Corrosive media and dry running vacuum pumps
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Contaminated waste disposal during a manufacturing process is a significant cost to the pharmaceutical industry. Uwe Gottschlich, business manger for dry running vacuum pumps at Sterling Fluid Systems, argues that vacuum pumps with a uniform temperature control throughout give the best solution
Dry-running designs of vacuum pump are increasingly becoming the dominant choice of vacuum generating machine within the pharmaceutical industry. The decision to adopt such technology is generally made on the basis of reduced effluent/abatement costs. No service liquid or internal lubricant within the machine results in the elimination of contaminated waste disposal costs.
Corrosive gasses and vapours, of a particularly wide-ranging nature, can only be handled by machines that offer universal compatibility. Corrosion resistant metals and polymers provide a degree of protection, but can be susceptible due to the thermal conditions that are particularly arduous within dry running vacuum pump technology. Additionally, high nickel coatings on ferritic base metals can accelerate galvanic corrosion as soon as any flaws appear.
It is widely recognised that highly corrosive media can be handled without the need for such susceptible exotic materials. On the basis that machines specifically designed for reliable temperature control ensure that corrosive media remains in the vapour phase, ductile irons provide a good metallurgical solution. Machines with a uniform temperature profile throughout, offer the ability to control heat-accelerated degradation/polymerization.
Market requirements
Generally, vacuum is utilized within the chemical process industry to ensure that product degradation does not take place with heat sensitive media. Therefore applications such as distillation, vaporization, and drying are undertaken at relatively low temperatures. As a rule, there is a precondenser located upstream of the vacuum pump in order to reduce the volumetric flow rate by condensing the vapour into a liquid. The purpose of the vacuum pump is, therefore, not really to cope with the excessive free vapour and gas load, but rather to handle an optimum flow of pour saturated gas.
The pharmaceutical and fine chemical industry are particularly reliant on multi-purpose plants, ie manufacturing plants designed for continuously changing media and process conditions. The list of demands placed upon a vacuum pump that must be reliable, in this field, are extensive. Safe operation with flammable vapours means low gas temperature must be maintained with no mechanical ignition source. Handling of corrosive media must be problem free; that means working with elevated gas temperatures to avoid condensation (as corrosion only occurs in the liquid phase) and pumping thermally sensitive media/covering agents.
A reliable temperature profile is needed to ensure that gas temperatures are suitably low enough to avoid cracking, while high enough gas temperatures to avoid crystallization. Further, designs must allow for easy rinsing and flushing, as well as simple maintenance and clean-in-place operations.
From these requirements it is easy to deduce that the temperature condition inside of the vacuum pump is very important. It must neither be too cold nor too warm!
Optimum cooling for wide ranging applications
Dry-running vacuum pumps are characterised by almost adiabatic compression with very small mass flow rates. Since the mass flow rate is almost zero during operation at very low suction pressures, and there are no service liquids present, the compression-generated heat cannot be automatically dissipated. Furthermore, the heat created by the extremely high (1,000,000:1) compression ratios is exacerbated further because of the low density of the gas under such levels of vacuum.
The simplest way to remove the heat is through jacket cooling of the pump. But low gas density adversely affects heat dissipation through convection to the cooled jacket. In other words, the temperature distribution throughout the pump is relatively inhomogeneous. As the pump becomes larger, so does the heat transfer rate per displacement volume. Peak temperatures in excess of 200degC are commonplace with pump capacities in the region of 250m3/h. Since elevated temperatures can cause cracking products to suffer accelerated polymerization, degradation, or simple sublimation, there exists an increased danger of an active ignition source. In other words, as the clearances within the machine are consumed by product deposition, friction can generate hotspots to the extent of localized ignition.
Lowering the peak temperature is only possible if the coolant temperature is extremely low. Along with practical constraints of providing coolant at such low temperatures, problems occur with condensation at the inner wall of the jacket. The cold inner surface of the casing chills any corrosive vapours, as they enter the pump, and causes condensation that leads to corrosion.
Consequently, it can be deduced that dry-running vacuum pumps in excess of 250m3/h must employ a heat dissipation mechanism more advanced than simple jacket cooling, if they are to be suitable for multipurpose plants.
Internal rotor cooling provides a solution to this problem when utilized in conjunction with the typical jacket cooled system. Importantly, the jacket temperature can subsequently be increased in order to remove problematic quench zones, whilst maintaining internal temperatures of less than 200degC.
As discussed earlier, however, the heat transfer rate per displacement volume becomes detrimentally affected as the vacuum pump size is increased. Moreover, peak temperatures below 200degC are seen to be achievable only with a suction capacities less than 400m3/h. The third, and most effective, way of cooling is through direct gas cooling. This system feeds cold gas directly into the compression cycle, and permits heat transportation through the unit. Such effective cooling is not only achieved through the mixing of hot and cold gas, but also by two additional aspects: Firstly, the coefficient of heat transfer (Cp) is improved with an increase in gas density, and secondly the increased mass flow enhances heat dissipation. The result is a very homogeneous temperature profile that allows further elevated jacket temperatures without reaching detrimentally high internal gas temperatures.
With regard to the pumping of corrosive media, this solution offers additional advantages. As stated earlier, corrosion will not occur in whilst it remains in the gas/vapour phase. It will only take place if the media is allowed to condense. Furthermore, condensation will only take place if the partial pressure of the condensable component reaches saturation during compression from vacuum to atmospheric pressure. The dilution effect of the Direct Gas Cooling system reduces the partial pressure of the condensable(s) within the pump and actually prevents condensation.
Should the vacuum pump be equipped with a post (abatement) condenser, relatively cold gas can be taken from the exhaust flow. Importantly, this downstream condenser must be constructed from suitable corrosion-resistant materials. The cold gas is injected into the machine at a point where compression is greatest. Because this point is significantly downstream of the pump suction, neither suction pressure nor flow rate is compromised.
24 July 2006
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