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Steam Ejectors 

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 converting 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.

Nash is globally recognized for assembling the most cost-effective steam jet and air ejectors. Application engineers ensure maximum 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.

How Steam Ejectors Work - Operating Profile

  • High pressure, low velocity motive steam enters the steam chest and exit through the steam nozzle at low pressure and high velocity (supersonic).
  • The high momentum flow entrains process vapor into the suction chamber. During the entrainment and mixing, motive fluid velocity is reduced and process fluid is accelerated.
  • The resultant mixture reaches a sonic velocity at some point in the diffuser throat and a stationary sonic-speed shock wave is formed producing a sharp rise in absolute pressure at this zone in the diffuser throat.
  • In the discharge cone, residual velocity energy is converted to additional pressure energy.

Advantages of a Steam Ejector:

  • No moving parts
  • Simple in construction
  • Relatively easy to maintain
  • Available in a wide variety of materials
  • Low investment, high utility cost
  • Good for low MW gases

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
  • Interstage ejector could be renozzled to optimize interstage pressure and minimize steam flow.

Nash Ejector

Trouble Shooting Ejector Systems

There are two basic types of malfunction in an Ejector system:

  1. Caused by External influence.
    • Determine weather any changes have been made to the process equipment served by vacuum system adding pressure drop etc.
    • Determine weather any changes have been made to the process itself adding load to the vacuum system.
    • Determine whether the pressure or temperature of the steam or cooling water have changed with respect to system specifications.
    • Determine the problem developed gradually or suddenly. Usually gradual changes are due to deterioration of vacuum system, sudden loss of vacuum is due to changes in process, utilities, excess back pressure or system leakage.
  2. Caused by Equipment Supplied (Ejector’s, Condenser’s or pumps).
    • To pinpoint malfunction a step by step procedure should be followed to assess each component.
    • Blank-off the inlet of first stage ejector (isolate system from process) and take shut-off reading of each ejector.
    • If shut-off readings at no load matches close to performance testing results on test floor. There is nothing wrong with ejectors.
    • Check pressure drop on condenser’s, cooling water inlet and outlet temperatures, vapor out temperatures of each condenser when system is running on process.
    • If results match design conditions, vacuum system is performing O.K, troubleshoot vacuum system upstream the ejectors.

 

Best practice for customer’s when they receive new equipment.

  1. Take shut-off reading at no load of each ejector and keep as base reading in safe place.
  2. Whenever system gives any problem compare shut-off reading at no load with base reading. If they match, most probably there is nothing wrong with ejector system.
  3. Collect inter condensers data like pressure drop, cooling water inlet and outlet temperatures, vapor out temperatures when first run on full load and keep as base reading.

TROUBLE SHOOTING A MULTI STAGE SYSTEM NEEDS PATIENCE, SKILL AND LUCK. ABOVE GUIDELINES WILL HELP YOU GET CLOSER TO RESOLVING PERFORMANCE PROBLEM AND HELP CUSTOMERS.


Nash Ejector

Installation

  • 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 hot well.
  • If condenser’s cannot be mounted at correct elevation, low NPSH pump has to be used.

Common Terms

  • Dry air equivalent – is the equivalent mass flow of 70 Degree F air and is industry standard for ejector capacity rating.
  • Compression Ratio – Ratio of discharge to suction pressure, both in absolute.
  • Capacity – The maximum PPH of load that the ejector can stably pump over the design pressure range.
  • Performance Guarantee – is the statement or condition which the ejector system must meet. Usually it is one point condition.
  • Motive Pressure – commonly steam, is the pressure existing in ejector steam chest which is the pressure directly upstream side of the motive nozzle.
  • Cooling water temperature – is the maximum temperature available at the condenser inlet throughout the operating life of the unit.
  • Shut-off pressure – is the suction pressure achieved by the ejector at zero load.
  • Stability – Stable operation is without fluctuation in suction and discharge pressures. In an unstable performing ejector the flow at the suction momentarily reverses and pressures cycle.
  • Break Pressure – Refers to either the minimum motive fluid pressure at which the operation is stable or the maximum process discharge pressure at which operation is stable. Varying pressure below (Motive fluid pressure) / above (discharge pressure) will cause the ejector to break
  • Pickup Pressures – The motive fluid pressure or discharge pressure which must be attained to restore a “broken” ejector to stability. The motive fluid pickup pressure will be above the break pressure. The process discharge pickup pressure will be below the process break pressure.
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