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Boiler

Boiler is an enclosed pressure vessel that provides means for combustion heat to be transferred into water until it becomes hot water or steam. The hot water/steam under pressure is then usable for providing heat for an industrial process.

Depending on the type of boiler different forms of fuel or energy used are coal, diesel oil, furnace oil, rice husk, firewood etc.

Design and Operation

A boiler system comprises three parts:

  1. A feed water system
  2. A steam system
  3. A fuel system.

The feed water system provides water to the boiler and regulates it automatically to meet the steam demand. 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 the equipment used to provide fuel to generate the necessary heat.

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 quantity of the steam produced is indicated in tons of water evaporated to steam per hour.

Boiler Types and Classifications

There are virtually infinite numbers of boiler designs but generally they fit into one of two categories:

Fire tube or "fire in tube" boilers; contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be converted to steam circulates.

Water tube or "water in tube" boilers in which the conditions are reversed with the water passing through the tubes and the hot gasses passing outside the tubes.

 

Figure 1:  Schematic view of fire tube boiler (left) and water tube boiler (right)

Besides above classifications, boilers are classified also classified based on the number of passes - the number of times the hot combustion gases pass through the boiler, fuel feeding systems, applications, etc.  

 

Boiler Efficiency

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.

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.

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

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:

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 pressure (in kg/cm2(g)) and superheat temperature (oC), if any
  • The temperature of feed water (oC)
  • Type of fuel and gross calorific value of the fuel (GCV) in kcal/kg of fuel

Where :

hg - Enthalpy of saturated steam in kcal/kg of steam

hf - Enthalpy of feed water in kcal/kg of water

Indirect Method

Indirect method is also called as heat loss method. The efficiency can be arrived at, by subtracting the heat loss fractions from 100.

The principle losses that occur in a boiler are:

  • Loss of heat due to dry fluegas
  • Percentage heat loss due to evaporation of water formed due to H2 in fuel
  • Percentage heat loss due to evaporation of moisture present in fuel
  • Loss of heat due to moisture in combustion air
  • Loss of heat due to combustion of hydrogen
  • Loss of heat due to unburnt
  • Loss of heat due to radiation and other unaccounted loss

Payback of energy saving options

Many Experience from the past have shown that upgrading the boiler can reduce fuel costs considerably. The following table gives some examples for the payback of such investments.

Table 1: Payback of investment for improving boiler efficiency

 

 

Improving Boiler Efficiency

Energy Saving Tips

  • Ensuring proper fuel storage, handling, and preparation, for achieving good combustion conditions.
  • Avoiding partial load operations of combustion equipment.
  • Operating with minimum excess air for fuel economy.
  • Operating with lowest the possible stack temperature for fuel economy.
  • Operating with variable speed options for fan motors, if capacity control is needed (rather than inefficient damper control operations) in order to achieve power savings.
  • Establish a boiler efficiency-maintenance program. Start with an energy audit and follow-up, then make a boiler efficiency-maintenance program a part of your continuous energy management program.
  • Preheat combustion air with waste heat. Add an economizer to preheat boiler feed water using exhaust heat.
    (Every 22°C reduction in flue gas temperature increases boiler efficiency by 1%.)
  • Use variable speed drives on large boiler combustion air fans with variable flows instead of damper controls.
  • Insulate exposed hot oil tanks.
  • Clean burners, nozzles, and strainers regularly.
  • Inspect oil heaters to ensure proper oil temperature.
  • Close burner air and/or stack dampers when the burner is off, to minimize heat loss up the stack.
  • Introduce oxygen trim controls (limit excess air to less than 10% on clean fuels).
    (Every 5% reduction in excess air increases boiler efficiency by 1%; every 1% reduction of residual oxygen in stack gas increases boiler efficiency by 1%.)
  • Automate/optimize boiler blowdown. Recover boiler blowdown heat.
  • Inspect door gaskets for leakage avoidance.
  • Inspect for scale and sediment on the water side.
    (Every 1mm-thick scale (deposit) on the water side could increase fuel consumption by 5%–8 %.)
  • Inspect heating surfaces for soot, fly-ash, and slag deposits on the fire side.
    (A 3mm-thick soot deposition on the heat transfer surface can cause an increase in fuel consumption of 2.5%.)
  • Optimize boiler water treatment.
  • Recycle steam condensate to the maximum extent.
  • Study part–load characteristics and cycling costs to determine the most efficient combination for operating multiple boiler installations.
  • Consider using multiple units instead of one or two large boilers, to avoid partial load inefficiencies.