Vacuum Dewar flasks

At very low temperatures with hquid air and similar substances, the tank may have double walls with the interspace evacuated. The weh-known Dewar flask is an example. Large tanks and even pipe hues are now built this way. An alternative is to use double walls without vacuum but with an insulating material in the interspace. Perlite and plastic foams are two insulating materials employed in this way. Sometimes both insulation and vacuum are used. 

This technique is based on the Dewar flask, which is a donble-walled vessel with reflective surfaces on the evacuated side to reduce radiation losses. Figure 11-66 shows a typical laboratory-size Dewar. Figure 11-67 shows a semiportable type. Radiation losses can be further reduced by filling the cavity with powders such as perlite or silica prior to pulling the vacuum. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Immersion well (quartz or pyrex), fitted with inlet (a) and outlet (6) for cooling liquid e.g. methanol), and with ground joint (c) to Reaction flask, fitted with outlet d) to vacuum system ( 10 mmHg) Liquid nitrogen Dewar flask... 

When 5.00 mL of ether has been delivered by the syringe pump, the pump is shut off The reactor is allowed to run an additional 15 min before the fluorine and the mercury arcs ate shut off. The preaerosol furnace, the evaporator heater unit, and the coolant pump are shut off. Once the system approaches ambient conditions, all the helium carriers are shut off and the product trap valves are closed The product trap and its Dewar flask filled with liquid nitrogen are removed to the vacuum line where the trap is evacuated... 

The work which led to the modern vacuum flask has been described by J. Dewar (1896). Vacuum jacketed flasks are used extensively in scientific laboratories and can be constructed relatively easily tubing is chosen to give a flask of the required size and, after the inside tube has been rounded off at one end, a Dewar seal to the outer tube is made at the other end (Figure 51,7). If a flask with a narrowed neck is required the parts must be prepared as in Figure 51, II, before the Dewar seal is made. 

In fact, a truly adiabatic system cannot be attained, since even the most insulatory materials will slowly conduct heat. The best approximations are devices such as a Dewar flask (sometimes called a vacuum flask ). 

Figure 3.6 Schematic representation of the bomb calorimeter for measuring the changes in internal energy that occur during combustion. The whole apparatus approximates to an adiabatic chamber, so we enclose it within a vacuum jacket (like a Dewar flask)...

Figure 1. Diagram of the apparatus used in the s3mthesis and isolation of V(CO)g. Components (A) round-bottom flask, 500 mL (B) connecting tube, (C) specially adapted Schlenk tube receiver (D) stopcock (E) Dewar flask (F) water bath (G) butyl rubber vacuum tubing connection to vacuum/argon manifold. Dimensions a =165 mm b = 130 mm c = 170 mm d = 75 mm e = 20mm od.

Specifically, the glass connection between the Schlenk tube receiver (C) and stopcock (D) should extend about 130 mm below the top of the 24/40 standard taper inner joint and approximately 50 mm into Dewar flask (E) to prevent loss of the especially volatile V(CO)e to the vacuum line during the reaction and sublimation. The Schlenk tube receiver is cooled to about —70°C with dry ice/acetone or dry ice/isopropyl alcohol. The level of coolant should be near the top of the Dewar so that the glass connection between the stopcock (D) and the Schlenk tube is entirely covered. The apparatus is evacuated very slowly and cautiously... 

A Dewar flask (vacuum-jacketed bottle) is used as a calorimeter, and the following data are obtained. Measurements in parts (a) and (b) are made to obtain the heat capacity of the calorimeter, and parts (c) and (d) are performed on an unknown metal. 

After the photolysis is completed, the reaction mixture is transferred under dynamic vacuum to a trap at — 195°C. The small amount of nitrogen formed in the reaction is expelled by the pumping system. The mixture is allowed to warm slowly to room temperature (an empty Dewar flask cooled to — 195°C. with liquid nitrogen is convenient), and a trap-to-trap separation is performed by using traps at —140 and —195°C. The first trap contains N204 and Br2. The photolysis vessel contains a white solid, probably (NO)2SiF6. 

Figure 2.89 illustrates a commercially available Quickfit lyophiliser (Bibby Science Products) accommodating a single flask of such a size that the volume of aqueous solution to be treated is one-quarter its total capacity. The charged flask is rotated in a dry ice-acetone bath so that an even layer of frozen solution is obtained over the inside. The flask is immediately attached to the refrigerant chambers which are filled with a Cardice-acetone mixture. An oil vacuum pump is connected to the refrigerant chamber via the supplementary trap, which if possible should be immersed in a Dewar flask filled with liquid nitrogen such a cooled trap provides maximum protection for the vacuum pump. Vacuum is... 

One of the consequences of Dewar s work was his invention of the vacuum flask to minimize heat loss. It was expensive and time-consuming to liquefy gases hence, Dewar designed a container where, once liquefied, gases could be kept for as long as possible. Still known as the Dewar flask among chemists, it is more widely known as the Thermos, named after the company that obtained the patent for the flask and to whom Dewar lost an ensuing court case. 

Vacuum-jacketed glass apparatus should be handled with extreme care to prevent implosion. Equipment such as Dewar flasks should be taped or shielded. Only glassware designed for vacuum work should be used for that purpose. 

The temperatures of the sample inside the reactor and of the heating element in the furnace are controlled by thermocouples with an accuracy of +1°C. Low-temperature measurements are performed by submersion of the reactor into the Dewar flask with liquid nitrogen (T = 77 K). The vacuum system has been assembled on a basis of the universal vacuum unit VUP-5. 

A water pump can reach pressures of 1 Torr. An oil vacuum pump can reach 20 mTorr. A turbomolecular pump can reach pressures of 10 10 Torr (10-8 Pa). A sorption pump can reach pressures of 10-2 Torr by exposing the system to a porous zeolite cooled to liquid nitrogen temperature with a Dewar flask placed on the outside. 

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