Example 3-9 Process Vacuum System

For this process example, again using water for convenience, a low pressure, low temperature water is emptied into a vented vessel, and then pumped to the process at a location at about 3000 feet altitude (see Appendix A-6) where atmospheric pressure is approximately 13.2 psia. Water SpGr is at 200°F = 0.963. 

---

Example 3-6 NPSH Available in Vacuum System, 191 Example 3-7 NPSH. Available in Pressure System, 191 Example 3-8 Closed System Steam Surface Condenser NPSH Requirements, 191 Example 3-9 Process Vacuum System, 192 Reductions in NPSHr, 192 Example 3-10 Corrections to NPSHr for Hot Liquid Hydrocarbons and Water. 192 Example 3-9 Process Vacuum System, 192 Example 3-10 Corrections to NPSHr for Hot Liquid Hydrocarbons and Water, 192 Example 3-11 Alternate to Example 3-10, 194 Specific Speed,... 

In many cases design modifications can substantially improve the productibility of the products and reduce their cost with improved product quality. As an example if voids exist the problem can usually be corrected by modifying or changing the plastic s composition and/or the use of a vacuum system during the casting. Understanding the effects of the process on the product is essential in making successful products. 

In addition to the vacuum valves, which perform solely an isolation function (fully open - fully closed position), special valves are needed for special functions. Typical are variable leak valves, which cover the leakage range from 10" ° cm /s (NTP) up to 1.6 10 cm /s (NTP). These valves are usually motor driven and suitable for remote control and when they are connected to a pressure gauge, the process pressures can be set and maintained. Other special valves fulfill safety functions, such as rapid, automatic cut-off of diffusion pumps or vacuum systems in the event of a power failure. For example, SECUVAC valves belong to this group. In the event of a power failure, they cut off the vacuum system from the pumping system and vent the forevacuum system. The vacuum system is enabled only after a certain minimum pressure (about 200 mbar) has been attained once the power has been restored. 

The vacuum system must be able to attain the required pressures reliably despite these high gas loads. In the example shown, the system is evacuated with a combination of a backing and Roots pump. A diffusion pump along with a cold surface forms the high vacuum pump system. The cold surfaces pump a large portion of the vapor and volatile substances emitted by the plastic parts while the diffusion pump basically removes the non-condensable gases as well as the noble gas required for the sputter process. 

Suppose that we wish to calibrate the volume of trap E connected to manometer D on the vacuum line in Fig. 5.2. A known quantity of a gas, such as CO, is condensed into trap E using the calibrated bulb and the techniques just outlined in Example 5.3.G. The stopcocks are then turned so that trap E communicates with the central manometer D but is isolated from the rest of the vacuum system. At this point, the cold trap is removed from E, the trap is allowed to come to room temperature, and the pressure and room temperature are measured. The volume of the manometer-trap combination is determined from the known moles of gas, the pressure, and the temperature, using the ideal gas law. The process is repeated with successively larger samples of CO2, and a plot of volume versus pressure is constructed from the data. Since the bore of the manometer is of constant diameter, this plot should be a straight line. It also is possible to... 

The vacuum system must be able to maintain the chamber pressure at below the saturation vapour pressure (SVP) of ice at the chosen process temperature. Although it is not universally recognised, freeze-drying can also be performed without the need of a vacuum system. The laundry model in which, even on the coldest of days, frozen laundry on a washing line can be air dried, albeit at a low rate, has been cited as an example. 

---