5 replies to this topic

Here's one weird question as follow:
when 1 kg/hr 11kg saturated steam is reduced to 6kg through a PRV ,
What is the mass flow rate of saturated 6 kg steam?

known parameter:
11kg(G) STM =>sensible enthalpy=189.9 kcal/kg
latent enthalpy=474.6 kcal/kg
6kg(G) STM =>sensible enthalpy=165.7 kcal/kg
latent enthalpy=493.8 kcal/kg
my idea is as follows
assume if 11K saturated STM->6K saturated STM completely
according to mass balance,we will get 1 kg/hr 6K saturated STM
but in the view of enthalpy balance, it is apparently wrong because 1*(189.9+474.6) ≠ 1*(165.7+493.8)
this assumption is wrong

so I assume that after pressure reducing to 6 kg,
we'll get x kg/hr saturated steam and y kg/hr saturated liquid water
by mass and enthalpy balance
mass balance: x + y = 1
enthalpy balance: (493.8+165.7)*x + 493.8*y = 1*(474.6+189.9)
x=1.03 kg/hr
y=-0.03 kg/hr
the result makes no sense,too
Is anything wrong here?
thanks!!

[rabindra]

You will get Superheated steam. Since the pressure reduces, the saturated enthalpy of the steam decreases. Decrease in enthalpy is used to superheat the saturated steam at 6.0 kg/cm2. You can calculate the delta increase in temperature by dividing the "Saturated steam enthalpy difference at the two pressures" by "Specific heat of steam at 6.0 kg/cm2".

Hope the above clarifies your doubt.

[Art Montemayor]

Sorry to say this, but the statement and question:

"Here's one weird question as follow:
when 1 kg/hr 11kg saturated steam is reduced to 6kg through a PRV ,
What is the mass flow rate of saturated 6 kg steam?"

-- Are not "weird"; they are logically wrong.

When one expands 1 kg/hr of 11 kg/cm²g saturated steam to a pressure of 6 kg/cm²g of saturated steam, the result is 1 kg/hr of superheated steam - not saturated steam. This is all clearly seen on the steam Mollier Diagram.

[breizh]

Take a look at this document ( in french ) - in particular to slide B16 diagram T-S. The Mollier Expansion at J = cte; i.e., enthalpy = cte.

As explained by Art, you will get superheated steam.

Hope this helps


Breizh

[Art Montemayor]

All:

I continue to be an avid collector of Breizh submittals to our Forums. I wish everybody would do the same. He must have an envious library of great information where he is located - and he shares it freely with all who come calling.

This subject of free saturated steam expansion is one that I consider very important - especially for young grads - to thoroughly understand and dominate when out in industry. The basic thermodynamic principles apply across the board for all thermodynamic fluids - be they water or refrigerants. That is why I am submitting my humble representation of what is happening within the Mollier Diagram and how this Diagram clearly explains the path of the expanding steam we are discussing.

I hope this helps out.

Attached Files

•  Steam Mollier Diagram.xls   1001KB   56 downloads

[Technical Bard]

Key issue that should be left as a warning about the letdown of saturated steam is that at pressures above 600 psi (4500 kPa), isenthalpic letdown (i.e. across a valve) will result in condensation.