How Steam Ejectors Work
Steam ejectors use steam or gas instead of moving parts to compress a gas. In a jet or ejector, a relatively high-pressure gas, like steam or air, expands through a nozzle. The steam or air converts that pressure or potential energy to velocity or kinetic energy. The jet of high-velocity steam or gas entrains the gas to be evacuated or pumped in the suction of the ejector. The resulting mixture enters the diffuser where velocity energy is converted to pressure at the ejector discharge.
Ejectors that use air as motive are often called air ejectors or air jets. Air is often employed on small ejectors when steam is not available. When paired with a NASH liquid ring vacuum pump, they can use the air from the room or the pump exhaust as motive air to increase the vacuum level the pump is capable of reaching. This is often used in applications such as deaeration when the vacuum system must be able to pull down to the vapor pressure of the water that is being degassed. Air jets of this type are convenient because no steam or pressurized air source is required to make it work, just the vacuum pump.
Steam ejectors can also be combined with liquid ring vacuum pumps to create a hybrid capable of deep vacuum that ejectors can deliver but with lower energy use. Nash is globally recognized for assembling the most efficient steam jet and air ejectors and ejector vacuum systems. Application engineers ensure maximum efficiency and performance benefits while optimizing a hybrid system customized to processes, applications, and technology requirements. NASH steam jet and air ejectors minimize greenhouse gas emissions and operational efficiency while improving system stability.
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Operating Profile Of Steam Ejectors
Steam ejectors pass steam through an expanding nozzle. The nozzle controls the expansion of steam and converts pressure into velocity; thus, creating a vacuum to transfer gases. An ejector operates on a mass basis, not by displacing volume. Therefore, ejectors are better-suited for handling gases with low molecular weights and when operating at low absolute pressures. These systems are ideally suited to high vacuum applications but are only marginally useful as compressors.
A jet of motive fluid is supersonic velocity entrains the inlet stream and raises its velocity to the speed of sound as the two flows mix. A stationary sonic shock wave forms in the throat of the diffuser, and absolute pressure rises sharply at this point. More pressure rise occurs along the discharge cone as flow slows down. The most common motive fluid is steam at 80 PSIG (6 bar abs.) to 400 PSIG (28 bar abs.). Other fluids can be used whenever there is a good reason to avoid mixing steam with the product.
Steam ejectors or steam jets, ejectors that use steam as motive gas, are by far the most popular type of ejectors. A single ejector can be designed to create as much as 27 in. Hg vacuum (or about 76 mm HgA). To create a deeper vacuum, ejectors can be ‘staged’ or installed in series. Steam ejectors are favored for this because the motive gas – steam – can be condensed between some of the stages to minimize load (and motive steam) to the following stage. Steam ejectors have been used in some industries to reduce the pressure of a vessel to the point that water freezes. They can be staged to reach a suction pressure of less than 0.1 mm Hg absolute.
How To Improve Ejector System Efficiency
- Combine ejector strengths with liquid ring vacuum pump strengths
- Last stage jet and after-condenser are eliminated and replaced with a high-efficiency liquid ring vacuum pump
- Interstage condenser pressure is optimized and cooling water load is typically reduced
- An interstage ejector could be nozzled to optimize interstage pressure and minimize steam flow
Installation Of Steam Ejectors
- Ejectors can be mounted in any direction, precaution must be taken to properly drain the system
- Barometric condensers/shell and tube condenser drain leg must be mounted high enough to gravity drain the water and avoid flooding in the condenser
- Ejectors can discharge into the hot well
- If condensers cannot be mounted at the correct elevation, a small capacity NASH pump has to be used
Advantages Of Steam Ejectors
- No moving parts
- Simple in construction
- Easy to maintain
- Available in a wide variety of materials
- Low investment, high utility cost
Ideal Solution For Demanding Applications
Steam ejectors and ejector/vacuum pump hybrids are the ideal solutions for the most demanding applications in oil & gas, chemical, electric power, and food & beverage.
- Reactor Vacuum (Chemical Industry) – Vacuum can allow the plant to reduce reaction temperature and save energy. It can also be used to avoid polymerization, undesired reactions, and avoid thermal degradation.
- Drying of Solids in Batch or Continuous Processes (Chemical/Food & Beverage Industries) – Using vacuum allows the solid to be dried at a lower temperature. It can be beneficial when processing heat-sensitive materials, to improve drying rates, and to produce very low final moisture concentration.
- Vacuum Distillation (Chemical/Oil & Gas Industries) – Pulling vacuum on a product and condensing it allows the separation of two or more volatile components with different boiling points.
- Evaporator Vacuum (Chemical/Food & Beverage Industries) – Concentrate materials in a liquid state by boiling off solvent (water). Much of the work is done by a condenser. Using vacuum can reduce energy costs and avoid damaging heat-sensitive products.
- Sugar Vacuum Pans (Food & Beverage Industry) – After the sugar juice is concentrated through evaporation (see evaporator vacuum), the vacuum is used to convert the syrup to a state where crystals begin to form.
- Bleaching & Deodorizing (Food & Beverage Industry) – Using vacuum to remove color and contaminants from edible oil.
- Condenser Exhausting (Electric Power Industry) – To optimize the expansion of the steam through the turbine requires the turbine condenser to be maintained at its optimum vacuum. This vacuum must be maintained by removing air that leaks into the condenser.