should be considered when selecting a boiler to meet the application needs. The
and standards requirements
2. Steam or hot water
3. Boiler load
4. Number of boilers
5. Performance considerations
6. Special considerations
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
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
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.
have established a maximum temperature at which water can be discharged to the
sewer. In this case, a blowdown separator aftercooler is required.
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.
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
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.
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.
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.
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.
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.
for maximum efficiency, taken from the average system load requirement.
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.
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.
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
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
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
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
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.
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
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 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.
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
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
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:
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.
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.
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.
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
- 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.
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.
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.
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.
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.
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.
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.
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.
Considerations: Replacement Boilers
If the boiler is to be placed in an existing facility, there are a number of considerations:
- Floor space
- Total space
- Access space
- Size and characteristics
of the boiler to be replaced, including location of existing piping, the boiler
stack and utilities.
- Boiler weight
- 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.