CAS Newsletter June 2011
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Conversion to SI and other common units
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Within a boiler system, the operating steam pressure and hot water temperature must always be maintained at a constant level regardless of the load demand placed on the system. “In a multi-boiler plant, demand-load management optimizes the distribution of steam demand among the units and adjusts the overall output to meet working requirements.
By adopting a proper demand load scheduling technique, one can easily cut down the running cost of the boiler plant since via this technique, firing of boilers take place only in case of actual need. On the other hand, demand-load scheduling can be used to run each boiler for exactly equal periods of time.
Major features of demand-load management in boiler systems are listed below:
Base-load or Modulating Operation
In order to achieve perfect demand load sharing, boilers can be operated in two ways i.e. base-load operating mode or modulating operating mode. In the former mode, the entire load demand placed on the system gets distributed amongst all the base-loaded boilers based upon the user input whereas in the latter mode the load sharing gets automatically done without any user interference. Actually, out of total load demand, the demand which doesn’t get fulfilled by the boilers operating in base load mode, gets enforced upon the modulating boilers and distributed among them depending upon their capacities. Effective load allocation is based on real-time calculations that account for operating safety margins, load fluctuations, shutdown characteristics and boiler capacities.
Parallel or Series Demand Sharing
In parallel mode of demand sharing, the entire load demand gets distributed among the boilers operating at same firing rates. With the increase in load demand, the firing rate of all boilers rises by same amount till the point where the demand load necessitates an extra boiler. Now, for making adjustment for the extra boiler, the firing rate of the previous boilers needs to be decreased. This method of demand sharing is frequently applied for steam boilers.
In series method of demand sharing, the firing rate of modulating boiler tends to rise till the point where an extra boiler comes into play. To meet load demand, this newly added boiler occupies the place of modulating boiler. This method of demand sharing is mainly put into use for heated water systems and meeting rapidly changing steam demands.
In this method, the boilers which are always active are known as lead boilers whereas the other boilers are referred to as lag boilers. The boilers showing effective performance tends to start first while less efficient boilers always stop first.
Boiler banking keeps boilers in hot standby mode by intermittently firing unused boilers, thus maintaining a required pressure. Boiler banking acts as a warm-start facility, improving the plant’s response to sudden load changes.
Steam boilers are frequently employed in various industrial and domestic applications where saturated or superheated steam is required. Steam generated via steam boilers is put into use for heating, cooking and reboiling purposes, mainly because of its heat energy. On the other hand, the pressure energy associated with steam primarily finds its use in reciprocating steam engines and steam turbines.
Steam boilers can produce steam by utilizing heat from variety of sources such as burning of fuel, nuclear reactions, solar energy and heat dissipated from several other processes. A steam boiler should be designed in such a way that it can handle exceedingly high pressure steam generated at immensely high temperatures. In order to achieve this, special care must be taken regarding steam boiler construction. Following factors must be specifically considered while designing a steam boiler:
In residential applications, one-pipe steam systems are usually employed. In these systems, a temperature controlling device called thermostat is used which basically generates a signal of heat requirement. Upon receiving this signal, the steam boiler commences its operation and steam starts to spread out in the whole system. As the steam expands, it displaces air contained in the pipes and radiators, and that air must be allowed to escape through air vents typically located at the top of each steam radiator. As soon as the steam arrives at the radiator, these air vents close up while the steam is locked inside. The heat energy from the steam is then transferred via radiators. After this, the steam gets condensed and returned back in the form of water towards the boiler. The water flows back due to gravity action via the same pipe, hence it is known as single pipe steam system. A typical single pipe steam heating system is shown in the figure below.
Two-pipe steam systems consisting of a separate condensate-return pipe are also available but they are rarely put into use for residential purposes.
Types of Steam Boilers
In early days, the boiler configurations were very simple which included only iron tanks placed over a wood fire. However, nowadays, advanced boiler designs have been developed with following two major configurations:
Both configurations mentioned above consist of a fire box inside which burning of fuel takes place via continuous introduction of fuel and air. The hot exhaust gases released from the fire box are then utilized for heating the water and converting it into steam. The steam generated is then passed via radiators to provide necessary heat.
Following are the major components incorporated inside a steam boiler system:
A steam boiler is capable of generating superheated steam having temperature more than 212 degrees F. However, this temperature can go beyond unsafe limits and result in fire breakout, if adequate safety controls are not incorporated within the boiler system. Hence, for safe and sound operation of steam boilers, following safety controls must always be provided:
Following are the major problems associated with the use of steam boilers:
They are also referred to as hydronic boilers. They are frequently applied in residential & commercial buildings for heating purposes. They are usually manufactured in small sized portable units for domestic applications whereas large sized units are put into use for industrial applications.
One can chose among variety of fuels such as propane, electricity etc. for hot water boilers operation but the most commonly used fuel source happens to be natural gas owing to its cost effectiveness. These boilers are extremely durable and offer long operational life. Besides, their usage involves less complication as compared to other heating systems. However, the installation procedure of hydronic boilers is quite expensive.
Just like steam boilers, hot water boilers also exist in two different configurations i.e. the fire tube boiler configuration and the water tube boiler configuration. The fire tube boilers are also referred to as shell type boilers owing to their construction.
Below mentioned are the key components used in the construction of hot water boilers:
In a typical hot water boiler system, the fuel is introduced into a pressurized tank where combustion process takes place. A temperature controlling device called thermostat is included in the system which keeps a check on the fuel temperature. Within the pressurized tank, the water combined with a regulated quantity of air is supplied which initiates the fuel burning process. The products of combustion are then passed via pipe towards the cylinder which contains the water. Due to the heat supplied by the hot gases, the water inside the system gets heated. The resulting hot water is then finally distributed by means of an electric pump. The heated water is sent via another pipe to all the parts in the building requiring heat.
In hot water heating systems, the whole system is usually divided into different heating zones in a building. This zoning method offers following benefits:
Radiant heating is one of the oldest techniques for generating hot water heating. It is usually applied owing to its highly efficient heating effect. With the advent of new technologies, radiant piping systems have been made extra robust as well as inexpensive. Plastic tubing (which is a more economical choice than other piping materials) has made it conveniently possible for homeowners to have their floors, walls, driveways and pools heated hydronically. Distribution of heated water inside a hydronic boiler system can take place via following techniques:
Types of Hot Water Boiler Systems
There are four major types of hot water boiler systems available which are mentioned below:
Following are the significant features associated with the use of hot water boiler systems:
It is very important to carry out periodic maintenance of a typical hot water boiler system; otherwise the boiler may burst out resulting in fires or explosions. The cylinder used inside the boiler system must be regularly drained and cleaned to avoid clogging of pipes because of surplus mineral deposits. This mineral build up takes place due to the nitrate components present in the water.
Oiling of pipes must be regularly done in order to provide lubrication to them. Also, the pipes must be carefully inspected at regular intervals for detecting leakage points. Moreover, the pressure gauge incorporated within the system needs to be cautiously checked for any type of irregularity in the pressure. To ensure proper boiler operation, one must get the boiler inspected and certified by a qualified boiler professional.
The design of water tube boilers is strictly opposite to that of fire tube boilers design. In water tube boiler systems, the water to be heated is enclosed inside the boiler tubes whereas the hot combustion gases exhausted by the burner keep circulating around the tube surfaces. The water within the tubes then gets heated via the hot flue gases and finally converted into steam. The diameter of the boiler tubes is kept very small in these boiler systems to provide them the capability to withstand higher pressures for the equivalent amount of stress. Water tube boiler designs are particularly suitable for high steam output requirements. Hence, their use is mainly preferred in industries for various types of process applications over home heating systems.
Natural water circulation, also referred to as “thermo-siphoning” is the major working principle for almost all water-tube boilers. This principle is illustrated via the diagram shown below.
Cooler feedwater is introduced into the steam drum behind a baffle where, because the density of the cold water is greater, it descends in the ‘downcomer’ towards the lower or ‘mud’ drum, displacing the warmer water up into the front tubes. Due to continuous heating, steam bubbles gets created in the front side of boiler tubes. In the boiler drum, these bubbles get automatically separated from the heated water and finally carried out.
As soon as the pressure rises inside the boiler, the deviation between the water density and saturated steam density tends to reduce resulting in less circulation. To keep the same level of steam output at higher design pressures, the distance between the lower drum and the steam drum must be increased, or some means of forced circulation must be introduced.
Key features of water tube boilers are listed below:
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.
Owing to their superb working properties, the use of water tube boilers is highly preferred in following major areas:
Besides, they are frequently employed in power generation plants where large quantities of steam (ranging upto 500 kg/s) having high pressures i.e. approximately 160 bar and high temperatures reaching up to 550°C are generally required.
Major advantages of water tube boiler systems include:
Few limitations associated with the usage of water tube boiler plants are given below:
A boiler may be defined just as a pressurized vessel in which heating of water takes place. It can be used for production of either hot water or steam. The operating principle of hot water boilers i.e. hydronic boilers and steam boilers is more or less same. However, there are few significant points of distinction lying in their equipment and processes which are mentioned in the section below.
In general, both hydronic boilers and steam boilers results in heating of water and generation of steam by burning of fuel. The heated water is then passed via boiler tubes while the steam generated is circulated in the building with the help of radiators. Though everything works in a similar fashion yet sometimes, the steam produced in steam boilers gets collected in a separate container. The burning fuel employed in a hydronic boiler is typically the same as used in steam boilers. However, owing to cost-effectiveness, use of natural gas is sometimes preferred in hot water boilers.
Steam boilers and hydronic boilers tend to find some differences in their application areas too. Since steam boilers are more robust, they are frequently put into use for industrial applications where hot water or steam is required whereas the use of hot water boilers is generally limited to domestic heating applications.
Some important point of differences between steam boilers and hot water boilers are mentioned below:
In hot water boilers, the intensity of water boiling is extremely low. Their heating temperature usually doesn’t go beyond 180 or 200 degrees Fahrenheit. On the other hand, in steam boilers high intensity of water boiling takes place in order to convert water into steam. Hence, their heating temperature is very high i.e. around 212 degrees Fahrenheit.
A hot water boiler typically requires extra boiler accessories as compared to a steam boiler, for example, a flow check valve, a circulator, an expansion tank and especially a motorized pump.
Water Level and Circulation
A hydronic boiler must always be completely filled with water whereas in steam boilers, there is no such requirement. Besides, a hot water boiler requires an electric pump for circulation of hot water whereas in steam boilers, natural expansion of steam takes place which automatically fills pipes.
Efficiency and Safety
The operation of hot water boilers is usually considered to be safer as compared to that of steam boilers. Also, the efficiency of steam boilers is found to be less in comparison to hot water boilers. This is due to the fact that steam boilers don’t employ motorized pumps for circulation. However, the boiler efficiency also depends upon the kind of boiler.
The cost involved with steam boilers and hydronic boilers along with their accessories is approximately equal. The basic cost difference depends upon the choice of required fuel which can be either gas or oil.
Water Levels & Controls
Unlike steam boiler systems, water level in hot water boiler systems is usually maintained very high i.e. upto top of the water jacket. On the other hand, the steam boilers are not completely full of water; their water level is normally maintained below the top of the water jacket in order to facilitate steam generation. Steam boiler designs generally incorporate a float valve along with a sight glass which automatically controls the level of water. Steam is distributed in a building by means of radiators and eventually returned back to the boiler utilizing the gravity effect. In this way, the whole boiler cycle keeps on repeating without need of any special control devices or fittings such as circulators, flow check valves or automatic bleeders etc.
However, hot water boilers require several fittings and accessories for their proper operation. They incorporate a special temperature regulating device known as an aquastat which basically meets two major boiler objectives. It controls the temperature the boiler operates at, and provides a low voltage transformer and relay to control the circulator. If more than one circulator is employed then extra relays or a multiple control panel would be required for a hot water boiler operation. The pressure of water is usually kept in check by means of a valve which is referred to as a regulator. A special device known as check valve or backflow preventer is also required to be incorporated into hydronic boiler systems for preventing the migration of boiler water back to the supply water in case the pressure within the boiler rises above the domestic water pressure. Besides, a temperature-pressure relief valve is employed to keep the system temperature and pressure within maximum limits. Also an expansion tank is needed to facilitate expansion and contraction of water depending upon heating or cooling effect applied.
Due to the impracticality of hermetically sealing the heating system, air bleeders whether it be automatic or manual should always be incorporated into the water circulating loop of the boiler. For circulation of water in the heat loop, a motorized pump is typically put into use. There are basically two means by which hot water can be generated for domestic heating purposes. A tank-less coil (a coil of copper tubing with aluminum fins attached) can be immersed into the water jacket to create allow a heat exchange. A hot water maker is a storage tank with a coil inside through which hot water from the boiler is circulated for the heat exchange. Since heated water produced by a hydronic boiler may consist of anti-freeze or corrosion preventing chemicals, it is not permissible to draw hot water directly out of the boiler system.
Scale Build-up & Corrosion
Steam boilers and hot water boilers are usually subjected to diverse water treatment techniques.
Steam boilers are mainly provided with treatment which avoids build up of scale and corrosion. Scale build-up takes place inside a boiler due to the accumulation of dissolved minerals present in the boiler feed water. This boiler water loaded with minerals tends to replace the clean steam and condensate leaks. Strategies to prevent scale attempt to keep the components of scale such as calcium and magnesium suspended in the boiler water or to reduce their concentrations in the boiler water.
One of the common techniques used to trim down the concentration of solids inside a boiler is known as bottom blowdown. In this method, the water simply gets discharged from the bottom of the boiler. To avoid scale deposits, the sludge build up must also be frequently discharged from the boiler system. Now for eliminating corrosion, oxygen scavenging chemicals are generally mixed up into the boiler water. Finally, to reduce the possibility of scale build-up and corrosion inside a boiler, the regular checking of boiler water is required to be carried out.
The problem of scale build up is typically not encountered in hot water boilers. It doesn’t mean that the water fed inside a hydronic boiler is free of dissolved minerals. Actually the only difference is that the hydronic boiler systems are capable of discharging equal quantity of dissolved minerals as much is entering the system. Hence, the possibility of mineral accumulation inside the boiler gets totally eliminated. However, the hydronic boiler systems still face the problem of corrosion which can be solved via addition of oxygen scavenging chemicals only.
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