Tuesday, September 27, 2011

Enhance system vacuum. Install Everest Boosters to get reduce process time & higher purity

Mechanical Vacuum Boosters are dry pumps  that meet most of the ideal vacuum pump requirements.  They work on positive displacement  principle and are used to boost the performance of water ring /oil ring /rotating vane /piston pumps and steam or water ejectors.  They are used in combination with any one of the above mentioned pumps, to overcome their limitations.  Vacuum booster pumps offer very desirable characteristics which make them the most cost effective and power efficient option. 

The major advantages are:-

(a) Can be integrated with any installed vacuum system such as Steam Ejectors, Water Ring Pumps, Oil Sealed Pumps, Water Ejectors, etc.

(b) The vacuum booster is a Dry Pump as it does not use any pumping fluid. It pumps vapor or gases with equal ease.  Small amounts of condensed fluid can also be pumped.

(c) Vacuum boosters are power efficient. Very  often a combination of Vacuum Booster and suitable backup pump results in reduced power consumption per unit of pumping speed.  They provide high pumping speeds even at low pressures.

(d) Boosters increase the working vacuum of the process, in most cases very essential for process performance and efficiency. Vacuum Booster can  be used over a wide working pressure range, from 100 Torr down to 0.001 Torr (mm of mercury), with suitable arrangement of backup pumps.

(e) It  has very low pump friction losses, hence  requires relatively low power for high volumetric speeds. Typically, their speeds, at low vacuums are 20-30 times higher than corresponding vane pumps / ring pumps of equivalent power. 

(f) Use of electronic control devices such as Variable Frequency Control Drive allow to modify vacuum boosters operating characteristics to conform to the operational requirements of the prime vacuum pumps.  Hence they can be easily integrated into all existing pumping set up to boost their performance. 

(g) Vacuum boosters don’t have any valves, rings, stuffing box etc., therefore, do not demand regular maintenance.

(h) Due to vapor compression action by the booster, the pressure at the discharge of booster (or inlet ofbackup pump) is maintained high, resulting in advantages such as low back streaming of prime pump fluid, effective condensation even at higher condenser temperatures and improvement of the backup pump efficiency.

The table below gives a rough estimate of how the boosters enhance the working vacuums of the processes when installed in combination with various types of industrial vacuum pumps currently used in the industry. They can effectively  replace multistage steam ejectors, resulting in considerable steam savings and reduced loads on cooling towers. Mechanical Vacuum Boosters are versatile machines and their characteristics depend largely on backing pump. Various types of backing pump can be used, depending upon the system requirement and ultimate vacuum needs.  However, the final vacuum is governed by the suitable selection of the backing pump and booster arrangement.The table below gives a broad range of vacuum achieved with various backing pumps combinations

For example, if a process is using water ring Pump, the estimated working vacuums would be of the order of about 670-710 mmHg gauge (90-50  mmHg abs.), largely depending on the water temperature and pump design. When a Booster is installed prior to the water ring pump, in series, the vacuum levels of the order of 5-10 Torr  can be easily achieved. In a Multi-Stage booster installation, vacuum levels of the order of 0.5  Torr & better can easily be expected. Mechanical Boosters offer a completely dry pumping solution and do not add to any vapor load, unlike steam ejectors, and therefore, do not require large inter stage condenses.    At low vacuums, higher pumping speeds are required to maintain the through-put, since the specific volume increases with the increase in vacuum. Vacuum boosters enhance the pumping speeds by about 3-10 times depending upon the  selection by virtue of which one can expect higher process rates and through-puts. The drawbacks of steam ejector system such as sensitivity to motive fluid pressures and discharge pressure are overcome easily by the Mechanical Boosters, since the volumetric displacements/pumping speeds are insensitive to the inlet & outlet working pressures. 

Calculating the Pump Capacity: -
Based on the fundamental gas laws PV= RT, an expression can be derived for Volumetric Flow
Rates required for pumping different vapors/gases. Based on the Mass flow rates one can estimate the
pump capacity required.
Booster Operation:
Power Constraints restrict the total differential pressures across the booster. This demands to ensure the total differential pressure across the Booster must not exceed the rated limits. This can be ensured by any of the following means;-

1.)Manual method:- Initially the fore pump is switched on until the required cut in pressure is achieved and there-after the booster is switched on.

2.)Auto method:- Installation of mechanical By-pass arrangement across the booster or hydro kinematic drive or Variable Frequency Drive (VFD). In this arrangement, the booster and fore pump can be started simultaneously from atmosphere.

Advantages of using Electronic Variable Speed Control Device Electronic A.C Variable Frequency Control Drives are most preferred devices used to regulate the Booster speed to match the varying load conditions of the process. These drives enhance the overall performance of the Boosters and offer various advantages for the trouble free operation. 
The major advantages are: -

1. Booster can be started directly from atmosphere.
2. No need for separate pressure switch, by pass line or offloading valves.
3. Considerable savings in power.
4. Prevents over-heating of Boosters.
5. Protects the Booster against overload and excessive pressures.
6. Offers complete protection to motor against over voltage, under voltage, over current, Over-heating, ground fault.
7. Eliminates the needs of separate starter and overload relays for the Motor.
8. Automatically adjusts the speed of Booster between low and high range set giving high
pumping speeds with relatively low input power.

The Electronic Variable Frequency Control Drive is a microprocessor based electronic drive which is specially programmed to meet the demands of the Booster allowing it to operate directly  from atmosphere along with suitable fore pump.  Conventionally, Boosters can be started only after achieving fore vacuum in the range of 30 – 100 Torr, as they are not recommended for direct discharge into the atmosphere.  Use of Pressure Switch, Hydro kinematic drive and by pass valves is necessary to prevent the overloading of the  Booster.  However with the installation of Electronic Variable Frequency Control Drive all the conventional methods can be bypassed since the drive is programmed to regulate the Booster speed automatically, keeping the load on motor within permissible limits.  This allows the Booster to start simultaneously with backup pump

When the backup-pump and Booster are started the drive reduces the Booster speed to the pre-set levels and as the vacuum is created the Booster speed picks up, reaching the final pre-set speed,  giving most optimum performance over the entire range. Since all the parameters are easily programmable, one can adjust the booster pumping speeds to match the system requirements easily and quickly.  The drive limits the current to the motor and safeguards the motor against over voltage, under voltage, electronic thermal,  overheat ground fault…. i.e. protects the motor against all possible faults. External computer control over all aspects of booster performance is possible via RS485 serial interface built into the drive electronics. This enables the Booster to be integrated into any computer-controlled operating system. 

Thursday, September 22, 2011

Vapor recompression to recover low pressure waste steam

Increasing energy cost and  pressures on improving process efficiency are forcing process engineers to minimize wasteful losses. Efforts are continually being made to minimize all such losses. In many industrial processes low pressure spent steam is let off into atmosphere and goes off as waste heat. Thermal separation processes such as evaporation and distillation are energy intensive. The need for reducing energy costs led to multieffect plants, then to thermal vapor compression and finally to use of mechanical vapor compression systems. Under steady  state conditions, sum of all energy and enthalpy inputs must equal the sum of all energy and enthalpy outputs.  It, therefore, becomes important to ensure that energy imparted to the vapors is recovered back/reused. The following options are generally  adopted in the industry for recovery of
a) Multi-effect Evaporation
b) Vapor Recompression  
  • Thermal vapor recompression
  • Mechanical Vapor recompression
Multi-effect Evaporation:-    
In a multi effect evaporation plant, the vapors produced in the first effect are utilized as the heating medium of the second effect and so on. This effectively reduces steam consumption in proportion to the number of effects. Ideally unit mass of vapor on condensation can evaporate  unit mass of liquid. The vapors generated at the first effect are condensed in the  second stage to further evaporate the liquid from the second stage and so on. A temperature gradient of about 7-10°C is maintained between stages for maximum efficiency. So a triple effect evaporator would consume only 35-36% of the energy in comparison to a single effect system.

Vapor Recompression:   
In vapor recompression arrangement the heat of condensation of the evaporated vapor is recovered in single effect only by raising the pressure and temperature of the generated vapor and then their condensation in the same evaporator. The vapor compression can be done by Thermal Vapor Compression or Mechanical Vapor Compression Process.
Thermal Vapor Compression:  In thermal vapor recompression steam jet ejectors are used to raise the pressure and temperature of the generated vapors. The motive steam mixes with the vapor and to maintain the steady flow heat balance some of the vapor steam mixture has to be taken to second effect for full recovery of latent heat of vapor and, therefore, excess vapor is to be conveyed to next effect for recovery.

Initially, heating steam is used to initialize evaporation. The vapors evaporated are compressed to higher pressure and temperature by steam jet ejector, condensed back for heat recovery and the residual vaporsare taken to second stage for condensation / heat recovery. The amount of surplus energy contained in the residual vapor corresponds to the amount of energy supplied for steam jet ejector operation. This is taken as additional heat input / work done for recovery of large heat content of the evaporated vapors.

Mechanical Vapor Compression:  In mechanical vapor compression, positive displacement compressors or multi stage centrifugal compressors are generally used to raise the pressure and temperature of the generated vapors.  Since mechanical compressors do not require any motive steam, all vapors can be compressed to elevated pressure and temperature eliminating the need for subsequent recovery system. The energy supplied to the compressor constitutes the additional energy input  to vapors. After compression  of vapor and subsequent condensation of the same, hot condensate leaves the system. A typical mechanical vapor recompression cycle would be as illustrated in figure below:

For mechanical vapor compressors, the  specific energy input depends upon the compression ratio (ratio of input pressure to discharge pressure). Compression ratio, therefore, must be maintained to the lowest required. 
The compression ratio is influenced by:
1. The boiling point elevation of the liquid to be evaporated. Higher the boiling point rise higher is the compression ratio required.
2. Minimum differential temperature gradient required for effective heat transfer. Indirect condensers require a minimum temperature gradient across the fluids exchanging heat. The condensers should be designed for least ∆T operation.
3. Total system pressure drop in the piping and valves. Adequate size of piping and valve selection should be done for minimum pressure drop during transfer of fluid through them.

The working cycle of Everest mechanical compressor for steam, as fluid handled, is explained under.

Tuesday, September 6, 2011

Enhance the performance of liquid ring pumps using Everest mechanical vacuum boosters

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.

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.