The design of water-tube and the fire-tube boilers have been perfected with time and suitably modified for the increasing industrial applications. Considering the rapidly decreasing fossil fuel availability, increased emphasis is progressively laid onincreasing the heat utilization factor in the boiler. Several energy conservation devices and accessories have been evolved for the boilers to achieve the economy of heat utilization. Before studying these devices let us first discuss different types of heat losses from the boilers which could cause the decrease in thermal efficiency of the boiler and during the transmission of steam from one location to another.
i. Heat losses
These losses occur on account of many factors, some of which have been listed below.
i) Loss of unburnt or partially burnt fuel in the fuel bed or in the exhaust gases from the chimney.
ii) Poor heat transfer from hot flue gases to water due to formation of soot later on the flue gas side and water scale on the waterside.
iii) Inadequacy of insulation on steam pipe lines and removal of condensate from the pipe lines resulting in loss of heat energy during transmission of steam
iv) Heat energy losses with stack gases due to excessive temperature difference between steam and the stack gases temperature.
The greatest amount of heat loss in a boiler is the heat carried away by the hot flue gases up the chimney. A certain amount of heat loss is unavoidable as the hot gases are required to be hotter than the water in the boiler. However some of the heat being carried away by the hot flue gases can be recovered and sent back to the boiler, if the energy conservation accessories like economiser, air pre-heater and steam-superheater are installed.
ii. Economiser
It is an accessory installed with the boiler to utilize a portion of heat from the flue gases for preheating the feed water to the boiler. It consists of a series of vertical tubes through which feed water passes on its way from feed water pump to the boiler water space. These tubes are placed in the path of hot flue gases after the combustion chamber. The waste flue gases flow outside the economiser tubes. The feed water flowing into the boiler while passing through these tubes gets heated up after getting heat from the flue gases. This way a portion of heat is recovered from the flue gases, which would have otherwise gone waste to the atmosphere. The outside surface of these tubes is always in contact of hot flue gases and is prone to deposition of soot. Hence the outer surface is kept clean and free from soot by means of mechanized scrapers.
The advantages gained by installing an economiser are:
i) Fuel economy
ii) Long life of the boiler
iii) Increase in steaming efficiency
The saving of fuel affected by an economizer is proportional to the amount of heat recovered in the feed water. The average percent saving is approximately 1% for every 5.5° C increase in feed water temperature.
iii. Air Pre-heater
The function of an air pre-heater is to extract heat from the flue gases and transfer it to air entering the boiler furnace. It is installed between the economizer and the chimney. The commonly available design of air pre-heater is tubular type, in which the tubes are so placed that the flue gases pass through them. Air being heated is made to make a number of passes across and around the outside of these tubes.Designs are also available in which the hot gases flows outside the tubes and air inside.
The installation of air pre-heater increases the overall efficiency of the boiler plant.This increase in efficiency varies between 2 to 10 per cent. In addition of above advantage of increase in plant efficiency, the pre-heating of air also have additional benefits of improving the heat generation and transfer in the furnace. Some of these are:
i) It creates higher furnace temperatures.
ii) It accelerates combustion and increases the percentage of CO 2 in the furnace gases by ensuring the complete combustion.
iii) The pre-heating of air facilitates the burning of poor grade fuel.
iv. Steam Superheater
We have already read that if the saturated steam is separated from the contact of water and further heat is provided to it, keeping the steam pressure same, then the heat content of steam increases. Such steam is known as superheated steam. Due to higher heat contents the superheated steam effects the improvements and economy in the following ways:
i) Reduces the steam consumption for a given process.
ii) Reduces the condensate losses in steam pipe lines.
iii) Increases the capacity of the plant.
iv) Eliminates friction in steam lines.
Steam Superheater is the most important accessory of a boiler. It is a device in which steam is superheated. A steam superheater is a set of pipe line coils through which steam is passed after it is separated from contact of water. These coils are placed in the path of flue gases so that a part of heat in the flue gases is utilized to superheat the steam. The superheater tubes are usually 5 cm in diameter and are generally made up of carbon steel or chrome nickel alloy to withstand high temperatures. Saving in steam consumption by use of superheated steam is about 1.5 to 2% for each 5° C of superheat.
V. Soot Blowers
We have read in this unit that the accumulation of soot layer on the flue gas side of the boiler tubes reduces the heat transfer from hot gases to water. This is because the soot is bad conductor of heat. It is thus of utmost importance that the external surfaces of the tubes be kept free from soot by brushing or by mechanical blowing. The mechanical device used for removing soot is known as soot blower.In economizer the soot is removed by scrapers, which travel slowly and continuously up and down the tubes Sometimes special free flowing powders are used which when introduced in the furnace get Vapourized. These powders contain a catalyst, which reduces the ignition temperature of the adhering carbon in the soot, and an oxidizing agent in the powder provides combustible oxygen to burn this carbon. This way the deposition of soot on the surface is reduced.
i. Heat losses
These losses occur on account of many factors, some of which have been listed below.
i) Loss of unburnt or partially burnt fuel in the fuel bed or in the exhaust gases from the chimney.
ii) Poor heat transfer from hot flue gases to water due to formation of soot later on the flue gas side and water scale on the waterside.
iii) Inadequacy of insulation on steam pipe lines and removal of condensate from the pipe lines resulting in loss of heat energy during transmission of steam
iv) Heat energy losses with stack gases due to excessive temperature difference between steam and the stack gases temperature.
The greatest amount of heat loss in a boiler is the heat carried away by the hot flue gases up the chimney. A certain amount of heat loss is unavoidable as the hot gases are required to be hotter than the water in the boiler. However some of the heat being carried away by the hot flue gases can be recovered and sent back to the boiler, if the energy conservation accessories like economiser, air pre-heater and steam-superheater are installed.
ii. Economiser
It is an accessory installed with the boiler to utilize a portion of heat from the flue gases for preheating the feed water to the boiler. It consists of a series of vertical tubes through which feed water passes on its way from feed water pump to the boiler water space. These tubes are placed in the path of hot flue gases after the combustion chamber. The waste flue gases flow outside the economiser tubes. The feed water flowing into the boiler while passing through these tubes gets heated up after getting heat from the flue gases. This way a portion of heat is recovered from the flue gases, which would have otherwise gone waste to the atmosphere. The outside surface of these tubes is always in contact of hot flue gases and is prone to deposition of soot. Hence the outer surface is kept clean and free from soot by means of mechanized scrapers.
The advantages gained by installing an economiser are:
i) Fuel economy
ii) Long life of the boiler
iii) Increase in steaming efficiency
The saving of fuel affected by an economizer is proportional to the amount of heat recovered in the feed water. The average percent saving is approximately 1% for every 5.5° C increase in feed water temperature.
iii. Air Pre-heater
The function of an air pre-heater is to extract heat from the flue gases and transfer it to air entering the boiler furnace. It is installed between the economizer and the chimney. The commonly available design of air pre-heater is tubular type, in which the tubes are so placed that the flue gases pass through them. Air being heated is made to make a number of passes across and around the outside of these tubes.Designs are also available in which the hot gases flows outside the tubes and air inside.
The installation of air pre-heater increases the overall efficiency of the boiler plant.This increase in efficiency varies between 2 to 10 per cent. In addition of above advantage of increase in plant efficiency, the pre-heating of air also have additional benefits of improving the heat generation and transfer in the furnace. Some of these are:
i) It creates higher furnace temperatures.
ii) It accelerates combustion and increases the percentage of CO 2 in the furnace gases by ensuring the complete combustion.
iii) The pre-heating of air facilitates the burning of poor grade fuel.
iv. Steam Superheater
We have already read that if the saturated steam is separated from the contact of water and further heat is provided to it, keeping the steam pressure same, then the heat content of steam increases. Such steam is known as superheated steam. Due to higher heat contents the superheated steam effects the improvements and economy in the following ways:
i) Reduces the steam consumption for a given process.
ii) Reduces the condensate losses in steam pipe lines.
iii) Increases the capacity of the plant.
iv) Eliminates friction in steam lines.
Steam Superheater is the most important accessory of a boiler. It is a device in which steam is superheated. A steam superheater is a set of pipe line coils through which steam is passed after it is separated from contact of water. These coils are placed in the path of flue gases so that a part of heat in the flue gases is utilized to superheat the steam. The superheater tubes are usually 5 cm in diameter and are generally made up of carbon steel or chrome nickel alloy to withstand high temperatures. Saving in steam consumption by use of superheated steam is about 1.5 to 2% for each 5° C of superheat.
V. Soot Blowers
We have read in this unit that the accumulation of soot layer on the flue gas side of the boiler tubes reduces the heat transfer from hot gases to water. This is because the soot is bad conductor of heat. It is thus of utmost importance that the external surfaces of the tubes be kept free from soot by brushing or by mechanical blowing. The mechanical device used for removing soot is known as soot blower.In economizer the soot is removed by scrapers, which travel slowly and continuously up and down the tubes Sometimes special free flowing powders are used which when introduced in the furnace get Vapourized. These powders contain a catalyst, which reduces the ignition temperature of the adhering carbon in the soot, and an oxidizing agent in the powder provides combustible oxygen to burn this carbon. This way the deposition of soot on the surface is reduced.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.