IPAC · FAQ

WATER

Water is one of the most widely used heat transfer media and is used in many industrial processes and in air conditioning systems. It normally exists in three physical forms, namely solid (ice), liquid (water) and gas (steam).

Water in its purest form is a tasteless, odorless, and colourless liquid. Normally there are two sources of water, ground water and surface water. Ground water (well and spring water) normally picks up impurities as it passes through the rock strata and the "filtering" effect of the rocks and sand will eliminate suspended solids. Surface water (rivers and lakes) is normally "softer" than ground water but is more prone to land slides, industrial and environmental pollution which result in higher suspended solids and organic matter.

At atmospheric pressure, water boils at 212 degrees F. In order to transform from a liquid phase (water) to a gaseous phase (steam) we need an extra 970 Btu/lb of energy, this is termed as the Latent Heat of Vapourization. The boiling point of water will vary with the pressure that it is subjected to. The water quality will depend on its source and environment.The impurities will include dissolved minerals, organic matter, suspended solids, color, dissolved gases and micrbiological contamination.

Dissolved minerals are normally calcium, iron, manganese and silica. When they exceed their solubility in water under specific temperature and pressure, they will form troublesome scale deposits and result in heat transfer problems.

Dissolved gases are commonly nitrogen, oxygen, carbon dioxide and hydrogen sulfide. Oxygen is the engine that drives the corrosion mechanism and nitrogen is inert. They are found in abundance in surface or aerated water. Carbon dioxide is a corrosive gas and is present in most natural water bodies. Normally ground water will contain more soluble carbon dioxide than surface water. The presence of hydrogen sulfide will present a rotten egg smell in the water. It renders the water distasteful and corrosive to most metals.

Color is measured in terms of "color units" and by comparing with a standard reference usually a platinum cobalt solution. It runs from colorless to a deep brown color. It is normally from decaying vegetation, runoff and industrial contamination.

Microbiological contamination occurs mostly in surface water, sometimes in ground water, but rarely in deep wells. Some of the microorganisms are algae, bacterial slimes, diatoms, molds, sulfate reducing bacteria, iron and manganese bacteria. Aside from presenting a health problem in drinking water, it can also provide a corrosive environment and clog up the circulation systems.

Suspended solids are mainly sand, silt, clay and corrosion by products. They can be removed by filtration. Turbidity is a measurement of the optical obstruction of light as it passes through a water sample. It is caused by suspended solids in a finely divided state. Sand, silt, clay, organic foulants and microorganisms all contribute to the turbidity of water.

The impurities in the water will present corrosion, scaling, fouling and biological contamination problems. The objective in water treatment is to minimize the impurities, which in turn will result in reducing fuel cost, water consumption, downtime and increasing the life span of the equipment.

Scale Inhibition Versus Removal

What is the difference between scale inhibition (prevention) and scale removal? The build-up of calcium and barium scale is prevented by the addition of low concentrations of a scale inhibitor. In the absence of a scale inhibitor, crystals grow in an orderly fashion to produce a hard and highly crystalline scale.

The mechanism of a scale inhibitor is to interfere with this orderly growth of the crystal. The inhibitors are of such molecular dimensions that they adsorb onto the growth plane of the crystal, causing the micro-crystal to either stop growing or to grow in a distorted form and generate a soft sludge instead of a hard scale.

Once scale is laid down, the best the inhibitor can do is prevent further build-up. Calcium carbonate can be removed by the use of an acid, generally hydrochloric or sulphamic acid, which contains an inhibitor to prevent attack on the metals in the system. Other scales are more difficult to remove. There is an organic acid based calcium sulphate scale remover, but those hard-to-remove organic/inorganic scales found in evaporators can only be removed with a combination of alkali, chelants and a wetting agent.

Dust Control

How does dust control work? Dust is defined as fines (very small particles) which become airborne at some point in the process. A dust control chemical is applied at , for example, a transfer point in a conveyor system. The product must“wet” the surface of the substrate, and cause the fines to adhere to the larger particles or to agglomerate into larger particles. This adhesion/agglomeration should be persistent enough that re-dusting does not occur later in the process.