Treating Boiler Water

Steam generation has been an area of interest in manufacturing since the industrial revolution. The method of steam production has changed from the first pot and kettle type boiler to the two basic types of boilers we have today. Fire tube boilers are designed to permit improved water circulation and to increase the heating surface area. Water tube boilers produce steam faster and at higher pressures than fire tube boilers.

The process of making steam is familiar to us all. At atmospheric pressure, water boils at 212F. In a closed vessel, the pressure increases as steam is produced due to the large increase in volume when water is changed from the liquid to a gas. As pressure increases the boiling point of water increases.

A BTU ( British thermal Unit) is the amount of heat required to raise one pound of water 1F. The heat value of fuels is expressed as BTU's per pound gallon or cubic foot. Fuel oils average 150,000 BTU's whereas natural gas is measured at 1,000 BTU's cubic foot.

When fuels is burned, the heat given off is transferred through the boiler tubes to the boiler water. If there are any deposits on the waterside or fireside heat transfer is impeded thus resulting in additional heat to generate steam. Overheating boiler tubes can result in failure.

Convection and radiation are the mechanisms by which heat transfer takes place. Heat is absorbed from the hot combustion gases as they pass through the fireside of the boiler. Each 40 to 50F rise in stack gas temperature results a one-percent decrease in thermal efficiency.

Water circulation is obtained naturally when feedwater is added to the boiler. The feedwater temperature is lower than the boiler water temperature. Therefore the feedwater tends to fall as the warmer water with steam bubbles rise. Baffles and other separation devices are installed in the boiler to minimize potential carryover. This makes the steam change direction, thus causing the entrained boiler water to be left behind as the steam leaves the boiler. As steam leaves the boiler it is saturated with water. For most applications this is acceptable. However, if dryer steam is needed more heat must be applied to the steam. This task is accomplished by using a superheater. A superheater is a heat exchanger that utilizes heat from the stack gas and passes it to the saturated steam. This extra heat forces the moist steam to a fully vaporized state.

As steam is produced, any natural contaminants not removed during feedwater pretreatment are left behind. The term for this is cycling. Cycling is measured by the concentration of TDS (Total dissolved solids) or conductivity over the concentration of TDS or conductivity in the feedwater. If these suspended or dissolved solids reach the point of saturation, deposition of the heat transfer surfaces result. Boiler cycles are controlled by a continuous blowdown situated near the top of the boiler where then most concentrated boiler water exists.

Pre-treating the feedwater can control the solids in the boiler. There are certain minerals in the incoming water that cause severe deposition when they come in contact with heat transfer surfaces.

Possible science experiment for students

1. Weigh an empty 1000 ml beaker

2. Add 1000 ml water the weigh again.

3. Boil water until completely evaporated.

4. weigh the beaker again.  subtract the initial weight from the final weight.

5. Write conclusions