Boiler superheaters 1

[Fahad3608]

greetings all,

It is my pleasure that I joined a such excellent community for chemical engineers and knowledge sharing.

Regarding the natural circulation boilers, I am just wondering, why there are two super heaters and water spray valve is located between them ? is one superheated not enough to handle the system ?

Thanks,

 

[MJCronin]

The questions that you seem to be asking are:

- Why do boilers require desuperheaters ? (they are also called interstage attemperators)

- Why are their two segments to the superheater part of a boiler with a desuperheater between ?

Desuperheaters are required in boilers to control final temperature at partial loads.

They have other uses in steam distribution systems just upstream of heat exchangers that operate better with saturated steam.

http://www.s-k.com/pages/pro_08_00.html

There is an excellent discussion of desuperheaters in the classic "Steam" by B&W



MJCronin
Sr. Process Engineer

 

[crshears]

The OP seems to assume that there are ALWAYS two superheaters; in some applications there don't need to be...but where there are, the general reason is to allow quite precise final steam temperature.

Saturated steam cannot be attemperated; it has to be somewhat superheated to start with...but if there were only one superheater it would be necessary to superheat the steam well above the desired final temperature before attemperation. This would work, but a single superheater like that would have to be made of more expensive materials to withstand the higher operating temperatures.

Bottom line: add some initial superheat to the steam from the drum, then attemperate as required before admission to the second superheater so as to achieve the desired final temperature. Result? Precise steam temperature control without having to use any more expensive of a superheater material than necessary.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Fahad3608]

many thanks gents.

 

[SepiaTones]

Superheaters can have anywhere from one up to (I've seen) five stages. Not each stage requires an attemperator. For example, a single stage will not have any attemperator, that would make no sense at all, for reasons given by crshears. The fact that there is necessarily a stage after the final attemperator makes the temperature control more difficult, and not as good as could be obtained by having the attemperator after the final S/H stage. The dead time between water addition and steam temperature measurement further hampers control. Within +- 5°C would be good control.

In addition to compensating for partial load as mentioned, many boiler tend to foul over time, affecting the S/H outlet temperature. Attemperation is useful in smoothing this out.

 

[waqasmanzoor]

Well in my view, an attemperator though solves the problem of controlling steam temperature, but it is this point where undissolved solids are also added to steam which cause impingement on steam transportation piping and other equipment consuming steam including steam turbines' rotor.

We had a very bad experience of a damaged rotor of a 1.5 MW steam turbine due to low quality steam at 360 C and 38 barG.

Steam quality does matter guys

 

[Fahad3608]

I second you