NCEES 2008 #129

[Firefly]

Help!

OK, on this problem you're given steam properties at 600deg F and 1atm and water at 60degF and 1atm.

YOu're trying to find what the quality of the mix would be using equal parts of steam and water.

Finding the enthalpy of the steam is easy b/c we know its super heated. However, the question comes for water. Which table do you use for water (Temp or Pressure) since they don't match up?

The solution is to take the enthalpy for water at T=60deg F and mix with enthalpy of superheated steam to get your mixed enthalpy.

THen you're suppossed to use the hf and hfg at P=1atm for sat steam to get quality.

I"m assuming that this solution is such, b/c you're initially dealing with water, so you ignore the pressure and go to the T=60deg F table. Since the mixture is some sort of a steam, you use the P=1atm b/c both products are at same pressure.

Is this the right understanding?

 

[gaidox]

Help!OK, on this problem you're given steam properties at 600deg F and 1atm and water at 60degF and 1atm.

YOu're trying to find what the quality of the mix would be using equal parts of steam and water.

Finding the enthalpy of the steam is easy b/c we know its super heated. However, the question comes for water. Which table do you use for water (Temp or Pressure) since they don't match up?

The solution is to take the enthalpy for water at T=60deg F and mix with enthalpy of superheated steam to get your mixed enthalpy.

THen you're suppossed to use the hf and hfg at P=1atm for sat steam to get quality.

I"m assuming that this solution is such, b/c you're initially dealing with water, so you ignore the pressure and go to the T=60deg F table. Since the mixture is some sort of a steam, you use the P=1atm b/c both products are at same pressure.

Is this the right understanding?

The water here is subcooled liquid; T=60degF is less Tsat (based on P=1 atm).

By merm ch.24 sec.26, for prop. of subcooled liquid it says"unless the pressure of liquid is very high,

the various thermodynamic prop. can be considered to be functions of only the liquid's temp."

Hope it helps on the water part.

Your correct on using Pmixture (1 atm) for getting quality for sat.steam.

 

[Firefly]

Excellent. Thanks Dox.(I foresee this conversation benefitting me on the test!)