Six criteria should be considered when selecting a boiler to meet the
application needs, The criteria are
Codes and standards requirements
Steam or hot water
Number of boilers
Codes and Standards
There are a number of codes and standards, laws, and regulations covering boilers and related equipment that should be considered when designing a system. Regulatory requirements are dictated by a variety of sources and are all focused primarily on safety. For more information on how the various rules affect boiler selection and operation, you may want to contact your local Cleaver-Brooks authorized representative. Here are some key rules to consider:
The boiler industry is tightly regulated by the American Society of Mechanical Engineers (ASME) and the ASME Codes, which governs boiler design, inspection, and quality assurance. The boiler's pressure vessel must have an ASME stamp. (Deaerators, economizers, and other pressure vessels must also be ASME stamped).
The insurance company insuring the facility or boiler may dictate additional requirements. Boiler manufacturers provide special boiler trim according to the requirements of the major insurance companies. Special boiler trim items usually pertain to added safety controls. Some industries, such as food processing, brewing, or pharmaceuticals, may also have additional regulations that have an impact on the boiler and the boiler room.A UL, ULC, cUL, CSA or CGA listing, or Canadian Registration Number (CRN) may be required. State, local, or provincial authorities may require data on the boiler controls or basic design criteria.
Most areas have established a maximum temperature at which water can be discharged to the sewer. In this case, a blowdown separator aftercooler is required.
Most state, local or provincial authorities require a permit to install and/or operate a boiler. Additional restrictions may apply in non-attainment areas where air quality does not meet the national ambient air quality standards and emission regulations are more stringent.
For all new boilers with inputs over 10 MMBtu/hr, U.S. Federal emission standards apply, including permitting and reporting procedures. Limits on fuel sulfur ******* are frequently set at 0.5% maximum.
A full-time boiler operator may be required. Operator requirement depends on the boiler's size, pressure, heating surface or volume of water. Boilers can be selected which minimize the requirements, either by falling under the requirements and being exempt or with special equipment that gives the operator more freedom in the facility.
Most states or provinces require an annual boiler inspection. There may be other requirements on piping as well.
Steam or Hot Water
Now that you have a general overview of the types of boilers and code and standards requirements, it's time to look at the facility's application in order to see how the boiler will be used. Keep in mind, the primary purpose of the boiler is to supply energy to the facility's operations - for heat, manufacturing process, laundry, kitchen, etc. The nature of the facility's operation will dictate whether a steam or hot water boiler should be used. Hot water is commonly used in heating applications with the boiler supplying water to the system at 180 °F to 220 °F. The operating pressure for hot water heating systems usually is 30 psig to 125 psig. Under these conditions, there is a wide range of hot water boiler products available. If system requirements are for hot water of more than 240 °F, a high temperature water boiler should be considered.
Steam boilers are designed for low pressure or high pressure applications. Low pressure boilers are limited to 15 psig design, and are typically used for heating applications. High pressure boilers are typically used for process loads and can have an operating pressure of 75 to 700 psig. Most steam boiler systems require saturated steam.
Steam and hot water boilers are defined according to design pressure and operating pressure. Design pressure is the maximum pressure used in the design of the boiler
for the purpose of calculating the minimum permissible thickness or physical characteristics of the pressure vessel parts of the boiler. Typically, the safety valves are set at or below design pressure. Operating pressure is the pressure of the boiler at which it normally operates. The operating pressure usually is maintained at a suitable level below the setting of the pressure relieving valve(s) to prevent their frequent opening during normal operation.
Some steam applications may require superheated steam. It should be noted that superheated steam has a high enthalpy, so there is more energy per pound of steam and higher (drier) steam quality. One example of an application where superheated steam may be required is with a steam turbine. The turbine's blades require very dry steam because the moisture can destroy the blades. When very high pressure or superheated steam is required, an industrial watertube boiler should be selected.
In addition to the system load considerations provided in this section, many excellent reference manuals are available to help further define specific load details and characteristics. For more information, refer to the ABMA Firetube Engineering Guide, the ASHRAE Handbook, or contact your local Cleaver-Brooks authorized representative.
System load is measured in either Btus or pounds of steam (at a specific pressure and temperature). When discussing the system load, we will include references to both steam and hot water. However, not all situations or criteria apply to both. It would be nearly impossible to size and select a boiler(s) without knowing the system load requirements. Knowing the system load provides the following information:
The boiler(s) capacity, taken from the maximum system load requirement.
The boiler(s) turndown, taken from the minimum system load requirement.
Conditions for maximum efficiency, taken from the average system load requirement.
Determining the total system load requires an understanding of the type(s) of load in the system. There are three types of loads: heating, process, and combination.
A heating load is typically low pressure steam or hot water, and is relatively simple to define because there is not a great deal of instantaneous changes to the load. And, once a heating load is computed, the number can easily be transferred into the equipment size requirements. A heating load is used to maintain building heat. Cooling loads, using steam to run an absorption chiller, also are included when computing a heating load. Characteristics of a heating load include large seasonal variations but small instantaneous demand changes. The boiler should be sized for the worst possible weather conditions, which means that true capacity is rarely reached.
A process load is usually a high pressure steam load. A process load pertains to manufacturing operations, where heat from steam or hot water is used in the process. A process load is further defined as either continuous or batch. In a continuous load, the demand is fairly constant -
such as in a heating load. The batch load is characterized by short-term demands. The batch load is a key issue when selecting equipment, because a batch-type process load can have a very large instantaneous demand that can be several times larger than the rating of the boiler. For example, based on its size, a heating coil can consume a large amount of steam simply to fill and pressurize the coil. When designing a boiler room for a process load with instantaneous demand, a more careful boiler selection process should take place.
Many facilities have a mixture of loads - different types of process loads and combination of heating and process loads. The information just given on heating and process loads should be taken into consideration when dealing with a combination load.
Defining Load Variations
Loads vary and a power plant must be capable of handling the minimum load, the maximum load, and any load variations. Boiler selection is often dictated by the variation in load demand, rather than by the total quantity of steam or hot water required. There are three basic types of load variations: seasonal, daily, and instantaneous.
Seasonal Variations. For a heating system, seasonal variations can mean no demand in the summer, light demand in the fall and spring, and heavy demand in the winter. Manufacturing operations often have seasonal variations, because the demand for production may vary. When selecting boiler equipment, the minimum and maximum load for each season should be determined.
Daily Variation. Daily variation can occur due to variations in the work hours, or the heat required at various times of the day or weekend. Minimum and maximum seasonal variations mentioned earlier may already reflect these changes if they occur daily. If not, the minimum and maximum daily loads should be included.
The seasonal and daily variations define the size of the load that the boiler(s) must handle. Seasonal and daily variations also help define the number of boilers and turndown requirements.
Instantaneous Demand. Instantaneous demand is a sudden peak load change that is usually of short duration. These types of loads are sometimes hidden. Many machines or processes are rated in pounds of steam per hour or Btu/hr as running loads, under balanced operating conditions, and there is no recognition given to "cold startup," "peak" or "pickup loads." The instantaneous load demand is important to consider when selecting a boiler to ensure that these load variations are taken into account. If the instantaneous demand is not included in the system load calculations, the boiler(s) may be undersized.
System Load Summary
The load demand matrix shown in Table I1-3 can be used as a work sheet in determining the minimum, maximum, and average system loads.
Load tracking is the ability of a boiler to respond to changes in steam or hot water demand. Most often associated with process loads, load tracking focuses on the boiler's ability to supply a constant volume of steam at the required pressure.
The ability of the boiler to track a variable load depends on the boiler type, burner turndown capability, feedwater valve control, and combustion control design. If the analysis of the load shows highly variable load conditions, a more complex control package may be necessary. This type of control is achieved with sophisticated boiler management systems. For more information on these types of systems contact your local Cleaver-Brooks authorized representative.
If the application has instantaneous load demands, whereby a large volume of steam is required for a short period of time, a boiler with a large energy storage reserve, such as a firetube, should be considered. If the application dictates large variances in load demand, where the load swings frequently for long periods of time, the best choice is probably a watertube type boiler, because it contains less water and can respond to the variances more rapidly.
In all cases, operation of the burner should be taken into account in selecting a boiler(s) to meet system demand. The burner will require proper operating controls that can accurately sense the varying demands and be capable of the turndown requirements. The boiler feedwater valve and control design are also critical if load swings are expected.
Number of Boilers
when selecting the boiler(s), consideration should be given to backup equipment to accommodate future expansion, emergency repairs, and maintenance. There are a number of considerations for a backup boiler.
Type of Load
Heating systems and non-critical loads that do not result in a sudden loss of production generally have little or no backup. While this is not recommended, it is still common practice. These types of applications rely on the ability to make repairs quickly to reduce downtime. The risk involved in having no backup is a total loss of heat when the boiler is not in service.
When process or heating loads use multiple boilers during peak times, and one boiler during most other times, the availability of an additional boiler to provide full backup during maximum demand should be considered.
In applications with critical steam or hot water requirements, laws or codes may dictate a backup. Even if laws or codes do not dictate a backup, there are many cases where the operation cannot tolerate downtime. For example, a hotel uses hot water 24 hours a day, seven
days a week. During periods of maintenance or in an emergency, a backup boiler is required.