Vapour tolerance

The maximum pressure of vapour that a vacuum pump, under ambient conditions of T— 20 °C, p = 1013mbar, can take in and transport is called the vapour tolerance of the pump. Since water vapour is the most commonly encountered vapour, pump manufacturers usually quote the water vapour tolerance for their pumps. The vapour tolerance can be obtained by rearrangement of Equation (3.3) ... 

Ethanol is to be pumped with an oil-sealed rotary pump with an exhaust pressure of 1.3 bar. Calculate the vapour tolerance to ethanol if the pump temperature is about 60 °C and atmospheric pressure is 1000 mbar. 

Moisture is to be removed, in a drier, from compacted material with a microcrystalline structure. The drier, fitted with a heating jacket and a regulator to maintain a constant temperature, is charged with 100 kg product containing 30% water. The final product must have a residual moisture of 0.1% or less. Because of the nature of the product, its temperature should not exceed 50 °C and, to achieve the required dryness, a pressure of 2 mbar is required. It is proposed to carry out the process with a single-stage oil-sealed rotary pump (Seff = 100 m3 hr1, water vapour tolerance = 50 mbar at 40 °C and pult (with gas ballast) = 1 mbar). 

The water vapour tolerance of a pump is the maximum pressure of pure water vapour that the pump can take in and transport. Example 3.3 indicates how this is calculated. 

The most effective method of increasing the level of vapour tolerance in an oil-lubricated rotary vane vacuum pump is that of providing a gas ballast, invented by Gaede. In the case of operation with a gas ballast, it is in connection with mechanical vacuum pumps not necessary to aim for a very high temperature level. Even so, an operating temperature level of 60-80 C and a temperature spread which is as uniform as possible should preferably be maintained so as not to provoke condensation at any point. 

In accordance with DIN 28 426 Part 1, the vapour tolerance of a mechanical vacuum pump which shall pump the taken in vapours without condensing during the compression phase from vacuum to atmospheric pressure, is defined as follows ... 

Ppy= vapour tolerance (mbar) P = gas ballast quantity (m h ) 5 = pumping speed of the vacuum pump (m h ) and P = vapour pressure of the pumped product at operating temperature of the vacuum pump (may, owing to solubilities in the pump s operating medium be subject to a lowering of vapour pressure). 

From Eq. (5.1) it is apparent which parameter setting ensures a high and thus desired vapour tolerance Pp ... 

Table 5.1 Vapour tolerances of a modern rotary vane pump with a water vapour tolerance of 40mbar.

The vapour tolerance is also negatively influenced by a too high partial pressure of the pumped vapour in the gas ballast. This means that a fresh gas ballast (without any gas return from the exhaust side) will here be most beneficial, but will under certain circumstances also increase the exhaust emissions. 

Given in Table 5.1 are some typical vapour tolerance values for a rotary vane pump with circulatory oil lubrication. 

G enerally a value of40 - 60 mbar maximum is aimed at for the water vapour tolerance limit. This results from the fact that a condenser operated upstream of the vacuum pump and with normal circulating coohng water will reduce the partial pressure of the water vapour down to 40 mbar and up to 60 mbar. 

As in these vacuum pumps the oil passes only once through the pump, the purity of the separated oil is of far less significance compared to rotary vane pumps with circulatory lubrication. Even so, also in the case of fresh-oil-lubricated pumps, the aspect of vapour tolerance must be taken into account because here too the oil may suffer from dilution or the oil film in the pump chamber may be even washed away with the result of very rapid and considerable wear within the pump chamber giving rise to high repair costs. 

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