Steam Generator, well known as boiler is an equipment designed to boil liquid to steam. Steam is produced by means of transfer of heat to water and combustion, this is provided by an enclosed vessel acknowledged as a boiler. The functional steam or hot water under pressure is aimed at transferring the heat to a process. Heat is transferred to a process using water as a cheap medium. The volume of water upsurges about 1,600 times when it is boiled, the force produced is just about the similar as that of an explosive of gunpowder. At this point the boiler becomes really dangerous tools that has to be used with caution. Evaporation come about when a liquid is heated up to a vaporous phase.
The boiler system is made up of:
1. Feed water system
2. Steam system
3. Fuel system
Water is provided inside the boiler by means of feed water and is automatically directed to meet the steam demand. The feed water is the water that is supplied to the boiler and then is vaporized to steam. The sources of water feed are the returned condensed steam from the process and raw water coming outside plant processes and boiler room. Produced steam is collected and directed by the steam system. The steam is then controlled using a piping system to the point of use. Pressure gauges are then used to check steam pressure and is controlled using valves.
Working principle of a flame tube boiler
The fuel is burnt inside a furnace in a flame tube boiler. Passing through the fire tubes is the hot gas produced in the furnace. The heat energy of gases is transferred to the water surrounds them. This results to a steam generation in the water, it comes up naturally and is stored upon the water in same vessel of fire tube boiler. Then the utilizable steam for desirable purpose is taken out from the steam outlet, through the feed water inlet water is fed into the boiler.
Advantages of a flame boiler
• Easy to operate, use, clean and maintain
• Can be used in small scales industries
• The water is supplied in shell and outside tubes while hot gas is supplied inside tubes so the water volume cannot be shaken easily when the fire tube boiler is running
• It is cheaper when compared to water tube boiler
Disadvantages of fire tube boiler
• There is a limit for steam generation capacity
• Steam cannot be produced at high pressure
• Heat exchanger does not use thermal radiator thus the efficiency of steam is bad
• In case of leakage of hot water or gas collected inside occurs it may cause serious accident
• The time to raise a temperature and pressure is less than that required to fill water, from the furnace side.
Types of flame tube boilers:
• Cornish boiler
• Lancashire boiler
• Locomotive boiler
• Scotch marine boiler
• Admiralty-type direct tube boiler
• Horizontal return tubular boiler
• Immersion fired boiler
• Velcon boiler
• Vertical fire-tube boiler
Water tube boiler
Water tube boiler is another type of a boiler in which the heating of water happens inside the tubes and hot gases surrounding them. An opposite of flame tube boiler where by hot gasses pass through tubes that surrounded by water.
Working principle of water tube boiler
The working principle of this type of a boiler is easy. It has mainly two drums, the lower drum known as mud drum and the upper drum know as steam drum. Riser tube and down-comer are tubes that connect these two drums. The steam is produced by heating water in the mud drum and riser tube connected to the mud drum and the steam naturally comes to the upper drum. Steam is then separated from water naturally and store above the surface of water. At the upper drum cold water is fed through the feed water inlet. Cold water is heavier than hot water in the lower drum and in the riser, hot water is then pushed upwards through riser by cold water.
Advantages of water tube boiler
These types of boilers are essentially used in large thermal thus they have a number of different advantages.
• There more numbers of water tubes used they help in archiving larger heating surfaces
• Movement of water is much faster than that of a flame tube boiler because of conventional flow, thus the rate of heat transfer is high which results into higher efficiency
• Very high pressure in order of 140 kg/cm2 can be obtained smoothly.
Types of water boilers
• Babcock & Wilcox boiler
• Stirling boiler
• La-Mont boiler
• Benson boiler
• Yarrow boiler
• Loeffler boiler
Boiler water treatment
There mainly three objectives that a treatment and conditioning of boiler feed water must satisfy:
• Corrosion protection
• Continuous heat exchange
• Production of high quality steam
External treatment is important because it’s when impurities are removed from the water. When the boiler system question is in and there are high impurities to be tolerated the external treatment is done. There is a number of external treatment including softening, evaporation, deaeration, membrane contractors etc. they can be used clean feed water.
Internal treatment in is a type of treating where by impurities inside the boiler system. Inside the boiler proper or the feed lines it is where reactions occur. The use of internal treatment is independent to the external treatment but it can also be used in conjunction with the external treatment. Its aim is to prevent boiler water foaming and act against feed water hardness, condition sludge, scavenge oxygen.
Feed water is made up of daerate and the water treatment facility purity. Pure vapour is sometimes produced by pre-treating water by evaporation and is condensed and used for boiler feed purposes. Different types of evaporators are used to mention one which is very simple is a tank of water through which coils of steams pass to heat water to the boiling point. Passing through coils in a second tank of water of the vapour to produce additional heating and evaporation it helps to increase the efficiency. When a steam is ready to use evaporators are suitable. If solids dissolved in the raw water are very high, evaporators have certain advantages over demineralization.
Internal treatment is able to establish different treatment when boilers operate at low or moderate pressure, when good quality raw water is available and when large amounts of condensed steam are used for feed water. The aim of an internal treatment is to prevent any feed-water hardness from precipitating on the boiler metal as scale, makes matter such iron oxide to be non-adherent to the boiler metal. In prevention of boiler corrosion internal treatment plays a part by eliminating the oxygen from water whilst providing alkalinity. In other words or in few words internal treatment is aimed at preventing corrosion and topping of feed water system and act against corrosiveness in the steam condensate systems.
There are different types of used for internal treatment of water mainly used, include phosphate-dispersants, polyphosphate-dispersants also called softening chemicals. Natural and synthetic dispersants, sequestering agents, oxygen scavengers, anti-forming.
Phosphate-dispersants also referred to as softening chemicals, in order to neutralize the hardness of water these chemical they form tricalcium phosphate, but first they react with alkalinity of the boiler water. Tricalcium phosphate can be easily removed from the bottom of the boiler by blowing it down and be disposed regularly on daily basis.
Natural and synthetic dispersants, these products they upturn the dispersive properties of the habituation products. These dispersive come in many form they can be natural polymers (tannins) or they can be synthetic polymers (polyacrilates, polystyrene, sulphonates, etc.)
Sequestering agents, the function of these products is to inhibit and implement the threshold effect.
Oxygen scavengers, the function of these products is to remove dissolved oxygen and oxides. They also reduce the chemical reactivity of metal surfaces. When choosing a product and the dose it will depend if the deaerating heater is used or not.
To amend the surface tension of a liquid and to protect against carrying over of fine water particles in the steam and to remove the foam, anti-foaming agents play this above mentioned role.