Heat is a form of energy a type of energy caused by the random movements of molecules in a material, naturally heat moves from hot to cold
(b) Units of heat.
(i) Calorie- The amount of heat required to raise the temperature of one gm of water through 1°C (15° to 16°C)
(ii) Kilocaloorie
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The amount of heat required to raise the temperature of 1 Kg of water through 1°C'(15° to 16°0C), 1 kilocalorie = 1000 Calories
(iii) British thermal Heat unit (B.T.H.U.)
The quantify of heat required to raise the temperature of 1 Lb (pound) of water through 1°F (60 D° to 61°F)
(iv) Centigrade Heat unit (CHU)
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The quantity of heat required to raise the temperature of 1 Lb (pound) of walkthrough 1°C( 15° to 16°C)
1 CHU = 4f53.6 Calorie = 0.4536 Kcal
(c) Types of heat-
(i) Sensible heat is measured by the thermometer. Amount of sensible heat = mass x specific heat rise in temperature.
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(ii) Latent heat
The heat which changes the physical state of a substance without changing the temperature
(iii) Latent heat of fusion. Heat required melting ice from (0°C) 32°F to water at 32°EF- (0°C). 80 k cal./ kg.
(iv) Latent heat of evaporation or vaporization
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Heat required changing water at 100°C (212°F) to steam at 100°C (212°F) 536 K cal/Kg.
(v) Total heat of steam = Sensible heat + latent heat of vaporization.
Ideal or stoichiometric Combustion
During combustion process, carbon reacting with oxygen to form Carbon-dioxide (C02) releasing heat energy to the tune of 8084 Kcal/Kg. of carbon.
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Under certain circumstances, carbon reacts with oxygen to form carbon monoxide (CO). This results in releasing smaller quantity of heat (2430 Kcal/Kg. Of carbon) and thus 5654 Kcals of heat is lost.
The heat released by the combustion is transferred to water to form steam in the boiler and to the thermic fluid in thermopac, the combustion products (flue gates) leave the boiler thermopac through the chimney, which also carries away certain quantity of heat with them.
For proper combustion, the oil and air should be supplied at correct pressure, temperature and in correct quantity. Also, the proper mixing of the fuel and air is essential.
Excess air:
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In actual practice, since mixing is never perfect, certain amount of excess air is needed to complete combustion and to ensure the release of the entire heat contained in fuel oil.
If air is less, it would lead to incomplete combustion and smoke. If air is too much, then surplus air will carry away heat in flue gases. Hence, the optimum excess air level should be maintained for optimizing fuel usage.
Combustion Efficiency
(i) How to determine combustion efficiency:
In ideal combustion, the flue gases contain carbon-di-oxide and Sulphur-di- oxide together as 15. 76% by volume and there is no oxygen in flue gases.
As excess air increases, the concentration of C02+S02 decreases and the concentration of oxygen increases in the flue gases.
By measuring the content of carbon-di-oxide (C02) or Oxygen (02) in flue gases, we can find out the excess air in flue gases. By measuring the temperature of flue gases at the point of sample, we can know flue gas losses ( stock losses ).
(ii) Smoke also indicates combustion efficiency:
Black smoke indicates incomplete combustion + fuel wastage White smoke indicates too much excess air + fuel wastage Hazy brown smoke indicates proper combustion.
Step-By-Step Procedure for efficient Operation of Burner –
(1) Start up:
(a) Check for correct sized burner/nozzle.
(b) Establish air supply first (start blower). Ensure no vapour/gases are present before light-up.
(c) Ensure a flame from a torch or other source is placed in front of the nozzle.
(d) Turn On the preheated, oil supply (Before start-up drain off cold oil).
(2) Operation:
(a) Check for correct temperature of oil at the burner tip (consult viscosity- vs temperature chart).
(b) Check for the correct air pressure for the LAP burners (63.5 cm to 76.2 cm. water gauge air pressure is commonly adopted).
(c) Check if oil is dripping near the burner.
(d) Check for flame fading /flame pulsation.
(e) Check for positioning of burner (ensure no flame impingement on refractory walls).
(f) Adjust the flame length to suit the conditions (ensure the flame does not loop out of the furnace).
(3) Load changes:
(a) Operate both air and oil valves simultaneously (if it is a self proportioning burner, operate the self-proportioning lever. Do not adjust valve only oil line).
(b) Adjust burners and damper for a light brown (hazy) smoke from chimney and at least 12% Carbon dioxide.
(4) Shut Down:
(a) Close oil line first.
(b) Shut the blower after a lapse of few seconds (ensure gases are purged out of the combustion chamber).
(c) Do not expose the burner nozzle to the radiant heat of the furnaces. (When oil is shut off, remove burner/nozzle or interpose a thin refractory between nozzle and furnace).
Burner Operation & Maintenance:
The purpose of burner is to atomise fuel oil i.e., to convert fuel oil into mil lions of small droplets. The oil & air pressures should be maintained at burner as recommended by manufacturers of boiler to get the best results.
The oil that is atomised by the burner is brought up to the ignition temperature in a burner block.
In burner block atomisation, mixing with air, evaporation of the oil from the surface of the drops, cracking of oil and the start of combustion are completed.
It is necessary to maintain the shape of the block and repair, where it is damaged; otherwise it will disturb the shape of the flame and result in poor combustion.
It is a good practice to dismantle and clean burner once in a shift of 8 hours.
First of all soak the nozzles in kerosene and after a while, when deposits are well soaked, remove the carbon particles by rubbing them away. Use some soft wire like copper. Never use steel wire as this would damage the nozzle.
Always keep spare burners ready. While tuning the boiler, if you are not able to achieve at least 12% C02 (or 02 is reduced to 3 to 4%) by regulating the air supply without causing smoke, your burner may be defective.
Use of Instruments for Excess Air Control
(1) Portable Absorption Analyser: Fyrite – C02/02:
Portable Absorption Analyser is used to determine the quantity of C02/02in flue gas by chemical absorption method and helps to run the boiler for optimum excess air.
It is quite handy and simple to operate. A sample of flue gases is sucked through a filter into the analyser, which contains caustic soda in C02 analyser and alkaline pyrogallol in 02 analyser.
(2) Controlling Boiler Losses:
Boiler losses are –
(a) Flue Gas Losses:
This loss is largest in a boiler. It represents the heat carried away by the hot flue gases.
These losses can be lowered by:
(1) Excess Air Control: By analysing flue gases with the help of instrument and maintaining CO2 at least 12% or 02 around 3 to 4% in flue gases.
(2) To keep chimney temperature as low as possible. It is required to log temperature of flue gas, for that; there should be a dial type thermometer at the base of chimney near sample point.
When flue gas temperature raises about 38 Deg. C above the normal, it is time for removing the soot deposits.
It is estimated that a coating of 3 mm thick soot on the heat transfer surface, can cause an increase in fuel consumption as much as 2.5%.
(b) Blow down Losses:
The water evaporates to form steam in boiler leaving behind the salts in the boiler which form scales. Regular blowing down reduces water side scale formation on the sides. A scale formation of 1 mm thickness on the water side would increase fuel consumption by 5-8%. On the other hand, excessive blow down also wastes flue.
Unburnt in Flue Gases:
If however, combustion is complete, the carbon must be converted into carbon monoxide which results in a liberation of only 52% of the total heat in the fuel.
The key to complete combustion is the proper filtration, preparation of fuel oil and proper operation of burners.
(c) Radiation Losses:
It depends on the temperature of the boiler’s external surfaces. When a boiler is operated at low loads, radiation losses account for high portion of the total boiler losses.
The insulation and design characteristics play important role for radiation losses.
It is important to allocate the total load among the different boilers judiciously when numbers of boilers are used to meet the steam demand to reduce total losses.
Typical radiation losses of package boiler:-.
10% at full load &
2 0% at 50% load & 3.0% at 25% load
Tuning of Boilers:
The aim is to optimise excess air level by analysing flue gases.
First determine excess air level by measuring the percentage of C02 /02 in the flue gases, when the boiler is running at normal firing rates.
Sample point on chimney should be as close as to boiler, but it should not be located immediately downstream of bends, dampers, or induced draft fans.
Also, ensure that there is hot air in filtration at the upstream of the sample point.
To start with, deliberately operate the boiler at higher excess air leaves so as to achieve a low C02 reading (typically 6 to 7%). Reduce air quantities slowly and monitor C02 readings. Continue reducing the combustion air until you got good C02 reading (preferably between 12 to 14%) in the flue gases. This can be ascertained when a light brown haze at the top of the chimney is achieved.
This method should be followed to achieve high C02 (less 02) readings at all the firing rates encountered in the boiler.
Importance of Condensate Recovery:
» The uncontaminated steam condensate from process departments is a valuable source of heat which can be used in the boiler as feed water
» The condensate is like distilled water and needs no water softening treatment, hence, it reduces water softening treatment costs.
» For every 6 Deg. C rise in boiler feed water temperature through condensate return, there is 1% saving of fuel.
Flue Gas Heat Recovery:
(a) In an economizer, the waste heat is used to increase the boiler feed water temperature.
For every 6 Degree. C rise in feed water temperature through an economiser, there is 1% saving of fuel.
(b) In an Air Pre-Heater, the waste heat is flue gases are used to heat combustion air.
For every 20 Deg. C rise in combustion air temperature through an air pre- heater, there is 1% saving of fuel.
Direct Trial Of Boiler:
The sure way to find out the efficiency of boiler performance is by direct tracery
In direct trial, the fuel oil and the feed water consumed is measured for about 4 hours.
The fuel oil service tank and water tank are calibrated first; condensate recover- is to be stopped. In the beginning, the oil service tank and water tank are filled.
During the direct trial time, no receipt of oil and water is taken in the tanks. At normal load, the readings of oil and water are taken every hour for four hours in this trial. The water consumed represents the steam produced.
Result Areas for Efficient Steam Utilisation:
(1) Avoiding steam leakage
A 3 mm hole on a pipeline carrying 7 Kg./cm2 steam would waste 32.65 KL of fuel oil/year.
(2) Providing dry saturated steam for process
Since dry saturated steam is required for process equipment, due attention must by paid to the boiler operation and logging of the pipelines.
(3) Utilising steam at the lowest practicable pressure for process.
(4) Insulation of steam pipeline & hot process equipment.
A bare steam pipe 150 mm in diameter and 100 meters in length, carrying saturated steam at 8 Kg/Cm2 could waste 25 KL of furnace oil in one year.
(5) Proper utilisation of directly injected steam
For direct injection of steam, the inlet steam pressure is to be kept very low around 0.5 Kg/Cm2 and in any case it should be less than 1 Kg/Cm2 to avoid wastage.
(6) Proper Air Venting.
(7) Minimising barriers to heat transfer.
(8) Condensate Recovery.
(9) Flash Steam Recovery.
(10) Proper selection & maintenance of steam traps.
(11) Proper sizing of steam & condensate pipelines.
(12) Reducing the work to be done by steam.