Pumps vacuum

The vacuum pumping system in a freeze drying plant has to fulfil two tasks  

During the operation with gasballast, such an amount of air will succeed into the pump house (after the pump house is separated from the vacuum chamber) that the water vapor at 

a two-stage vacuum pump with a pumping capacity of 100 nf/h, operated with gasballast 2, a two-stage vacuum pump with a pumping capacity of 

vacuum valve behind the condenser 2, vacuum gauge 3, roots pump 4, vacuum valve between roots- and backing pump 5, backing pump 6, exhaust filter. 

During the operation with gas ballast, such an amount of air will enter the pump house (after the pump house is separated from the vacuum chamber) that the water vapor at the operating temperature of the pump cannot condense during the compression phase of the pump. Consider an example water vapor is pumped at a partial pressure of 0.5 mbar and the temperature of the pump is +70 °C. Under these conditions, the water vapor will condense if the compression exceeds -310 mbar. If the pressure in the pump house is increased by air from 0.5 mbar to e.g. 50 mbar, the compression needs only to be 1000/50 = 20. The original water vapor at 0.5 mbar is compressed by a factor of 20, and the water vapor pressure reaches only 0.5 X 20 = 10 mbar. No condensation can take place at 70 °C in the pump house. 

The following equation can be applied to calculate the pumping speed of a vacuum pump set for a given unit size  

T = evacuation time from PA (atmospheric pressure) to PSUB (sublimation pressure (h) 

Sw = surface of the chamber walls door shelves condenser walls evaporator (m2) PRS = water vapor released from the metal surfaces (e.g. 1 x 10 4) (mbar L/s m2) 

A variety of tasks in organic chemistry require provision of a vacuum source, but different tasks require different levels of vacuum. Vacuum supplies can be loosely divided into three categories  

A low vacuum of between 20-50mmHg is sufficient for rotary evaporation of most solvents, filtering under vacuum, distillation of relatively volatile oils, and similar tasks. 

A medium vacuum of about 10-12mmHg is ideal for a variety of tasks including rotary evaporation, distillations, and for serving double manifold-type inert gas lines. 

The main components of a vacuum system are the pumps. The types of pumps most commonly used in low-temperature experiments are  

In the following sections, the functioning principles of the single pumps used in cryogenics will be described. 

The two principal operations in an organic preparative laboratory which require the use of a vacuum pump are those of filtration and distillation under reduced pressure. The effectiveness of the vacuum attained by a pumping system may be quoted as centimetres/millimetres of mercury, or as a torr value, although recently the use of the millibar (mbar), has become widespread. The interrelationship of these units is  

The high-pressure water supply is employed for the operation of the ordinary filter pump , which finds so many applications in the laboratory. Several types of water-jet pumps of glass, plastic or metal construction are available from most laboratory suppliers. These are often fitted with a suitable non-return valve to prevent the apparatus being flooded as a result of fluctuating water pressure. Connection to the water tap in the case of the metal pump is by a direct screw-threaded joint with the glass or plastic models high-pressure tubing of suitable bore is wired to the tap and to the pump. 

It is routinely desirable to interpose a large pressure bottle A (Fig. 2.121) fitted with a rubber bung between the pump and the apparatus to act as a trap in the event of failure of the non-return valve and to serve as a pressure equalising reservoir. Connection to the apparatus and to a manometer (see also Fig. 2.124) is via a three-way tap B which allows for the release of the vacuum as required the two-way tap C permits the manometer to be isolated from the system when necessary. 

Theoretically, an efficient water pump should reduce the pressure in the system to a value equal to the vapour pressure of the water at the temperature of the water supply mains. In practice this pressure is rarely attained (it is usually 4-10 mm, or 5.3-13.3 mbar, higher) because of the leakage of air into the apparatus and the higher temperature of the laboratory. The vapour pressure at 5, 10, 15, 20 and 25 °C is 6.5, 9.2, 12.8, 17.5 and 23.8 mm (or 8.2, 12.3, 17.1, 23.3 and 

7mbar) respectively. It is evident that the vacuum obtained with a water pump will vary considerably with the temperature of the water and therefore with the season of the year. The water pump vacuum is routinely used for filtration, for removal of solvent using a rotary evaporator, and for many distillations under reduced pressure. 

Many chemical plants, particularly distillation columns, operate at low pressures, and pumps are required to create and maintain the vacuum. Vacuum pumps take in gas at a 

As a guide, a single-stage unit gives a vacuum to 13.5 kN/m2, a double stage from 3.4 to 13.5 kN/m2, and a three-stage unit from 0.67 to 2.7 kN/m2. 

For very low pressures, a diffusion pump is used with a rotary pump as the first stage. The principle of operation is that the gas diffuses into a stream of oil or mercury and is driven out of the pump by molecular bombardment. 

A fluid will flow of its own accord so long as its energy per unit mass decreases in the direction of flow. It will flow in the opposite direction only if a pump is used to supply energy, and to increase the pressure at the upstream end of the system. 

The work done on unit mass of fluid is — Vkv, and the total rate at which energy must be transferred to the fluid is —GWS, for a mass rate of flow G. 

The lowest pressure is the extreme low pressure which can be achieved by a vacuum pump at its inlet. This pressure is mainly dependent on the working principle of a vacuum system and the type of pumps used in such a system. For a given vacuum pump, the lowest pressure is determined either by the leakage pressure in the pump itself, at which point the pump becomes ineffective in maintaining the desirable pressure, or by the vapour pressure of the fluid used. 

The pressure range is the pressure difference of the maximum pressure and the minimum pressure in which a vacuum pump can work efficiently. It is necessary to combine the different types of pumps together to generate high vacuum pressure. 

Pumping speed is defined as the volume of gas evacuated by a pump within the unit time. It is expressed by 

The exhaust pressure is the pressure at the outlet of a pump. If the maximum pressure of a pump is lower than the atmospheric pressure (e.g. roots pump and oil diffusion pump), it must be backed with a mechanical rotaiy pump. In addition, some types of pumps have no outlet, such as ionisation and sorption pumps. 

Vacuum pumps are generally divided into 13 categories according to the working principle, as listed in Table 2.5. They include water jet pump, water ring pump, steam ejector, oil-sealed rotaiy pump, Roots pump, vacuum diffusion pump, oil vapom booster pump, sputtering-ion pump, radial field pump, titanium sublimation pump, sorption pump, molecular pump and cryopump [9], 

Electric shock is the major electrical hazard. A relatively low current of 10 iruUiamperes (mA) poses some danger, and 80 to 100 mA can be fatal. In addition, if improperly used, electrical equipment can serve as an ignition source for flammable or explosive vapors. Most of the risks involved can be minimiz by regular, proper maintenance and a clear understanding of the correct use of the device. 

All 110-volt (V) outlet receptacles in laboratories should be of the standard design that accepts a three- 

Standard design for a three-wire grounded outlet. 

Receptacles that provide electric power for operations in hoods should be located outside the hood. This location prevents the production of electrical sparks inside the hood when a device is plugged in or disconnected, and it also allows a laboratory worker to disconnect electrical devices Irom outside the hood in case of an accident. Cords should not dangle outside the hood in such a way that they can accidentally be pulled out of their receptacles or tripped over. Simple, inexpensive plastic retaining strips and ties can be used to route cords safely. For fume hoods with airfoils, the electrical cords should be routed under the bottom airfoil so that the sash can be closed completely. Most airfoils can be easily removed and replaced with a screwdriver. 

Equipment plugged into an electrical receptacle should include a fuse or other overload protection device to disconnect the circuit if the apparatus fails or is overloaded. This ova-load protection is particularly useful for equipment likely to be left on and unattended for a long time, such as variable autotransformers (e.g., Variacs and powerstats), vacuum pumps, drying ovens, stirring motors, and electronic instraments. Equipment that does not contain its own built-in overload protection should be modified to provide such protection or replaced with equipment that provides it. Overload protection does not protect the worker from electrocution, but it does reduce the risk of fire. 

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Mercury is extensively used in various pieces of scientific apparatus, such as thermometers, barometers, high vacuum pumps, mercury lamps, standard cells (for example the Weston cell), and so on. The metal is used as the cathode in the Kellner-Solvay cell (p. 130). 

Any commercially available vacuum pump is perfectly fine for the underground chemist s needs but the best kind to buy is a diaphragm pump, which is more resistant to the often-harsh chemical vapors that are sucked through it. Most vacuum pumps cost about 100- 200. However, the stronger the vacuum the better. If a chemist is looking to pull 1mm of Hg (don t ask) like the girls in the chemistry papers do then she can be looking at a turbovac that can run well over 5000. 

The Z-spray inlet/ionization source sends the ions on a different trajectory that resembles a flattened Z-shape (Figure 10.1b), hence the name Z-spray. The shape of the trajectory is controlled by the presence of a final skimmer set off to one side of the spray instead of being in-line. This configuration facilitates the transport of neutral species to the vacuum pumps, thus greatly reducing the buildup of deposits and blockages. 

The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode, in which it allows all ions in a beam to pass through, whatever their m/z values. In doing so, the ion beam is constrained, so it leaves the hexapole as a narrow beam. This constraint is important because the ion beam from the inlet system tends to spread due to mutual ion repulsion and collision with residual air and solvent molecules. By injecting this divergent beam into a hexapole unit, it can be refocused. At the same time, vacuum pumps reduce the background pressure to about 10 mbar (Figure 22.1). The pressure needed in the TOF analyzer is about 10 ... 

Of course, some substances are sufficiently volatile that a heated inlet line can be used to get them into a mass spectrometer. Even here, there are practical problems. Suppose a liquid or solid is sufficiently volatile, that heating it to 50°C is enough to get the vapor into the mass spectrometer through a heated inlet line. If the mass spectrometer analyzer is at 30°C, there is a significant possibility that some of the sample will condense onto the inner walls of the spectrometer and slowly vaporize from there. If the vacuum pumps cannot remove this vapor quickly, then the mass... 

The Z-spray source utilizes exactly these same principles, except that the trajectory taken by the ions before entering the analyzer region is not a straight line but is approximately Z-shaped. This trajectory deflects many neutral molecules so that they diffuse away toward the vacuum pumps. 

The beam of tiny drops passes from the exit nozzle across an evacuated space and into another small orifice (skimmer 1). In this evacuated region, about 90% of the originally injected helium and solvent is removed by vacuum pumps to leave a stream of droplets so small that they are called clusters. 

In a world increasingly conscious of the dangers of contact with chemicals, a process that is conducted within the walls of a vacuum chamber, such as the VDP process for parylene coatings, offers great advantages. Provided the vacuum pump exhaust is appropriately vented and suitable caution is observed in cleaning out the cold trap (trace products of the pyrolysis, which may possibly be dangerous, would collect here), the VDP parylene process has an inherently low potential for operator contact with hazardous chemicals. 

Fig. 38. Caustic purification system a, 50% caustic feed tank b, 50% caustic feed pumps c, caustic feed preheater d, amonia feed pumps e, ammonia feed preheater f, extractor g, trim heater h, ammonia subcooler i, stripper condenser j, anhydrous ammonia storage tank k, primary flash tank 1, evaporator reboiler m, evaporator n, caustic product transfer pumps o, purified caustic product cooler p, purified caustic storage tank q, ammonia stripper r, purified caustic transfer pumps t, overheads condenser u, evaporator v, evaporator vacuum pump w, aqueous storage ammonia tank x, ammonia scmbber y, scmbber condenser 2, ammonia recirculating pump aa, ammonia recycle pump. CW stands for chilled water.

This reduction in permeabiUty due to cake consoHdation or coUapse may be so large that it may nullify or even overtake the advantage of using high pressures in the first place and there is then no reason for using the generally more expensive pressure filtration hardware. While a simple Hquid pump may be cheaper than the vacuum pump needed with vacuum filters, if air displacement dewatering is to foUow filtration in pressure filters, an air compressor has to be used and is expensive. 

The so-called hyperbar vacuum filtration is a combination of vacuum and pressure filtration in a pull—push arrangement, whereby a vacuum pump of a fan generates vacuum downstream of the filter medium, while a compressor maintains higher-than-atmospheric pressure upstream. If, for example, the vacuum produced is 80 kPa, ie, absolute pressure of 20 kPa, and the absolute pressure before the filter is 150 kPa, the total pressure drop of 130 kPa is created across the filter medium. This is a new idea in principle but in practice requires three primary movers a Hquid pump to pump in the suspension, a vacuum pump to produce the vacuum, and a compressor to supply the compressed air. The cost of having to provide, install, and maintain one additional primary mover has deterred the development of hyperbar vacuum filtration only Andrit2 in Austria offers a system commercially. 

Du Pont called this new lubricant material Krytox (64,65) and initially it had such extraordinary properties that it sold for 200/kg ( 187kg ca 1993). Krytox was and is used ia most of the vacuum pumps and diffusion oil pumps for the microelectronics iadustry ia this country and ia Japan because it produces no hydrocarbon (or fluorocarbon) vapor contamination. It has also found important appHcations ia the lubrication of computer tapes and ia other data processiag appHcations as weU as military and space appHcations. 

Hafnium hydride is brittle and easily cmshed to very fine particle sizes. It is usually produced as an intermediate in the process of making hafnium powder from massive hafnium metal. The hydrogen can be removed by high vacuum pumping above 600°C. 

Halogenated hydrocarbons that are inexpensive sometimes are used alone or in blends with phosphate esters as fire-resistant hydrauHc fluids. Other halogenated fluids are used for oxygen-compressor lubricants, lubricants for vacuum pumps that are in contact with corrosive materials, solvent-resistant lubricants, and other lubricant appHcations where highly corrosive or reactive materials are being handled. 

The primary sources of contamination in ion implantation come from metal atoms that may be etched off reactor fixtures, such as reactor wads, wafer holder, cHps, and so on. The pump oils used by the vacuum pumps may be a source of hydrocarbon contamination. The dopant sources themselves are not a significant source of contamination because unwanted ions are separated out from the beam during beam analysis. 

The process operates at 1 kPa (10 mbars) and 450 kW of power. When the condenser temperature reaches 580°C, the power is reduced to 350 kW. Cooling water is appHed to the condenser, throughout distillation, by means of sprays. Normally distillation takes 10—12 hours and the end point is signified by an increase in furnace temperature and a decrease in vapor temperature to 500—520°C. At this point the power is turned off and the vacuum pump is shut down. Nitrogen is then bled into the system to prevent oxidation of 2inc. 

The dezincing chamber is set first in the drossed lead bath, then water connections are immediately made in order to prevent the formation of steam within the water jacket. While the temperature is being raised, the vacuum pump is placed in operation and the agitator started. The temperature is then raised to 600°C and held throughout the operation. 

Fig. 2. Flow sheet of lecithin producing unit. Crude soybean oil is heated in the preheater, 1, to 80°C, mixed with 2% water in the proportion control unit, 2, and intensively agitated in 3. The mixture goes to a dweUing container, 4, and is then centrifuged after a residence time of 2—5 min. The degummed oil flows without further drying to the storage tanks. The lecithin sludge is dried in the thin-film evaporator, 6, at 100°C and 6 kPa (60 mbar) for 1—2 min and is discharged after cooling to 50—60°C in the cooler, 8. 9 and 10 are the condenser and vacuum pump, respectively.

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