BoilerIntroduction;A boiler is a closed vessel in which water or other fluid is heated under pressure. The steam or hot fluid is then circulated out of the boiler for use in various process or heating applications..The boiler system includes two prime processes

Defination;A boiler is defined as "a closed vessel in which water or other liquid is heated, steam or vapor is generated, steam is superheated, or any combination thereof, under pressure or vacuum, for use external to itself, by the direct application of energy from the combustion of fuels, from electricity or nuclear energy."

Processes;To generate heat by combustion, or using some other process (nuclear, solar, waste heat).

To transmit energy by heat, transfer to the fluid being heated.

Boiler Operation;Energy is continuously added to water in a closed system (vessel or piping) array and the water evaporates to steam and the pressure and temperature increases as long as the strength of the containment is not exceeded. The relationships between temperature, pressure and volume of the contained fluid are shown in steam tables. There will be a point where the energy supplied is just sufficient to maintain steam conditions and evaporation will stop....If the contained steam is released to drive a turbine or engine the pressure will tend to fall allowing increased evaporation. So long as the temperature is maintained constant the pressure will also be constant as it supplies steam, under steady conditions, to the plant being driven. The operation of the boiler is controlled to ensure a sufficient supply of water to make up for the steam being supplied and to ensure sufficient energy input to maintain constant pressure in the steam system.In modern boilers over 80% of the heating value of the fuel is transmitted to the fluid in the boiler. The rest of the heating value is lost in the exhaust gases or is lost by conduction radiation and convection on the external surfaces of the boiler system..A vital part of any boiler system is the system designed to release the steam pressure to ensure safe conditions are maintained. The safety system is generally based on using high integrity safety relief valves.The basic working principle of boiler is very very simple and easy to understand. The boiler is essentially a closed vessel inside which water is stored. Fuel (generally coal) is bunt in a furnace and hot gasses are produced. These hot gasses come in contact with water vessel where the heat of these hot gases transfer to the water and consequently steam is produced in the boiler. Then this steam is piped to the turbine of thermal power plant. There are many different types of boiler utilized for different purposes like running a production unit, sanitizing some area, sterilizing equipment, to warm up the surroundings etc.

Efficiency;

Combustion efficiency

The efficiency of the heat generation side is known as the combustion efficiency.

Transmission efficiencyThe efficiency of the heat transfer process is known as the transmission efficiency.

Boiler EfficiencyThe overall efficiency is known as the boiler efficiency.

The boiler efficiency = Energy transmitted to Fluid / Heating value or calorific value of the fuel

The boiler efficiency is based on the calorific value of the fuel used and the enthalpy change from water to steam. Notes on the calorific value of the fuels are provided below...

m s = Mass flow rate of steam (kg/s)

m f = Mass flow rate of fuel (kg/s)

h = specific enthalpy of generated steam (kJ/kg)

h w = specific enthalpy of feed water (kJ/kg)

Types of BoilersThere are two principle types of Boiler (Also called Steel Boilers)

1.Boilers, fire tube boilers.

2.Water tube boilers.

Fire Tube Boilers.

Fire-tube boilers force hot combustion gases through tubes submerged in water.Fire-tube boilers are usually preferred in low-pressure applications between 0,5 kg/s (approx. 75kW) and 5kg/s (approx. 750kW boiler ) of steam. In sizes below 0,5kg/s and above 5 kg/s, water-tube units are often preferred.Fire Tube Boilers are widely used in small installations to heat buildings and to provide power for factory processes. Fire-tube boilers are also used in steam locomotives Fire-tube boilers have the advantage of being easy to install and operate. The main disadvantage of the fire tube boiler is that the steam is generated in a large containment vessel which is at risk of catastrophic failure due to the high stresses developed at high pressures....

The older fire-tube boiler design, in which the water surrounds the heat source and gases from combustion pass through tubes within the water space, is a much weaker structure and is rarely

used for pressures above 2.4 MPa (350 psi). A significant advantage of the water tube boiler is that there is less chance of a catastrophic failure: there is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

Fire Tube Boiler

Water Tube Boilers

Water Tube Boiler circulate water in tubes within a furnace enclosure. Hot flue gases pass over the tubes, heat the water, and then exit through a stack.Water-tube boilers are usually specified in all situations where operating pressures of 20 bar or greater are required because of their greater structural integrity.Water tube boilers were developed to satisfy the the demand for large quantities of steam at pressures and temperatures far exceeding those possible with fire-tube boilers. The tubes are outside the steam drum, which has no heating surface and is much smaller than in the fire-tube boiler. For this reason, the drum of the water tube boiler is better able to withstand higher pressures and temperatures. A wide variety of sizes and designs of water tube boilers are used in power stations, ships and factories.Water tube boilers have the following advantages.

Low risk of disastrous explosion compared to fire box boiler Space saving Rapid steam raising Ease of transportation Large capacity and pressure units available

Water tube boilers have the following disadvantages.

The supply water has to be pure and specially treated to protect the steam tubes Water tube boiler systems are often complicated require special maintenance procedures Water tube boiler cannot serve as steam reservoirs because the fluid volume is minimized The water feed system has to be very accurate - (small fluid volume)

Hydronic Boiler System

A boiler is used to heat water that is circulated through a closed loop piping system for general facility and service water heating. Low-temperature systems generally operate below 200° Fahrenheit Medium-temperature systems generally operate at temperatures between 200 and 250° Fahrenheit.A feature of hot water systems ('''Figure 6)''' is an expansion tank to accommodate the expansion of the water in the system as the water is heated. The expansion tank, when piped into the system on the suction side of the circulating pumps, also pressurizes the system to prevent flashing in the circulating pump, piping, and piping components. In many low- and medium-pressure systems, pressurization is maintained by flash steam in the expansion tank. In a few hot water systems, pressurization is maintained by maintaining a compressed gas blanket above the water level in the expansion tank.

'''Figure 6: Hydronic (Hot Water) Boiler System'''High-temperature hot water systems, which operate above 250° Fahrenheit, are basically the same as hot water systems that operate below 250°F. High-temperature systems are generally installed when a process requires the higher temperature, a number of locations require small quantities of low-pressure steam that the high-temperature hot water can generate in a local converter, or high-temperature drop equipment can be used at end use points to minimize the size of water circulation piping required.Most facility boiler systems are fired using a combustible gas (typically natural gas or propane) or fuel oil. In many facilities, the boilers are designed to fire both a combustible gas fuel and a fuel oil. In these facilities, the combustible gas fuel is generally natural gas that is considered the primary fuel, and fuel oil is considered to be the backup fuel.

Condensing boilers

Condensing boilers are by far the most common boiler type in UK homes, and are far more energy-efficient than older mains gas boilers, using around 90% of their heat.

A condensing boiler works by passing hot gas through a central chamber that heats up water but, cleverly, a second chamber uses remaining heat to warm up water coming back into the unit from the heating system.Of all the condensing boiler types, the combi-boiler is the most popular. A combi-boiler includes the hot water unit and cold water tank in the same unit, which means all your hot water and heating come from the same unit. This makes it easy to install.It also means you get a steady supply of hot water through your taps as you don’t have to wait for the tank to fill and don’t have to worry about lots of space for different tanks. The downsides lie in the fact that, as they are a small unit, you will only get maximum pressure through one tap at a time, and they struggle to produce large quantities of hot water.Heat-only boilers, in comparison, have a more traditional approach by offering hot water only, whilst cold water is supplied separately. This does away with some of the problems of supply and water pressure. But, because the hot- and cold-water systems are separate, they take up more space, and are less energy-efficient.

Oil boilers

Oil boilers represent an alternative to the estimated four million homes across the UK that are not connected to mains gas. This isolation comes at a cost however, and an oil boiler will typically be a few hundred pounds a year more expensive to run than gas.In terms of the mechanics, oil boilers are fairly similar to traditional boilers. Instead just using oil rather than gas to heat the inner pipes and, thereby, the water.The main difference comes in when you try and get your hands on the oil itself. The oil has to be delivered to your home, making oil boilers more logistically challenging. Prices do also vary for heating oil or kerosene, so you may have to shop around to get a good deal.Maintenance can also be an issue because, if you run into any problems, you will need to get an Oftec registered engineer in to inspect it (rather than a Gas Safety Register engineer for conventional gas boilers).

Biomass boilers

A biomass boiler, or wood boiler, is the other main alternative and, as the name suggests, relies on wood pellets, chips or logs to generate heat. Wood pellet boilers are very cheap in comparison to the other alternatives, costing an estimated £600 a year to run, and are energy-efficient.

The carbon dioxide given off by wood pellet boilers is similar to that absorbed by new plants, so it is a sustainable fuel. Pellets are the most practical solution for biomass boilers even though logs are cheaper, as they can be automatically fed in to the system. You will have to find a supplier for your pellets though. The Energy Saving Trust estimates installation costs between £7,000 and £13,000, and a tonne of wood pellets can cost under £200.

They emit around 3 tonnes fewer of carbon dioxide a year compared to a gas boiler, but there are some things to be aware of.