Rotary pumps pressure

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

Rotary Pumps In rotary pumps the liquid is displaced by rotation of one or more members within a stationary housing. Because internal clearances, although minute, are a necessity in all but a few special types, capacity decreases somewhat with increasing pump differentia pressure. Therefore, these pumps are not truly positive-displacement pumps. However, for many other reasons they are considered as such. 

In general, rotary pumps with discharge pressure of up to 100 psi are considered low-pressure pumps. Rotary pumps with pressure between 100 and 500 psi are considered moderate-pressure pumps. Pumps with pressure beyond 500 psi are considered to be high-pressure pumps. 

Positive-displacement pumps can be divided into two major types rotary and reciprocating. All rotary pumps use some form of rotating element, such as gears, vanes, or lobes to increase the discharge pressure. Reciprocating pumps use pistons or wobble plates to increase the pressure. 

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. 

The pressure in the foreline must be kept below a value called critical forepressure. If this pressure is exceeded, oil vapour will be injected into the vacuum chamber in great amounts. A dumping of the pump can occur. For this reason, the size of the rotary pump must be properly chosen. 

Of course, the pressure p3 at the inlet of the rotary pump must be less than 0.08 torr, say 0.04 torr. Hence the speed S3 at the inlet of the rotary pump must be at least ... 

Exposure of the sample chamber to various ambients was controlled by use of a manometer system with evacuation obtained using a rotary pump. The lowest pressure obtainable using this system was 20 mtorr (base pressure of the rotary pump) with the highest pressure being 800 torr (maximum allowable on the manometer). 

Water vapor is frequently removed by pumps that operate with water or steam as a pump fluid, for example, water ring pumps or steam ejector pumps. This depends considerably on circumstances, however, because the economy of steam ejector pumps at tow pressures is generally far inferior to that of rotary pumps. For pumping a vapor - gas mixture in which the vapor portion is large but the air portion is small, the vapor can be pumped by condensers and the permanent gases, by relatively small gas ballast pumps (see Section 2.1.5). 

In this region the vrater vapor pressure exceeds the admissible partial pressure at the inlet. The gas ballast pump must, therefore, have a condenser inserted at the inlet, which is so rated that the water vapor partial pressure at the inlet port of the rotary pump does not exceed the admissible value. The correct dimensions of the condenser are selected depending on the quantity of water vapor involved. For further details, see... 

Positive-displacement pumps and those that approach positive displacement will ideally produce whatever head is impressed upon them by the system restrictions to flow. The maximum head attainable is determined by the power available in the drive (slippage neglected) and the strength of the pump parts. A pressure relief valve on the discharge side should be set to open at a safe pressure for the casing and the internal components of the pump such as piston rods, cylinders, crankshafts, and other components which would be pressurized. In the case of a rotary pump, the total dynamic head developed is uniquely determined for any given flow by the speed at which it rotates. 

A vacuum system composed of a combination of a rotary pump and an oil diffusion pump is frequently used. Although use of a rotary pump alone can reach low pressures of 1—10—2 Torr, use of both rotary and oil diffusion pumps is desirable because less gas remains in the reaction chamber. 

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