2 replies to this topic

[AFFAN]

Hi

Usually we use saturated steam for heating purposes, also the correlations for finding the condensing steam coefficients are commonly available.

What about Super heated steam- Water heat transfer Coefficients, are there any different formulas to be used for this purpose, or we should use the simple convection formulas, with different properties

Also is there any rule of thumb values available for Super Heated Steam Heat Transfer Coefficients.

Plz advise...

[vinay]

Dear AFFAN,

My 2 cent worth of opinion is given below :

Normally with steam system most of the heat is transferred by condensation of steam rather than sensible heat transfer ( unless degree of superheat is very high). Normally Kern recommends taking steam side heat transfer coeff as 1500 Btu/hr/ft2/F. Off course you can calculate this too if you want by calculating condensing steam heat transfer coefficient at your pressure /temperature!!!

Logically your method of estimating steam side HTC using properties makes sense but I am not sure it is worth it unless you have high degree of superheat. You can find out this based on calculating the sensible heat transfer from steam by Q1=M*Cp*DT method where DT is the difference between Superheat temp till the saturation temperature at given pressure. Now balance heat transfer is by latent heat (Q2=M*Lambda) , where Lambda is latent heat of steam . Compare the heat transfer by desuperheating steam & the amount of heat transfer by condensing steam , if the ratio is very low you need not worry about it.

In case the sensible heat transfer is a sizeable amount then you need to calculate the heat transfer coeff for desuperheating steam based on vapor properties , calculate the steam & other side heat transfer coeff , estimate the area requirement for the sensible heat transfer .

Then calculate the condensing heat transfer area requiremnt , add up the two areas to estimate total heat transfer area requirement.

I am not an expert on heat transfer , the experts on the forum can guide you/correct me on this opinion of mine. However I thought of putting my opinion on this issue as i am also faced with this kind of situation many time but i had let our software estimate this thing for me.

Regards,
Vinay Mathur

[AFFAN]

Dear Friend

Thks for replying me in good detail

Question infact was that can we apply condensing heat transfer coefficient correlations for super heated steam as well?

or there are some other relations

In my case yes super heated steam heat load is more than 25% of condenation heat load, which usually is considered as mininum critera

But the second factor is wall temperature, and in my case wall temp is far below the saturation temp, so vapors will condesne directly, with out desuper heating

so u see one condition is asking me to do calculate the super heated steam coefficients separately and other one is suggesting me to just ignore it...

also in saturation and super heated zone we use enthlpies and latent heat rather than cp.

plz guide me about sampel cooler design corelations for super heated steam, if u have some references

Regards

Affan




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