Dry mechanical boosters – replace steam ejector

Steam ejectors find wide use in vacuum pumping applications – so called dirty application such as in Vapour extraction, Chemical processing, Evaporative Cooling, Vacuum distillation, Vegetable oil de-odourization, Vacuum Refrigeration, Drying etc. In spite of the fact that steam ejectors have poor overall efficiency and relatively high energy consumption, they are popular in vacuum applications because of their simplicity and ease of operation. Its high time now when the industry should realize the disadvantages associated with it and switch over to efficient alternatives – Dry Mechanical Vacuum Booster being one of them. Mechanical Vacuum Booster offers an efficient replacement to steam ejector, for most of the applications, as they overcome major drawbacks associated with steam ejectors. The major advantages of Mechanical Booster being :-

• Mechanical Vacuum Boosters are more energy efficient.
• Minimum of auxiliary equipment is needed; unlike for steam ejectors, which need large condensers, cooling towers, re-circulation pumps etc.
• Mechanical Vacuum Boosters are dry pumping system and don’t give rise to water and atmospheric pollution.
• Startup time for mechanical booster is very low making them ideal for Batch process operation where immediate startup and shut down is essential for energy conservation.

Apart from the above, the operating costs for mechanical vacuum systems are low, resulting in extremely short pay back period. For example, when operating in the range of 5-10 Torr the operating cost of mechanical pumping system would be about one tenth of the equivalent steam ejector system.

Steam Ejectors:

Steam ejectors comprise of converging – diverging nozzle through which high-pressure steam (motive fluid) is forced through. (Fig.1). The ejector nozzle converts the high-pressure head of the motive fluid into high velocity stream as it emerges from the nozzle into the suction chamber. Due to increase in velocity head, there is a drop in pressure head causing partial vacuum in the suction chamber. Pumping action occurs as the fluid / vapors present in suction chamber are entrained by the motive fluid and are carried into the diffuser, by

viscous drag process.

The capacity of steam ejector is directly proportional to the weight of the motive fluid. Generally, the ratio of motive fluid to the gas pumped is high, especially under low vacuum and results in excessive demand of steam in multi-stage systems. The overall performance of steam ejector is sensitive to changes in operative parameters such as motive steam pressure and discharge pressure. A slight variation in operating parameters weighs heavily on the system capacity. Multi steam ejectors require inter-stage condensing as each stage adds to the pumping load for the succeeding stage and for reason of economy, condensation becomes important. The heat gained during condensation i.e. latent heat of vaporization, adds to the need for additional equipment such as re-circulation pumps, cooling towers etc. so that the same can be dissipated. In a steam ejector, steam comes in direct contact with gas/vapour pumped and many a time, this mixture of pumped vapour and water needs elaborate treatment before it  can be discharged / re-used. Steam ejectors, especially multistage not only require steam generation facilities but also raise demand for auxiliary equipment such as D.M. plant for boiler feed water, condensing units, re-circulation pumps, cooling towers, effluent treatment plant etc. thereby increasing total energy consumption and maintenance costs. Steam ejectors are, therefore, no longer popular as they were once because of dramatic increase in cost of steam generation, auxiliary power and effluent treatment problems. It is for this reason many steam ejector installations have been replaced by mechanical Vacuum Pumps which use far little energy for the same service and require no additional auxiliary power, cooling tower nor give rise to effluent.

Vacuum boosters for drying applications

Drying is a process of removal of a liquid from a solid mixture by thermal means. Under this article, we shall not consider mechanical methods, such as filtration, centrifuging, pressing, etc. of liquid removal from solids. Various drying process & techniques are extensively used in the various Process industry, Pharmaceutical industry, Food processing industry, Dye & Chemical industry, Perfumes & Permitted Food additive industry etc primarily to achieve one or more of the following,

• Product concentration.
• Purification by removal of unwanted volatile elements.
• Solvent recovery.
• To increase shelf life and to facilitate further processing and permit proper utilization of the final product.
• To reduce shipping costs by reducing weight of the product.
• To reduce the rate of biological decay.
• To enhance the value of by products of a process.

Drying is an important and widely used process in the industry. Often it is a major cost center in process operations. The reason for this is the high-energy requirement for the removal of water. Typically, to remove 1 kg of water, we require 540 kcals of energy [latent heat of vaporization of water] plus at least another 60 kcals to take care of sensible heat requirements. Hence, regardless of the nature of process, we must supply at least 600 kcals for every kg of water removed from the material. This, therefore, demands high-energy inputs and for this reason the process efficiency must be maintained as high as possible.

The cheapest energy source is the sun. That is why many industries take recourse to sun drying. For example, in small food industry, chillies, ginger, etc., are all sun-dried; in the textile industry, fabrics and yarn are often sun-dried; in the ceramic industry, freshly moulded bricks and blocks are sun dried. Very often, most process requirements demand continuous working, independent to the weather conditions or time of the day. So there is a vast body of ovens, dryers, drying tunnels, dehydrators, etc., to take care of industrial drying processes. While these techniques are well known, their use is now being replaced by Low-pressure, Low temperature drying techniques, which have the advantage of enabling the drying process to be carried out at low temperatures. This process results in optimum energy utilization, lesser thermal exposure & damage to the product and very often, improved quality. In fact, it is possible to dry a product at sub-zero temperatures (Freeze drying) by simply reducing the pressure to an appropriate value for carrying out the process. Freeze Drying, most widely adopted process today in food processing industry is based on the same principle. Amongst the many advantages of low pressure drying techniques over oven-techniques, some are: -

• Drying time is accelerated drastically.
• Solvent recoveries are possible, resulting in substantial savings.
• Reduces pollution.
• Minimizes oxidation losses and product degradation due to reduced Thermal exposure.
• Wide range of operating temperatures can be selected to suit the product/process requirements.

For example, conventionally Katha (an essential ingredient of paan masala and paan), is dried by traditional cold room drying process. It is kept in cold rooms for over a period of about 24 days, with cold dry air blowing over it. The moisture levels are reduced from typically 50% + to about 12-15%. The temperature in the cold room is maintained at slightly above 0 degrees C, throughout the process time. However, by dopting low pressure drying techniques, this period an be reduced substantially. This would not only save energy requirements but also reduce the huge inventory hold-ups. Katha is a high-priced product and shortening of the process time ould result in substantial savings otherwise involved. Shorter process times also reduce possibility of fungus/moulds/bacteria attacks. Similarly Drying of various other products, such as Gelatin, Meat, Milk products, Green bodies etc, can also benefit by this fast process.

A vacuum Booster, when used in conjunction with any of the above, over comes all the associated limitations and increases the overall process efficiency by increasing the vacuum and pumping speeds with relatively very little extra energy.

Optimise vacuum to improve plant performance

Vacuum Pumps and systems are widely used in the chemical process industry for various applications such as drying, solvent recovery, distillation, short path distillation (Molecular distillation), concentration etc. It is therefore, essential that the vacuum principles are understood which can be employed to maximize process throughputs, product purity and quality & minimize power consumption.

Success has been achieved in many industries such as food product, essential oils, aromatics, solvent recovery and steam jet replacements. Wide range of pumps and vacuum equipment is being used in the industry to achieve the desired vacuum and pumping speeds. The understanding of their advantages and limitations can result in optimizing their performance.

What is Vacuum?

Vacuum is simply a pressure below atmosphere. To create vacuum in a system, a pump is required to remove mass (gas/vapor) from the system. The more mass is removed, lower is the pressure that exists inside the system. Various vacuum levels are defined depending upon the ultimate vacuum as:

􀂉 Coarse Vacuum 10 – 760 Torr

􀂉 Medium Vacuum 0.001 – 10 Torr

􀂉 Fine Vacuum 10^-3 – 10 ^-7 Torr

􀂉 Ultra High Vacuum < 10^-7

Generally, the chemical industry operates in Coarse and Medium vacuum range. In this range the vacuum is generally measured in mm Hg gauge or Torr (absolute pressure). Measurements from datum as atmosphere are gauge reading, whereas the measurements referred to absolute zero are expressed in Torr. For example at sea level (atmospheric pressure 760mmHg), a system maintained under vacuum of 700mmHg, as indicated by vacuum Boudorn gauge, is said to have absolute pressure of 60 Torr. Vacuum gauges, mercury manometers, transducers etc. indicate gauge pressure and their reading when subtracted from atmospheric pressure gives absolute pressure. It is important to under stand the above since all vacuum principles and calculations are based on absolute pressure units.

Pumping speed: It is the volumetric rate of exhausting, generally expressed in Lts/min., m3/hr or cfm. It is the rate at which the inlet of the pump actually removes the gas / vapor load. It should not be confused with Displacement of the pump. Displacement of a pump is the geometric volume swept by the pump per unit time at rated operating speed. For most of the pumps, pumping speed is close to displacement value at no load conditions (FAD-Free air delivery) and changes with inlet pressure, reaching to zero where the pressure attained is said be pumps

Ultimate pressure: The Curve below, gives pumping speed for different type of pumps.

It is evident from the curve that pumping speed drops with drop in pressure. This must be taken into consideration while selecting a pump. The inlet pressure at which the pump’s speed falls to zero is termed as “Ultimate pressure or Blank-off pressure” of the pump. It is a pump characteristic, dependent on the type of pump/ pump construction. The ultimate pressure/Blank off pressure of a pump can be easily checked by measuring the inlet pressure, with inlet of the pump blanked off. At Blank-off pressures, the effective pumping speed of the pump is zero. This means that a process can never achieve vacuum better than the blank off vacuum of the pump. While selecting a pump, desired process vacuum and that achievable by a pump must be verified. Ultimate vacuum is the pump type characteristic and general conception that using a bigger Pump (of the same type) would yield better vacuum is false. The process engineer’s should establish desired process vacuum and the selection of the pump should be made accordingly. To get better working vacuum and higher pumping speed, Boosters are invariably used in combination. In most cases much higher speed and lower pressure can be achieved with a fraction of extra power, when Booster combination is used.