Why deaerate a steam system?
The elimination of noncondensable gases in steam systems is a largely neglected aspect of steam system operation, even though it is a crucial one. Deaeration has three main goals:
- decreasing corrosion throughout the steam system
- improving the heat transfer at the process level
- providing a more even product temperature
The presence of noncondensable gases in a steam system increases corrosion, which bring about costs associated with excessive concumption of anti-corrosion chemicals and frequent repairs. Noncondensable gases also cause a decrease in steam pressure and therefore in temperature. Energy transfer being less efficient, the pressure in heat exchangers must be raised in order to obtain the target temperature. Consequently, it takes more fuel to heat the product at the required temperature.
These are avoidable expenses that justify deaeration.
What are noncondensable gases?
Composition of dry air
All these noncondensable gases are grouped in the word ‘air’. These gases are called ‘noncondensable’ because temperatures in the cryogenic range (around -150 oC) are required to condense them.
How noncondensable gases enter a steam system
All steam systems are filled with air when they are not under load. Every time the system or part of it stops, it fills with air. This is explained by the fact that condensation of the steam in the system induces a vacuum which pulls air in through all the defective joints or the smallest leaks. Even the best systems, equiped with drain valves to prevent freeze-up and vacuums, can’t prevent air from entering.
Air can also be introduced into a steam system through vacuum breakers.
Finally, air can enter the system in solution in the feedwater. At 80°C, water can dissolve a quantity of air corresponding to about 0.6% of its volume. Carbon dioxide has a higher solubility, roughly 30 times greater than oxygen. When the water is heated in the boiler, the gases are released with the steam and carried into the distribution system.
Since air is present everywhere in all systems, it is necessary to plan a good deaeration throughout the steam system.
What are the impacts of noncondensable gases?
Corrosion is one of the most common problems in steam systems. It causes premature degradation of equipments and can be responsible for production downtime. In a badly-designed steam system, it spares no kind of equipment: heat exchangers, steam traps, pumps, pump traps, condensate return piping, deaerator, etc.
Oxygen (O2) that enters a steam system through air oxidizes and corrodes the metal parts of the system. Oxygen corrosion, also known as “pitting”, creates small, but deep holes that can eat through a steel pipe in less than two years.
When carbon dioxide (CO2) gets in contact with condensate, it dissolves in it and deteriorates into carbonic acid, a highly corrosive compound which attacks the steel of pipes and equipments. Acidic condensate eats through pipes, causing a characteristic trough in the bottom of pipes. By-products of corrosion are then returned to the boiler where they can create iron deposits.
Oxygen is the main cause of corrosion in steam systems, but if carbon dioxide is also present then the pH will be low, the water will tend to be acidic, and the rate of corrosion will be increased.
Poor heat transfer
Air, including nitrogen and other gases, is the best insulator, or worse conductor: 1/100 inch of air isolates as well as 11 feet of copper, 15½ feet of steel or 1/5 inch of water. That is why all good isolating materials are made of air. Knowing that, it is easy to see why it is extremely important to remove air that could concentrate on heating surfaces.
Figure 2 shows how steam must cross a layer of gas before condensing on the heating surface of a heat exchanger. This slows down the heating process.
Furthermore, the presence of noncondensable gases in an exchanger lowers the partial steam pressure and thus lowers the temperature. The pressure gauge shows the total pressure in the exchanger, and not only the partial steam pressure. If the presence of noncondensables, the real temperature in the exchanger, which is determined by the partial steam pressure, will be inferior to the temperature inferred from the pressure reading at the gauge, since the gauge shows the total pressure.
The following equation illustrates this phenomenon:
|Pressure reading at the gauge||= total pressure (Pt)|
|= partial steam pressure (Pv) + partial gas pressure (Pg)|
If there is no air in the steam, Pt = Pv. At a pressure of 100 psig, the steam table indicates that the temperature is 170 oC.
If there is 20% air in the steam, then 100 (Pt) = 80 (Pv) + 20 (Pg) and the temperature corresponding to a steam pressure of 80 psig is 162 oC.
Consequently, it is necessary to raise the pressure in the exchanger in order to reach a temperature of 170 oC and heating the product to the desired temperature involves higher fuel costs.
Uneven product temperature
If non-condensable gases accumulate unevenly on the heating surface of an exchanger, the result is an uneven temperature on the product side of the heating surface.
Removing non-condensable gases from exchangers can have a positive impact on product quality. This is especially critical with exchangers such as rotary dryers and hot plates, where an extremely even surface temperature is essential to a quality product. Evenness of temperature is also important when heating some liquid products.
Figure 3 shows that, at a constant pressure of 10 bar (150 psig) in an exchanger, the temperature decreases as the percentage of air in the steam increases. At 0% air, the temperature is 180 oC while it decreases to 160 oC when there is 50% air. One consequence of this is that the temperature in the exchanger is actually is lower then what the pressure gauge might lead one to think. That’s because the temperature is determined by steam pressure only, not total pressure!
Figure 4 shows that, for a given flow rate, the more air there is in an exchanger, the worse the heat transfer gets. We can also note that, as the flow rate gets faster, the heat transfer improves for a similar air percentage.