AIR VENTING FROM PROCESS EQUIPMENT

In process equipment, the air vents should be fitted in the steam lines connecting the steam main to the process equipment and be should be a part of the steam trap to remove residual air which is pushed into the trap by start-up steam and condensate.

To understand Air venting in process equipment we shall illustrate and explain air venting with few examples:

Vertical drying Range

In a vertical drying range, it is essential to remove air from the drying cylinders to achieve the desired drying rates. If the cylinders are not effectively vented it leads to low surface temperatures as air tends to rim along with condensate over the peripheral circumference cylinder. Productivity is also affected as the cylinder speeds cannot be raised. The most effective way of air venting in the drying range is to position an air vent on the header charging steam to cylinders and traps should compulsorily be equipped with an in-build thermostatic air vent. The air that is pushed towards the trap will be vented by the in-build thermostatic air vent.

 

 Jacketed pans

Jacketed pans are used for the heating process. The application and the liquid being heated decides the position of the inlet and outlet. The figure shows that the steam inlet is from the side and the outlet from the bottom. Equipping air vents at the top dead ends reduces the boiling time required.

For jacketed pan which has both inlet and outlet at the bottom of the vessel. Air gets collected at the ends on both sides and air vents should be fitted at both respective locations. On average removing the air collected at dead ends of a small pan reduces the cooking time by 35%.

Fig: jacketed pans

Large steam spaces

Applications that use equipment like autoclaves or large retorts experience direct exposure to steam with the material being processed. The materials in these processes need to attain a certain critical temperature and be held for a period at this temperature.

Thus, to achieve the same heating cycle the steam pressure will have to be increased further.

Because of the presence of air, the distribution of steam in the retort will not be uniform and hence the heat treatment too shall be hampered.

In autoclaves or large retorts, proper air venting is necessary for efficient heating. The following graph shows the effect of effective and poor air venting.

 

Graph: . Heating effect for poorly vented and efficiently vented autoclaves

With proper air venting, an ideal profile of heating is observed which is shown by (A-B-C-D-E), in the case of poor air venting the observed heating profile is shown by (A-F-G-H). The temperature achieved with proper air venting is 40 0C in 6 min time and the temperature is maintained for a certain time as shown by BC. This is important to maintain product quality. AF shows the time required to reach the temperature of 40 0C. It takes more than 4 min to achieve the same temperature. The fall and rise in temperature are observed due to ineffective air venting and the heating cycle is also prolonged.

Air venting is most effective if it takes place rapidly and does not cause pressure to build up in the retort or autoclaves during the venting period.

For large spaces, the fact that air is denser than steam, importance should be given towards the arrangement of air vent and inlet position. The steam inlet should usually be kept at the top and the air vent at the bottom. This arrangement leads to less intermixing of steam with air. In other traditional retorts/autoclaves steam is injected at the bottom and air vents at the top, but in certain large spaces, different arrangements may be needed.

The thermostatic filling should have minimum possible sub-cooling which ensures the least possible residual air in the autoclave.

The location of air vents is very much important as they can be useless if fitted in the wrong place.