What is the difference between a tower and a pressure vessel?

　　 A tower is a device for the transfer of substances between the gas phase and the liquid phase, or between two liquid phases. Based on structure, they are divided into two main categories: plate towers and packed towers. A plate tower is equipped with a certain number of plates, and the gas comes into contact with the liquid layer on these plates in the form of bubbling or spraying to facilitate mass transfer. The packing tower is filled with packing to a certain height; the liquid flows downward along the packing, while the gas flows upward and comes into countercurrent contact with the liquid film to facilitate mass transfer. It is commonly used in operations such as distillation, water absorption, and extraction. Pressure vessels are sealed devices used to hold gases or liquids and withstand certain pressures. Storage and transportation containers, reaction vessels, heat exchange vessels, and separation vessels all fall under pressure vessels. To distinguish them from ordinary containers (atmospheric pressure containers), only those containers that meet all of the following three conditions can be referred to as pressure vessels: For more detailed information: http://baike.baidu.com/view/966754.html?wtp=tt http://baike.baidu.com/view/41619.htm

Literally, it can be understood as a container under pressure, that is, a pressure vessel. Chinese regulations stipulate that only vessels operating at a pressure of 0.1 MPa or higher are considered pressure vessels; those with lower pressures do not qualify as pressure vessels and can only be regarded as vessels under normal pressure. The classification of countries varies slightly. As one type of container, a tower is considered a pressure vessel if its pressure meets the aforementioned requirements ； If it does not meet the requirements, it can only be considered a atmospheric pressure tower, that is, it is not a pressure vessel; therefore, all regulations applicable to pressure vessels do not apply to it, unless otherwise specified.

The two of you aren’t classified under the same category. Simply put, any structure that meets the criteria for a pressure vessel counts as a pressure vessel; pressure vessels can be various types of equipment, as long as they satisfy those criteria.