Need urgent help with thermo problems...


by eku_girl83
Tags: thermo, urgent
eku_girl83
eku_girl83 is offline
#1
Sep23-04, 07:09 PM
P: 90
Here are my problems:
1) A rigid tank contains 5 lb of a two-phase liquid vapor mixture of water, initially at 260 F with a quality of .6 Heat transfer to the contents of the tank occurs until the temperature is 320 F. Determine the initial mass of vapor, in lb, initially present in the tank and the final pressure, in psi.

For T=260, vf (specific volume of saturated liquid) = .01708 and vg (specific volume of saturated gas) = 11.77
I know quality is .6, so I can use the equation x=(v-vf)/(vg-vf) to find v.
v=7.07

For T=320, vf=.01765 and vg=4.919.
*Can I assume quality is the same and use the same equation as above to solve for v? How do I find initial and final pressures? How do I know the initial mass of the vapor?*
Hints please!

2) Water vapor is heated in a closed, rigid tank from saturated vapor at 160 C to a final temperature of 400 C. Determine the initial and final pressures, in bar, and the initial and final specific volumes, in m^3/kg.

*Here i'm having trouble discerning the relationships between p, v, and t. Can someone clarify for me?*

Thanks in advance,
eku_girl83
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Bystander
Bystander is offline
#2
Sep23-04, 10:36 PM
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P: 1,382
Quote Quote by eku_girl83
(snip)*Can I assume quality is the same
Nope.

and use the same equation as above to solve for v?
Yup. What's "unknown" in your second use of the equation?

How do I find initial and final pressures? How do I know the initial mass of the vapor?*
Hints please!
You've access to excerpts from the steam tables in your text (appendices), course materials (handouts), or library (CRC Hndbk, Int. Crit. Tables, JANAF tables, NBS500, yada-yada-yada). Phase rule: 1 component, 2 phases, you're given T, P is fixed --- it's the "vapor pressure." Given volumes and densities of the phases, do you need any other information to calculate mass?

2) Water vapor is heated in a closed, rigid tank from saturated vapor at 160 C to a final temperature of 400 C. Determine the initial and final pressures, in bar, and the initial and final specific volumes, in m^3/kg.
This looks like a freebie --- you probably want to check the problem statement again. As it stands, water vapor (saturation density at 160 C, and vapor pressure at 160 C) is heated at constant volume to a supercritical temperature, again single phase. Specific volumes don't change --- all you've got to do is pick the pressure from the tables, Molier diagram, whatever your instructor's pet representation of the steam tables happens to be.


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