8 replies to this topic

[alphanumerouno]

Hi All,

I need a little help with some desuperheating calculations.

I want to find out the mass of water needed to desuperheat a steam line.

The information I have is:
Steam supply temperature and pressure is 141.5 deg C and 275kPag.
Water supply temperature is 19.2 deg C.
Mass flowrate of steam is 1000kg/hr

I've tried using the enthalpy into process = enthalpy out of process formula

m(cw)h(cw) + m(s)h(s) = m(s)h(d) + m(cw)h(d)

m(cw) = mass of water
h(cw) = enthalpy of water
m(s) = mass of superheated steam
h(s) = enthalpy at superheat condition
h(d) = enthalpy at desuperheat condition

Thank you.

[Padmakar Katre]

Dear,
Check this spreadsheet which could help you.

Attached Files

•  DESUPERHEATER_BOILER_FEED_WATER_REQUIREMENT.xls   25.5KB   269 downloads

[alphanumerouno]

Dear,
Check this spreadsheet which could help you.

Thank you.

But I am wondering if 141.5 deg C is the saturation temperature, how would you determine the superheated temperature for steam at the same pressure of 275 kPag?

[DRS]

Conditions given in your first post are saturated and there is no superheat. So, no desuperheating.

[JoeWong]

But I am wondering if 141.5 deg C is the saturation temperature, how would you determine the superheated temperature for steam at the same pressure of 275 kPag?

If you wonder, please check the steam table.

Conditions given in your first post are saturated and there is no superheat. So, no desuperheating.

In my opinion...the saturation temperature for steam at 275 kPag is about 131.8 degC. The steam temperature is at 141.5 degC. It is superheated steam.


The following posts could be useful :

• Square-root-Square-root Formula Ease Saturated Steam-Codensate Temperature Prediction
• Conduct Steam-Water Balance MANUALLY using Water97_v13

[DRS]

In my opinion...the saturation temperature for steam at 275 kPag is about 131.8 degC. The steam temperature is at 141.5 degC. It is superheated steam.

Is there a possibility that your steam tables have pressure in absolute terms and you are using gauge terms? I am using steam tab which is based on IAPWS formulae.

[Andree]

In my opinion...the saturation temperature for steam at 275 kPag is about 131.8 degC. The steam temperature is at 141.5 degC. It is superheated steam.

Is there a possibility that your steam tables have pressure in absolute terms and you are using gauge terms? I am using steam tab which is based on IAPWS formulae.

simple energy balance: M_s*cp_s*(141.5-T_s) = M_w*cp_w*(T_s-19.2)+M_w*r_w
and solve for M_w

M_s - mass flow of superheated steam
cp_s - specific heat capacity of superheated steam (average in temp range from T_s to 141.5)
T_s - saturation temerature of steam at 275kPa
M_w - mass flow of water
cp_w - specific heat capacity of water (average in temp range from 19.2 to T_s)
r_w - heat of vaporization of water at T_s and 275kPa

[JoeWong]

In my opinion...the saturation temperature for steam at 275 kPag is about 131.8 degC. The steam temperature is at 141.5 degC. It is superheated steam.

Is there a possibility that your steam tables have pressure in absolute terms and you are using gauge terms? I am using steam tab which is based on IAPWS formulae.

Opps...Not really the gauge and absolute terms. I key in 2.85 bar abs instead of 3.75 bar abs...

My apologize. My mistake. You are right. I withdraw my comment.

Thanks for knocking my head for this silly mistake...

[Padmakar Katre]

Dear,
Check this spreadsheet which could help you.

Thank you.

But I am wondering if 141.5 deg C is the saturation temperature, how would you determine the superheated temperature for steam at the same pressure of 275 kPag?

Dear,
Please let us know that what is the final temperature of steam after PRDS(Pressure Reduction and Desupeheating station)? The reason to have a PRDS is to let down the steam from high pressure to Medium or Low pressure as per the requirement in the equipments in the unit like reboilers and subsequent desuperheating is done inorder to avoid the steam losses wherever saturated steam is required at the relative pressure. In your case there is hardly any or no degree of superheat. Already your steam is saturated steam. So first of tell us that what you want to desuperheat by using the desuperheating water.
Now moving to your second query how to calculate the superheated temperature the steam comes from the Boilers which have the fixed temperature of steam depending upon typy of boiler you have. The outlet temperature of the steam from Boiler is adjusted with the help of BFW injection in the attemptators of the boilers.
The spreadsheet which I have uploaded my prvious post you could use when you have some degree of superheat which you to remove by using the desuperheating water. Hope this could make some sense.