February 25, 2004
03-1
WATER Just as water is common in every day living, it is also common in vacuum applications. Unlike air, it can change between solid, liquid, and vapor phases in every day environments. In vacuum applications, water generally has a negative affect. Liquid water is highly corrosive, causing iron to rust, and it can mix with the lubricant of lubricated pumps, destroying its lubricating properties and causing abrasion, etc., of wearing parts. Therefore, it is important to either remove water prior to entering a vacuum pump, or to create a situation, whereby the pump can handle the water (vapor) without having the above-mentioned negative affects. While liquid water in a process will generally increase the pumpdown time to a particular pressure, there is the benefit of increased capacity if water vapor is condensed prior to, or as it enters a pump. This paper covers the basic properties of water, prepump removal of water vapor, pump operating considerations for water vapor, and the affect on pumpdown time. Note that all terms are defined in the Key. Water as a Pure Substance When water exists by itself, it is considered a pure substance. As mentioned above, water can exist in solid, liquid, or vapor phases, depending on pressure and temperature. Graph 1 shows the basic relationship. When the temperature and pressure are in the liquid region of this graph, only liquid water will exist. If the temperature and pressure of a liquid change in such a way that they cross the vaporization line, the liquid water will turn into water vapor. The process can also happen in reverse. If the temperature and pressure are actually on the vaporization line, then the water can be liquid, vapor, or a combination of both. To take a closer look at the process of changing from liquid water-to-water vapor, consider Figure 1. In this example, there is a closed piston cylinder arrangement in which the piston is designed to maintain a constant pressure of 760 torr in the cylinder. In Step 1, the water is at 70째F and a pure liquid. As heat is transferred to the cylinder in Step 2, the pressure remains constant, the temperature rises, and the volume slightly increases. When the temperature reaches 212째F at Step 3, a change in phase begins to occur, i.e., water vapor begins to form. During this process, the temperature and pressure remain constant and the volume rapidly increases. At Step 4, all of the liquid water has vaporized and the temperature is still 212째F. Further heat transfer causes the temperature to begin increasing again, the volume slightly increases, and the pressure remains constant. The temperature at which water begins to vaporize is known as the saturation temperature (at a given pressure). There is a definite relationship between saturation temperature and pressure for water, and these values can be found in the saturated steam table (see Appendix).