February 25, 2004
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).
Pressure and temperature define the state of water, except at phase change; in which case, density and either temperature or pressure would be required. When dealing with water vapor, it is generally treated as an ideal gas and it follows the ideal gas law: PV = n ℜ T = mRT Basic properties of water can be found in the Appendix. Water in a Mixture When water is found together with air, it is known as a mixture. In this case, water vapor will generally always exist, even if the temperature is below the saturation temperature at the total mixture pressure. In mixtures, the total gas pressure is considered to be made up of a contribution of “partial pressures” of its constituents, i.e.: P = Pv + Pa When a mixture reaches equilibrium and there is liquid present, the partial pressure of the water vapor is generally assumed to be equal to the saturation pressure from the steam table at the temperature of the mixture. If not at the saturation pressure, the partial pressure of water vapor in a mixture will generally be less than the saturation pressure at that temperature. Some common definitions associated with mixtures are: Dewpoint is defined as the temperature at which the water vapor of an air/vapor mixture will begin to condense when it is cooled at constant pressure. An example is when the night cools the air to a lower temperature and dew forms. Relative Humidity (phi) is defined as:
Pv x 100 Pvs
When air reaches 100% humidity, it is considered saturated, and is on the verge of producing condensate. Compressing an air/vapor mixture will increase the partial pressure of vapor, and this can lead to condensation in vacuum pumps by causing the partial vapor pressure to reach the saturation pressure. Humidity Ratio is defined as the ratio of the mass of water vapor to the mass of air in a mixture. On a 77°F day at sea level with 100% humidity, the humidity ratio is only about 3.2%, meaning that the air has only a small fraction of water in it. It should be noted that 100% humidity and saturated air have totally different meanings than a gas that is 100% water vapor.
The Dalton Model can be used to analyze the components of an air/vapor mixture. This model treats each constituent as if it occupies the entire given volume, in order to determine its partial pressure, i.e.:
P = Pv + Pa =
n v ℜT n a ℜT + V V
The Amagat Model is similar, but treats each constituent as having a partial volume, at the total mixture pressure. Important to note is that:
V Volume fraction v V
n mole fraction v n
Prepump Removal of Liquid Water If a process actually produces liquid water in the piping system approaching the pump, it is generally advisable that this liquid be removed before reaching the pump. The most common method for this is to use a liquid separator such as the Busch knock-out pot or the Busch clear liquid separator. These devices simply use directional control of the gas stream and gravity to cause the liquid and vapor to separate. The liquid will collect in the bottom of the vessel and must be drained before it reaches the point of overflowing the vessel. Prepump Removal of Water Vapor Water vapor by itself isn’t generally considered harmful, but condensed water vapor is. Unfortunately, water vapor that enters a pump can condense inside as its partial pressure is increased in the pumping process, and so it is sometimes determined that some water vapor must be removed before entering the pump. Liquid separators cannot remove water vapor. The only method for removing water vapor prior to a pump is to use a condenser. Condensers are devices that remove heat from a mixture and generally decrease the temperature of a mixture. If the temperature is reduced to or below the dewpoint temperature, condensation will occur. A large mass of water can be removed from the gas stream using a condenser, but the air mass in the gas stream remains constant, meaning that the humidity ratio has been greatly reduced, and the pump will have a much greater chance of handling the remaining water vapor. It should be noted that it is practically impossible to remove 100% of the water vapor using a condenser because a partial pressure of water vapor will exist, approaching the saturation pressure at the temperature at the exit of the condenser. For example, if a mixture enters a condenser at 150°F, 80% relative humidity, and 250 torr, and exits at 90°F, 100% relative humidity and 240 torr, then the water vapor occupies 61.7% of the volume entering the condenser, but only 15.2% exiting the condenser. In fact, a fringe benefit is that the total volume of gas entering the condenser has been reduced by 57.6% by the time it exits, due largely to the liquid water removed in the process. This is known as the “condensing effect” which may be thought of as boosting the capacity of a pump, or allowing a greater capacity than the pump itself to be achieved. Sizing and selecting a condenser to meet the desired conditions and determining if it is even practical to use a condenser are matters that should be deferred to engineering. Pump Operating Temperature If water vapor should enter a pump, then it is desirable to maintain the water in a vapor form throughout the pump, as water vapor isn’t generally considered harmful, but liquid water is. Unfortunately, the compression process not only raises the total pressure of a gas mixture, but proportionately increases the
partial pressure of water vapor. This tends to cause water vapor to condense if 100% humidity is reached. On the other hand, the compression process causes increased temperatures as well, which means that the saturation pressure is also increasing, and this tends to reduce the relative humidity, hopefully avoiding condensation. To prevent condensation, the following inequality should be satisfied:
Pv Pvsd < P Pd For example, if an R 5 pump pumps saturated air at 100 torr and 100°F, and has an internal back pressure of 860 torr, and the pump discharge temperature is 180°F, condensation will occur. Obviously, the warmer a pump operates, the less tendency there will be for condensation as the saturation pressure at the pump operating temperature is increased. Caution should be taken not to exceed the allowable temperature limits of a pump, though. It should also be noted that it will take time for a pump to reach operating temperatures once being cold, thus the typical recommendation to warm up a pump prior to pumping condensable vapors. If there is a situation where 100% water vapor is pumped, the above equation can be used to realize that the deeper the vacuum level, the less tendency there is for condensation. So, the “Water Vapor Tolerance” of a vacuum pump is defined as the highest inlet pressure at which a pump, without gas ballast, can pump 100% water vapor without having condensation. The Gas Ballast The gas ballast is a device that allows ambient air (relatively dry air) to enter the pumping chamber at a point beyond the inlet of the pump, in order to avoid a significant reduction in pumping speed. This additional air is combined with the process vapors, and because it is relatively dry, it reduces the humidity ratio, and simultaneously causes the total pressure to be increased prior to the final volumetric compression of the pumping chamber. This decreases the final partial pressure that is reached by the water vapor before reaching and exceeding the discharge pressure (which is necessary for the gas to discharge). In order to avoid condensation when a gas ballast is used, the following equation should be used:
GB •Patm Pvsd − GB •PdPvGB + PdPa Pd − Pvsd
For example, if an R 5 pump pumps saturated air at 100 torr, and 100°F, has an internal backpressure of 860 torr, an operating temperature of 180°F and a gas ballast capacity of 5% of the pump volume, in an environment that is 70°F with a relative humidity of 60%, then condensation will not occur. The Vacuum Relief Valve Another interesting point is that when a vacuum relief valve is used, it not only has the affect of a gas ballast, but it also prevents the pump from achieving a greater vacuum on the process. Creating a greater vacuum than necessary on a wet process will produce even more vapor, so the relief valve will aid by minimizing the amount of vapor that is produced. Of course, the relief valve must be set at a vacuum level that is deeper than the application requires. Purging and Other Considerations Prior to shutting off a pump that has been used on a wet process, a dry air purge, using ambient air should be used to dry and sweep out any condensation that occurred either in the pumping process, or in parts of the pump that are beyond the pumping chamber. A typical time period for drying a pump is about 20-30 minutes. A drip leg should always be installed on the discharge line in the case of a wet process in
order to prevent water vapor that was successfully pumped, but later condensed in the exhaust piping from reentering the vacuum pump. Pumps with silencers and low points prior to the discharge should also have drains. Coating in dry pumps can be used to aid in preventing corrosion as well. These coatings might include Teflon-based coatings and nickel-based coatings. Pumpdown Time When pumping down a closed vessel that has liquid water in it, as the air/vapor mixture above the liquid is pumped, the water vapor will constantly be replenished by the liquid, and the partial pressure of the water vapor will generally be maintained depending on the temperature. However, the air will not be replaced, and the partial pressure of air will be reduced, causing the total pressure to be reduced. The total pressure will be reduced until the vapor pressure of the water is approached, at which point constant pressure pumping will occur until all of the liquid water is vaporized. After vaporization is complete, the pumpdown process will continue, following the well known pumpdown equation. Until vaporization is complete the following equation may be used, assuming that the temperature and relative humidity remain relatively constant.
V P1 − φPvs ln Q P2 − φPvs
Conclusion Water is common in vacuum applications and can be damaging. With proper understanding, systems can be devised to reduce the amount of water that enters a pump, and to increase the ability of a pump to handle water vapor. Liquid water in a process will affect pumpdown time, and the loss can be determined. For all practical purposes, the considerations made in this paper can be used for most all condensable vapors. However, most other condensable vapors are chemically aggressive and require additional considerations.
Troy Bridges Product Engineering Manager Reference: Fundamentals of Classical Thermodynamics, 3rd Edition, Van Wylen & Sonntag, John Wiley & Sons, Inc., 1985 Key: P V n
= = = ℜ = R = T = Pv = Pa = φ = Pvs =
Pressure, Total pressure (torr) Volume, Total volume (ft.3) moles, Total moles Universal Gas Constant Specific Gas Constant Temperature (ΕR) Vapor pressure, partial vapor pressure at a given temperature (torr) Dry air partial pressure (torr) Relative humidity (%) Saturated vapor pressure at a given temperature (torr)
nv na Vv Pvsd
= = = =
Patm P1 P2 Q
= = = =
moles of vapor moles of dry air Partial volume of vapor (ft.3) Saturated vapor pressure at pump discharge temperature (torr) nternal pump discharge pressure (torr) gas ballast scfm as a percentage of inlet capacity (%) Atmospheric pressure (torr) Initial pressure, total (torr) Final pressure, total (torr) Pumping speed (acfm)
Appendix Universal Gas Constant
ℜ = 553.92 torr·ft.3/mole·°R Properties of Water/Water Vapor: Triple point Critical point Chemical formula Molecular weight Specific gas constant
Ratio of specific heats Density of liquid Surface tension Viscosity of liquid
32°F, 4.586 torr 705.5°F, 218 atms H2O 18.015 85.76 ft.·lbs./lbm·°R
1.329 62.4 lbs./ft.3 @ 60°F .00503 lb./ft. @ 60°F 1.21 x 10-5 ft.2/s
Properties of Saturated Steam (Steam Table): Temp. Sp. Vol. °F Torr ft.3/lb. 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
2.369 2.486 2.609 2.738 2.870 3.012 3.157 3.310 3.469 3.635 3.810 3.990 4.178 4.376 4.581 4.769 4.965 5.166 5.504 5.593 5.817 6.052 6.292 6.541 6.797 7.064 7.341 7.625 7.920 8.227 8.542 8.867 9.205
6210 5929 5662 5408 5166 4936 4717 4509 4311 4122 3943 3771 3608 3453 3305 3180 3062 2948 2839 2734 2634 2538 2445 2357 2272 2190 2112 2037 1965 1896 1829 1766 1704
Temp. °F 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
Torr 9.553 9.914 10.28 10.67 11.06 11.48 11.90 12.33 12.78 13.25 13.73 14.22 14.73 15.26 15.80 16.36 16.94 17.53 18.14 18.78 19.43 20.09 20.78 21.50 22.23 22.98 23.75 24.55 25.38 26.21 27.08 28.00 28.91
Sp. Vol. ft.3/lb. 1645 1589 1534 1482 1431 1383 1336 1292 1249 1207 1167 1129 1092 1056 1022 988.6 956.8 926.1 896.5 686.0 840.5 814.0 788.4 763.7 740.0 717.0 694.9 673.5 652.9 633.0 613.8 595.3 577.4
Temp. Sp. Vol. °F Torr ft.3/lb.
Temp. Sp. Vol. °F Torr ft.3/lb.
84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116
117 118 119 120 121 122 123 124 125 126 127 128 129 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 212
29.85 30.84 31.83 32.84 33.91 35.00 36.12 37.26 38.46 39.65 40.89 42.19 43.49 44.83 46.23 47.65 49.10 50.60 52.15 53.72 55.35 57.00 58.70 60.45 62.23 64.08 65.96 67.89 69.88 71.91 73.97 76.10 78.28
560.1 543.3 527.2 511.6 496.5 482.0 467.9 454.3 441.1 428.4 416.1 404.2 392.7 381.5 370.7 360.3 350.2 340.4 331.0 321.8 312.9 304.4 296.0 288.0 280.2 272.6 265.3 258.2 251.3 244.6 238.1 231.8 225.7
80.52 82.83 85.17 87.58 90.04 92.56 95.17 97.79 100.7 103.3 106.1 109.0 112.0 115.0 131.3 149.5 169.8 192.4 217.5 245.3 276.1 309.9 347.5 388.4 433.6 483.1 537.0 596.1 660.4 730.3 759.99
219.8 214.1 208.6 203.2 197.9 192.9 187.9 183.2 178.5 174.0 169.6 165.4 161.3 157.3 138.9 123.0 109.1 97.04 86.50 77.27 69.17 62.04 55.77 50.22 45.31 40.96 37.09 33.64 30.57 27.82 26.80