In this article we will discuss about the performance test and efficiency calculation of boiler.
Feed Water:
The measurement of flow rate of feed water can be done by different methods. One of the best methods, if it can be arranged, is for the volume of feed water to be measured by means of two vented calibrated tanks of relatively large capacity arranged one above the other.
The feed water is run through a quick-closing valve into the top tank, which is provided with an external gauge glass. This tank is calibrated beforehand and the gauge glass marked with divisions corresponding to definite volumes of water. By this method great accuracy can be obtained in the measurement of the feed water used.
When measuring the water by volume, its temperature must of course be noted to enable volume to be converted to weight this can be done by the immersion of a mercury-in-glass thermometer. The bottom tank should be at least 25 percent larger than the top one. The water passes from the top tank to the bottom through a valve before being sucked out by the boiler feed pump.
ADVERTISEMENTS:
A full tank is generally used over the period of the boiler trial. The time of the trial will invariably be measured to the nearest minute. As volume of water and not time is the deciding factor, the test cannot be planned over a definite number of hours.
This method might have its disadvantages as required for manipulation of the tank system. Nevertheless, the tank method has many good points, the most important of which is the greater accuracy which can be obtained as to the weight of water used.
An alternative method of measuring the feed water is by means of a rotary meter inserted in the pipe line. Where reciprocating feed pumps are used this type of meter should be placed as far from the pumps as possible to eliminate errors due to pulsation effects.
When this alternative method is used the temperature of the feed water should be obtained correctly. The thermometer pocket inserted in the pipe line for this purpose should be after the meter, and should not be so large as to restrict or upset the water flow.
ADVERTISEMENTS:
A constant and steady water level should, if possible, be maintained in the boiler throughout the test; this is important, as it eliminates the necessity for priming, and the poor quality of steam that results from fluctuations.
Steam:
The measurement of flow rate of steam can be made by various flows measuring instrument. To maintain a check on the water evaporated, the steam generated by the boiler can be measured by some suitable means (e.g., orifice or venturi meters), provided that all the steam generated is measured. The fact that this cannot always be done satisfactorily, owing to the number of auxiliary supplies fed by the boiler, is usually the sole reason why steam measurement is not more widely adopted in testing industrial plants.
For purposes of calculation, a test carried out on a boiler without superheaters assumes the generation of dry saturated steam. This is impossible to obtain. The steam, when leaving the boiler, is in contact with water at boiler level; its heat content, as used in the calculations, is therefore too high and leads to an error of 1 or 2 percent, depending upon the degree of moisture present in the steam. To compensate for this error accurately, it is necessary to measure the dryness of the steam by means of a calorimeter.
In calculating a net economic heat balance the steam used by boiler auxiliaries such as steam-driven feed pumps, furnace jets, etc., must be taken into account, and the proportion of steam lost must be deducted from the total heat generated by the boiler.
Fuel:
ADVERTISEMENTS:
The solid fuels are generally used in boiler.
The analyses can be carried out on at least two samples of the fuel as fired-the first to be analyzed for its chemical constituents and the estimation of its gross and net calorific values and the second to be analyzed for free moisture.
In addition, it is necessary to take samples of ashes, clinker, and riddling, to analyze for unburnt carbon.
It is most important that the raw coal samples be taken in the correct manner to obtain as accurately as possible an average of all the fuel burned. The samples should be taken, say, every 30 minute during the boiler trial.
Duration of Boiler Test:
ADVERTISEMENTS:
If only one boiler is being tested and the steam load is absolutely steady over a long period the test need occupy no longer than six to eight hours provided settled conditions are obtained beforehand. If, on the other hand, two or more boilers are being tested together and/or the load is fluctuating, the minimum time for the test will be decided by the length of the steam demand cycle.
In other words, if for example the steam cycle fluctuates over twenty-four hours this should be the minimum period of the test; for greater accuracy, of course, the test should include the steam cycle several times.
Flue Gas Sampling:
As the chemical composition of the flue gases is constantly changing, it is best to measure the CO2, CO, and O2 by means of a recorder. This type of instrument will give a continuous record of analysis over the duration of the trial and will consequently provide more accurate average. The instrument like orsat apparatus, NDIR, Gas Chromatography can be used.
The samples for analysis should be taken at the boiler exit, at the entries and exits of the auxiliary plant (i.e. economizer and/or air heater) if fitted; and certainly at the nearest predictable point to the fuel bed.
ADVERTISEMENTS:
It is usually assumed that if the CO2, CO, and O2 quantities are measured and the balance to 100 percent taken as nitrogen, then the analysis is complete. Such is not always the case- when the CO2 content reaches about 15 percent upwards during firing, varying amounts of methane (CH4) and hydrogen (H2) may appear in the gases leaving the boiler.
The presence of these combustible gases can make a difference of 1 or 2 percent to the figure for the “unaccounted” gases leaving the boiler. If greater accuracy is required in the heat balance, therefore, these gases must be measured.
Flue Gas Temperature Measurement:
In the boiler there is a thermal equilibrium, in which several factors contribute to this unbalanced state, e.g., the cooler surfaces surrounding the hot gas; the size, shape, and position of the temperature measuring instrument; the velocity and stratification of the gas; combined with radiation effects, etc.
The metal-sheathed thermo-couples are inserted in the side flues of a Lancashire boiler. Here, the thermocouples record a temperature between that of the gases and that of the surrounding boiler steel shell and brick walls; i.e., though heat is absorbed by the couples from the gases, a certain proportion is lost by radiation from the couples to the cooler surrounding steel and brick walls and by conduction from the thermo-couple leads.
From this it is evident that the smaller the temperature difference between the gases and surrounding walls, the smaller will be the error in temperature measurement. The errors in measurement can be eliminated in thermo-couples to a great extent by using the thinnest wires possible and by shielding them with metal of low emissivity so that the minimum of heat is lost to the surrounding walls.
Air:
The temperature of the air supplied for combustion should also be measured. This can easily be obtained by an ordinary mercury-in-glass thermometer, special care being taken to place the thermometer in such a way that it is shielded from direct radiation from the boiler. It is also advisable to measure the outside air temperature and a comparison of the two temperatures is an indication of the amount of useful radiation.
Draught:
The draught measurements afford the simplest and the most effective means of controlling the dampers, as they make it possible accurately to regulate the amount of excess air supplied for combustion. During the actual boiler trial the draught should be adjusted to the minimum required for the load. An ordinary vertical or inclined water gauge will be sufficient to give all the information required.
External Heat Losses from Boiler and Brick Work:
Undoubtedly the best method of calculating the external heat losses from the boiler brickwork and other exposed surfaces is by measuring the temperatures by means of a surface pyrometer. The total heat loss by radiation and convection can then be easily computed. When the velocity of the air currents over the exposed surfaces is also measured, the effect of forced convection is calculated.
If a surface pyrometer is not available, some indication of these losses can be obtained by careful measurement of the surface temperatures with a mercury thermometer protected from the air currents.
Auxiliary Plant:
Additional plant attached to the boilers (e.g. superheaters, economizers, and air heaters) needs to be tested if the overall efficiency of the boiler plant is required.
Superheaters fitted to shell boilers invariably receive their heat by radiation from flames and brickwork. It is not possible, therefore, to measure temperature drops on the gas side; hence the only temperatures that can be measured are those of the steam entering and leaving the superheater.
The addition of superheaters to a boiler will, on an average, effect a saving in fuel of about 1 percent for every 40°C rise in steam temperature above that of the steam generated at boiler pressure.
Where economizers and/or combustion air pre-heaters are installed, six additional observations are necessary, i.e., the temperatures of the gases and water and/or air entering and leaving the plant and also the pressure losses through these auxiliaries.
It is important to obtain as accurately as possible average temperatures of the gases and air; to achieve this it is often necessary to traverse the flue and duct sections to find out what fluctuations in temperature occur.
A gas analysis at the gas entry and exit of these auxiliaries will also be helpful, as any reduction in CO2 content will indicate air leakage. Also, any increase of draught loss on the waste gas side would probably point to clogging up by deposits.
The surface heat loss can also be accounted by measuring the external surface temperatures and air currents which increase the loss by forced convection.
Air Infiltration and By-Passing:
The heat loss by air leakage, especially through brickwork settings is also important. While the CO2 content of the flue gases by analysis should indicate whether it is occurring. As a check, an inspection of the settings should be carried out by means of a naked light if there are serious leaks, they will become apparent.
Another common source of leakage is to be found on the gas side in Lancashire boiler setting, between the down take and the side flues at the back of the boiler.
During a boiler test, however, special care should be taken to avoid all air leakages in the flues, as these will prejudice the gas analyses and the temperature measurements.
Performance Test:
The purpose of the performance test is to determine actual performance and efficiency of boiler. It is an indicator of operational conditions of boiler and basis for improvement of performance.
These are two parameters such as:
(1) Efficiency of boiler.
(2) Evaporation ratio.
Thermal Efficiency of Boiler:
The efficiency is defined as ratio of heat output to the heat input in percentage.
Some controversy exists at present among engineers as to whether the boiler efficiency should be expressed in terms of the gross calorific value of the fuel as fired, or its net calorific value, or “by difference”.
The gross calorific value is, of course, the total heating value of the fuel per unit of weight or volume, including the heat of oxidation of the hydrogen in the fuel to water (steam) plus the latent heat of the total moisture in the fuel. On the other hand, the net calorific value of the same fuel is the total heating value of the combustible elements only, and excludes the heat of formation of water from the hydrogen and the evaporation of the moisture in the fuel to steam.
Some engineers prefer to quote their boiler efficiencies “by difference”, which means that the figure is obtained by subtracting all the accounted heat losses from a hundred. The boiler efficiency expressed in this way includes all the losses which cannot be determined; thus it gives an exaggerated value. It also assumes that all the measured losses have been correctly determined.
Evaporation:
Evaporation, frequently termed the capacity of the boiler, is the amount of steam, expressed in kg or tonne which it can raise per hour at full load. The grate area, heating surface and storage capacity are all determined from the boiler evaporation.
In obtaining the grate area from the capacity, the designer has to employ experimental data for the consumption of coal per square metre of grate area per hour. This quantity is greatly influenced by the draught.
Evaporation Ratio:
It is the ratio of amount of steam generated to the amount fuel consumed by boiler.
Boiler Performance:
It may be expressed in terms of quantity of steam produced per kg of coal. Alternatively, the efficiency of the boiler may be taken as a measure of its performance.
The comparison between any two boilers is possible only when they use the same fuel, have the same feed water temperature and working pressure. Actual conditions vary considerably in this respect and it is necessary to adopt standard reference feed temperature and working pressure. The feed temperature adopted is 100°C and the working pressure 1.01325 bar the pressure of the atmosphere at sea level.
Under these conditions, evaporation of 1 kg of water requires the latent heat of 1 kg of steam, the value of which is 2256.9 kJ/kg. This quantity is known as ‘Standard Evaporation Unit’.
Equivalent Evaporation:
In a boiler raising steam, the feed water temperature and steam pressure conditions are different from the standard. A certain amount of heat say H1 kJ is required for raising a given quantity of steam, say m kg.
The same quantity of heat applied to feed water at the standard temperature 100°C produces a mass me of steam at the standard pressure and temperature adopted. This quantity of steam me is said to be equivalent to the quantity of steam m actually raised and ,is known as the equivalent evaporation.
The specific enthalpy of evaporation at a pressure of 1.01325 bar corresponding to saturation temperature of 100°C is 2256.9 kJ/kg.
If m be the mass of water evaporated per kg of coal, then me will be the equivalent evaporation per kg of coal.
This is a fair indication of the performance of the boiler, but suffers from the defect that the quality of the fuel is not taken into account.
Reference Standards:
The British standard BS845- 1987 which describes the methods and conditions under which the boiler is tested and efficiency of boiler is determined.
The efficiency of boiler is determined as a percentage of the total energy available by burning the fuel on gross calorific value (GCV) basis.
The ASME standard PTC 4-1 power test for consists of two methods:
(1) Direct method.
(2) Indirect method.
As per Indian standard IS 8753 for boiler efficiency testing the above two methods are to be used for determination of efficiency of boiler.
Direct Method of Testing:
This method is also known as input-output method because it uses heat output to heat input ratio.
Fig. 6-1 shows the details of input and output going to the boiler.
Indirect Method of Testing:
The efficiency of boiler can be measured by measuring all the losses occurring in the boiler.
In the boiler method all the lasses occurring in the boiler are accounted and then remaining part which is useful work is accounted.
Fig. 6-2 show various losses in the boiler.
Measurements Required for Direct Method of Testing:
The heat input and output has to be measured. The heat input is calculated by mass flow rate of fuel and calorific value of fuel. Heat output can be measured by mass flow rate of steam and heat supplied to convert water to steam.
Advantages:
(1) It is easy to determine efficiency.
(2) Require less parameter for computation.
(3) Few instruments for maintaining the boiler.
(1) No dues to operator if the efficiency is low.
(2) Do not show effect of various parameters in losses.
(3) Evaporation ratio and efficiency may not be accurate.
Heat Losses in Boiler:
There are various losses after combustion of fuel in the boiler.
(1) Heat Loss Due To Dry Flue Gas:
This is the highest loss in boiler.
(2) Heat Loss Due to Evaporation of Water Formed Due to H2 in fuel:
The combustion of hydrogen causes heat loss because of the product of combustion is water.
(3) Heat Loss Due to Moisture Present in Fuel:
The moisture entering the boiler with the fuel leaves as a superheated vapour.
(4) Heat Loss Due to Moisture Present in Air:
Vapour is present in from of humidity in the incoming air. The mass of vapour which air contains can be obtained from psychrometries charts.
Boiler Heat Balance:
The heat balance sheet for boiler can be prepared as:
Boiler Trial:
Objective of a Boiler Trial:
(a) To estimate steam raising capacity of the boiler when working at a definite pressure
(b) To determine the thermal efficiency of the boiler when working at a definite pressure
(c) To draw up a heat balance sheet for the boiler.
Plant:
(a) Line Diagram of a Plant:
This diagram should include the main components of the boiler plant and the energy inside and outside the plant should be shown. Fig. 6-3 shows a line diagram for a boiler plant which consists of an economizer, a boiler and a superheater.
(b) Energy Equation and Energy Stream Diagram:
When the steady state of operation is reached, the rate of flow of energy into the boundary must be equal to the rate at which energy flows out of the boundary.
The energy enters into the boundary as:
(i) Potential energy of the fuel
(ii) Sensible heat of entering air
(iii) Sensible heat of feed water entering the boundary
(iv) Radiation of heat from the surrounding air into the boundary.
The energy flows out of the boundary as:
(i) Total heat of steam leaving the boundary
(ii) Sensible heat of flue gases leaving the boundary
(iii) Radiation of warmer air.
The energy steam diagram is prepared from above considerations.
Heat utilized for steam generation
Hs = m (H – A)
where, m = amount of steam, generated in kg from one kg of fuel,
H = enthalpy of one kg of steam leaving the boiler plant and
h = the enthalpy of one kg of water entering the boiler plant.
Heat carried away by flue gases
Hg = mg Cpg (tg1 – tg2)
mg = mass of flue gases per kg of fuel fired
Cpg = specific heat of dry flue gases
(tg1 – tg2) = rise in temperature of flue gases.
Rise in temperature of flue gases means the difference between the temperature of atmospheric air and the temperature of the flue gases leaving the boiler.
Heat carried away by moisture of combustion,
Hw= mm x (ts – ta) x Cpw + hfg + Cp (tf – ts)
where, mm = mass of moisture of combustion product per kg of fuel
Cpw = specific heat of water
ts = saturation temperature corresponding to the partial pressure of water vapour in the flue gases
ta= temperature of air entering the boiler plant
hfg = latent heat corresponding to the partial pressure of water vapour in the flue gases
Cp = specific heat of superheated steam
tf = temperature of flue gases leaving the boiler plant.
Heat lost to surrounding air and in unburnt fuel
This item has been found out as the difference of the energy supplied and the heat utilized for steam raising and heat carried away by flue gases.
(c) Description of Plant and Method of Testing:
The description of plant should give in brief the idea regarding the boiler, boiler house equipments and boiler accessories. Method of testing should give us the idea of how the various readings are taken during the experiment. Simultaneous observations should be made on all the instruments and apparatus every 10 minutes or 15 minutes and mean values should be taken for calculation purposes.
Report Sheet on Boiler Trial:
The following items should be included in the report sheet on boiler trial:
(i) Boiler data
(ii) Date of trial
(iii) Duration of trial (The duration of trial should be at least six hours to get better results).
(iv) Quantity of fuel supplied
(v) Ultimate analysis of fuel
(vi) Higher calorific value of fuel
(vii) Feed water pumped per hour
(viii) Temperature of feed water entering the boiler plant
(ix) Pressure of steam generation in bar
(x) Quality of steam generated (wet or superheated) (If the steam is wet then dryness fraction should be stated and if it is superheated then temperature of superheated steam should be included)
(xi) Temperature of steam leaving throttling calorimeter
(xii) Pressure of steam after throttling
(xiii) Temperature of steam after throttling
(xiv) Temperature of flue gases leaving boiler
(xv) Temperature of air in the boiler house
(xvi) Specific heat of dry flue gases
(xvii) Dry flue gas analysis % by volume
CO2 O2 CO N2 (by difference)
(xviii) Chimney draught.
(xix) Heat supplied to boiler
(xx) Heat transmitted to water and steam
(xxi) Heat carried away by dry flue gases
(xxii) Heat carried away by water vapour in flue gases
(xxiii) Heat lost to surrounding air and in unburnt fuel (by difference)
(xxiv) Boiler efficiency.
Deductions:
(xxv) Equivalent evaporation from and at 100°C
(xxvi) Equivalent evaporation per sq metre of heating surface
(xxvii) Amount of coal burnt per sq metre of grate area per hour
(xxviii) Theoretical quantity of air required per kg of fuel
(xxix) Actual quantity of air entering combustion chamber per kg of fuel
(xxx) Ratio of actual to theoretical air
(xxxi) Excess air percentage
From the observed readings, the calculations are made for the energy account of heat supplied and the heat which has been given out.
Graphical Representation of Results:
The Heat Balance Sheet results can be represented graphically in various styles.
In fig. 6-5(a) of heat balance sheet for a boiler trial and in fig. 6-5(b) heat transmitted in various components of boiler plant are represented in area chart, while in fig. 6-6(a) it is represented in the 3-D pie chart and in fig. 6-6(b) it is represented in bar chart with 3-D visual effect.
