Rotary oil pumps

Rotary oil pumps will provide a reliable vacuum down to about O.OlmmHg (at the apparams) and are thus one of the most valuable pieces of equipment in the lab. Ideally every research worker should have his/her own high vacuum pump, but in many cases cost prohibits this and pumps are shared. If a pump is shared, it is common to have it mounted on a trolley which has all the ancillary devices (traps, gauge etc.) mounted on it. Alternatively, a shared pump may be fixed in one place, but attached to a communal manifold, distillation set-up or other piece of apparatus. A good two-stage pump is suitable for most high vacuum requirements in an organic chemistry lab. 

A high vacuum pump must be fitted with efficient solvent traps to prevent the pump oil from becoming contaminated. The traps are simply cold finger condensers incorporated into the vacuum line before it enters the pump. It is preferable to use a double condenser system, as shown in Fig. 8.2, with the traps being cooled by either liquid nitrogen, or solid CO2-acetone. If a single condenser system is used liquid nitrogen must be used as the coolant because C02-acetone will be ineffective. 

If the trapping system incorporates a cone joint, it can be connected directly to a manifold such as that shown in Fig. 4.1 (Chapter 4). 

Alternatively, a piece of high vacuum tubing can be used to connect the traps to a distillation apparatus, a double manifold (Fig. 4.4, Chapter 4), or any other piece of equipment. 

Do not immerse the solvent traps in liquid nitrogen unless they are evacuated, as this will cause liquid oxygen to condense. The result of this can be a violent explosion, caused by re-vaporization or by oxidation of organic solvents etc. 

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Almost all the high vacuum systems with which we will be concerned have at one end a rough pump, usually a rotary oil pump, capable of attaining ca. 10 Torr. This is followed by a high vacuum pump which can attain ca. 10 Torr, which is followed by a cold trap, the purpose of which is to condense out any volatile matter to prevent it entering, and possibly... 

Fig. 2.2. Diagram of the rotary oil pump. Its functioning is explained in the text.

The principal types of vacuum pumps will be discussed in this section. The water aspirator is a crude but useful pump for many routine operations. Rotary oil pumps are used for pumping on refrigerant baths and as the forepump for backing low-pressure pumps. For most high-vacuum work, diffusion pumps are utilized to achieve pressures... 

Schematic diagram of a rotary oil pump similar to the Welch Duo-Seal based on the Gaede oil pump design.

To achieve ultra-high vacuums (0.1-0.001 Pa, corresponding to 10-3-10-s Torr) it is necessary to couple a diffusion pump to the rotary oil pump. 

A manometer to measure the pressure (vacuum) in the apparatus, since the boiling point of a liquid varies with pressure. Two types are in common use (Fig. 15.3) the Anschutz manometer, which gives a constant reading of the vacuum throughout the distillation, and the Vacustat , which is used to take a sample of the vacuum at a given instant. Vacustats are very accurate and are more often used in eombination with a rotary oil pump. 

The pumping speed of a rotary oil pump is a measure of the volume swept out per minute by the vanes. Small pumps, of the type used in laboratory work, have a speed of about 22 l./min. Large pumps, such as are used for many industrial purposes, have a speed of 250 l./min, measured at normal pressure. 

The planning of a vacuum system should be undertaken in consultation with the person who will use it, or with his supervisor. The type of work to be done with the apparatus and the ultimate vacuum required will determine whether a rotary oil pump or a diffusion pump must be used. The inclusion of a refrigerated trap or traps and the refrigerant to be employed will depend on the materials to be handled, and on their vapour pressures. The measurement of low pressures is not always necessary, but it is advisable to make provision for a vacuum gauge by including a side-arm to which the gauge may be attached should it prove essential at any later time. 

A metal desieeator fitted direetly to the rotary oil pump inlet and provided with a vacuum release valve, a eonnection for a tilting MeLeod gauge and one for the manifold or apparatus, is very convenient if pressures of not less than 10 torr are adequate. 

The successful operation of any vacuum system is the result of experience and familiarity, rather than of instruction. The best results are usually obtained by an operator who has been using the system for some time who knows the pressure conditions in any part of the vacuum line because he knows what has been done who can estimate, from the sound of a rotary oil pump, when it is time to bring the diffusion pump into operation, or whether there is a leak in the system or not. For example, an experienced neon mbe processor can evacuate and degas the glassware and electrodes of such a tube, fill it with some 20 mmHg pressure of spectrally pure neon gas and seal the tube from the vacuum manifold, all in about 10 min. The only measurement he will make will be the pressure of the filling gas. 

FIG. 10.5. (a) A glass moisture trap containing a boat for phosphoms pentoxide. (b) A metal moisture trap in section, showing the trays for phosphoms pentoxide. (c) Oil reservoir, fitted to tlie vacuum connection on a rotary oil pump. Tlie two-way stopcock is opened to tlie atmosphere when the pump is stopped. 

FIG. 10.7. Stopcock anangement for by-passing a diffusion pump. Such an anangement allows tlie rotary oil pump to recover the low pressure required before the diffusion pump can operate and eliminates the need to discontinue the supply of heat to tlie pump boiler. The diffusion pump can be made to function again by opening the stopcock and nmiiiig stopcock handle through 180 °. 

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