Classification and working of boilers

  There are two general types of boilers: ''fire-tube'' and ''water-tube''. Boilers are classified as "high-pressure" or "low-pressure" and "steam boiler" or "hot water boiler." Boilers that operate higher than 15 psig are called "high-pressure" boilers.

     A hot water boiler, strictly speaking, is not a boiler. It is a fuel-fired hot water heater. Because of its similarities in many ways to a steam boiler, the term ''hot water boiler'' is used.

Hotwater boilers that have temperatures above 250? Fahrenheit or pressures higher than 160 psig are called ''high temperature hot water boilers''.Hotwater boilers that have temperatures not exceeding 250? Fahrenheit or pressures not exceeding 160 psig are called ''low temperature hot water boiler''s.

     Heating boilers are also classified as to the method of manufacture, i.e., by casting (cast iron boilers) or fabrication (steel boilers). Those that are cast usually use iron, bronze, or brass in their construction. Those that are fabricated use steel, copper, or brass, with steel being the most common material.

Steel Boilers

'''Steel boilers''' are generally divided into two types: ''firetube'' and ''water-tube''.

Fire-tube Boilers

      In fire-tube boilers, combustion gases pass through the inside of the tubes with water surrounding the outside of the tubes. The advantages of a fire-tube boiler are its simple construction and less rigid water treatment requirements.

     The disadvantages are the excessive weight-per-pound of steam generated, excessive time required to raise steam pressurebecause of the relatively large volume of water, and inability to respond quickly to load changes, again, due to the large water volume.

The most common fire-tube boilers used in facility heating applications are often referred to as ''scotch'' or ''scotch marine'' boilers, as this boiler type was commonly used for marine service because of its compact size (fire-box integral with boiler section).

     The name "fire-tube" is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes ('''Figure 2''') that are surrounded by the fluid to be heated. The body of the boiler is the pressure vessel and contains the fluid. In most cases, this fluid is water that will be circulated for heating purposes or converted to steam for process use.

'''Figure 2: Fire-tube Boiler Gas Flow'''

      Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass." So, a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A four-pass boiler will have four sets and the stack outlet at the front.

Fire-tube boilers are:

Relatively inexpensiveEasy to cleanCompact in sizeAvailable in sizes from 600,000 btu/hr to 50,000,000 btu/hrEasy to replace tubesWell suited for space heating and industrial process applications

Disadvantages of fire-tube boilers include:

Not suitable for high pressure applications 250 psig and aboveLimitation for high capacity steam generation

Water-tube Boilers

     In a water-tube boiler ('''Figure 3'''), the water is inside the tubes and combustion gases pass around the outside of the tubes. The advantages of a water-tube boiler are a lower unit weight-per-pound of steam generated, less time required to raise steampressure, a greater flexibility for responding to load changes, and a greater ability to operate at high rates of steam generation.

'''Figure 3: Water-tube Boiler'''

     A water-tube design is the exact opposite of a fire-tube. Here, the water flows through the tubes and is encased in a furnace in which the burner fires. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum.

     Large steam users are better suited for the water-tube design. The industrial water-tube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications. The best gauge of which design to consider can be found in the duty in which the boiler is to perform.

Water-tube boilers:

Are available in sizes far greater than a fire-tube design , up to several million pounds-per-hour of steamAre able to handle higher pressures up to 5,000 psigRecover faster than their fire-tube cousinHave the ability to reach very high temperatures

Disadvantages of the water-tube design include:

High initial capital costCleaning is more difficult due to the designNo commonality between tubesPhysical size may be an issue

Cast Iron Boilers

     Cast iron boilers ('''Figure 4''') are made in three general types: horizontal-sectional, vertical-sectional, and one-piece. Most of the sectional boilers are assembled with push nipples or grommet type seals, but some are assembled with external headers and screw nipples. Horizontal-sectional, cast iron boilers are made up of sections stacked one above the other, like pancakes, and assembled with push nipples. Vertical-sectional, cast iron boilers are made up of sections standing vertically, like slices in a loaf of bread. One-piece cast iron boilers are those in which the pressure vessel is made as a single casting.

'''Figure 4: Cast Iron Boiler'''

Steam and Condensate Boiler System

     Boilers are generally used to provide a source of steam or hot water for facility heating and process needs.

     In steam and condensate systems ('''Figure 5'''), heat is added to water in a boiler causing the water to boil and form steam. The steam is piped to points requiring heat, and as the heat is transferred from the steam to the building area or process requiring heat, the steam condenses to form condensate. In some very low-pressure saturated steam heating applications, the steam distributionpiping may be sized to slope back to the boiler so that the steam distribution piping also acts as the condensate return piping (single-pipe system).

'''Figure 5: Steam and Condensate Boiler System'''

     In other low-pressure applications, there may be steam supply piping and condensate return piping (two-pipe system), although the condensate system is open to the steam system. In typical packaged steam boiler operations, the boiler system may generate steam at about 150 psig for distribution throughout the facility and may be lowered to the operating pressure of equipment supplied through point-of-use pressure reducing stations. As heat is transferred from the steam, condensate is formed which collects in discharge legs until enough condensate is present to operate a trap that isolates the steam distribution system from the condensate system. In common facility heating applications, the condensate system is at atmospheric pressure and the system is arranged to drain the condensate to a central condensate receiver, or into local smaller receivers that pump the condensate back to the central condensate receiver.

Hydronic Boiler System

     A boiler is used to heat water that is circulated through a closed loop piping system for general facility and service water heating. Low-temperature systems generally operate below 200? Fahrenheit Medium-temperature systems generally operate at temperatures between 200 and 250? Fahrenheit.

     A feature of hot water systems ('''Figure 6)''' is an expansion tank to accommodate the expansion of the water in the system as the water is heated. The expansion tank, when piped into the system on the suction side of the circulating pumps, also pressurizes the system to prevent flashing in the circulating pump, piping, and piping components. In many low- and medium-pressure systems, pressurization is maintained by flash steam in the expansion tank. In a few hot water systems, pressurization is maintained by maintaining a compressed gas blanket above the water level in the expansion tank.

'''Figure 6: Hydronic (Hot Water) Boiler System'''

     High-temperature hot water systems, which operate above 250? Fahrenheit, are basically the same as hot water systems that operate below 250?F. High-temperature systems are generally installed when a process requires the higher temperature, a number of locations require small quantities of low-pressure steam that the high-temperature hot water can generate in a local converter, or high-temperature drop equipment can be used at end use points to minimize the size of water circulation piping required.

     Most facility boiler systems are fired using a combustible gas (typically natural gas or propane) or fuel oil. In many facilities, the boilers are designed to fire both a combustible gas fuel and a fuel oil. In these facilities, the combustible gas fuel is generally natural gas that is considered the primary fuel, and fuel oil is considered to be the backup fuel.

Boiler System Major Components

     Boiler systems are comprised of the major components described below and shown in '''Figure 7'''.

'''Figure 7: Boiler Room Schematic'''

Feedwater Heaters

     Feedwater heaters are energy recovery devices generally found only in large steam generating plants where all of the steam generated is not reduced to condensate by the steam user. This "waste steam" is reduced to condensate for return to the boiler in the feedwater heater. The boiler feedwater is used as a cooling medium to reduce the steam to condensate, which increases the temperature of the feedwater and, thereby, increases the thermal efficiency of the boiler.

Fuel Heater

     Many boilers firing heavy fuel oil require fuel heaters to reduce the fuel viscosity, so the fuel can be atomized by the burner system for complete combustion.

Deaerators

     A deaerator is a special case of feedwater heater that is designed to promote the removal of non-condensable gases from the boiler feedwater. The principal gases of concern are oxygen, carbon dioxide, and ammonia, which are major contributors to boilers, and steam and condensate piping corrosion problems. In small steam plants, a portion of the steam generated by the boiler is used to operate the deaerator if "waste steam" is not available. Failure to maintain and properly operate the deaerator can lead to early failure of the boiler, steam using equipment, and the steam and condensate piping.

Pumps

     In most hot water systems, the system circulating pumps are electric motor-driven, end suction centrifugal pumps. In steam systems, the condensate return pumps are typically electric motor-driven, end suction, centrifugal or turbine-type pumps. Feedwater pumps are generally electric motor-driven, multiple-stage, end suction centrifugal pumps. The shutoff head of the pump must be greater than the steam or hot water system operating pressure.

Combustion Air Blowers

     In many packaged boiler installations, the combustion air fan is designed and provided by the boiler manufacturer and is integral with the boiler housing. In installations where a stand-alone fan is provided, low-pressure centrifugal blowers are commonly used. An important characteristic of the blower is the ability to maintain a relatively constant air pressure over a wide range of airflows.

Flue

     Flues (boiler firebox exhaust duct or boiler discharge stack) must be large enough to conduct the products of combustion away from the boiler with a minimum of duct friction loss. Flues may be fabricated from any material suitable for the operating temperature and pressure. Common materials of construction associated with packaged boiler installations are carbon steel and stainless steel.

Economizer

     An economizer is an energy recovery device that uses the hot exhaust gases from the boiler (waste heat) to heat combustion air or feedwater.

Steam Traps

     Steam traps are installed throughout steam systems to remove condensate (spent steam), air, and non-condensable gases from the steam system. There are five types of steam traps in general use today, as described below.

The heart of a '''balanced pressure thermostatic trap''' is the flexible bellows that moves the valve head from its seat to discharge the condensate. The bellows is filled with a volatile fluid and hermetically sealed. The fluid has a pressure-temperature relationship that closely parallels, but is approximately 10 degrees Fahrenheit below that of steam.The '''liquid expansion steam trap''' has for its operating element a liquid-filled cartridge. Within this cartridge is a hermetically sealed bellows which is attached to the valve head and plunger.'''Float and thermostatic steam traps''' provide immediate and continuous discharge of condensate, air, and non-condensables from a steam system as soon as they reach the trap. The trap consists of a ball float connected by a leverassembly to the main valve head. As condensate reaches the trap, the ball float rises, positioning the valve to discharge the condensate at the same rate as it reaches the trap.The '''inverted bucket steam trap''' is a type of trap with an inverted bucket attached to the valve head by a lever mechanism and operates to open and close the trap. When condensate enters the trap, a water seal is formed around the bottom of the inverted bucket which, since it is filled with air, becomes buoyant and rises and closes the trap. A small hole in the top of the inverted bucket allows air to escape with condensate taking its place inside the bucket. The inverted bucket loses its buoyancy and sinks to the trap bottom, opening the valve to discharge the condensate.'''Thermodynamic steam traps''' are a type of steam trap that responds to differences in kinetic energy between steam and condensate to open and close the valve for discharging condensate.

Piping

     Piping two-inches and smaller used in steam and hot water systems is typically Schedule 80, American Society for Testing and Materials (ASTM) A 106, Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service (1999), Grade B, steel pipe with threaded joints and carbon steel fittings. Piping larger than two-inches is typically standard weight, ASTM A 106, Grade B, steel pipe with flanged joints and carbon steel fittings.

Boiler Room Definition/Terminology

     There are many terms used in a discussion of boilers, the following is a list of some of the most common terms. There is a glossary provided that covers some of the other terms that may be also used. This section also contains some of the basic valves that are used on boiler and boiler systems, along with some of the common HVAC and piping symbols.