CAS Newsletter May 2011
IN THIS ISSUE
Boilers are the primary source of steam and hot water generation in industrial processing plants where steam demands vary very frequently according to the requirements. Hence, a well designed boiler must be capable of giving immediate response to these load demand variations. While doing so it must also maintain its efficiency and safety features. In order to achieve this, various techniques of boiler control are available. Some of these are meant as local boiler controls whereas the more advanced ones are said to be direct digital controls i.e. DDC.
Local boiler controls are generally employed for boiler systems functioning in stand alone manner. It is very necessary to set right water levels in the systems such as dry fired boilers owing to the risk involved with their operation.
On the other hand the direct digital controls are mainly applied in situations where multiple boilers are used. DDC controls are individual systems which are capable of handling multi- boiler staging as per demand changes. Moreover, they can control hot water multi-pump functional sequencing as well as home hot water services. Also, they help in collecting and analyzing surplus amount of boiler data. In addition to all, they are provided with an exclusive hot water reset schedule feature which results in smaller cycle rates than exterior temperature of air. These control techniques offer high adaptability and reduced running costs in boiler plant management.
In domestic applications a special device named aquastat is employed for boiler control whereas in commercial boiler applications involving large horsepower boilers far advanced control systems are used. While selecting a boiler control type for a particular application, one must consult experienced control manufacturers and engineers.
Types of Boiler Controls
Based upon the system specifications and user requisites, local boiler controls are directly manufactured inside the factories and made available outside as complete systems. Following are the major controls available for boilers:
Single-element Drum-level Control
This is a very simple kind of drum level control design. It needs single analog input for its operation and in return provides single analog output. Hence, it is named single element control. Because there’s no relationship between drum level and steam or feedwater flow, it can be applied only to a single feed pump on a single boiler supplying a relatively stable load. Its performance is not as effective as compared to other two level control designs. A typical drum level control with single element module is shown in the figure below.
Two- element Drum-level Control
This drum level control design is particularly suitable in case of single drum boilers where the feedwater is available at a constant pressure. Two-element control includes the same level element used for the single-element configuration but has an added steam-flow element that provides a density-corrected mass flow-rate signal to control the feedwater flow. In this design, presence of double control elements tends to provide rigid drum level control. A typical drum level control with two element module is illustrated in the figure below.
Three-element Drum-level Control
Three-element drum-level control is suited for handling variable feedwater pressure or multiple boilers with multiple feedwater pumps. In this design, three elements are used, each for controlling level, steam and feedwater flow respectively. This system offers far better and advanced drum level control as compared to all other systems. For best control, correct flow values of both steam as well as feedwater must be maintained with regard to density. A typical drum level control with three element module is shown in the figure below.
Boiler Efficiency is a term which establishes a relationship between energy supplied to the boiler and energy output received from the boiler. It is usually expressed in percentage. As a general rule,
Types of Boiler Efficiency
The efficiency of a boiler may be classified into following three major types:
Amongst all the three above mentioned boiler efficiencies, the fuel-to-steam efficiency is considered to give the most accurate representation of boiler efficiency on the whole. This is due to the fact that fuel-to-steam efficiency takes into account, the radiation and convection losses while performing efficiency calculations. Typically, it is the job of the boiler manufacturer to define boiler efficiency so that any type of economic analysis could be done properly.
Combustion efficiency generally gives an idea about the fuel burning capability of a burner. This type of efficiency is determined by the quantity of fuel which is left unburned in the boiler along with the surplus exhaust air. For getting high boiler efficiency, their burners should be well designed to provide low quantities of unburned fuel and excess air.
Combustion efficiency tends to vary with the types of fuel sources. In general, gaseous and liquid fuels result in very small amount of unburned fuel as well as 15% surplus air levels; hence they offer highly efficient burning as compared to solid substances. By operating at only 15% excess air, less heat from the combustion process is being used to heat excess air, which increases the available heat for the load.
This type of boiler efficiency is only used to assess the performance of heat exchanger units used in boilers. It basically determines the efficacy via which a heat exchanger would convey heat generated by burning process to the fluid in the boiler. While doing so, it does not take into account the radiation and convection losses occurring in the boiler sections. Hence, thermal efficiency is not considered valuable for economic analysis since it doesn’t reflect correct fuel consumption of a boiler system.
Fuel-to-steam efficiency is helpful in determining the overall efficiency of a boiler since it takes into consideration both the thermal efficiency i.e. heat exchanger effectiveness and the radiation and convection losses. This is the type of boiler efficiency which is ought to be used for making all types of economic assessments.
Methods of Determination
The two major methods employed to find out the fuel-to-steam efficiency of a boiler are explained below:
This method of efficiency determination largely depends upon the input-output ratio determination of the boiler. In this method, the output of the boiler derived in BTUs is divided by the boiler input supplied in BTUs and then the resulting number is multiplied by 100.
Heat Loss Method
It is also referred to as heat balance efficiency measurement method. This method of efficiency determination takes into account all kinds of heat losses occurring inside the boiler. The true boiler efficiency is calculated by summing up the percentage of all stack, radiation and convection losses and then finally deducting the resultant sum from 100 percent. This entire calculation will provide actual fuel-to-steam boiler efficiency.
Types of losses
Two major types of losses which take place inside a boiler system are mentioned below:
The stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler and reflects the energy that did not transfer from the fuel to the steam or hot water. In other words, it gives an indication about the quantity of heat energy lost due to dry exhaust gases and moisture loss. It is found valuable in determining the true efficiency of a boiler. A lower value of the stack temperature is always preferred for gaining well efficient heat exchanger performance and greater fuel-to-steam boiler efficiency.
Radiation and Convection Losses
Radiation losses are defined as the losses which occur due to radiation i.e. emission of heat energy out of the boiler whereas convection losses are the losses happening due to the air circulating around the boiler. Nearly all kinds of boilers experience these two significant losses. Radiation and convection losses, expressed in Btu/hr, are essentially constant throughout the firing range of a particular boiler, but vary between different boiler types, sizes, and operating pressures.
Boiler Design Criteria
To achieve high efficiency out of a boiler system, it must be designed in such a way that it meets all the required design criteria which mainly includes:
Some of the important boiler-efficiency deciding factors are explained in brief below:
It is also referred to as flue gas temperature. It is defined as the temperature level at which the hot exhaust gases make their way out of the boiler. The flue gas temperature must be a proven value for the efficiency calculation to be reflective of the true fuel usage of the boiler. Lower than real stack temperature values must always be used for boiler efficiency calculations.
The specification of a fuel source can immensely affect the efficiency of a boiler system. For example, if the hydrogen content within a gaseous fuel source is comparatively high, extra water vapors get generated in the burning process. These water vapors tend to consume heat energy from the boiler for shifting their physical state during combustion. The efficiency of the boiler generally drops if huge loss of water vapors takes place. Due to this, the fuel oil offers greater boiler efficiency as compared to natural gas. To get an accurate efficiency calculation, a fuel specification that represents the jobsite fuel to be fired must be used.
Excess air is defined as the amount of surplus air provided to the burner which is more than the necessary air needed to carry out combustion process. This given excess air mainly acts as a safety air reservoir for combustion in difficult situations such as inadequate air conditions.
However, at the same time, this more than required air tends to consume heat energy produced by combustion which in turn affects the heating efficiency of the boiler. Seasonal changes in temperature and barometric pressure can cause the excess air in a boiler to fluctuate 5% – 10%. A realistic excess air level for a boiler in operation is 15% if an appropriate safety factor is to be maintained.
The efficiency of a boiler also depends upon the ambient air temperature surrounding the boiler. For every 40 degree shift in ambient temperature, the efficiency of a boiler can get affected by at least 1%. Since all the boiler rooms are maintained at moderately warm temperature, majority of the efficiency computations takes upon 80 deg. F as the ambient temperature value.
Radiation & Convection Losses
These are the losses which emerge due to radiation of heat energy from the boiler. To eliminate the effect of these losses, boiler systems are usually shielded with some sort of insulation material. The presence of these losses extremely influences the efficiency of a boiler. In cases where these losses are not taken into consideration while performing efficiency calculations, accurate fuel consumption value can never be attained.
A boiler must always be designed in such a way that the radiation and convection losses get minimized. These losses tend to increase in proportion to the wind or air velocity prevailing around the boiler. Hence, the boiler systems located in open atmosphere experience more radiation and convection losses as compared to room boilers.
All boiler systems employ a fuel mechanism which basically comprises of the apparatus needed to supply fuel for heat generation. The design of the apparatus in use varies according to the kind of fuel employed in the system. Variety of fuels is available for application in boilers, each having different chemical properties. Major chemical characteristics of boiler fuels include:
Their energy measurement unit is either BTUs i.e. British Thermal Units or KWs i.e. Kilowatts. One can convert between the two units by means of conversion factor i.e. 3.46 BTU/W.
Each and every fuel whether it is in solid, liquid or gaseous form is explosive and can prove hazardous if not used according to the recommended safety guidelines. When monitoring the efficiency of a combustion process, it is important to know the fuel being burned since this information will help not only determine a boiler’s optimal working conditions but also maximize the boiler’s efficiency.
Commonly Used Fuels
The list of major fuels which are employed in boiler systems is given below:
Electricity is an alternative source to gaseous fuels employed in traditional boiling systems. It tends to give a heating value around 3.4kBTU per kWh. Use of electricity in boiler systems offers various advantages which are mentioned below:
LPG (Liquid Petroleum Gas)
In practice, LPG is mainly used in applications where the availability of natural gas is either very limited or costly. LPG works as a boiler fuel in similar manner as natural gas. However, the boiler in use must be capable of conversion features so that it could be made compatible with LPG.
Major solid fuels used for burning in a boiler include coal and wood. They were the only fuel sources available for use in boiler systems prior to the emergence of heating fuel oil. These are the cheapest means of boiler fuels which are getting exhausted day by day due to uncontrolled use. However, boiler systems employing coal or wood as fuel source call for some extra precautions and care due to the reasons mentioned below:
Different forms of coal are available for burning process, most common among them are listed below:
With the advent of fuel oil, solid fuels such as coal and wood increasingly got replaced in nearly all parts of the world owing to its cleaner and ash free combustion process. Oil fuels generally consist of less carbon content as compared to solid fuels such as coal, which in turn results in less emission of carbon dioxide upon combustion. On the other hand, oils contain higher carbon content as compared to natural gas producing high amounts of carbon dioxide due to burning.
The fuel oil employed for boiler use is mainly manufactured from a mix of extremely heavy hydrocarbons, which tend to contain relatively high amounts of hydrogen content in comparison to coal. Burning of a fuel oil usually produces same kind of pollutants as produced with burning coal. Heating oil is a boiler fuel which is widely employed in the northeastern areas of the United States whereas in other parts of the world, it is facing severe competition with the gaseous boiler fuels available.
Diversity of oil fuels is available for heating such as oil #2, oil # 4, and oil # 6. Fuel oil #2 is popularly referred to as the home heating fuel. It is almost identical to the diesel oil fuel which is largely employed in vehicles and automobiles. Its energy value is found to be approximately 139 kBTU per gallon.
Boiler and heating systems that employ oil for its operation happen to be more expensive than gas powered boiler systems since they need complicated burner mechanism as compared to their gas counterparts for efficient firing. However, at the same time, this difficulty of ignition (or firing) in case of heating fuel turns out to be a great plus point since it results in safer storage of fuel oil in comparison to gas. Otherwise, a leakage in the fuel tank could prove to be very costly and hazardous.
Natural gas is the key fuel source for boilers which is widely employed in United States and Europe for home heating needs. It is largely prepared from methane along with a mixture of few other gases in small proportions. Natural gas has an energy content of about 100kBTU per therm or 103kBTU per ccf (100 cubic feet). However, anecdotal evidence suggests that the actual heating value of “Natural Gas” coming out of the distribution pipe may vary from as little as 60kBTU up to 160kBTU per ccf.
Natural gas can be conveniently put into use for boiler applications since it can be transported easily via gas pipelines when in gaseous sate and trucks or ships when in liquid state. Very less amount of air is needed for burning of natural gas owing to its unique C/H2 ratio. This fuel contains quite low values of carbon and high values of hydrogen because of which the combustion of natural gas results in production of less greenhouse gases which are considered to be highly responsible for global warming. Also, the burning of natural gas is found to be very clean as compared to the burning of oil and solid fuels oil.
In general, an equivalent amount of natural gas burns to generate approximately 30% and 45% less carbon dioxide than heating oil and coal respectively. Besides, carbon dioxide gas, burning of natural gas emits an ingredient called NOx whereas the quantity of sulfur dioxide i.e. SO2 and other emission particles is almost insignificant. However, if the burning of gas takes place facing scarcity of combustion air, there is a possibility of volatile hydrocarbons generation which is very unsafe to human health and surroundings. Hence, care must be exercised to avoid these hazardous possibilities.
Nowadays, natural gas fuel reserves are getting exhausted at a rapid rate. Hence, substitutes for this boiler fuel need to be discovered very soon.
Propane is a boiler fuel which is basically manufactured out of refining process carried out for petroleum goods. It is generally carried and delivered to the usage point with the help of pressurized gas containers. Unlike natural gas, the energy content of propane is measured in gallons instead of cubic feet. It usually consists of 91 kBTU per gallon. In general, propane is available at a fairly higher price as compared to natural gas. Just like natural gas, it offers simple and economic integration into the forced-air heating systems, predominantly in the US market.
Renewable Energy Sources
Variety of renewable energy sources is available for use as boiler fuels. Most common of them includes solar energy and wind energy. The energy from solar radiations can be utilized in many devices such as solar water heating systems and solar cell systems whereas wind energy can be easily used in wind generators. This category of fuel resources proves to be extremely useful for people residing in distant areas where other fuel sources are difficult or costly to achieve. Hence, places which get abundant supply of sun heat are the justified locations for usage of solar energy. Moreover, photovoltaic systems operating via solar energy can be used for production of electricity to meet the power requirements in these areas.
Some other fuels commonly used for consumption in boilers are listed below:
All these fuels have unique combustion characteristics which tend to influence the performance efficiency and emissions of the burning process.
Both hydronic boilers as well as steam boilers require a water source for their operation. This boiler feed water can be obtained either in the form of city water supply or well water that is provided by the pump. Barring any leaks, drips, or weeping in your system, the supply from the well or city is necessary to keep the water level at desired levels.
A boiler fired in its dry state may lead to hazardous results. Hence, it is very crucial to maintain appropriate water levels in a boiler system. In case of hydronic boilers, the system must be entirely filled with water whereas in steam boilers, the level of water should not touch the mains otherwise the system will not work properly. In steam boilers, a control at the water supply must always be maintained to limit the water levels.
A gate or ball valve is usually employed to stop the supply of water in periods of maintenance or water leaks. When the water supply is put to a halt, the power and fuel source supply should also be cut from the boiler system. After the shut off valve there should be a back flow preventer, however there may be some older systems that do not have back flow preventer’s. Current local and national codes require back flow preventer’s to keep the supply water from being contaminated by back flow water from the hot water loop. Subsequent to the back flow preventer, a pressure reduction valve should always be mounted for the purpose of bringing down the supply water pressure upto 12 P.S.I. After the pressure reducer, the source supply water should be fed into the return or supply loop depending on the application and type of system.
The boiler feed water must undergo various treatment and conditioning processes before entering the boiler system due to following major reasons:
Two major ways via which the boiler water supply can be purified include: External water treatment & Internal water treatment methods.
External Water Treatment
In this method, the boiler feed water is treated and purified outside the boiler system. This method of water treatment is normally chosen when the levels of impurities present in feedwater reaches so high that they can damage the inside structure of the boiler system in use. Hence, the impurities must be removed from feed water before water enters inside the boiler via external method.
Internal Water Treatment
In this method, the treatment & conditioning of water impurities is carried out inside the boiler only. The necessary reactions tend to take place either in the water feed lines or exactly in the boiler. This method of water treatment can be employed either alone or in combination with external water purification techniques. Internal water treatment method basically fights with following feed water problems:
An engineering unit falls under Power group, commonly used by automation professionals.
Conversion to SI and other common units
1 Horsepower (boiler) equals to:
Communication Protocol Support
This engineering unit is supported by the following communication protocols:
1. CIP (includes Ethernet/IP)
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