Online course and simulator for engineering thermodynamics

## Boilers

There are two main types of boilers, known from the fluid that circulates inside the tubes: fire tube boilers, and water tube boilers.

• In the first, the flame develops in a corrugated tube, then the flue gases pass inside tubes, in one or more passes, water being at the outside;

• Within the second type, water circulates by natural or forced convection between two drums placed one above the other, through a network of tubes. The flame develops in a furnace lined with tubes that absorb the radiation. A second tube bundle receives heat by convection from the flue gases. The water rises in the tubes subjected to radiation, and falls by the convection assembly.

Fire tube boiler

### Water-tube boilers

The fire tube boilers can achieve flue gas temperatures lower (220 to 250 °C) than water-tube boilers (300 °C) without an economizer, which gives them a slightly better efficiency.

However, the former are limited to capacities lower than the latter, for reasons of mechanical strength and safety (with a very large volume of water under pressure).

Their main area of use is the supply of saturated steam under low pressure (&lt;15 bar), and represent over 60% of the French fleet of boilers, against 20-25% for water tube boilers, which are well suited for the supply of superheated steam at medium and high pressure.

A boiler has three successive functions:

• heat pressurized feedwater (in the economizer) to the vaporization temperature corresponding to the pressure;

• vaporize steam;

• and finally superheat steam at the desired temperature.

Water-tube boiler

### Thermodynamic characteristics

As a first approximation, boilers can be considered as isobaric . The pressure drops are in fact generally relatively low.

When the combustion is not stoichiometric, it can be characterized in several ways:

• either by excess air e, which as its name implies, represents the amount of air in excess

• or by the air factor lambda, which is the term multiplying the air in the combustion equation

• or by the richness R, ratio of the number of moles (or mass) of fuel in a quantity of mixture, to the number of moles (or mass) of fuel in the stoichiometric mixture.

For stoichiometric mixture lambda = 1, with excess air lambda > 1, with an excess of fuel (lack of air) lambda <1

These three variables are linked by simple relationships: lambda = 1 + e, and R = 1/lambda

In Thermoptim, a boiler is represented either by a combustion chamber represented by the icon:

or by a triple interchange involving exchange processes represented by the icon:

The setting of combustion screens is done using lambda.

• the first is to give the value of lambda and calculate the end of combustion temperature and the flow-rate of fuel required

• the second is to give the value of the end of combustion temperature and to calculate lambda and the flow-rate of fuel required

• the third is to give the value of the fuel flow-rate and to calculate lambda and the end of combustion temperature

### Book reference

An excerpt of the textbook chapter is freely downloadable with the agreement of CRC Press

### Available Diapason sessions

content

steps

soundtrack duration

S15En

16

9 mn 50 s

S16

Technologie des chambres de combustion et des chaudières

9

4 mn