During the period of shutdown, steam and condensate are removed from the system and the space inside is occupied by air. A very small quantity of air and other condensible gases enters the system along with the steam. During the next start-up, air and non-condensible gases should be removed from the system as they have many adverse effects on processes and process equipments. Not removing the air and other non-condensible gases can have many adverse effects including lengthy warm up times and reduced plant efficiencies.
Air-vents can effectively remove the air trapped in the distribution network and process equipments. For complete and fast removal of the air from the system, air venting should be done properly. Improper air venting will have following adverse effects:
1. Reduced heat transfer rates and productivity
Air offers much higher resistance to heat transfer compared to metal wall. Air is more than 1500 times resistant to heat transfer than iron or steel and no less than 13000 times more resistant than copper. This means that a 0.5 mm film of air will offer the same resistance to heat transfer as a 6.5 m copper. It can be seen from the figure below that even a thin layer of air can hamper the heat transfer drastically.
2. Increased corrosion
Air contains gases like oxygen and carbon dioxide which are responsible for the corrosion of the metal walls and equipments. Carbon dioxide when dissolved in water forms highly corrosive carbonic acid. Corrosion many times goes undetected and comes up as an unexpected failure.
3. Trap malfunctioning
Presence of air in condensate/steam lines affects the performance of traps. Condensate discharge capacity of steam traps is affected on account of presence of air and other non-condensible gases in the condensate/steam lines. Reduced condensate removal may cause water logging which in turn affects process timings and product quality. Water logging can also significantly raise the number of rejections. Traps with air venting facility should always be preferred over other traps to avoid above mentioned issues.
4.Reduced actual steam pressure
The pressure exerted by any gas or mixture of gases is always governed by Dalton’s law of partial pressure. According to Dalton’s law, The total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture. This can be best illustrated by the example below -
If the total pressure of a steam / air mixture at 2 bar (absolute) is made up of 3 parts steam to 1 part air by volume, then: Partial pressure of air= ¼ x 2 bar a= 0.5 bar a Partial pressure of steam= ¾ x 2 bar a= 1.5 bar a Total pressure of mixture= 0.5 + 1.5 bar a= 2 bar a (1 bar g)
As it can be seen, in the example discussed above, the actual steam pressure is lesser than the pressure reading displayed by the gauge. If air is present inside the steam, the actual pressure obtained at the process end will be lower than that shown by the gauge. As a result of this, the temperature of the system will be lower than that required for the process. Unavailability of required pressure results in longer batch timings and increased number of rejections.
Role of air vents
Air vents are thermo static devices which remove the air from the system. Air mixed with steam lowers the temperature of the mixture which enables the air vent to remove the air.