June 2011 Newsletter

• Boilers Part 1
• CAS BACnet Explorer 2 For 1 Sale
• Engineering Units – Btu (IT) per hour
• Demand Load Management
• Steam Boilers
• Hot Water Boilers
• Fire Tube Boilers
• Water Tube Boilers
• Steam Boilers Vs. Hydronic Boilers
• Sixth Sense Technology

Engineering Units – Btu (IT) per hour

1. The “IT” suffix indicates that this is the International Steam Table definition.
2. It is also known as Btu per hour, Btu/hour, btu/hr. This unit is commonly used in the UK, US unit systems. British Thermal Unit (IT) Per Hour (Btu (IT)/h) has a dimension of ML2T-3 where M is mass, L is length, and T is time. It can be converted to the corresponding standard SI unit W by multiplying its value by a factor of 0.29307107.

Conversion to SI and other common units

Conversion to SI and other common units

Communication Protocol Support

This engineering unit is supported by the following communication protocols:

1. BACnet

2008 specification.
Enumeration=50

2. CIP (includes Ethernet/IP)

Enumeration=0×2607
Base Unit=2.930711 • 10^-1 • W

Demand Load Management

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.

Main Features

Major features of demand-load management in boiler systems are listed below:

1. Base load or modulating operation
2. Parallel or serial demand sharing
3. Boiler banking operation and programmable sequence selection

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

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

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:

• Building materials
• Construction methods
• Mechanisms like consistent heat-up and cool-down

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:

1. Fire Tube Boiler Configuration
2. Water Tube Boiler Configuration

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.

Main Components

Following are the major components incorporated inside a steam boiler system:

• Steam valves: These valves are mounted upon radiators to allow the expulsion of air from the pipes and keeping the steam inside. However, in cases where steam starts escaping from the system, a white calcium layer is formed around the steam valves. Hence, this situation can be easily detected and corrected.
• Gauge glass: This device is basically used to indicate the level of water inside the boiler. Typically, the average water level is in the center of the gauge glass. But once the burner starts operating, the level of water changes to small extent.
• Low-water cutoff: According to code requirements, this device is generally used to stop the burner in situations where the level of water drops below the required water level limits.
• Automatic water feeder: This is an automatic device which is used for the purpose of feeding water into the boiler when the level of water decreases. Use of this device is optional and entirely depends upon the preference of the homeowner. However, a homeowner must always keep a periodic check on the water level in boiler even if an automatic water feed is employed.
• Pressure relief valve: This valve is used for safety purpose since it protects the boiler system from getting over pressurized.
• Steam limit control: This device is also referred to as pressuretrol. Its sole purpose is to cut off the burner as soon as the adequate steam pressure is build up.

Safety Controls

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:

• Limit switches: These are the safety switches which are basically included in the system to detect the increase of temperature beyond a certain limit. As soon as the temperature inside the boiler crosses the defined limits, a limit switch operates resulting in the shut down of the whole system. It is usually mounted near the draft diverters. It proves to be a useful device in preventing fire breakouts and plugging up of chimney or flue gases. It helps in avoiding carbon monoxide poisoning or any other dangerous situation related to temperature rise.
• Low water cut-off: This safety feature should always be incorporated in the boiler system for keeping a check on the level of water in the boiler. The normal level of water is in the half-way in the gauge/ sight glass which should always be maintained. For carrying out a regular visual inspection of water level, the sight glass indicating the level must be kept clean and free from dirt. Lower than average water level in the boiler may lead to undesirable results.
• Automatic water feeder: This device is generally employed for automatically feeding water into the boiler system whenever the water level drops below a certain point. Appropriate water level will result in generation of high quality steam heat. However, small adjustments are needed in the system to maintain correct water level i.e. neither too much high nor too much low. Since excess water will not heat up the system whereas scarcity of water may cause hazardous situation.
• Pressure cut-off control: Various devices such as pressure relief valves and blow-down valves etc. are employed in the boiler system for exercising control over excess steam pressures.

Major Problems

Following are the major problems associated with the use of steam boilers:

• Water Hammer or Banging Pipes: This is a very frequent problem which mainly takes place due to the mixing of condensed water with the steam. Owing to the cold nature of water and hot nature of steam, the pipes in the system tend to expand and contract very quickly. Because of this pipe action, a sound resembling to pipe beating with a hammer somewhere down in the basement is produced, hence it is named water hammer problem. The problem can be raised in the system when the condensed water flows back via the Hartford loop towards the boiler. It could mean a strainer or trap is plugged with sediment and limiting the return of condensate water returning to the boiler. The automatic water feed employed in the system will continuously work to maintain the proper water level in the boiler. However, the water level in the system will become too high that the mains will overflow. Hence, to avoid this problem, it is necessary to maintain the efficiency of return water piping system. It is very difficult to totally eliminate this problem but it can be reduced to a great extent with the help of a professionally qualified HVAC consultant and use of appropriate devices.
• Leakage of Water via Steam Vents: In case of single pipe steam systems, steam radiators are usually not kept on level. Hence, when the floor tends to settle, the radiator results in leakage of water from the steam valve. To avoid this, a shim can be positioned beneath the radiator legs opposed to the pipe which will enable the condensed water discharged from the radiator to flow back towards the boiler via the return piping system. If the problem persists even after employing a shim in the system, an HVAC professional must be consulted.
• Radiators not heating: There can be various reasons lying behind this problem based upon the type of radiator employed in the system i.e. whether it is two-pipe or single-pipe radiator. To detect the cause of this problem, first of all one should verify that the valve is in open position at the bottom of radiator except in cases where a thermostatically controlled valve is used. In case of single pipe steam systems, the thermostatically controlled valve is located at the steam vent. On the other hand, in case of two-pipe systems, this valve is usually mounted upon the supply pipe approaching towards the radiator. If the thermostat is completely turned down the valve will close whereas if the thermostat is altogether turned up, the valve will accordingly open up. Even if after applying all these techniques, nothing solves the problem then HVAC professional needs to be consulted for advice.
• Total Dissolved Solids (TDS): This problem usually occurs in the boiler when a proper and regular blow-down of the boiler is not being performed. Blow-down always needs to be done at regular intervals and a pre-defined frequency. In a boiler system, when water converts into steam, the dissolved solids are left behind in the boilers. If these solids are not timely discharged, then they tend to build up and result in various problems. Besides, they affect the efficiency and safety of the boiler system. This is something usually done by the homeowner or maintenance person (if it is a business) but should be done according to the advice of an HVAC professional as far as blow down frequency is concerned.
• The build-up of carbonic acids can eat through hydronic piping and needs to be addressed by either venting oxygen and CO2 or having water treatment.

Hot Water 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.

Major components

Below mentioned are the key components used in the construction of hot water boilers:

1. Thermostat which is mainly employed for controlling heat and temperature inside the system.
2. A gas valve for regulating fuel gas flow
3. A pressure gauge which is basically used to indicate the pressure of the water in the boiler.
4. Water feed valve for adding water into the boiler system.
5. Pressure reduction valve for lowering pressure inside the system.
6. An air vent which allows the escape of redundant air from the system.
7. An expansion tank which enables expansion of water upon heating.
8. A flow control valve for controlling the flow of boiler water
9. A pressure relief valve for pressure control
10. A circulator which is usually a motorized electric pump used for circulation of water throughout the system.
11. A drain valve

Working

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:

1. It results in efficient heating.
2. It makes the living extremely comfortable.
3. It makes the boiler operation very simple.
4. It offers extremely cost-effective heating solution.

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:

1. Radiators
2. Baseboards
3. Convectors and
4. Vents, also referred to as hydro-air systems

Types of Hot Water Boiler Systems

There are four major types of hot water boiler systems available which are mentioned below:

1. Closed System: In these types of systems, the water which gets evaporated and converted into steam is again put back into use by condensing the steam back to liquid form. This means that the 100 percent reutilization of the water takes place in closed systems.
2. Open System: In these systems, the water is heated up but the evaporated water is not put back for reutilization inside the system.
3. Single-pipe System: In these systems, two pipes are utilized. One pipe is used for carrying the heated water to the required location whereas the second pipe is used for returning the cool water back towards the boiler by means of a motorized pump.
4. Gravity System: These are the older hot water boiling systems in which the water is carried back after heating via the gravity effect. Hence, in these systems, circulators are not required.

Main Features

Following are the significant features associated with the use of hot water boiler systems:

• Although the installation of hot water boiler systems is extremely costly yet they are frequently employed for heating applications owing to their high efficiency and cost in comparison to forced-air boiler systems.
• Since the hot water boiler systems are made up of aluminum fins and copper pipes, they tend to employ less metal for their construction and hence occupy comparatively smaller area.
• Hydronic boilers typically offer small and uniform values of temperature as compared to forced-air boilers. Moreover, in these systems, the on-off switching action is usually prevented because of the ability of baseboard pipes to grasp the heat and liberate it over longer periods of time.
• Another important feature regarding hot water boilers is that they dry the air inside the system to a very small extent only.
• A Hydronic boiler does not allot allergens, dust, combustions or mold by any products that enter living space. This is always an advantage, especially for a family that is sensitive with allergies.
• The piping mechanism within a hot water boiler system is very exceptional and its correct implementation is very much responsible for the proper operation of the boiler.
• A single hydronic boiler system per home is usually found adequate to produce necessary heating effect, unless it’s a big triple story house.
• Hot water boilers are mainly employed for domestic heating applications, generally in the northern parts of Europe.
• Hydronic boiler systems are mainly applied as central heating system inside buildings to give heating effect to the areas, which are otherwise very cold.
• A typical hot water boiler operates throughout the winter with a water temperature determined by the Aquastat, or low limit, setting (an Aquastat is like a thermostat that senses the water temperature inside the boiler and shuts off the burner when the water is hot enough).

Maintenance

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.

Fire Tube Boilers

They are popularly known as Shell boilers since in these boilers all the surfaces contributing towards heat transfer are enclosed inside a shell made up of steel. They can be also called as Smoke tube boilers. As the name suggests, in fire tube boiler systems, the hot exhaust gases emanating out of combustion reaction are passed through the boiler tubes. These tubes are in turn surrounded by the water which needs to be heated. Heat is then transferred to the circulating water via the hot gases produced by the burner. In firetube boilers, the combustion gases pass inside boiler tubes, and heat is transferred to water on the shell side. In this way the water gets converted into steam for utilization into various process applications. The most widely used boilers for industrial applications are Scotch marine boilers.

Scotch Marine Boiler

Scotch marine boilers are typically cylindrical shells with horizontal tubes configured such that the exhaust gases pass through these tubes, transferring energy to boiler water on the shell side.

This type of fire tube boiler is considered to be an industry workhorse owing to its several advantages. Most common of them include:

• They are highly proficient and strong.
• Their use proves to be very economical because they involve very low initial price.
• These types of boilers usually hold quite huge quantities of water due to which they are capable of retorting to load changes occurring with reasonably small variations in pressure.

However, this category of boilers suffers from few limitations too which are listed below:

• Since, these types of boilers consist of large amount of water; they consume somewhat extra time to set off steaming operation and put up with variations occurring in steam pressure.
• In these types of boilers steam generation takes place in the shell portion which offers a big surface area. Due to this fact, the quantity of pressure generation tends to be less.
• Typically, the use of Scotch marine boilers is not recommended in areas requiring high pressures (normally beyond 300 psig).

Main Features

Key features of fire tube boiler systems include:

• Firetube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water.
• Each pass enables the hot combustion gases to pass through the boiler tubes in the different direction. Every time, for conceiving next pass, the flue gases in the boiler make a shift of 180 degrees and go back via the shell.
• A typical three-pass boiler consists of three sets of boiler tubes whereas a four-pass boiler contains the four sets. The stack outlet in case of former one is normally positioned at the back end of the boiler while the latter one will have the stack outlet located at the front end.
• Various types of boiler tube arrangements are available for fire tube boiler systems depending upon the number of passes made by the hot flue gases emerging from the boiler furnace till the time they get cleared out of boiler system.
• In case of boilers made up of plain steel, the hot combustion gases going in to the reversal chamber are required to be cooled down to a temperature of 420°C prior to their entrance whereas in case boilers having alloy steel construction, this temperature should be at least 470°C. If the temperature of combustion gases happens to be more than recommended then problems like overheating and cracking of boiler tubes may take place.
• Since a fire tube boiler system consists of a considerable quantity of water within it maintained at saturation temperature point, it usually serves as significant energy storage ground to deal with very quick short term load applications. However, this fact can pose a major limitation also since re-buildup of energy reserve would be required which may consume large time.
• Nowadays, the largest shell boilers offering 1,500 boiler horsepower i.e. approximately 50,000 lbs/hr have been made available.

Different Configurations

Fire tube boilers come in variety of configurations. The two most common configurations include:

1. Dry back boiler configuration
2. Wet back boiler configuration

The above mentioned two-pass boiler configuration types are shown in the figure below.

In the first configuration i.e. dry back boiler configuration, the reversal of hot combustion gases is done via a refractory lined chamber located at the external covering of the boiler.

The another one is wet back or water back boiler configuration which is considered to be extra efficient than the former one since in this kind of arrangement the main chamber for reversal of hot gases is totally enclosed inside the boiler. No refractory lining is required in this configuration since the turn around zone is water cooled in this case. Hence, it makes available larger area for transfer of heat. Also, it helps in heating boiler water at the side of the chamber wall where the heat generated from the furnace is usually found to be of maximum intensity.

Applications

The fire tube boiler systems mainly find their use in following areas:

• For operation of steam locomotives
• In small factories and industrial processes
• For home heating systems

Advantages

Following are the major advantages offered by fire tube boiler plants:

• The entire plant may be purchased as a complete package, only needing securing to basic foundations, and connecting to water, electricity, fuel and steam systems before commissioning. In this way, the installation cost of fire tube boiler systems gets reduced.
• Also, due to their availability as packaged systems, they can be easily relocated from one place to another.
• The construction and maintenance routine of a typical fire tube boiler happens to be very simple.
• Fire tube boilers usually consist of a single furnace tube and a burner. Hence, the control systems meant for heating purpose inside these boilers is kept very simple.
• Due to their comparatively low operating pressures, the accessories needed to support fire tube boiler systems can be obtained at very economical rates.
• These types of boiler systems are found to be very fuel efficient. Besides, their operation is very easy.
• They provide very cost-effective heating solutions.
• Their cleaning procedure is very simple.
• They are available in compact size and form. They generally exist in size ranging from 600,000 btu/hr to 50,000,000 btu/hr
• They are designed in such a way that replacement of shell boiler tubes is very simple
• They are widely put into use for space heating purposes and variety of industrial applications.

Disadvantages

Major disadvantages associated with fire tube boilers are mentioned below:

• As a general rule, the maximum output generated out of fire tube boiler is around 27 000 kg / h. In case of high steam requirements, one needs to combine multiple boilers together.
• Besides, the fire tube boiler systems are not capable of working with applications which involve high operating pressures i.e. beyond 250 psig. This limitation is experienced due to the large diameter cylindrical construction of fire tube boilers. For getting higher pressures, water tube boilers are generally employed.
• Also, these boiler systems are not considered suitable for processes where high capacity steam is required.

Water Tube Boilers

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.

Operating Principle

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.

Main Features

Key features of water tube boilers are listed below:

• They usually exist in larger sizes as compared to fire tube boiler designs. The size of a typical water tube boiler can go upto quite a few million pounds per hour of steam generated.
• They are proficient in dealing with steam pressures which can be as high as 5,000 psig.
• Their recovery is much quicker than their firetube counterparts.
• Besides, they are designed to attain extremely high temperatures
• Water tube boilers which are smaller in size can be easily prepared and made available in the form of single packaged units similar to packaged firetube boiler units. On the other hand, large sized water tube boiler units are unable to be manufactured in packaged form; hence the large sections are carried to the work site where they are finally assembled.
• A typical water tube boiler system consists of several burners positioned on the walls, each having a choice of either horizontal or vertical firing. Also, temperature control option is available in many sections of the boiler. These features must be specifically considered when the boiler contains an integrated superheater whose temperature is required to be controlled for getting adequate superheated steam.

Applications

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:

• Variety of process applications in industries
• Chemical processing divisions
• Pulp and Paper manufacturing plants
• Refining units

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.

Advantages

Major advantages of water tube boiler systems include:

• These boiler systems contain very small quantity of water hence; their response is fairly quick towards variations in load and heat supply.
• The small diameter tubes and steam drum mean that much higher steam pressures can be tolerated, and up to 160 bar may be used in power stations.
• Water tube boiler systems are mainly applied in applications requiring high steam pressures (i.e. upto 3,000 psi) owing to their capability to tolerate higher pressures built up inside the boiler tubes.
• Moreover, watertube boiler designs provide highly efficient performance due to which they can be even employed to produce saturated or superheated steam.

Disadvantages

Few limitations associated with the usage of water tube boiler plants are given below:

• Unlike fire tube boilers, these boiler systems can not be manufactured in packaged form. Hence, their installation procedure at the job site tends to be comparatively difficult and time consuming.
• The installation and initial cost of these boiler systems happens to be very high.
• Although one can employ several burners together to attain a flexible system yet the use of more than 30 burners in power generating plants is generally avoided due to increased complexity.
• As opposed to fire tube boiler systems, the water tube boilers are not available in compact physical sizes. Besides, their cleaning process is quite complicated.
• Moreover, replacement of tubes is not possible in water tube boilers since there is no accord between tube designs.

Steam Boilers Vs Hydronic Boilers

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.

Major Differences

Some important point of differences between steam boilers and hot water boilers are mentioned below:

Heating Points

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.

Accessories

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.

Cost

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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