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Boiler Selection

Six criteria should be considered when selecting a boiler to meet the application needs. The criteria are:

1. Codes and standards requirements
2. Steam or hot water
3. Boiler load
4. Number of boilers
5. Performance considerations
6. Special considerations

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.

The American Society of Mechanical Engineers (ASME) and the ASME Codes tightly regulate the boiler industry. ASME governs boiler design, inspection, and quality assurance. All boiler pressure vessels must have an ASME stamp, including deaerators, and economizers.

The company insuring the facility or boiler may dictate additional requirements. Boiler manufacturers provide special boiler trim components 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, CSA or CGA listing, or Canadian Registration Number (CRN) may be required. State, local, or provincial authorities may also 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.

State, local or provincial authorities normally 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 content 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 and license requirements. 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. Additionally, most states or provinces require an annual boiler inspection.

Steam or Hot Water
The facility's operation or process will dictate whether a steam or hot water boiler will 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 designs, 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 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.

System Load
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 or the ASHRAE Handbook.

System load is measured in either Btu's or pounds of steam (at a specific pressure and temperature). Included are 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.

System Load: Heating Load
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. 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.

System Load: Process Load
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.

System Load: Combination Load
Many facilities have a mixture of loads - different types of process loads and combinations 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.

Defining Load Variations: 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.

Defining Load Variations: Daily Variation
Daily variation can occur due to fluctuations 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.

Defining Load Variations: 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.

Load Tracking
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.

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 boiler 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: Backup Boilers
When selecting a boiler(s), consideration should be given to backup equipment to accommodate future expansion, emergency repairs, routine and mandated maintenance. There are a number of considerations for a backup boiler:

1. 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. 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.

2. Downtime
Another way to determine whether a backup boiler is a wise decision is to compute the cost of downtime to the owner or the user, as shown in the following three examples:

  • A chemical company manufactures dry cell battery compound in a batch process. The process temperature must be maintained within 2 degrees. The boiler shuts down on a flame failure. They have 20 minutes to recover steam or the batch is scrap. The value of the product is $250,000.
  • A Midwestern insurance company building has comfort heat supplied by one boiler. There are over 2000 workers in the building. The boiler shuts down due to a failed gas valve. Outside, it's 11°F. Inside, the temperature continues to drop and at 1:30 in the afternoon all 2,000 workers are sent home.
  • A meat processing company makes its entire packaged ham line in a Southern plant. It operates 24 hours a day, every day. A single boiler provides heat for curing, sterilizing, and cleaning. The boiler goes down due to a lack of feedwater. Each hour of steam loss results in four hours of lost production.

Boiler Turndown
Boiler turndown is the ratio between full boiler output and the boiler output when operating at low fire. Typical boiler turndown is 4:1. For example, a 400 horsepower boiler, with a 4:1 turndown burner, will modulate down to 100 horsepower before cycling off. The same boiler with a 10:1 turndown burner will modulate down to 40 horsepower.

The ability of the burner to turn down reduces frequent on and off cycling. Fully modulating burners are typically designed to operate down to 25% of rated capacity. At a load that is 20% of the rated capacity, the boiler will turn off and cycle frequently.

A boiler operating at low load conditions can cycle as frequently as 12 times per hour, or 288 times per day. With each cycle, pre- and post-purge airflow removes heat from the boiler and sends it out the stack. Keeping the boiler on at low firing rates can reduce the energy loss. Every time the boiler cycles off, it must go through a specific start-up sequence for safety assurance. It requires about one to two minutes to place the boiler back on line. If there's a sudden load demand, the start-up sequence cannot be accelerated. Keeping the boiler on line assures the quickest response to load changes. Frequent cycling also accelerates wear of boiler components. Maintenance increases and, more importantly, the chance of component failure increases.

As discussed earlier, boiler(s) capacity requirement is determined by many different types of load variations in the system. Boiler oversizing occurs when future expansion and safety factors are added to assure that the boiler is large enough for the application. If the boiler is oversized, the ability of the boiler to handle minimum loads without cycling is reduced. Therefore, capacity and turndown should be considered together for proper boiler selection to meet overall system load requirements.

Performance Considerations
Three important considerations pertain to fuels, emissions, and efficiency. All three have an important impact on boiler performance, and can affect long-term boiler operating costs.

Performance Considerations: Fuels
From an operating perspective, fuel costs typically account for approximately 10% of a facility's total operating budget. Therefore, fuel is an important consideration. Normally, the fuels of choice are natural gas, propane, or light oil. Increasingly stringent emission standards have greatly reduced the use of heavy oil and solid fuels such as coal and wood. Of the fossil fuels, natural gas burns cleanest and leaves fewer residues; therefore less maintenance is required.

It can be advantageous to supply a boiler with a combination burner that can burn two fuels independently - for example, oil or natural gas. A combination burner allows the customer to take advantage of the lowest cost energy by merely switching as fuel prices change throughout the year. Dual fuel capability also is beneficial if the primary fuel supply must be shut down for safety or maintenance reasons.

Some waste streams can be used as fuel in the boiler. In addition to reducing fuel costs, firing an alternate fuel in a boiler can greatly reduce disposal costs. Waste streams are typically used in combination with standard fuels to ensure safe operation and to provide additional operating flexibility.

Performance Considerations: Emissions
Emission standards for boilers have become very stringent in many areas because of the new clean air regulations. The ability of the boiler to meet emission regulations depends on the type of boiler and burner options.

Special Considerations: Replacement Boilers
If the boiler is to be placed in an existing facility, there are a number of considerations:

  • Floor space required.
  • Total space requirements.
  • Access space for maintenance.
  • Size and characteristics of the boiler to be replaced, including location of existing piping, the boiler stack and utilities.
  • Boiler weight limitations.
  • With little access to the boiler room, the cast iron boiler and some bent-tube type boilers can be carried into the boiler room in sections or pieces and easily assembled, with no welding required.
  • Electric boilers should also be considered, especially since they do not require a stack.
  • Vertical firetube boilers have a small floor space requirement.