superheated steam heat transfer coefficient 2

[jacket46]

I have an application where a client wishes to use superheated(SH) steam in a coil to melt sulfur. We realize that heat transfer from SH steam is inefficient and ultimately the latent heat released by condensing the steam will provide the bulk of the heat required. My problem is that I need to estimate the SH steam heat transfer coefficient in order to determine the transfer area required to cool the SH steam to its saturation point.

 

[Latexman]

jacket46,

I believe this topic has been discussed and argued many times in the past in this forum. Please use the website's search functionality (under the title to this post) or the Google Custom Search (top left side) and you should get many relevant hits.

Good luck,
Latexman

 

[ione]

Here you’ll find some hints

http://www.spiraxsarco.com/resource...iples-and-heat-transfer/superheated-steam.asp

 

[jacket46]

Dear LATEXMAN,

I am new to this process and perhaps am doing something wrong.

I received no hits from web site's search engine.

I received one page of hits from google but they were not relevant.

How do I improve search to target heat transfer coefficient?

Thanks

 

[zekeman]

What form is the sulfur and what is your present method , if any, of doing this melt.
If you are using a dedicated boiler for this, I would consider pumping hot water at pressure to do it, but much depends on the nature of the sulfur and it removal after melting.

 

[speco]

Jacket46,

I will try to answer your question, although similar questions have been discussed in earlier threads. I am assuming that the coil you are describing has the steam flowing through tubes. Generally, the inside (tube) heat transfer coefficient for superheated steam is pretty low, since it can be treated as a dry gas. However, at some point, the tube wall temperature may be at the saturation temperature of the steam. At that point, you can use a condensing coefficient, but the effective steam temparature is the saturation temperature. It all depends on the velocity of the steam, and the temperature of the tube wall, which is very dependent on the temperature of the sulfur around the tube.

Regards,

Speco

 

[rmw]

"Speco" has it just right. The data presented in the link by "ione" will give you some good estimating numbers.

I searched this site (advanced search) on two words, " coefficient & superheat " (without the quotation marks or the ampersand) and got 7 hits.

rmw

 

[Latexman]

Copy the title of your post. Click Search. Paste your post's title into Keywords. Select AnyWords and Subject Lines. Click Search.

Search results say "Hundreds of records matched your query of superheated steam heat transfer coefficient."

Play around with it to narrow your search. Look at the posts with stars first, but don't ignore the ones without stars.

Good luck,
Latexman

 

[ione]

jacket46,

What kind of superheated steam are you talking about (pressure and temperature)?

Sulphur melting point is about 115 °C (a relatively low temperature). The process does not ask for particularly high “wall” temperature, so I cannot really understand the use of superheated steam for this application, unless your customer has got superheated steam already available (power generation purposes).
The use of superheated steam makes the sizing procedure a tougher challenge. The equipment (including control valves) could require more expensive materials.
IMO it worth the effort to evaluate whether desuperheating can be an option for your client. Restoring saturated steam conditions would allow you to deal with convenient stuff.

 

[25362]

Be caredul with the saturated steam pressure because at its melting point the viscosity of the molten sulfur is about 100 cP, but it has the strange characteristic of increasing viscosity as it is heated, reaching a maximum of about 500,000 cP at about 200^oC, falling thereafter, until at the b.p. it becomes again a mobile, dark red liquid as a result of the presence of some % of S₃ and S₄ species.

Although S₈ rings persist up to about 193^oC, the viscosity changes are due to ring cleavage and the formation of catenated species of a length 5-8[×]10⁵ atoms at 200^oC.

 

[ione]

The point raised by 25362 should be taken into consideration. The threshold above which the viscosity of liquid sulphur steeply increases is around 160 °C. Usually liquid sulphur is kept at a temperature around 140 °C as at this temperature is handled more easily (lowest viscosity).