Steam flow calculation 2

[f1ori]

I have a question.

I need to size a steam flow control valve at a pre-heater. The only information I have are: steam inlet pressure (337 psig), steam inlet temp. (290C), product density (1.010 g/cm3)and product specific heat (2.46 J/g).
How can I calculate the steam flow throught this pre-heater?

 

[maddocks]

What's the duty? Is the steam condensate exiting at saturated conditions? Throw us a little more info....

 

[JimCasey]

Yep, Maddocks is right.
Aside from small losses through the insulation, the energy taken from the steam will go into the product.

You need the mass flow rate of the product, the inlet temperature, and the desired outlet temperature. You already have
THat gives you the specific heat but I think you meant the units to be j/g degreeC.
so X grams per hour x 2.46 J/g C x Y Degrees C temp rise gives you how many joules you need from the steam.

Assume the steam is condensing (constant temperature process) and it is unlikely that it will be cooled to a temperature cooler than the discharge temperature of the product. Enthalpy of the inlet steam (dry, I hope) minus Enthalpy of the discharge condensate gives you how many joules per gram of condensed steam. Match the joules needed for the product to the joules supplied by the steam and that tells you the grams per time of the steam required.

 

[f1ori]

Yes, I didn't include the steam condition. I am sorry. The steam is a superheated steam, and the temp for the product are: inlet product (45C) and outlet(225C).

JimCasey, I am not sure if I understood.
steam mass = (inlet steam enthalpy)- (outlet steam enthalpy)/(energy required to heat the product).

Can you please correct me?

Thank you both.

 

[f1ori]

Can I have a little more explanation on this calculation, please?

 

[Homayun]

You need to first calculate how much steam is needed to heat up your stream from 45C to 225C (exchanger duty). What is confusing is that your Pressure unit is in PSIG but your Temperature unit is in SI!

After you have established what the duty is, then you simply divide that duty by the Condensation Enthalpy of your steam. It's that easy...But you also need the flowrate of your process stream...

Q (duty) = Mass flowrate x Heat Capacity x (T2-T1)

Steam flowrate = Q / Steam Condensation enthalpy

 

[TBP]

If you're taking saturated steam, and reducing the pressure, then the bit of superheat you get is typically of no consequence. If you're dropping the pressure on superheated steam, you'll likely have trouble. You very much do NOT want any amount of superheat for heat exchange applications. You have to get past the bit of heat that is superheat to get to the latent heat, which is where almost all of the action is. Until you get past the superheat, your heat exchanger will act as if it is airbound.

In any event, to determine the amount of steam required, you need: cold side inlet & outlet temps, and the fluid characteristics. That will give you the heat input required per hour. Once you have that value, you only need the steam input characteristics and that will provide the steam flow number. From there, it's only a matter of looking at the various valve vendors catalogues or sizing programs.

 

[TBP]

I don't believe I included mass flow for the cold side fluid in my previous post. I should have. Sorry.

 

[f1ori]

I would like to thank y'all. It helped a lot.

 

[Drexl]

Hi,

337 psig = 23.24 bar(g)
Saturation temperature at 23.24 bar(g) is 222 degC.
If your steam is 290 degC it is almost 70 degC superheated and not suitable for heating unless you are inserting the steam into the other medium.

 

[fronten]

Hello Drexl,

Can I ask why you say that 70 degC of superheat is not suitable for heating with out direct injection?

 

[MJCronin]

Drexl is correct..... steam with 70C degrees of superheat is not suitable for process heating.

In order to use that particular steam source, you must either 1) install a massive HX with adequate area that will bring the steam down to saturation, then heat the process stream or 2) install a de-superheater.

http://www.eng-tips.com/viewthread.cfm?qid=198506&page=30

See page five of the following document:

http://www1.eere.energy.gov/industry/bestpractices/pdfs/heattransfer.pdf

-MJC