Liquid Ring Pumps are used throughout process industry. These pumps provide legitimate alternative to steam jet ejectors in applications requiring rugged pump that can tolerate entrained liquids, vapors and fine solids. These Pumps operate in a liquid environment, generally water and are capable of handling vapors along with non-condensable loads. They are extensively used in industrial processes such as filtration, drying, solvent recovery, distillation etc. Unfortunately they suffer from two major limitations that restrict the process
performance. They are:
• The final vacuum achievable, as it is largely dependent on the vapor pressure of the pump fluid corresponding to the working temperatures. For example, for water sealed pump, the lowest practical operating pressure for two-stage design would be in the range of 40 – 60 Torr (720-700mm Hg) for exit water temperature at 30-32 Deg. C.
• Their energy consumption per unit of gas pumped is higher since most of it is lost in handling pump fluid.
Mechanical vacuum boosters (MVB) overcome these limitations of liquid ring pump (LRP). A properly matched MVB – LRP Combinations can result in:
• Higher working vacuums – any where the range of 50 Torr – 1 Torr (710-760mmHg) or better is achievable.
• Very high pumping speeds – generally to the order of 4-8 times higher.
• Vapor/gas compression at the inlet of the water ring pump allowing use of
higher water temperature in the pump.
• Relatively very low energy consumption per unit of pumping speed.
Figure1 gives typical two stage WRP speed curve. The pumping speed is equal to the rated speed(displacement) during initial pumping and thereafter drops rapidly reaching to zero at its ultimate (690 – 720 mm Hg). In most of the chemical processes the process vacuum is in the range of 680-700mmHg where the pumping speed of WRP is merely 15-20% of it’s full rated capacity. This demands installation of much larger WRP loosing on one time pump cost and recurring energy charges. The power consumption, however, is largely constant throughout the range that makes LRP relatively less energy efficient in comparison to MVB-LRP Combination.
Curve2, Fig.1 gives a typical MVB–LRP (water-two stage) speed curve. As the WRP vacuum drops to the range of 60-100 Torr (660-700mm Hg), the Mechanical Booster boosts the effective speed manifold. As can be seen from the curve the booster exhibits relatively flat pumping speed curve in the region 10-1 Torr (750 –760mm Hg), high pumping speeds and better process vacuum is achieved, overcoming the limitations of LRP in this range. The power consumption of the Mechanical Vacuum Booster is relatively low in this range as compared to any other conventional vacuum pump. Therefore, with little extra energy, the overall pumping speed and ultimate vacuums can be greatly enhanced. In many applications, replacing WRP with a smaller one can easily offset the extra energy of MVB.
Installation of MVB undoubtly results in high pumping speeds and better vacuums. However, to get the best results in process its location is important. It can be effectively located between the condenser (Post condenser installation) and the WRP or between the kettle/evaporator and the condenser followed by WRP (Pre-condenser installation). To enable to determine most effective location process parameters play an important role.
POST CONDENSER INSTALLATION
performance. They are:
• The final vacuum achievable, as it is largely dependent on the vapor pressure of the pump fluid corresponding to the working temperatures. For example, for water sealed pump, the lowest practical operating pressure for two-stage design would be in the range of 40 – 60 Torr (720-700mm Hg) for exit water temperature at 30-32 Deg. C.
• Their energy consumption per unit of gas pumped is higher since most of it is lost in handling pump fluid.
Mechanical vacuum boosters (MVB) overcome these limitations of liquid ring pump (LRP). A properly matched MVB – LRP Combinations can result in:
• Higher working vacuums – any where the range of 50 Torr – 1 Torr (710-760mmHg) or better is achievable.
• Very high pumping speeds – generally to the order of 4-8 times higher.
• Vapor/gas compression at the inlet of the water ring pump allowing use of
higher water temperature in the pump.
• Relatively very low energy consumption per unit of pumping speed.
Figure1 gives typical two stage WRP speed curve. The pumping speed is equal to the rated speed(displacement) during initial pumping and thereafter drops rapidly reaching to zero at its ultimate (690 – 720 mm Hg). In most of the chemical processes the process vacuum is in the range of 680-700mmHg where the pumping speed of WRP is merely 15-20% of it’s full rated capacity. This demands installation of much larger WRP loosing on one time pump cost and recurring energy charges. The power consumption, however, is largely constant throughout the range that makes LRP relatively less energy efficient in comparison to MVB-LRP Combination.
Curve2, Fig.1 gives a typical MVB–LRP (water-two stage) speed curve. As the WRP vacuum drops to the range of 60-100 Torr (660-700mm Hg), the Mechanical Booster boosts the effective speed manifold. As can be seen from the curve the booster exhibits relatively flat pumping speed curve in the region 10-1 Torr (750 –760mm Hg), high pumping speeds and better process vacuum is achieved, overcoming the limitations of LRP in this range. The power consumption of the Mechanical Vacuum Booster is relatively low in this range as compared to any other conventional vacuum pump. Therefore, with little extra energy, the overall pumping speed and ultimate vacuums can be greatly enhanced. In many applications, replacing WRP with a smaller one can easily offset the extra energy of MVB.
Installation of MVB undoubtly results in high pumping speeds and better vacuums. However, to get the best results in process its location is important. It can be effectively located between the condenser (Post condenser installation) and the WRP or between the kettle/evaporator and the condenser followed by WRP (Pre-condenser installation). To enable to determine most effective location process parameters play an important role.
POST CONDENSER INSTALLATION
Processes such as distillation of high boilers (kettle temp. are generally above 125°C), processes using chilled water condenser, processes having direct discharge of vapors to WRP, processes demanding vacuum close to condensate vapor pressure are generally the applications where post-condenser installations can give boost to the process, resulting in higher yields, lower process time and better product quality.
In drying applications where water vapor is exhausted from the dryer and cooling water of 10°C or lower is available in the condenser, post condenser installation would be a good choice. Since the vapor pressure of condensate (Water) at 10°C is about 9 Torr, (refer graph below) the condenser working vacuum can be estimated to about 20 Torr. Double stage WRP having fluid temperature in the range of 30-35°C would not be able to deliver working vacuum below 50-60 Torr (710-700 mm Hg). However on installation of Mechanical Booster between the condenser and the WRP would very conveniently pull down vacuum to the range of 15-20 Torr (745-740 mm Hg). Still better vacuums can be possible if the condenser & condensate temperatures are lowered further.
No comments:
Post a Comment