Introduction
In chemical process industries vacuum is very oftenly used for various purposes e.g.
Vacuum distillation of high boiling organic compounds. At atmospheric distillation, the products may deteriorate due to higher temperature, but in vacuum distillation temperature is lower.
For transferring a material from on place to another, pressure differential can be created by applying vacuum.
In chemical reactions where gases are generated negative pressure that is vacuum is applied for scrubbing gases so that leakage to atmosphere is minimised.
In filtration and drying operations also vacuum is used very frequently.
For creating vacuum common devices used are:
Ejectors
Venturi scrubbers Vacuum pumps Ejectors
These are the most common equipments used for creating vacuum because of their simple design, No moving parts.
Basic principle Type of ejectors
Points to be considered while purchasing steam jet ejector Performance factors
Steam flow through ejector nozzle Information required for ejector selection Cost factors
Basic principles of steam jet ejector
Steam ejectors are pumps without moving parts. Construction and operation are extremely simple in as much as only three main processes are involved.
The main parts are head, the driving nozzle and the diffuser. The main processes are expansion of driving steam in the driving nozzle, mixing of the steam jet thus produced with the medium to be drawn off ( air, gasses or vapors) and the conversion of velocity of this mixture into pressure in the diffusers.
Steam jet ejectors operate at very high velocities. The velocity of the driving steam jet is nearly always many times that of the speed of sound. The large volumes under vacuum can therefore be easily handled. This is the reason why stem ejectors are eminently suitable as vacuum pumps.
These are simple is construction, constructed from wide range of material of construction, simple in operation, robust in design, long working life, extremely safe operation.
Type of ejector
Single stage
To compress from about 80 torr to atmospheric pressure. It is used generally for compression ration < 10. It is suitable as pre-evacuator.
Two-stage
Without intercondensation, to compress from about 30 torr to atmospheric pressure. It is used for small suction quantities and as two-stage pre-evacuator.
Three- stage
With intercondensation to compress from about 10 torr to atmospheric pressure. It is mainly used to evacuate large condensers. These ejectors have a smaller steam and water consumption than the two stage steam jet ejectors with intercondensation, if the working conditions are same for both the units.
Four-stage
With surface condenser for vacuum from about 3 torr to about 30 torr when the drawn off medium must not come into contact with the cooling water or if the condensate is to be recovered.
These ejectors are used extensively in the mineral oil industries.
Five stage
With mixing condensers for vacuum from about 0.1 torr to about 3 torr. For suction quantities from about 0.5 kg/hr upto about 1000 kg/hr condensable and in condensable vapor and gases.
These ejectors are used for freeze drying where large quantities of water vapor are drawn off from a vacuum of about 1 torr. Also used in steel de-gassing where large quantities of incondensable gases must be drawn off.
Six-stage
With intercondenser and an after condenser, for vacuum from about 0.05 torr to about torr.
These are used in the manufacture of synthetic fibres where vacuum between about 0.1 torr and 0.3 torr is required. An after condenser is always used, if the exhaust from the final stage can not flow direct into the atmosphere.
2 Steam stage + water ring vacuum pump
For vacuum from about 0.5 to 5 torr. This combined pump is particularly suitable if barometric erection is not possible.
1 Stem stage + water jet
This combination is used for vacuum upto 10 torr.
2 Steam stage + 1 water jet
This combination is very famous in the chemical process industries. This gives vacuum in the range of 2 to 5 torr.
Points to be considered while purchasing steam jet ejector:
Suction capacity
The steam consumption of a stem jet vacuum pump does not depend upon whether the whole or only part of the suction is ejected. No steam is saved when a large capacity pump is operating below full load. Hence it is important to make the plant to be held under vacuum as air tight as possible, to determine the suction capacity as accurately as possible and to design the pump for this capacity. However the capacity should always be chosen with a safety margin so as not to endanger safe working.
The suction capacity is best calculated by weight that is Kg/hr. The suction capacity be weight is made up of:
Air leakage For calculating the air leakage rate click here Air leakage rate
This leakage is that which enters through gaps in sealing. ( A hole of 1 mm² lets in approximately 1.8 lbs/hr ).
Gases, vapor & air released from the material handled in the plant
The condensable part must be distinguished from the noncondensables ( Inert gas ). The molecular weight & temperature must be taken into consideration. For discontinuous operation, it is important to know at what vacuum the gas or vapor quantities are released.
Gases & air from the cooling water
The air tightness of the vacuum plants can vary greatly depending upon whether the plant is mainly welded or whether it has many flange connections, valves, cocks, inspection glasses, stuffing boxes etc. The type of sealing material, the condition of the sealing surfaces and the degree of the use of the fillings are important. However it is possible to give a rough guide for a practical evaluation of the suction capacity ( Not including of course vapors or gases released in the chemical process ).
Vacuum
The vacuum should not be chosen higher than the absolutely necessary. Too high a vacuum leads to unnecessarily large suction pipelines, unnecessarily large steam jet ejectors, excessive steam consumption and excessive cooling water consumption.
Pre-evacuator
If a vacuum plant is in constant operation, evacuating time on start-up is generally not an important factor and it is not necessary to provide special pre-evacuator ( Start-up ejector ). If however the vacuum plant has to be started up frequently, a short evacuating time is desirable. To achieve this a pre-evacuator is used.
A pre-evacuator is normally a single stage steam jet ejector with a large suction capacity, which is put to operation simultaneously with the steam jet ejector. Together with the ejector it evacuates the plant very quickly to an intermediate vacuum, say upto 150 torr. At 150 torr almost 80 % of the original volume of air in the plant has already been pumped out. The pre-evacuator is then shut-off and the steam jet ejector alone evacuates in the remaining time to the required working vacuum.
The pre-evacuator has a high steam consumption. However this is not of great importance since it operates only for 10-15 minutes during the start-up of the plant. The steam consumption of pre-evacuator decreases considerably with increasing stem pressure. The steam consumption at 10 atm is only 40 & of that at 3 atm.
MOC
A steam jet vacuum pump should be constructed of a material at least resistant to corrosion as the plant to which it will be attached. The various MOCs available are: MS, SS-304, SS-316, Graphite, MS/FRVE lines, PP etc.
The jet comes in various MOCs such as: SS-304, S-316, Titanium, PTFE etc.
Type of condenser
Mixing condensers are simple, reliable and inexpensive. Mixing condensers like steam jet ejectors can be manufactured from many different materials and also make the best use of cooling water.
With surface condenser, the cooling water does not enter the vacuum. The cooling water is separate from the condensate. Surface condensers must be used when
The cooling water must not be contaminated by the condensate of the vapors drawn off.
The condensate is to serve as cooling water for units such as steam turbine condenser.
The condensate is to be reclaimed.
Ammonia vapors are contained in the drawn off vapors because insoluble condensate are quickly formed in mixing the condensates which would cause breakdowns, that is, blocking of the water outlet etc. Even small amount of ammonia are dangerous.
Method of installation
Barometric erection
If possible steam jet ejectors should be erected barometrically because then the water flows from the condensers without pumping. A column of 760 mm Hg ( 29.92” ) high balances the pressure of the atmosphere. Since water is 13½ times lighter than mercury, the
corresponding height of water is 10.33 m. This height of 10.33 m is called the barometric height. According to the pressure in difference condensers at reduced height may be sufficient to ensure a free outlet of the cooling water. At a barometric height of 760 mm Hg and a condenser pressure of 300 torr. A height of 6.3 m ( 20.7” ) is necessary.
Since the water may be mixed with same air and hence will no longer have a sp.gravity of 1, it is safer to assume a full barometric height of 11 meters.
Non barometric erection
In many cases it is not possible to install a steam jet ejector 11 m ( 36 ft ) above the water drain. There are various alternatives depending on the height available.
Performance factors:
Steam jet ejectors can be designed for driving steam from about 15 psia to 600 psig. A steam jet ejector operates at the driving steam pressure for which it is designed. It is necessary to ensure that the designed pressure is maintained otherwise a breakdown is possible. On the other hand operating above the design pressure results in steam wastage. In this case it is necessary to install a pressure regulator.
It is essential to check what working steam pressure is available at the site of steam jet ejector.
The ejector should be designed for that pressure. ( This pressure is often much lower than the boiler pressure ). Steam jet ejectors operate most efficiently with dry saturated steam or superheated steam. Low superheat can be disregarded. Wet steam is not desirable at all.
The motive steam design pressure must be selected as the lowest expected pressure at the ejector steam nozzle. The unit will not operate stably on steam pressures below the design pressure.
Recommended steam design pressure = Minimum expected design pressure at the ejector nozzle – 10 psi.
This design basis allows for stable operation under minor pressure fluctuations. An increase in steam pressure over design pressure will not increase vapor holding capacity for the usual “ Fixed capacity ejector “. The increased pressure usually decreases capacity due to the extra steam in the diffuser. The best ejector steam economy is attained when the steam nozzle and diffuser are proportionated for a specified performance.
For a given ejector an increase in steam pressure over the design value will increase the steam flow through the nozzle in direct proportion to the increase in the absolute steam pressure. The higher the actual design pressure of an ejector the lower the lower the steam consumption. This is more pronounced on one or two stage ejectors. When this pressure is above 350 psig, the decrease in steam requirement will be negligible. As the absolute suction pressure decreases
advantage of high pressure steam becomes less pronounced. In very small units the physical size of steam nozzle may place a lower ceiling on the pressure.
For the ejectors discharging to the atmosphere, steam pressure below 60 psig at the ejector are generally uneconomical. If discharge pressure is lower in multistage units, the steam pressure at inlet can be lower. Single stage ejectors designed for pressure below 200 mm Hg (abs) cannot operate efficiently on steam pressures below 25 psig. The first stage for two of a multistage system can be designed although perhaps not economically to use stem pressure below one psig.
To ensure stable operations the steam pressure must be above a minimum value. This minimum is called motive steam pick-up pressure, when the pressure is being increased from unstable region.
Effect of wet steam
Wet steam erodes the ejector nozzle and interferes with performance by clogging the nozzle with water droplets. The effect on performance is significant and is usually reflected in fluctuating vacuum.
Effect of superheated steam
A few degree of superheat (5-15°C) is recommended, but if superheated steam is to be used its effect must be considered in the ejector design. A high degree of superheat is of no advantage because the increase in available energy is offset by the decrease in the steam density.
Suction pressure
The suction pressure of an ejector is expressed in absolute units. The suction pressure follows the ejector capacity curve, varying with the non-condensable and vapor load to the unit.
Discharge pressure
All types of ejectors are sensitive to discharge pressure just as they are to the steam supply pressure. Normally designed ejectors are suitable for operating against a pressure only slightly in excess of atmospheric. In most cases ejectors can be designed to discharge at a pressure of 5 psi, provided a considerable increase in steam consumption can be permitted. It must be
appreciated, however, that the discharge of an ejector is contaminated with incondensable gases and is therefore in many cases unsuitable for further use.
The pressure drop through discharge piping and aftercooler must be taken into consideration.
Discharge piping should not have pockets for condensation.
Cooling water
The cooling water temperature is of great importance.
Well water : 10 to 15°C
River water : 5 to 25°C
Re-circulated water : 10 to 28°C
Seawater : 15 to 30°C
Steam jet ejectors must be designed for the maximum cooling water temperature available.
Steam and water consumption are greatly dependent on the design temperature as can be seen from the following example.
A unit to eject at 5 torr using steam at 45 psig.
The steam consumption at a cooling water temperature of 25°C is double that of 15°C. If the cooling water temperature varies to a large extent throughout the year, it is advisable to adjust the steam consumption in relation to the water temperature by suing different nozzles, or by altering
the steam pressure ( The lower in only possible at high pressure ). The pressure of the cooling water should not oscillate as this may affect the vacuum.
With missing condensers it is not necessary to have any particular water pressure. With water jet condensers a minimum pressure of 30 psig is required. With surface condensers the pressure loss in the tubes must be overcome.
In general the cooling water outlet temperature must not exceed 45°C to 50 °C, otherwise chalk deposits will cause breakdown.
Amount of gases released from cooling water, W = G/t1, where W = Air liberated lbs/hr
T1 = Inlet temperature °C G = Cooling water, gal/min Ws X L
GPM of cooling water required = ---, where tw X 500 Ws = Lbs of steam to condense
L = Latent heat of vaporisation, usually taken as 1000 BTU/lb for process application and 950 BTU/lb for turbine exhaust steam.
tw = Cooling water temperature rise °F
Dry and saturated air
When an ejector is maintaining a condenser vacuum, the air extracted by the ejector is saturated with water vapor. The amount of water vapor entrained by the dry air which leaks into the
condenser depends upon the temperature of the mixture and the vacuum at the ejector suction.
Gas and vapor densities
When an ejector or thermo-compressor is required to handle chemical gases or vapors, it is necessary that the gas density ( For failing this the temperature & molecular weight ) be known.
The compression of a given weight of heavy gas requires less operating steam than the same weight of light gas. As a simple example on pound of air is more easily handled by an ejector than one pound of water vapor. The correct proportioning of the ejector & thermo-compressor
therefore necessitates detailed knowledge of the properties of the gas or vapor to be compressed.
Steam flow through ejector nozzle
The quantity of steam passing through any ejector nozzle varies directly as the steam pressure, but decreases with an increase of steam temperature. It is therefore essential that the maximum steam temperatures be stated to enable the ejector to be correctly designed.
The weight of steam flowing through an ejector nozzle may be obtained from the following simple formula.
S = KPD2, where S = Steam flow in lbs/hr
P = Steam pressure at the upstream of nozzle in psi (abs) D = Diameter of nozzle bore in inches.
K = A constant which depends on the steam pressure and temperature.
Information required for ejector selection
To enable the manufacturer to design most suitable type of vacuum system, it is essential that correct data be available upon which to base proposals & designs. Whenever possible the following information should be supplied with any enquiry.
Brief description of purpose for which equipment is required.
Weight of air or vapor to be handled in lbs/hr. If a mixture of air, vapor and /or other non-condensable gases. Their approximate proportions should be stated
Lbs/hr of condensable vaporous & lbs/hr of non-condensable gases, either dissolved, injected or carried in process formed by reaction, air leakage etc.
The absolute pressure required at the ejector suction in inches mercury, or millimeters mercury.
If ejector is required to discharge at other than atmospheric pressure, the maximum discharge pressure must be stated.
If normal steam pressure and also the minimum steam pressure at which the plant is required to operate. If steam is superheated give maximum temperature.
Maximum temperature of cooling water.
Cost factors
Of all the classes of Chemical Engineering Equipments, ejectors are unique in that the actual operating cost is usually much greater than the installed equipment cost. Ejectors have low first cost because they are simple in design, have no moving parts and occupy a small space.
In general operating cost increases as the ejector steam pressure increases. If an ejector is supplied with steam above the design pressure, steam consumption will be increased in direct proportion to the ratio of actual operating pressure to the design pressure. It is good practice to install a stem pressure regulating valve to maintain a uniform steam pressure at the ejector.
The other things which affect the performance of on ejector are.
Discharge pressure
Wet and superheated steam
Load variation
Back pressure
Capacity and suction pressure
Number of stages
Condenser requirements
Material of construction Water jet ejectors
As the name suggests it water jet ejector the motive fluid is water instead of steam. Ejectors using water as the motive fluid are designed for reasonable non-condensable load together with large condensable flows. Water pressures as low as 10-20 psig are usable, while pressures of 40 psig and higher will maintain a vacuum of 1-4 inches of Hg (abs) in a single stage unit. Combination of water & steam ejectors are used to effectively handle a wide variety of vacuum situations. The water ejector serves to condense steam from the steam ejector.
Water ejectors and water jet eductors are also used for mixing liquids, lifting liquids and pumping
Water ejectors and water jet eductors are also used for mixing liquids, lifting liquids and pumping