Liquid Ring Designs

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Today two main types of liquid ring vacuum pumps can be found, the radial flow or conical pump and the axial flow or flat port plate pump. Inside the two main designs three main types of liquid ring vacuum pump sub-designs can be found, the true single stage, the single stage variable port plate and the two-stage pump design.

The true single stage axial flow design is pictured above with its large inlet port on the right and smaller discharge port pictured on the left side.  It features a suction port, impeller and discharge port. The final vacuum the pump can achieve is designed through the distance between the inlet and discharge port. True single stage pumps can achieve roughly 26"Hgv or 4"HgA and if the discharge is roughly atmospheric, say 30"HgA then the compression ratio is 30:4. As the gas enters the pumps inlet its conveyed and compressed until it reaches the beginning of the discharge port where almost all of the gas is then exhausted. Of course not all of the gas escapes and some finds its way back to the pumps inlet where it re-expands to the operating pressure.These rough vacuum pumps are one of the most durable pumps ever made and are used on a multitude of rough and wet vacuum applications.

The single stage variable port plate design is only available with the axial flow pump. It's construction includes the suction port, impeller and a valved discharge port shown above. If the final vacuum is determined by the total distance between the inlet and discharge port, then increasing the distance between them is the solution to increasing the compression ratio. The variable port plate only allows as much air to discharge as required while maintaining good efficiency through out the low pressure curve. In general a single stage flat port plate liquid ring vacuum pump can achieve 28.9"HgV or roughly 1"HgA and with an atmospheric discharge our compression ratio is roughly 30:1. As the gas is conveyed and compressed it enters the discharge area but it's not allowed to escape all at one time (as with the true single stage) as the port plate has multiple small slots (as shown below) that are covered by individual teflon valves. As the gas enters the compression cycle, only the valves that need to react will operate creating a more efficient Hp/cfm ratio, and lower service liquid usage than traditional designs. The final horizontal port is not valved as this is the final fail safe for all the gas to be exhausted before intake is reached in the compression cycle.

The third and final liquid ring is the two-stage design pump wich uses two impellers in series to achieve a final pressure of 28.9"Hgv. The first stage is 3 times as large as the second stage and if we assume the first stage has a displacement of 100 and the second stage has a displacement of 33 then our staging ratio is 100/33 or 3:1. For example, if our pump is "blanked off" generating the best vacuum it possibly can produce, then our compression ratio can be said to be roughly 30:1. The theoretical interstage pressure is then found by examining the 1st stage inlet pressure and the final stage discharge pressure, or in our example 5.5:1.  Some two-stage liquid ring pumps can be favored over single stage valve and conical pumps for increased capacity per Hp at approximatly 27"Hgv to 28.9"Hgv.

Two-stage liquid ring machines should not be operated below 27"Hgv due to the designed staging ratio of the pump. For example,assume the pump is now operating at 20"HgA at the inlet and maintain our 30"HgA discharge condition as before, then our compression ratio becomes 30/20 or 1.5r. The first stage displacement of 100cfm/1.5r = 67cfm discharge into the inlet of the second stage, which carries a displacement of 33cfm, so our capacity can be choked by the 33cfm second stage limitation and over compression results. The interstage pressure may be found by examining the inlet pressure and staging ratio, or 60"HgA at the interstage. Interstage bypass valves are available to compensate for operating pressures above 27"Hgv to relieve over compression and reduce the operating horsepower.

Laboratory Vacuum Pumps

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Most Laboratory vacuum applications operate at partial vacuum levels between 1 Torr and 277 torr(29.88"HgV & 19"HgV). This pressure range is where most common lab applications operate and also represents the rough range of vacuum.

Common lab applications can be defined as vacuum filtration, liquid aspiration and similiar applications that use vacuum to move liquids and are best served in the 60- to 277 Torr range. Partial vacuum below 60 torr can be detrimental to filtration and aspiration applications as vapor pressure concerns come into play. If the partial vacuum is allowed to move as low as the vapor pressure rating of the material this will result in the evaporation of the filtrate and can prove unfavorable for both the application and the vacuum pump. These common laboratory applications are typically served by a "House Vacuum System" or HVS, and can be designed to operate anywhere in the 60- to 277 Torr range.

Laboratory applications including evaporation, distillation, gel drying, and vacuum ovens require a partial vacuum range of 1- to 60 Torr. HVS can also be designed to accommodate these requirements. This pressure range crosses into the use of smaller individual laboratory vacuum pumps and is a perfect fit for dry diaphragm pumps.

Drying/freeze drying, molecular distillation, and glove box applications all require a partial vacuum from 1- to 10-3 Torr. These low pressure applications cannot be served by the typical HVS but require the use of 2-stage oil flooded rotary vane vacuum pumps.

Gas chromatography/mass spectrometry (GC-MS) type lab applications require a partial vacuum as low as 10-8 Torr and are served by turbo-molecular vacuum pumps.

Definition:
SCFM is measured at standard conditions (Air at 68F, 29.92"Hg or 14.pais)
ACFM is measured at actual inlet conditions

When sizing for typical medium sized labs, as a rule of thumb, use 1.0 scfm per outlet and a 50% usage factor. Because the majority of vacuum pump manufacturers rate their vacuum pumps capacity in ACFM a simple method of converting these flow rates has been developed in the lab vacuum industry. The method is based on Boyle's gas law which describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a closed system.

Example: A small laboratory vacuum system requires 20scfm of air at a partial vacuum level of 25"HgV at sea level, convert the mass flow rate to a volumetric flow rate and select a vacuum pump. If the pressure in the system decreases then the volume of the gas occupied will increase proportionally according to the following formula:                                                      P1V1=P2V2

                                                V2= 29.92x20/4.92 = 121.6acfm @ 25"HgV
                                                V1= 4.92x121.6/29.92 = 20scfm @ 25"HgV

Pump Selection: After determination of the pressure and flow required for the house vacuum system, specific application requirements need to be considered before final pump and system selection including;

  1. A close look at the process constituents may include simple water, saline or biological media thats non-corrosive but very wet. Another application may include more corrosive material such as bleach and require a thorough examination of the pump system construction, its operating fluids if applicable, pre-filters or condensers/traps may may have to be evaluated. Pre and discharge manifold arrangements including discharge drains may require consideration.
  2. Where is the vacuum pump system to be installed? Is there an area classification? It's hot in Houston and it's well known that heat is a major enemy of mechanical and electrical equipment. That mechanical room that used to be 75 degrees back when there were only 3 or 4 pieces of equipment in it keeps growing, and the ambient temperature right along with it. Consider the type of vacuum pump technology, can the technology be water cooled? does the facility offer chilled water? It's estimated that for every 15 to 18 degrees F above the maximum rated operating temperature (180F) of oil in a rotary vane vacuum pump, the oil degradation rate doubles. As a rule of thumb regarding electric motors, for every 18 degrees F rise above the highest allowable stator winding temperature reduces the motor insulation useful life in half.
  3. Other factors such as space requirements, operating sound levels and operating budgets must also be considered.

Common HVS Vacuum Pump Types:

  1. Oil sealed liquid ring vacuum pumps use light weight oil in the pumping chamber to create compression. They are one of the most reliable vacuum systems available and be configured for air or water cooling. Liquid ring pumps operate with the lowest sound pressure compared to any other vacuum pump available in the market today. Pump operating temperatures can be selected form 165F to 190F in order to match the expected gas load to be handled. Since the oil in the compression chamber is not used as a lubricant, it can easily last for one year or more before an oil change is necessary. Oil can be selected to include anti-wear, anti-oxidants, and corrosion inhibitors to tolerate many mild corrosive gas and vapors. Liquid ring pumps are capable of producing pressures as low as 28.9"HgV (25.9 Torr) and can discharge up to 30psig with an atmospheric inlet depending on the pumps configuration. Pumps are available as either single or 2-stage designs depending on the pressure/capacity requirements and use single acting mechanical shaft seals.
  2. Water sealed liquid ring vacuum pumps use water in the pumping chamber to create compression. They are one of the most robust vacuum systems available and can tolerate up to 1/4" soft solids and large condensable loads. These systems can be configured as once through or total recirculated seal fluid to meet the application and customers requirements. Liquid ring vacuum pump systems also offer very low operating sound pressures. Water sealed liquid ring systems offer the coolest operating temperatures compared to any other vacuum pump in its class featuring near isothermal compression. Pumps are available as single or 2-stage designs to meet application requirements and use single acting mechanical shaft seals. Water sealed liquid ring vacuum pumps are capable of producing pressures as low as 28.9"HgV (25.9 Torr). Pump construction is available in cast iron, CI/stainless or all 316 stainless steel to handle very corrosive gas and vapors.
  3. Oil flooded rotary vane single stage vacuum pump systems can produce a partial vacuum as low as 1 Torr. Due to their internal design these pumps should only be selected for applications that require lower pressures from 75 Torr to 1 Torr. When operating oil flooded vacuum pumps above 75 Torr you can expect the pumps operating temperature to be hotter than normal. The elevated operating temperature leads to a shorter oil and oil mist eliminator life, shorter bearing, seal and vane life expectancy along with oil blowing out the pumps discharge port. In special cases the pump can be water cooled and secondary mist eliminators can be applied to manage the situation. In other situations an auto-purge system may be utilized to help control condensable pump loads from condensing inside the pump and creating vane and cylinder   damage. Rotary vane vacuum pumps are best applied to clean dry applications but over the years have been   adapted to operate beyond their intended capabilities. We want to pay close attention to the pumps rated water vapor handling characteristics before applying this machine to wet applications. Pump construction is cast iron with ductile iron rotor using 3 phenolic vanes, needle bearings and viton lip seals.
  4. All of the above vacuum pump technologies can be used in conjunction with a backing blower to achieve a lower working pressure. Rotary claw and liquid ring vacuum pumps can utilize a VFD.
  5. Dry rotary vane single stage vacuum pumps can produce pressures as low as 27"HgV. They are constructed of cast iron with ductile iron rotor using up to 6 carbon vanes, sleeve bearings and lip seals. These pumps offer low initial cost but higher life cycle cost than their oil flooded counterparts as vanes must be replaced every 1-2 years of operation. These pumps should not be used in wet applications due to the carbon dust accumulation in the pumping chamber. When the carbon dust comes into contact with water and vapor mist it has a strong tendency to transform into what looks like tiny BB shaped balls that can interfere with the pumps normal operation creating internal leak paths or catastrophic vane failure. Great care should be considered when specifying this technology to protect the pump from wet or dirty pump loads. Dry rotary vane vacuum pumps will produce one of the highest sound levels of any mechanical vacuum pump.     
  6. Rotary claw vacuum pump systems offer no fluids in the pumping chamber and up front seems  to allow a more environmentally friendly atmosphere compared to pumps that use oil or other fluids in the pumping chamber. The issue here is that the labyrinth seals are not 100% leak proof and over time oil from the gear box makes it’s way into the pumping chamber. Because there is no “clean up equipment” on the discharge side of the pump hot oil vapor is allowed to discharge directly into the atmosphere. The claw vacuum pump offers an ultimate vacuum level of 27"HgV and can discharge up to 32psig with an atmospheric inlet. The claw pumps operating temperature is hotter (350F> range) compared to pumps with fluids (190F<) in the pumping chamber and generally exhibits a higher sound level as it operates at 3500 rpm. The claw pumps internals are coated with molybdenum disulfide (MoS2) and can tolerate wet and mild corrosive gas and vapors. Because of the claws unique design it performs with the greatest Hp to cfm efficiency (depending on the operating vacuum level) than any other vacuum pump in its class.. Maintenance cost is about the same compared with pumps that use oil in the pumping chamber and still require regular oil changes in the gear box. The claw pump is only available in an air cooled configuration and uses labyrinth shaft seals.

Other HVS pump technologies are available to fit special lab requirements including the scroll vacuum pump
(0.1 Torr) and the twin screw dry vacuum pump (.075 Torr). The screw is available with several types of
protective coatings to handle corrosive gas and vapor. Fixed pitch screw pumps offer high discharge
temperatures that lend itself to handling many acids with low vapor pressures. Variable pitch dry screw
vacuum pumps offer lower discharge temperatures and can successfully pump potentially heat sensitive
reactive gases. Sound levels are high and require discharge silencers.

Vacuum Pressure Flow Through Orifices

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Many applications with respect to compressed gas and vacuum require the use of understanding and experience of applying orifices. Applications include pressure and vacuum measuring devices, control valves, anticavitation valves, various material handling processes, agitation of liquids and slurry's to mix, prevent settlement of solids, or accelerate oxidation or fermentation.

The flow of gas through pipes and orifices is usually measured in cubic feet per minute as this is the unit of measure used when designating compressors and vacuum pump capacities. The flow of air may also be measured in linear feet per minute (LFM), usually called velocity. Velocity is the rate of flow past a certain point. Just before a flowing fluid at a certain pressure and temperature reaches the orifice it's forced to converge, creating a higher pressure upstream of the orifice. As the fluid passes through the orifice it expands (velocity reduces) creating a lower pressure on the downstream side. When the flow rate through an orifice is subsonic (velocity less than Mach 1) a reduction of backpressure will increase the mass flow through the orifice until the pressure is lowered to a critical level. At this point of critical pressure differential the flow rate stops increasing all together. The flow is said to be choked, the velocity at the orifice has just reached sonic (Mach 1) and will no longer increase no matter how much further the downstream pressure is reduced.

The relation between cubic feet per minute and velocity is given by the formula: V= FxA
Where  V - Cubic feet per minute

F - linear feet per minute
A - Area of hole or pipe in square feet

Consider a hole 1/10 of a square foot in area and the air is flowing through the hole at a velocity of 500 ft. per minute, then we have a flow of (1/10 x500) 50 cubic feet per minute.

An orifice is a round hole with a sharp edge in a thin plate. There are two types of gas flow through an orifice depending on the pressure ratio as previously mentioned. When the ratio of downstream absolute pressure to upstream absolute pressure is above approximatly .5 (based on air from 0-250F), the flow is subcritical (subsonic) and will vary depending on the pressure. Flow (Velocity) through an orifice will gradually increase until the ratio equals .5, at this point the throat velocity of the orifice reaches the speed of sound (Sonic Velocity) and the flow becomes critical and will remain constant for any ratio below .5. The critical ratio can be calculated for any gas if the ratio of specific heats is known;

rc = P2/P1 = (2/K-1) k/k-1

Because many different final shapes of orifices are possible, and these different shapes have a large effect upon the flow of gas through them, coefficients have been assigned to approximate the flow. For example an orifice with a well rounded entrance has a .98 coefficient assigned and will pass nearly twice as much air per minute compared to an orifice with a sharp entrance which has a coefficient of .53. When dealing with a number of small orifices, the volume of air that will pass through them per minute is the same as will pass through one large orifice having an area equal to the sum of the areas of the small orifices.

The flow of gas through orifices depends upon the air pressure, the orifice diameter and the coefficient of the orifice shape. Many complicated formula have been devised for calculating the flow and one when simplified is approximately as follows:

Where W = Weight of gas in lbs./cu.ft.

Therefore it can be seen that the volume varies directly as the coefficient of the orifice, the square of the diameter, the square root of the pressure, and the square root of the reciprocal of the weight. Since air has a weight of .0766 and hydrogen .0053 lbs. per cu. ft. it can be seen from the above formula that the light hydrogen will flow about four times as fast as air.

Selection of Process Vacuum Pumps

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A Liquid ring vacuum pump? Dry Screw or dry claw? A Steam jet ejector? Rotary piston or rotary vane pump? Comparison of available pumping systems as they relate to specific applications can be difficult and time consuming.

The first step in evaluating alternatives is to eliminate from consideration pumps or pumping systems that can't meet process requirements. This involves a consideration of (1) required suction pressure and capacity, and (2) reliability and maintenance. Following elimination of those pumping systems that can't meet process requirements, the most economical system can be determined by considering (3) purchase and installation costs and (4) operating costs. Final selection is subject to constraints imposed by (5) environmental considerations.

The most important parameters affecting final selection of the vacuum pump comes down to the suction pressure (P2) and the throughput the pump must handle (V2). The suction pressure is calculated by subtracting the losses in the suction line from the system operating point back to the vacuum pump. Line Losses include losses across sections of manifold, bends, and losses across filters, KOP's, scrubbers and precondensers.

When calculating the load to the vacuum pump, it's important to first examine the process. What are the primary sources of vapor or gas load to the pump. Is evacuation time a major concern. The checklist below will provide a basis for calculating capacity requirements for a vacuum pump.

Sources of Vapor-Gas Load
1) Air Leakage
2) Vapors of Saturation
3) Evaporated Vapors         
4) Evacuation of Process Equipment
5) Decomposition
6) Reaction Products
7) Sparge Gas
8) Stripped Gas out of Process Material
9) Dissolved Gas
10) Purge gas from instrument lines

After the process design has been completed pump selection can be prepared through the following questions:

1) P1 - Starting pressure
2) P2 - Operating Pressure
3) Pd - Discharge Pressure
4) Tg - Gas Temperature
5) Tw - Temperature and type of available cooling water
6) Ts - Temp and type of sealing liquid in the case of a liquid ring
7) Non-Condensable Pump Load
8) Condensable Pump Load

A reality check is necessary at this point to determine if the presented process conditions are indeed realistic. For example if the condensable pump load is 50lb/hr of water vapor at an inlet gas temperature of 110F and the operating pressure (P2) is requested to be 45mmHgA, is this possbile? A quick look at a steam table chart indicates that at 110F. water has a vapor pressure 65.9mmHgV. This means that as the pressure is reduced on the water at 110F it will continue to flash off until all the water is removed. Not until all the water is removed will the pressure move lower than the stated vapor pressure of 65.9mm@110F.

I've created a general list of available capacities and operating pressure ranges for the most often used process vacuum pumps and systems.

Steam Ejectors, single stage: 10-1,000,000 cfm (50mm)
Steam Ejectors, two-stage: 0-1,000,000 cfm (4mm)
Steam Ejectors, three-stage: 10-1,000,000 cfm (800 microns)

Liquid Ring Vacuum Pumps, single stage: 3-18,000 cfm (25mm)
Liquid Ring Vacuum Pumps, two stage: 3-6,000 cfm (25mm)

Rotary Piston Pumps - single stage: 3-800 cfm (5 microns)
Rotary Piston Pumps - two stage: 3-800 cfm (.001 microns)

Rotary Vane Pumps - oil sealed once-thru two stage: 100-600 cfm (.5mm)
Rotary Vane Pumps - oil sealed recirculation two stage: 3-150 cfm (.001 microns)

Rotary lobe blowers - single stage: 30-30,000 cfm (400mm)
Rotary lobe blowers - two stage: 30-30,000cfm (60mm)

Dry Screw - single stage: 60-600 cfm (.0075mm)

Dry Claw - single stage: 30-350 cfm (75mm)

This list should help in your preliminary process of eliminating pumps that can't meet your process requirements. Further assessment of the pumps reliability to the process including tolerance for solids, liquid slugs, reactive gasses, response to surge in gas or air leaks, performance in pumping condensable loads or excess discharge pressures etc... must be evaluated before making the final selection.

Liquid Ring Vacuum Pumps

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In a liquid ring pump, an impeller with at least 12 or more fixed blades is mounted eccentrically in a drum shaped casing between 2 end plates. As the impeller rotates the sealing liquid moves out by centrifugal force to the wall of the casing forming the liquid ring. The process gas is drawn into the pump by the expanding pockets trapped between the liquid ring and the impeller hub. The air is then discharged slightly above atmospheric pressure after being compressed as the pockets between the liquid ring and the impeller hub decrease.

As a vacuum pump the liquid ring can generate pressures down to 28.9"HgV and as a low pressure compressor with an atmospheric inlet will produce up to 30 psig at the discharge. Liquid ring compressors are also available as double acting machines and can produce pressures over 200psig at the discharge with an atmospheric inlet.

Correction Of Vacuum Pump Capacity For Altitude

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All Vacuum Pump performance curves and technical data are referenced to barometric pressure at
Sea  level. When operating at higher altitudes, the barometric pressure is always lower, therefore
some calculations are required to correct for this pressure variation. Caution must be taken when specifying  a  pump at higher altitudes; the amount of vacuum desired cannot exceed the barometric pressure at that altitude.

Since the pump performance of a positive displacement pump is a function of pressure ratio:

P1/P2 at sea level = P1/P2 at altitude
(P2) at sea level = (P1) at sea level (P2) at altitude/(P1) at altitude

Example:  Select a vacuum pump for 200 CFM to be installed at an altitude of 2743 meters (approximately 9,000feet), and to operate at a vacuum of 15 in. Hg at that altitude using 60f.
water as service liquid.

Barometric pressure at 2743 meters is 551.5 mm Hg Abs, or 21.7 inch Hg absolute. Therefore an operating pressure of 15 inches at this altitude means an absolute pressure of 21.7 - 15.0 = 6.7 in. Hg Abs

The actual pressure ratio of the unit is:

P1/P2 at altitude = 21.7/6.7 = 3.238

Because pump performance basically is a function of pressure ratio, the selection should be made at the similar pressure ratio at sea level.

29.92/3.238= 9.24" Hg absolute

29.92 - 9.24= 20.68in. Hg vacuum or 9.24/29.92 x 760 = 235mm

Medical Surgical Vacuum System

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A medical vacuum pump system must meet NFPA99 design standards.  The minimum size is a duplex system where one vacuum pump can carry the entire load of the hospital and the 2nd pump (lag pump) then provides 100% backup. Vacuum serves a multitude of medical surgical operations such as chest drainage, gastro intestinal and pharyngeal/tracheal treatments. NFPA99 regulates the vacuum level at all hospitals shall be 19”HgG and the lowest allowable vacuum level at the furthest outlet form the source to be 12”HgG.  Capacity at all hospitals is given in SCFM and is generally defined as 70F air, 0% humidity and 14.7psia.

Vacuum pump manufacturers will use units of pump performance such as “HgG (inches mercury gauge) and Torr or mmHgA (millimeters of mercury absolute)   same measurement two different terms.   To convert 19”HgG to inches of mercury absolute (“HgA) 29.92”HgA – 19”HgG = 10.92”HgA. To convert from 10.92”HgA to say PSIA then 10.92”HgA/29.92”HgA x 14.7psia = 5.365 psia.

Because all vacuum pump curves are rated in ACFM from the manufacturer we need a way to convert from SCFM to ACFM so we can select a pump. In our example lets assume the hospital requires 25scfm @ 19”HgG, since air or gas under a partial vacuum is expanded our answer will always be a number larger than the 25scfm. Then 29.92”HgA/10.92”HgA x 25scfm = 68.5acfm @ 19”HgG.  Since medical vacuum shall always operate at 19”HgG then you may memorize the expansion factor of 29.92/10.92 = 2.74, this will always remain the same in medical surgical vacuum applications.

Controls on most medical vacuum pump systems are simply load/no-load. Medical vacuum controls most found today include load/No-Load, On/Off or VFD, rarely does any vacuum pump system use a modulated inlet valve. Typical vacuum pump will be single stage, standard materials of construction, common Hp size for a single pump will be anywhere from 5Hp to 40Hp range.

Liquid ring medical vacuum pump systems offer the greatest reliability and durability of any other type of mechanical vacuum pump and feature lowest noise level, a 40 year pump life, can compress up to ¼” soft solids, cool discharge temperatures and minimal maintenance. Water sealed liquid ring vacuum pumps offer the greatest safety and protection when considering WAG compression. Be cautions when considering technologies such as Claw vacuum pumps, especially for higher elevations as these machines are limited in maximum achievable vacuum levels (24”HgG max) and may not meet the NFPA 99 partial vacuum level (19”HgG) requirement. Other issues that should be considered with the Claw technology include high discharge temperatures (300F>) 3600rpm operations speed creates loud noise levels, and the inability of labyrinth seals to provide a 100% seal between the gear box and pumping chamber. Since claw technology does not include any type of clean up equipment on the pumps discharge it is possible to exhaust hot oil vapor directly into the discharge pipe.

The Power of Vacuum Condensers

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Process condensers and pre-condensers should be considered when large condensable loads are indentified in a vacuum pumping system. Imagine one cubic foot of water being heated; turning into steam that one cubic foot of water now occupies 1700cu/ft of space. The proper use of condensers will allow the use of a smaller vacuum pump, reducing total energy and capital costs. Condensers will also recover valuable process materials. Reducing the condensable pump load to a liquid ring vacuum pump translates into a smaller heat exchanger; less temperature rise due to condensation inside the liquid ring means a deeper vacuum level and higher capacity pump.  Reducing the condensable load to oil sealed or dry vacuum pump (pumps with a low tolerance to vapor loads) will mean the difference between consistent catastrophic pump failures and normal pump operation.

The use of a condenser is not always technically possible. If the dew point of a gas stream is less than the available cooling medium temperature, the condenser can’t be used to remove process vapors. In a direct contact condenser, a device that uses direct contact of the cooling medium to condense the vapor load, condensing will not be possible at pressures below the vapor pressure of the cooling medium. If the use of a condenser proves possible the question becomes one of economics. Will the potential of reduced capital and operating costs for the vacuum pump offset the new costs of the condenser, associated components, and utilities.  Valuable product recovery, an increase in pumping system reliability, smaller vacuum pump and related components, reduced operating costs and a number of other factors must be considered.

The proper sizing of both the condenser and the vacuum pump depends on an accurate estimation of the condensable vapor load.  Process gasses from many vacuum applications will contain 95 to 100 percent condensable vapors. When process condensers are not in use, the vapor load entering the pre-condenser or the vacuum pump can be considered large. This load should be treated as evaporated vapor because the flow rate of this type of stream is directly related to the evaporation rate in the process vessel.

Air leaking or intentional release of non-condensables into a process vacuum system will become saturated with vapors as they contact the liquid process stream. Condensable vapors that saturate non-condensable gases are called “vapors of saturation”. If these vapors are present they must also be accounted for in order to calculate the load to the condenser or vacuum pump.

In the ideal process conditions the condenser can be the most energy efficient, cost effective vacuum pump ever developed.
 
Example – Process Condenser using Simple Water

Customer requirements:

P1 - 760mm
P2 - 125mm
Pd - 760mm
Tg - 135f.
Tw - 65F.
Condensable pump load: 500lbs/hr water vapor
Non-condensable pump load: 50lbs/hr air

Step 1: Determine pump size without the implementation of a process condenser.

Step 1a. Determine volumetric requirement by using the ideal gas law PV=nRT

V= (n)(R)( 1/P)(T)

V= 1,368acfm @ 125mm

The type of mechanical pump selected must be a liquid ring vacuum pump due to the large condensable pump load.  The volumetric requirement calculated would indicate an initial pump size using a 125Hp motor. After performing a mass balance on the discharge side of the liquid ring it was found that the temperature rise exceeds 20f. and the next larger pump size was selected with a greater service liquid flow. The pump size now requires a 150Hp motor and carries an estimated system capital cost of $75,000.00.

Step 2: Determine pump size when implementing a process condenser.

Step 2a. Determine Process conditions at the inlet of the condenser

Inlet process conditions: P2 – 125mm
                                    Tg – 135f
                                         Condensable pump load: 500lbs/hr water vapor
                                         Non-condensable pump load: 50lbs/hr air

Step 3: Determine new process conditions at the vacuum pump inlet

P1 - 760mm
P2 - 120mm
Pd - 760mm
Tg - 80f.
Tw - 65F.
Condensable pump load: To be determined
Noncondensable pump load: 50lbs/hr air

Step 4: Determine amount of water in the air by using Dalton’s law of partial pressures. Assume 60f cooling water into the condenser and 70f out (70f +10f approach = 80f gas temp)

Wi = Wair Mi/29 (Xi Yi Poi/P-pc )  
Where Wi = mass flow rate of vapor
Wair  = mass flow rate of air
Mi  = Molecular weight of vapor

Wi = 10lbs/hr water vapor

The power of condensing becomes clear as we are able to reduce the 500lbs/hr of water vapor to 10lbs/hr.

Step 5: Determine volumetric requirement by using the ideal gas law PV=nRT

V = 95.5acfm @ 120mm

The volumetric requirement calculated would indicate an initial pump size using a 7.5Hp motor and carries an estimated system capital cost of $5,500.00.

Estimated capital cost of pump alone: $75,000.00
Estimated capital cost of pump when using process condenser: $5500.00

Estimated Vacuum Pump Capital Savings…$69,500.00

The example using simple water is very common on applications such as Uranium drying where very large condensable loads are encountered and the implementation of a process condenser saves thousands of dollars in capital and operating costs.