The purpose of a vapor withdraw tube, also known as a J-Tube, is:

The purpose of a vapor withdraw tube, commonly referred to as a J-Tube, is to facilitate the withdrawal of propane vapor from the vapor space above the liquid propane in a cylinder. This design ensures that when propane is drawn from the tank for use, it does so in vapor form, which is necessary for the proper functioning of propane appliances.

In propane tanks, there is a distinction between the liquid propane at the bottom and the vapor space above it. Since most propane appliances require vapor to operate – as liquid propane cannot be used directly in many systems – the vapor withdraw tube is strategically placed to capture this propane vapor right before it leaves the tank, thereby supplying a consistent and safe flow of vaporized propane to the system.

This operation is crucial for ensuring the efficient functioning of propane systems and preventing issues related to liquid propane entering the lines, which could cause operational failures. The other choices reflect misunderstandings of the vapor withdraw tube's function, such as limiting liquid flow or reversing the direction of vapor and liquid, which are not relevant to the tube's design or intended purpose.