Chemical Plant Workhorses: A Closer Look at Boilers

   Basics

    A boiler is a closed vessel where water is heated, steam is generated, or steam is superheated with heat from combustion fuels, electricity, or nuclear energy.  Boilers consist of two main parts:  the furnace and the convection sections.  The furnace section is where the heat is generated (we'll assume the more common combustion heat source).  This section is designed to accommodate the three "T's" of combustion: time, temperature, and turbulence.   For complete combustion, the fuels (usually hydrocarbons and air) need sufficient time and turbulence to mix properly.  The fuel must then have the correct ignition temperature to burn.  The convection section is where heat is transferred from the flue gases to the water.

Classification

    Boilers are generally classified as being one of two types:  firetube (as seen to the left) and watertube (below).

    Firetube boilers are so named because the combustion flames and flue gases are inside the boiler tubes with the water surrounding the tubes.

    Watertube boilers are designed to carry the water within the tubes while the flue gases surround the tubes and supply heat.

General Setup

    The deaeration of the condensate returning from the process ensures that the water is free of air bubbles that will inhibit heat transfer.   The chemical treatment is used to reduce the amounts of chemicals such as CaPO₃, Fe₂O₃, CaCO₃, CaSO₄, and silicates that can cause build up in the heat transfer equipment and reduce performance as shown by Figure 2.   The economizer is used to preheat the water entering the boiler with the flue gas.   This helps reduce the duty in the boiler and minimize the fuel cost, which is the primary operating cost of a boiler.

    Cleaning

    A pH test is commonly used to determine when it may be time to clean a boiler.  A pH reading under 7 (indicating acidity) is a sure sign that contaminants have unavoidably entered the water system.  The system of either steel or copper can easily be cleaned by circulating one of the following solutions:
1.  Trisodium phosphate (1 lb for each 50 gallons in the system)
2.  Sodium Carbonate (1 lb for each 30 gallons in the system)
3.  Sodium Hydroxide (1 lb for each 50 gallons in the system)

Fuel Chart