MTC 381: Fundamentals of High Vacuum Technology Date: 12-04-2000 Title: Applications and Operations that Involve Steam-Jet Ejectors

Outline:
  1. Introduction
  2. What is a Steam-jet Ejector
    1. Four Basic Types
      1. Single-stage ejectors
      2. Multi-stage non-condensing ejectors
      3. Multi-stage condensing ejectors
      4. Multi-stage non-condensing/condension ejectors
    2. Staging Ejectors
      1. Pressure range of the staging
    3. Condensers
      1. Condenser function
    4. Basic Construction of the Ejector
      1. Ejector materials
  3. Application of Steam-jet Ejectors
    1. Introduction
    2. Vacuum Refrigeration
    3. Vacuum Processing
    4. Miscellaneous Ejector Applications
      1. Space applications
      2. Recompression

Bibilography
A. Introduction:

 A steam-jet ejector is a device, which converts pressure energy in a fluid into velocity energy in the same fluid. It takes air from a process combine it with a high-pressure gas or steam to create a vacuum. The air is drawn in by suction and combined with the steam or gas. The combination gas is forced to a condenser or an evaporator, where the exhaust is dumped into a hotwell or cool to be released in the atmosphere.

A high-pressure fluid (steam or gas) enters the ejector chest through a nozzle located inside the ejector. The high-pressure fluid is then mixed with the air from the suction vacuum in the mixing section of the ejector. The mixing section of the ejector is located before the Venturi of the ejector. The fluid expands in the ejector chest. Then the fluid converts its pressure energy to fluid velocity. As the fluid velocity increases, the pressure of the same fluid decreases. The drop in pressure causes a vacuum that draws in air from a vacuum distillation, evaporation or drying process.

Figure 1: Cross Section of a Steam-Jet Ejector.

Figure 2: System with Steam-Jet Ejector.

The air and steam fluid mixture travels from the mixing section, through the Venturi, and exits to the diffuser section of the ejector. The diffuser section recompresses the mixed fluid stream to an intermediate (atmospheric) pressure. Then, it is sent either to a condenser or to another ejector depending on what type of system. If the mixed fluid stream is sent to a condenser, the steam is condensed at a low pressure and temperature (around 75 F), so the volume of the fluid can quickly decrease before it is released from the system. If the mixed fluid is sent into another ejector, the process starts all over again with the next ejector stage handling the fluid from the previous ejector and the new steam added to the existing fluid in that stage.

B. Types of Ejectors

1. The Different Types of Ejectors:

Ejectors are classified into four main types:

  1. Single-stage
  2. Multi-stage, non-condensing
  3. Multi-stage, condensing
  4. Multi-stage with both non-condensing and condensing stages

Single-stage ejectors are the simplest in construction and most commonly used in vacuum applications. The operation of a single-stage ejector was described at the introduction of the paper. The single-stage ejector has a range of pressures from atmospheric pressure to around 3 inches of mercury absolute.

Multi-stage, non-condensing ejectors are used when an application requires a pressure lower than what the single-stage ejectors could develop. A multi-stage ejector is a group of single stage ejector linked together in a series, which has a greater suction pressure than a single-stage ejector. This type of ejectors also requires more steam or gas than an individual single-stage ejector so it could handle the load of the vacuum for each stage the fluid goes through.

Multi-stage condensing ejectors are available in two or more stages, up to six stages. A surface or direct-contact inter-condensers are inserted between each stage. It condenses the steam from the ejector and reduces the load going into the next stage from the previous stage. A two-stage ejector has an absolute suction pressure that ranges from 4 inches of mercury to one-half of an inch of mercury. A three-stage ejector has an absolute pressure range from 25 mm of mercury to 2 mm of mercury. As more stages are added to the system, the pressure of the first stage becomes less. The system could handle up to six stages with the absolute suction pressure of 5 microns.

If a multi-stage-condensing ejector has a large amount of condensable vapors to handle, a booster condensers is inserted after a condenser and a two-stage ejector to handle the amount of condensable vapor to compress it at atmospheric pressure. If a system produces smaller condensable loads, a single intercondenser is used to condense the vapors after the ejectors.

2. Staging Ejectors:

Lower suction pressures are obtained when ejectors are placed in multiple stages. Staging multiple ejectors units creates a complex vacuum system, with some systems having up to six individual ejectors stages with intercondensers placed in between each stage. Placing condenser between stages is typical from putting together a multi–ejector system. There are many different ejector-condenser combinations available for the certain operations and applications. Condensers are used to condense motive steam and the suction vapor from each stage(s). It allows saturated non-compressibles to pass onto the next stages. Ejector stages can be divided into condensing units and non-condensing units.

A non-condensing unit is used when the fluid flow from a previous stage discharges directly into the next stage. Steam consumption in a non-condensing unit is normally higher than in a condensing unit. This is because the second stage must handle all of the fluid from the first stage. Non-condensing units are normally used when the inter-stage pressure is lower than could be obtained with the temperature of the cooling water available. Non-condensing units could be paired up to three stages without needing a condenser between one of the stages.

As more stages are added to a condensing ejector system, the overall pressure of the first stage becomes lower than the other stage. Compression ratios of the system also increase as more stages are added. In this type of arrangement, the pressure measurement for each stage goes as follows:

Figure 3: Staging Steam-Jet Ejectors.

3. Condensers:

A condenser is used in a steam-jet vacuum system to reduce vapor to its liquid state by removing the latent heat from the vapor. A condenser is placed before an ejector stage to remove condensable vapor from a prior stage, thus reduces the size of the ejector and the amount of steam required for the next stage to handle. The size and type of condenser is determined by the function of the air-vapor ratios, the temperature of the available cooling water, steam and water costs and the amount of contaminants in the first stage vapor.

There are different definitions for condensers according to their locations in the steam-jet vacuum system. Here is a list of the different condensers:

  1. Precondensers: A precondenser is used for direct condensing of vapors from a process. Non-condensable vapors are removed from the pre-condenser by one or more ejector stages. The absolute pressure of the process must be high enough to allow the condensation of the vapor with the available water supply.
  2. Booster Condensers: It is used to condense process vapor and motive steam from booster ejectors. The booster ejectors are used to compress vapors from a previous ejector and lower the absolute pressure of the fluid so the fluid could be condensed in the condenser.
  3. Intercondenser: It is used between ejector stages in steam jet system with two or more stages. The ejector stages are required to compress non-condensable fluids from the process. An intercondenser could also be used to compress the condenser pressure from a previous ejector stage to atmospheric pressure. Thus eliminating the necessity of the next ejector stage to handle the motive steam from the previous stage.
  4. Aftercondenser: An aftercondenser is used to condense steam at atmospheric pressures from the last stage of the steam jet system before releasing the condense steam out of the system. The aftercondenser releases non-condensable steam into the atmosphere.

There are two types of condensers that could be used in any of the above functions.

  1. Direct Contact: A direct contact condenser is an inexpensive way to condense motive steam in a steam jet system. This type of condenser could be made with non-corrosive materials that could be used in a corrosive operation. The condenser has no moving parts, so it requires virtually no maintenance.
  2. Surface Condenser: It permits main-condenser water to cool the motive steam from the ejectors before releasing the fluid out of the system. This type of condenser has a surface area as large as 1,400,000 square feet. Most systems do not need a very large condenser to condense the vapor from the motive stream.

4. Basic construction of the ejector:

Steam jet electors could be made from the following metals and alloys:

For corrosive processes, steam jet ejectors are also made from non-metals and plastics such as:

5. Advantages and Disadvantages:

Some advantage in using an ejector system instead other vacuum and refrigeration system such as mechanical vacuum pumps and refrigerators.

  1. Ejectors could be built from a variety of materials.
  2. Ejectors can be incorporated into harsh environment.
  3. No moving parts to adjust or repair.
  4. High reliability, because of the fact there are no moving parts.
  5. Easy installation, many ejector systems are compact and could be installed easily into an existing system.

Disadvantages are few but notable:

  1. Same with other high vacuum system, plant engineers tend not to have first hand knowledge into troubleshooting problems in the system.
  2. Because many ejectors are custom-built with high tolerances and very accurate dimension, spare parts are not readily available. Reworking of the parts are discouraged because reworking affect the performance of the system. Extra parts must be kept handy in case something happens to the system.
C. Applications for Steam-jet Ejectors:

Figure 4: Vacuum Syatems using Steam-Jet Ejectors

1. Vacuum Refrigeration:

A major application of steam-jet vacuum equipment is vacuum refrigeration in the food and chemical industry. Vacuum refrigeration is when an object for example, fruits and vegetables are placed in water and the water and vegetables mix will cool itself when a vacuum is applied to it. No other refrigerant or additional refrigerating processes are needed after the vacuum freezing of the vegetables. This system could be easily installed where a mechanical refrigeration system is used for the same process. A vacuum refrigeration system could handle the same load as a mechanical system could. The advantages of a vacuum system over a mechanical system is no other refrigerants (may be a harmful chemical) are used in the process. The vacuum system has no moving parts, noise or vibration. Therefore, this system requires little or no maintenance, and processes a product at an overall lower cost than a mechanical system. Some disadvantages are the system requires a large amount of condensed water and steam and in some areas a slightly higher utilities cost. However, the advantages of operating the system outweigh the disadvantages overall. Vacuum cooling can save a lot of money for a company who uses this equipment regularly. There is a larger savings for industries that process corrosive materials, such as phosphoric acid for fertilizer.

2. Vacuum Processing:

Evaporation is the one process that used more single and two-stage ejectors than other processes. Vacuum processing is very common in the food; chemical and other process that require vacuum evaporation systems. Some commodities require a very high vacuum at very low temperature in order to remove moisture from it. A substantial quantity of water vapor is able to be compressed an absolute pressure of .25 inches of mercury to 2 inches of mercury. Multi-stage ejectors are used in this process, with as many as six-stages in some applications. This permits evaporation of water vapor or other from the solid phase direct to the vapor phase at extremely low temperature.

3. Recompression:

Instead of flowing vapor directly to a condenser or a heater, it can be compressed in ejector and permit higher discharge temperatures. This type of ejector is called thermocompressors, but it also overlaps with the term booster ejectors. A thermocompressor is an ejector which handles vapor at a higher than sub-ambient temperatures. This type of system is applied to recompressing spent (waste) steam and low-pressure water vapor in order to recycle a large amount of vapor in order to reduce overall energy consumption. The operating pressure for a thermocompressor is around 15 psig with better efficiencies at lower pressures. Application for thermocompressors include milk evaporation or any application that needs a low pressure steam requirement.

4. Other Ejector Applications:

Other ejector applications include space simulation. An ejector system is used to create simulations in test station of air pressure at different altitude above of the earth. Some ejectors for this application are around 10 feet in diameter. These ejectors handle rocket motor exhaust at various degrees of vacuum, which simulate different altitudes above the earth. A multi-stage system is used in a wind tunnel to simulate wind fiction acting upon an object in high vacuum. Other space simulations also incorporate various multi-stage ejector systems.

Other applications include:

Bilibiography:

1. Steam Ejector Fundamental, Henry E. Huge, July 1998, Chemical Processing, Reprint, http://www.croll.com

2. Key Benefits, Schutte and Koerting, http://www.s-k.com/content/products/vac_sys/key.htm

3. Keep Ejectors Online, Russel Ojala, Croll Reynolds Company, http://www.croll.com/library/articles/vac/a_ejectors.htm

4. Vacuum Systems, Croll Reynolds Company, http://www.croll.com/library/v_ilib/v_iart4a.htm

5. Vacuum Equipment, R.M. Price, University of Mississippi, http://home.olemiss.edu/~cmprice/lectures/vacuum.html

6. A Chilling Idea, Mike Smith, Grimley Smith Associates LTD, http://www.gsa-ltd.co.uk/chilling1.html

Last updated: 03-17-2002 Copyright 2000, 2002 Christopher Pohorence