fluid system. According to their web site, the Cannon Superheater can be used on new boilers and retrofit installations. The Cannon Superheater can be used on watertube or firetube boilers in the 25 HP to 1,000 HP range. For more information, see www.cannonboilerworks.com. Boiler Stack Economizer Flue gases from large boilers are typically 450 - 650°F. Stack Economizers recover some of this heat for pre-heating water. The water is most often used for boiler make-up water or some other need that coincides with boiler operation. Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the boiler or large amounts of live steam are used in the process so there is no condensate to return.) The savings potential is based on the existing stack temperature, the volume of make-up water needed, and the hours of operation. Economizers are available in a wide range of sizes, from small coil- like units to very large waste heat recovery boilers.