10 replies to this topic

[pleckner]

This can take a rather long response but I'm going to make it quick. Let's see if anyone else wants to add to what I'm going to say.

A vacuum is simply pressure less than atmospheric. If you are at sea level, atmospheric pressure is 14.7 psia or 0 psig (gage). In a closed vessel, if you make the pressure less than this, say 13.7 psia or -1.0 psig, then you have a vacuum.

Vacuums are produced in a number of ways. The concept is to pump out the environment. If this vessel is filled with air, then by pumping out the air you create a vacuum. You can use various mechanicl devices to pump this out such as vacuum pumps, which are really compressors. You can use an eductor (kind of a rather large converging/diverging nozzle) where a motive fluid, such as steam, flows through at a high velocity and this produces a very low pressure area within the throat (the point where the convering/diverging sections intersect) of the ejector and sucks the air out of that vessel.

Advantages, well we use vacuums when we need to process something that might be temperature sensitive. At pressures of atmosphere or higher, the necessary operating temperature might be harmful to whatever is being processed. Using a vacuum allows us to perform the same processing but at lower temperature. Think about boiling water at seal level (212 F) versus boiling water in a city such as Denver, Colorado (somewhere about 5000 feet above seal level). Water boils at a significantly lower tempeature (can't remember what).

Disadvantages, well, the equipment we use can be rather expensive compared to a similar piece at a low positive pressure. They must be desgned to take into account the working vacuum. It takes a fair amount of energy to produce the vacuum, which translates into cost. And vapor/gas volumes are greater in vacuums so the piping associated with these systems are larger.

I hope this helps in giving you some basic understanding. Now use the Internet and do a search on vacuum and plan on spending the better part of the rest of your life reading all the hits you are going to get.

[aliadnan]

Hi
I have another question about the vacuum. How we maintain a constant flow throught a vacuum system.

[djack77494]

The simple answer to the last question is to exhaust the same quantity of fluid as is entering your system. This maintains steady conditions in your vacuum vessel. There are a variety of devices capable of performing this task.

[aliadnan]

Hi djack thanks for you reply . Please can you tell me the name of some of the devices used for this case and explain there working a little . Any reference from internet site can be provided so that I can read and understand it from there
Thank you
Regards

[djack77494]

aliadnan,
The hardware solution to the problem of maintaining the desired level of vacuum, like most engineering problems, will depend on many factors. Perhaps the most obvious is size. For very small systems, high vacuum can be achieved using laboratory vacuum pumps. (They're really compressors though they're called pumps.) Most mechanical vacuum pumps are positive displacement devices that basically trap a volume of the gas that will be exhausted and then "mechanically push" the gas out of the vacuum system. (Sort of like a car engine's piston/cylinder arrangement.) For large industrial systems, steam jet eductors are common. These work by converting the pressure energy of a motive fluid to high velocity but low pressure. Gases in the system being exhausted are then drawn into region of low pressure and rejected from the system. There are other entirely different approaches to the problem of how to produce and maintain vacuum, but I believe the vast majority fit into one of the above broad catagories. You will note that both concepts can provide devices that are able to continuously exhaust a given quantity of fluid from the vacuum system against a continuous vacuum.

I don't know if I've really said much more than Mr. Leckner. For more information, follow his advice and turn to the internet. For more specific hardware information go to the websites of the suppliers of vacuum producing equipment. Some have excellent tutorial information about vacuum systems. Do a google search for Croll Reynolds, Graham, and other major vendors.
Hope that helps,
Doug

[aliadnan]

Hi all
Is is possible to create vacuum in a tank containing water at a height of 33.91 ft above the ground by draining the water from the bottom of the tank under the action of gravity. The empty tank will be in vacuum when the water is fully drained? Please comment on this.

[djack77494]

Aliadnan,
Yes it is possible to generate a vacuum by filling an air-tight vessel with a liquid (water in your question) and then gravity draining the liquid. I think, however, that your question is more oriented to a total vacuum, since you specified a water height that would be needed to generate a full vacuum. In a system such as you describe, however, your main limitation will be the vapor pressure of the liquid. If your tank were filled with water at 80 degrees Fahrenheit (about 27 degrees Celcius) you would be limited to the vapor pressure of water at that temperature. That would be 0.507 pounds per square inch absolute (psia), and you could not generate a stronger vacuum than this.
Doug

[aliadnan]

Hi Mr. djack
Thank you very much for your reply. Now I have another question , you have said

If your tank were filled with water at 80 degrees Fahrenheit (about 27 degrees Celcius) you would be limited to the vapor pressure of water at that temperature. That would be 0.507 pounds per square inch absolute (psia), and you could not generate a stronger vacuum than this.

My question is that why the vacuum is dependent on the vapor pressure of the liquid (in this case water) & why we cannot get a vacuum stronger than the vapor pressure of the liquid at corresponding temperature. Please explain it a little since I don't have idea about it.
I will be very gratefull to you

Regards

[djack77494]

If you consider the operation of an industrial scale steam jet vacuum generation system, you will note that:
1) The exhaust from the steam jet typically flows to a condenser.
2) Cooling water in the condenser condenses the entering vapor mixture.
3) Usually, the entering vapor mixture is predominantly steam.
4) You can only condense the steam to the point where its vapor pressure is reached.
You are unable to condense to a lower pressure because, below its vapor pressure,
the condensed water will re-evaporate confounding your efforts to achieve lower pressure.
5) Beyond that, you cannot condense additional steam.
The above is true whether you use a direct contact (barometric) condenser or an indirect condenser. The only thing different is the approach to equilibrium.

If this is a bit hard to grasp, consider if you had a tank full of 80 degree water. As you reduced the pressure of the tank lower and lower, when you reach 0.507 pounds per square inch absolute (psia), any further attempts to lower the pressure will just cause the water to start boiling. You could not decrease the pressure below 0.507 psia until you had boiled away all the water or reduced its temperature further.
HTH,
Doug

[Art Montemayor]

Phil and Doug:

This has little to do with this thread's subject, but what the heck........

Nice to see Phil writing in to the Forum with his quality responses; Doug, that's some good shortstoping glove work on those vacuum queries.

Guys, I'm enjoying a very quality Christmas and New Years with my beautiful grandchildren here in Tucson and have only had time to read queries and responses from others. I want to wish you all a happy and prosperous New Year --- and may your days of health and meritorious rewards be continuous and enjoyable.

Chris, Milt, Guido, etc., etc., etc., I include all the guys & gals that have made this Forum so important and rewarding for students and would-be future engineers.

A toast to you all !!!

Art Montemayor