BOILERS & THERMIC FLUID HEATERS

1. INTRODUCTION

This section briefly describes the Boiler and various auxiliaries in the Boiler Room.

A boiler is an enclosed vessel that provides a means for combustion heat to be transferred to 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 inexpensive medium for transferring heat to a process. When water at atmospheric pressure 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 an equipment that must be treated with utmost care.


The boiler system comprises of: a 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 valve sand 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 twosources 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 roomand plant processes. For higher boiler efficiencies, an economizer preheats the feed waterusing the waste heat in the flue gas.


2. TYPE OF BOILERS

This section describes the various types of boilers: Fire tube boiler, Water tube boiler, Packaged boiler, Fluidized bed combustion boiler, Stoker fired boiler, Pulverized fuel boiler, Waste heat boiler and Thermic fluid heater.


2.1 Fire Tube Boiler
In a fire tube boiler, hot gases pass through the tubes and boiler feed water in the shell side is converted into steam. Fire tube boilers are generally used for relatively small steam capacities and low to medium steam pressures. As a guideline, fire tube boilers are competitive for steam rates up to 12,000 kg/hour and pressures up to 18 kg/cm². Fire tube boilers are available for operation with oil, gas or solid fuels. For economic reasons, most fire tube
boilers are of “packaged” construction (i.e. manufacturer erected) for all fuels.



2.2 Water Tube Boiler
In a water tube boiler, boiler feed water flows through
the tubes and enters the boiler drum. The circulated water is heated by the combustion gases
and converted into steam at the vapour space in the drum. These boilers are selected when the steam demand as well as steam pressure requirements are high as in the case of process cum power boiler / power boilers.
Most modern water boiler tube designs are within the capacity range 4,500 – 120,000 kg/hour of steam, at very high pressures. Many water tube boilers are
of “packaged” construction if oil and /or gas are to be used as fuel. Solid fuel fired water tube designs are available but packaged designs are less common.



The features of water tube boilers are:

1. Forced, induced and balanced draft provisions help to improve combustion efficiency.
2. Less tolerance for water quality calls for water treatment plant.
3. Higher thermal efficiency levels are possible

2.3 Packaged Boiler
The packaged boiler is so called because it comes as a complete package. Once delivered to a site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made to become operational. Package boilers are generally of a shell type with a fire tube design so as
to achieve high heat transfer rates by both radiation and convection.

The features of packaged boilers are:

1. Small combustion space and high heat release rate resulting in faster evaporation.
   
2. Large number of small diameter tubes leading to good convective heat transfer.
3. Forced or induced draft systems resulting in good combustion efficiency.
4. Number of passes resulting in better overall heat transfer.
5. Higher thermal efficiency levels compared with other boilers.

These boilers are classified based on the number of passes - the number of times the hot
combustion gases pass through the boiler. The combustion chamber is taken, as the first pass
after which there may be one, two or three sets of fire-tubes. The most common boiler of this class is a three-pass unit with two sets of fire-tubes and with the exhaust gases exiting through the rear of the boiler.

Source : Thermal Energy Equipment: Boilers & Thermic Fluid Heaters (Energy Efficiency Guide for Industry in Asia – www.energyefficiencyasia.org)