Vacuum-pressure process

As an alternative to the conventional polyethylene glycol (PEG) treatment of waterlogged wood see Chapter 8), attempts were made in 1966-1967 to apply a radiation-curing process (i, 2). The aim was to reduce treatment time and achieve more stability for treated objects subjected to interior climatic changes. The impregnation of waterlogged wood by immersion involves a liquid-liquid exchange process. Dry wood was directly bulked by monomers by a vacuum-pressure process. 

Green wood can be impregnated by immersion in aqueous PEG solution over a period of several weeks. Air-dried wood is best treated in a combined vacuum-pressure process. For example, beechwood can be made almost completely dimensionally stable by immersing it in an approximately 50% solution of PEG 1500 so that it absorbs about 45% PEG. The process has two advantages [116—119] ... 

Vacuum flash processes, which operate under the atmospheric boiling point of the solution, include the Uhde—LG. Farbenindustrie process and the closely related Kestner process (22). In these, ammonia, nitric acid, and recirculated ammonium nitrate solution are fed into the neutralizer. Hot solution overflows to an intermediate tank and then to a flash evaporator kept at 18—20 kPa (0.18—0.2 atm) absolute pressure. Partial evaporation of water at this point cools and concentrates the solution, part of which is routed to evaporation. The rest is circulated to the neutralizer. 

Dearation can be either vacuum or over pressure dearation. Most systems use vacuum dearation because all the feedwater heating can be done in the feedwater tank and there is no need for additional heat exchangers. The heating steam in the vacuum dearation process is a lower quality steam thus leaving the steam in the steam cycle for expansion work through the steam turbine. This increases the output of the steam turbine and therefore the efficiency of the combined cycle. In the case of the overpressure dearation, the gases can be exhausted directly to the atmosphere independently of the condenser evacuation system. 

Vacuum in process systems refers to an absolute pressure that is less than or below the local barometric pressure at the location. It is a measure of the degree of removal of atmospheric pressure to some level between atmospheric-barometer and absolute vacuum (which cannot be attained in an absolute value in the real world), but is used for a reference of measurement. In most situations, a vacuum is created by pumping air out of the container (pipe, vessels) and thereby lowering the pressure. See Figure 2-1 to distinguish between vacuum gauge and vacuum absolute. 

The operating principle of steam-jet ejectors is explained in Volume 1, Chapter 8. Then-specification, sizing and operation are covered in a comprehensive series of papers by Power (1964). Diffusion pumps are used where very low pressures are required (hard vacuum) for processes such as molecular distillation. 

TES are based on the steep temperature dependence of the resistance of superconducting metallic films. The useful temperature range is very narrow. These thermometers which may have a very low intrinsic noise, are fabricated by a vacuum deposition process at very low pressure and are patterned either by photolithography technique (see e.g. ref. [21]) or by micromechanical machining (see e.g. ref. [22]). The dimensionless parameter a = T/R-dR/dT defines the DC quality of a sensor. TES with a as high as 1000 have been built [23],... 

One practical advantage of pressure purging versus vacuum purging is the potential for cycle time reductions. The pressurization process is much more rapid compared to the relatively slow process of developing a vacuum. Also, the capacity of vacuum systems decreases significantly as the absolute vacuum is decreased. Pressure purging, however, uses more inert gas. Therefore the best purging process is selected based on cost and performance. 

The number of purge cycles is j = 5.6. Thus six pressure purges are required, compared to four for the vacuum purge process. The quantity of nitrogen used for this inerting operation is determined using Equation 7-7 ... 

The organic materials must evaporate without decomposing during the fabrication process. The typical deposition temperature range is between 150 and 450°C. Factors that contribute to the ultimate temperature used in addition to the physical properties of the material include the vacuum pressure, source to substrate geometry, and required deposition rate. 

Vacuum sublimation is a development of simple sublimation, which is particularly useful if the pressure at the triple-point is lower than atmospheric, where the transfer of vapour from the vaporiser to the condenser is enhanced by the increased driving force attributable to the lower pressure in the condenser. Iodine, pyrogallol, and many metals have been purified by vacuum sublimation processes in which the exit gases from the condenser are usually passed through a cyclone or scrubber to protect the vacuum equipment and to minimise product loss. 

In pressure-bubble vacuum snapback, the heated sheet is clamped and sealed against a pressure box. Air is forced through the female pressure box forcing the sheet to push outward from the pressure box. A male mold is then pressed against the bubble and as it pushes into the pressure box, excess air is forced from the pressure box forcing the heated sheet to take the shape of the male mold. The major difference between the vacuum snapback and pressure-bubble vacuum snapback is that in the vacuum snapback process vacuum from a female... 

During the thermoforming process (see Fig. 12), the sheet is heated above the glass transition temperature and below the melting point of the crystalline phase [35]. Afterwards, the hot sheet is formed into a chilled mold using vacuum, pressure and/or mechanical force. After a cooling step, the thermoformed containers are punched out and ejected. The skeleton (30-70% of the total volume) is recycled in the same application (Fig. 12). 

Many control problems can be better solved with a diaphragm controller. The function of the diaphragm controller (see Fig. 3.27) can be easily derived from that of a diaphragm vacuum gauge the blunt end of a tube or pipe is either closed off by means of an elastic rubber diaphragm (for reference pressure > process pressure) or released (for reference pressure < process pressure) so that in the latter case, a connection is established between the process side and the vacuum pump. This elegant and more or less automatic regulation system has excellent control characteristics (see Fig. 3.28). 

Fig. 3.30 Control of vacuum drying processes by regulation of the intake pressure of the vacuum pump according to the water vapor tolerance...

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