the boiler

Introduction to Boilers:

  A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat to a process. Water is a useful and cheap medium for transferring heat to a process. When water is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be extremely dangerous equipment that must be treated with utmost care. The process of heating a liquid until it reaches its gaseous state is called evaporation. Heat is transferred from one body to another by means of:

  1.             Radiation, which is the transfer of heat from a hot body to a cold body without a conveying medium
  2.             Convection, the transfer of heat by a conveying medium, such as air or water

III.            Conduction, transfer of heat by actual physical contact, molecule to molecule

Boiler Specification:

The heating surface is any part of the boiler metal that has hot gases of combustion on one side and water on the other. Any part of the boiler metal that actually contributes to making steam is heating surface. The amount of heating surface of a boiler is expressed in square meters. The larger the heating surface a boiler has, the more efficient it becomes. The quantity of the steam produced is indicated in tons of water evaporated to steam per hour. Maximum continuous rating is the hourly evaporation that can be maintained for 24 hours. F & A means the amount of steam generated from water at 100°C to saturated steam at 100°C.

Boiler Systems:

The boiler system comprises of: feed water system, steam system and fuel system. The feed water system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and repair. The steam system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. Throughout the system, steam pressure is regulated using valves and checked with steam pressure gauges. The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system

The water supplied to the boiler that is converted into steam is called feed water. The two sources of feed water are:

  1.             Condensate or condensed steam returned from the processes and
  2.             Makeup water (treated raw water) which must come from outside the boiler room and plant processes.

For higher boiler efficiencies, the feed water is preheated by economizer, using the waste heat in the flue gas.

boiler performance

boiler performance:

The performance parameters of boiler, like efficiency and evaporation ratio reduces with time due to poor combustion, heat transfer surface fouling and poor operation and maintenance. Even for a new boiler, reasons such as deteriorating fuel quality, water quality etc. can result in poor boiler performance. Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action.

Boiler Efficiency:

Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilized to generate steam. There are two methods of assessing boiler efficiency.

  1. a)The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
  2. b)The Indirect Method: Where the efficiency is the difference between the losses and the energy input.
  1. a)Direct Method

This is also known as ‘input-output method’ due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency. This efficiency can be evaluated using the formula

Boiler Efficiency = (Heat Output/Heat Input) * 100                     

Parameters to be monitored for the calculation of boiler efficiency by direct method are :

  • Quantity of steam generated per hour (Q) in kg/hr.
  • Quantity of fuel used per hour (q) in kg/hr.
  • The working gauge pressure (in kg/cm2) and superheat temperature (°C), if any
  • The temperature of feed water (°C)
  • Type of fuel and gross calorific value of the fuel (GCV) in kCal/kg of fuel

 

Boiler efficiency = Q x (hg-hf) ×100 /(q×GCV)

Where,

hg – Enthalpy of saturated steam in kCal/kg of steam

hf – Enthalpy of feed water in kCal/kg of water

It should be noted that boiler may not generate 100% saturated dry steam, and there may be some amount of wetness in the steam.

Advantages of Direct Method:

  • Plant people can evaluate quickly the efficiency of boilers
  • Requires few parameters for computation
  • Needs few instruments for monitoring

Disadvantages of Direct Method:

  • Does not give clues to the operator as to why efficiency of system is lower
  • Does not calculate various losses accountable for various efficiency levels
  1. b)Indirect Method:

There are reference standards for Boiler Testing at Site using indirect method namely British Standard, BS 845: 1987 and USA Standard is ASME PTC-4-1 Power Test Code Steam Generating Units’. Indirect method is also called as ‘’heat loss method’’. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The standards do not include blow down loss in the efficiency determination process. A brief procedure for calculating boiler efficiency by indirect method is given below.

The principle losses that occur in a boiler are:

  1. Loss of heat due to dry flue gas
  2. Loss of heat due to moisture in fuel and combustion air
  3. Loss of heat due to combustion of hydrogen
  4. Loss of heat due to radiation
  5. Loss of heat due to unburnt

In the above, loss due to moisture in fuel and the loss due to combustion of hydrogen are dependent on the fuel, and cannot be controlled by design. The data required for calculation of boiler efficiency using indirect method are:

  • Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)
  • Percentage of Oxygen or CO2in the flue gas
  • Flue gas temperature in °C (Tf)
  • Ambient temperature in °C (Ta) & humidity of air in kg/kg of dry air
  • GCV of fuel in kCal/kg
  • Percentage combustible in ash (in case of solid fuels)
  • GCV of ash in kCal/kg (in case of solid fuels)

With the help of these parameters the boiler engineers find the losses using standard approaches as specified by ASME and other boiler OEMs. Finally losses can be subtracted from the heat added and hence efficiency can be found.