Boiler Primer 1: Introduction

What is a boiler?
A boiler is used to either heat water, or to heat water with the purpose of making steam.

Boilers do this by contacting water with a hot surface.  There are two general types, fire-tube (left) and water-tube boilers (right).  Water-tube boilers have water running through tubes, and the outside of those tubes are heated by burning natural gas, to heat the water running through them.  More commonly seen are fire-tube boilers.  These involve burning natural gas within the tubes and radiating the heat out to water that surrounds them.

Hot-water boilers are used to heat water which is sent in a loop throughout a building typically, or through a process in general.  These are often used to heat buildings by blowing air over the tubes through air handlers.  This system can be thought to function similarly to how an old radiator does--the kind that tend to bang, and are found in old homes, schools or offices.

Steam boilers create pressurized steam by heating water.  The steam leaves the boiler through a header pipe and is passed to a process or wherever it is needed.  This steam that leaves is precious though, because it is mostly pure.  When water turns into steam, it leaves salts and most other chemical treatment behind in the boiler.  What happens after the steam deposits its heat of vaporization to a process, is that it condenses back into water because it lost the energy to be steam.  The condensed water is then very pure, because the steam was pure.  The condensate is then collected when possible, and fed back into the boiler.  Some boilers cannot do this because the steam is not part of a closed loop and goes elsewhere, but wherever possible, the pure water is collected and recycled, replacing the water that leaves as steam.

As you can see from the above diagram, there is an enormous amount of energy carried in steam.  This is the energy in steam that is applied to a process when the steam is allowed to condense.  That energy isn't free though, it comes from the combusted natural gas, but it allows us a more convenient way to pass energy along, using water/steam as a medium.