Intro To Vacuum Systems
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This was one of the very first CBT modules I developed. This presentation was imported into Captivate, where additional features such as mouse-over definitions were added.
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Intro To Vacuum Systems
- 1. FACT Vacuum Systems Introduction Vacuum System Components & Operation
- 20. Liquid Ring Vacuum Pump
- 45. Types of Steam Jet Ejectors Cooling Water High Pressure Steam Suction Connection Optional Liquid Ring Vacuum Pump
- 53. Condensers High Pressure Steam Process gas Intercondenser Cooling water
Notes de l'éditeur
- Welcome to FACT Vacuum Systems: Introduction to vacuum system components and Operation This module was developed specifically for the individuals working at the Cognis-OleoChemcials Plant in Cincinnati, OH. The purpose of this module is to provide an introduction to vacuum, equipment used to produce a vacuum and how the equipment operates. If you have any questions about any of the information presented in this module, contact your trainer. Click Start to begin.
- This module will discuss the equipment used to establish a vacuum and the principle of how the equipment operates. We will start with an overview of vacuum and then discuss specific equipment and how it works.
- Let’s go over some instructions for features you can use while watching this module. If you see a word written in bright blue text, that means there is a definition available. Use your mouse to move the cursor arrow over the pink box and the word's definition will appear. There is no need to click. Give it a try on this screen. At the bottom of the screen, you'll see a navigation bar. This bar works just like the controls on a VCR or DVD player. Click on the appropriate button to pause, rewind or forward. To make it even easier, you may click anywhere on the slide to continue to the next screen. To review the previous screen, simply click on the Back button. On the right side of the bar, you'll see a button that looks like a speaker. If you click this button, you can mute the audio and just read the information, if you prefer. To exit the module, close the browser window by clicking the X at the top right corner of your screen. At various points, you'll may see a white button with a procedure name like the one shown here. These buttons will link you to another document or procedure. Just click on the button, then click Open. The document will appear. When you are finished reviewing the document, click the red X at the top of the screen to close the document and return to the module. Try this link to see how it works. The audio for that slide will continue to play even if you open a document.
- In this first section you learn: The definition of vacuum and how operating under a vacuum changes the physical properties of a material; types of equipment that can be used to produce a vacuum and different industrial applications of vacuum and how vacuum provides a benefit to those operations. Let’s get started!
- What is a vacuum? Vacuum is considered any pressure below atmospheric pressure or pressure below 14.7 pounds per square inch absolute or below 760 mm HG. Vacuum is typically measured in units of mm of Mercury, inches of water or Torr. A strong or high vacuum is well below atmospheric pressure A weak or low vacuum is slightly below atmospheric pressure Atmospheric Pressure - Pressure on a planet's surface caused by the amount of gas in the atmosphere. The atmospheric pressure at the Earth's surface is 14.7 psia.
- Using a vacuum has many industrial applications, but the most common use is distillation. Because some products will degrade at higher temperatures, it is necessary to a use separation method at lower temperatures. By lowering the operating pressure below atmospheric pressure, the boiling point of the components will be lowered and separation can be accomplished without damaging product quality. Polymerize - To join together many small molecules called monomers to form giant molecules, called macromolecules or polymers.
- Some more examples of industrial application of vacuum include, solid-liquid separation and drying of solids. Using vacuum to separate liquids from solids allows for the operation to be continuous verses batch. It will also reduce the amount of time it takes to separate the solids from the liquid. Vacuum can also be used for drying products. Operating under vacuum minimizes the drying temperature and avoids thermal degradation of heat sensitive materials.
- In order to establish a vacuum in a system, mass must be removed. The majority of the mass that is removed is air and steam along with some process vapor. The more mass that is removed, the higher the vacuum and the lower the system pressure.
- Pulling a vacuum on a system can be accomplished using one or more vacuum pumps. Types of vacuum pumps include mechanical, high and ultra-high vacuum and venturi jet (also called a jet ejector)
- This table provides a comparison of different units of pressure measurement. Atmospheric pressure was used as an example and anything below that is considered a vacuum. For example, 14.7 pounds per square inch absolute is equal to 760 Torr and 407 inches of water.
- The information provided during the training will cover liquid ring vacuum pumps, steam jet ejectors (also called venturi jet pumps) and a system that uses a combination of steam jet ejectors and liquid ring vacuum pumps, known as a hybrid system. Let’s try a few questions for review.
- Let’s try a couple of questions for review. READ THE SLIDE
- Read the question.
- First we will discuss the Liquid Ring Vacuum Pump
- After completing Section 2, you will be able to identify components of a liquid ring vacuum pump and their functions, how a liquid ring vacuum pump operates, the functions of the seal liquid and why the temperature of the seal liquid is important. We will also discuss different configurations of the liquid ring vacuum pump.
- The liquid ring vacuum pump is the most widely used vacuum pump in the chemical process industry. Its design is simple and efficient. The liquid ring vacuum pump has one rotating part – an impeller that is offset from the center of the pumping chamber. The pump housing is partially filled with a liquid, typically water that during operation will create a liquid ring around the outside edge of the pump wall.
- The liquid ring will be equally dispersed around the outside wall of the pumping chamber to form a seal inside the pump. The area between the impeller blades is known as the impeller cell. Since the impeller is offset from the center, the degree to which the impeller is submerged in the liquid varies. This causes the volume of the impeller cell to decrease as the impeller rotates.
- The gas is sucked in as the volume of the impeller cell increases. The liquid ring traps and seals the gas inside the pump. As the impeller rotates, the volume of the impeller cell becomes smaller and the gas is compressed. When the gas is discharged a portion of the liquid is discharged also.
- This is another view of what happens to the gas inside the liquid ring vacuum pump. Gas molecules are drawn in at the suction, indicated by the yellow arrow, and then trapped by the sealing liquid. As the impeller rotates, the gas is compressed and then discharged, indicated by the red arrow, from the pump along with a portion of the sealing liquid at a pressure higher than at the suction. Remember, the gas is mostly air & steam.
- This diagram shows where the gas enters the pump and where it is discharged. Since a portion of the sealing liquid is discharged along with the gas, it must be replaced. The vacuum pump has an inlet for the constant replacement of the sealing liquid. The liquid ring provides two functions, it forms a seal to trap the gas and it also absorbs heat produced in the pump.
- Used alone and operating at maximum capacity, a single water ring vacuum pump will only be able to obtain a vacuum of 40-60 mm of mercury or 40-60 Torr. Let’s try a few questions about the liquid ring vacuum pump.
- READ THE SLIDE
- Read the question
- If the temperature of the sealing liquid reaches it’s boiling point, cavitation will occur. It is important that the make-up sealing liquid temperature is cool enough to prevent cavitation. Make-up refers to replacing what has been consumed by the process or removed from the process. Cavitation - A general term used to describe the behavior of gas or bubbles in a liquid occuring in a pump, propeller or impeller.
- A liquid ring vacuum pump can be configured in 3 ways. Once through or no recovery, where the sealing liquid is only used once and discharged. Partial recovery, where a portion of the sealing liquid is recovered and reused, and closed loop or total recovery, where the entire amount of the sealing liquid is recovered and recycled back to the pump. All liquid ring vacuum pump configurations contain 4 elements. (1) A source for the seal liquid, (2) A regulating device, such as a valve to control the flow of seal liquid, (3) A component to stop the flow of liquid when the pump is turned off and (4) A device to separate the seal liquid from the gas.
- One advantage of the Once-through configuration is that the seal liquid is completely replaced, so equipment to cool the seal liquid is not required. The disadvantage is the cost associated with replacing and discharging the seal liquid.
- With partial recovery, part of the seal liquid is recovered and mixed with fresh seal liquid. The amount of fresh seal liquid added is equal to the amount that is discharged. Make up - Material added (as in a manufacturing process) to replace material that has been used up or removed.
- This configuration recovers the entire amount of seal liquid discharged from the vacuum pump. The gas is separated from the liquid and then the liquid is cooled before being returned to the vacuum pump. This arrangement is the best option when the seal liquid may become contaminated.
- After the seal liquid and any vapor that mixed with the seal liquid leave the liquid ring vacuum pump, they are sent to a discharge separator. A non-condensable gas is a gas that will not undergo a phase change, even though other gas is condensed in the same location. These vapors exit the top of the separator and are discharged to the seal pot. The seal pot is located well below the separator. Non-condensible - Incapable of being liquefied; not condensible
- Liquid from the discharge separator will overflow to the seal pot. A liquid level is maintained inside the separator to prevent loss of vacuum. For units at the Cincinnati site where methanol is NOT present, the seal pot will overflow to a wastewater collection sump. If the potential for methanol contamination exists, the liquid in the seal pot is pumped to a methanol recovery system
- In summary, A liquid ring vacuum pump is used in industrial applications to create a vacuum in a system. The seal liquid traps the process gas and absorbs heat produced inside the pump. The seal liquid temperature must be maintained below it’s boiling point to prevent cavitation.
- Let’s try another question to check your understanding. READ SLIDE
- In Section 3, you will be presented information about Steam Jet Ejectors.
- By completing this section you will learn the parts of a steam jet ejector and how those parts work together, advantages of using a steam jet ejector, and the four types of steam jet ejector systems.
- A steam jet ejector, also called a venturi pump is another simple device used to produce a vacuum. It has no moving parts, thus there is no chance for vibration. It is virtually maintenance free and simple to operate. Steam is used as the “motive force” or in other words steam is used to produce or cause motion. Motive power - Something, such as water or steam, used to cause motion.
- As indicated by the graphic, the four basic parts of all steam jet ejectors are: (1) the steam chest (2) the steam nozzle (4) the mixing chamber and (5) the diffuser. Area 3 is where the gas from the process is drawn in to the ejector. Next we will discuss in detail how the steam jet ejector works.
- High pressure steam provides the energy for the ejector to operate. It enters through the steam chest (number one on the graphic), and when the steam passes through the nozzle (number 2) it expands into the mixing chamber (number 4). Recall from your previous training on “Plant Science” that as a gas expands into a larger volume area, the pressure of the gas decreases (volume increases/pressure decreases). Also recall that energy can NOT be created or destroyed, so the pressure energy of the steam is converted to velocity energy OR the speed at which the steam is moving will increase.
- The increased velocity energy causes the motive action of the steam. More simply stated, the steam causes the gas to “move” into the suction area of the ejector, labeled number 3 on the graphic. The vapor from the process stream mixes with the steam in the mixing chamber, Area 4. The vapors are compressed and discharged from the diffuser, Area 5 at a pressure higher than the pressure of the process gas, but lower than the inlet pressure of the steam.
- Let’s try a question about the steam jet ejector. READ SLIDE.
- There are four basic configurations of steam jet ejectors. Single stage, multi-stage with out condensing, multi-stage with condensing and a combination of condensing and non-condensing stages.
- A single stage ejector is very simple. Only one ejector is used and the discharge pressure is equal to or near atmospheric pressure. The capacity of the steam jet ejector is determined by it’s internal dimensions, so the throughput of a single ejector is limited.
- This configuration has the steam jet ejectors directly connected to each other. The discharge from the first steam jet ejector is feed for the second stage. The initial cost for this type of system is low, since condensers are not used. However, the operating costs are high due to a need for a large amount of steam.
- Ejectors that use an intercondenser, which is a condenser located between ejector stages, are called condensing ejectors. These systems require cooling water for the condensers, but require less steam to operate than non-condensing units. The ejectors can also be smaller due to the condensing of the gas.
- Here is a schematic of a four stage condensing steam jet ejector system. The liquid ring vacuum pump is optional and is often used during start-up to assist with initial evacuation of the system.
- This configuration uses two steam jet ejectors connected directly to each other followed by a condenser and another steam jet ejector with a condenser. The operating pressure of the first two ejectors may be too low for the gas to condense using water as the cooling medium.
- Let’s try another question. READ SLIDE
- When condensers are used, they are an integral and important part of the vacuum system.
- In this section, you will be provided information about the function of the condenser in a vacuum system, how the vapor load to the condenser is different to the vapor load to an ejector, names of condensers based on their location in the system, how the operating pressure of the condenser affects the ejector discharge pressure and why that is important. We will also discuss 2 specific types of condensers and how they differ.
- The condenser is an important part of many vacuum systems. The condenser reduces the amount of gas the next ejector must move. In fact, the vapor load to the condenser can be 10 times that to the ejector.
- A condenser is used to condense gas and vapors to the liquid phase. Recall that a lowered pressure will lower the boiling or vapor point of components. The temperature of the cooling water will limit how much vapor will condense.
- The operating pressure of the condenser must be high enough for condensation to occur at the temperature of the cooling water. Most vacuum system applications only use cooling tower water for condensers and this limits the operating pressure. A refrigerated water or glycol type of system could be used for condensation, if the application required. However, this type of system requires a great deal more energy to operate. The condenser operates at the discharge pressure of the upstream ejector and the suction pressure of the downstream ejector.
- The intercondenser operates at the discharge pressure of ejector number 1, which is also the suction pressure of ejector number 2. So the operating pressure of the intercondenser is equal to the suction pressure of ejector number 2 and the discharge pressure of ejector number 1. This operating pressure must be high enough for the phase change of the gas to occur at the temperature of the cooling water being used.
- Let’s try a question about condensers. READ THE SLIDE
- An ejector system can use a variety of condensers. These condensers are named based on their position in respect to the ejector.
- A precondenser is a condenser placed before the ejector system, but is considered part of the ejector system. A precondenser is preferably attached directly to the vacuum vessel in order to minimize any pressure drop. Again the operating pressure is important for condensation to occur at the temperature of the cooling water supply.
- An intercondenser is positioned between ejector stages. It is used to reduce the amount of gas the downstream ejector must handle. The first intercondenser is the most crucial to obtaining the desired vacuum. The operating pressure of an intercondenser is related to the maximum temperature of the cooling water.
- An aftercondenser is used after the last ejector stage and operates at atmospheric pressure. Aftercondensers do not improve the efficiency of the ejector system, but are used to recover the heat of the steam, to recover condensate and to minimize air emissions and odors.
- There are two categories of condensers used in vacuum systems. One is the direct contact or barometric condenser. With the barometric condenser the cooling water mixes with the vapor stream.
- The other type of condenser used in vacuum systems is the surface contact or shell-and-tube. With this type of condenser the vapor stream does not mix with the cooling water. Construction of the shell-and-tube condenser is similar to a shell-and-tube heat exchanger; however, the internal design differs significantly due to the presence of two-phase flow, non-condensible gas, and vacuum operation. The vapor stream is almost always shell side to minimize the pressure drop.
- Let’s try another question about condensers. READ SLIDE
- Operating at a pressure less than atmospheric will lower the boiling points of components; allowing for separation using distillation to occur at a lower temperature. This helps to maintain product quality and prevent degradation. We have discussed some of the equipment that can be used to obtain a vacuum and how they operate, including liquid ring vacuum pumps and steam jet ejectors. Condensers are often used as part of the vacuum system and correct operation of the condensers is just as important to obtaining the desired vacuum as correct operation of the steam jet ejectors and liquid ring vacuum pumps. In the next module we will discuss performance of the vacuum system and the process variables that affect performance.
- Now that you have completed this module you are required to pass a post test. You can access the post test by logging into NCMS. When the course list appears, select the button that says e-test. In the field labeled course, choose vacuum systems then select one of the two Introduction to vacuum systems tests. Each test consists of 20 questions and a score of 80% is required for successful completion. If you have any problems or questions, please contact your trainer.