9 replies to this topic

[Nikunj Gangar]

why is it that steam is always generated at high pressures and use at low pressures??

[pavanayi]

Nikunj,
That is a very broad and wrong generalisation you have taken/ someone else has told you. What prompted you to write the question?

[Art Montemayor]

Nikunj Gangar:

Careful! What you say isn't true.

Perhaps you use the term "always" loosely; but nevertheless, steam is generated at a certain pressure level for a very specific reason. Can you give us some specific cases that represent what you state?

[breizh]

Hi ,

I would encourage you and others to take a look at this link :
http://www1.eere.ene...pubs_steam.html

A lot of great information about steam.

Breizh

[Art Montemayor]

Breizh:

My compliments and Kudos to you for presenting such valuable and fundamental information to our Student members.

I strongly recommend every engineering student reading this thread to download ALL the information available in this website regarding steam and steam systems.

It is a deplorable shame that most (if not all) university engineering programs regard this type of learning as too basic for engineering study and exposure in their engineering programs. Most universities regard this as "stuff" meant only for "technicians" and not worthy for engineers. I am - and have been- a strong opponent of this type of "elitist" attitude because it deprives a future engineer of have the very basic of all energy tools available to him/her the moment they step into the real world. Steam is - and continues to be - the basis of most of the practical energy generated and consumed in the industrial world. It will be the first - and foremost - thermodynamic process encountered by the young engineer the moment he/she steps into a responsible industrial job.

I hope the students reading this recognize the importance and give you their respective thanks.

[Nikunj Gangar]

thank you sir for providing the link....but i am unable to open the link...do u have a copy of the page? tks,

nikunj

[kkala]

I cannot open the link by breizh, either; I will try it when home, having a broader bandwidth there.
A simple answer to the query could be that it is possible to pressure reduce steam, as it is usually realized in desuperheaters. The oposite is not practically possible.
A generous superheat would permit some compression at expense of power, but compression would be very limited for usual superheat levels; and power is more expensive than fuel used in boilers. So it looks preferrable to produce steam at the highest level needed and pressure reduce some of it either through back pressure turbines or desuperheaters. This general practice met in industries does not exclude separate production of a lower pressure steam. But the latter occurs rather seldom.

[breizh]

I've no problem to open it !
Breizh

http://www1.eere.ene...pubs_steam.html

[tarafdar]

Higher steam pressure has the following benefits:
1.Steam pressure plays an important role when trying to maximize the energy trasfer of steam.The higher the steam pressure, the higher the temp. and total energy content of a unit of steam.
2.As pressure increases, so does temp.Since temp. differences governs heat energy transfer, the higher the temp.,the easier it is to transfer heat.
3.Due to high pressure smaller dia steam mains are required,resulting in lower capital cost,for materials such as pipes,flanges,supports,insulation.
4.In plants where steam usage takes place at many different pressures,pressure reduction allows generation of steam at a single high pressure and subsequent reduction to the desired pressure at the point of usage.

But selecting higher than required steam pressure to a process application will result in a number of -ve effects.
1.High steam pressure have a higher steam temp. but is a lower usable (latent) energy.
2.A rule of thumb,a 10 psi drop in steam pressure can lead to a 1% reduction in energy costs.

Using the lowest effective steam pressure to achieve the process objective will result in the following:
1.Lower steam consumption.
2.Lower thermal expansion of components .
3.Lower flash steam volume,paricularly at the discharge of the traps.
4.Better temp. control, due to the closer proximity to the process heat sink.
5.Longer equipment life.

[kkala]

I've no problem to open it ! Breizh

I can open the link today, being actually very useful. Since I am using a mobile connection here, the mentioned previous problem might be to heavy web traffic in week end. If one tries during "quiet hours" the link will be probably accessed.
In addition to references on steam, a book of good fame (at least here) is "The effective use of steam" by Oliver Lyle, His Majesty's Stationery Office (Hmso), 1947 (my impression is that newer editions to 1972 are just reprints). Too detailed for students (intending to understand principles), yet useful to anyone looking for practical knowledge (Operations, Foremen, detail design on steam systems).
Besides leaflets of steam trap suppliers (e.g. Spirax Sarco, Armstron), now found in the Web, contain useful information.

Edited by kkala, 12 September 2011 - 09:44 AM.