Need help with thermo problems

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In summary, the conversation discusses two problems related to thermo. The first problem involves determining the initial mass of vapor and the final pressure in a rigid tank containing a two-phase liquid vapor mixture of water. The second problem involves finding the initial and final pressures and specific volumes of water vapor as it is heated in a closed tank. The conversation also touches upon using equations and steam tables to solve these problems.
  • #1
eku_girl83
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Need urgent help with thermo problems...

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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  • #2
eku_girl83 said:
(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.
 
  • #3


Dear eku_girl83,

I understand that you are in need of urgent help with some thermo problems. I will do my best to provide you with some guidance and hints to help you solve these problems.

1) For the first problem, you are given the initial temperature (260 F) and the quality (.6) of a two-phase liquid vapor mixture of water in a rigid tank. You are also told that heat transfer occurs until the temperature reaches 320 F. To solve this problem, you will need to use the following equations:

x = (v - vf)/(vg - vf) and q = m(hf + x*hfg)

Where x is the quality, v is the specific volume, vf is the specific volume of saturated liquid, vg is the specific volume of saturated gas, q is the heat transfer, m is the mass, hf is the enthalpy of saturated liquid, and hfg is the enthalpy of vaporization.

To find the initial mass of vapor, you can rearrange the equation for quality and solve for m:

m = q/(hf + x*hfg)

Once you have the initial mass, you can use the equation for specific volume to find the initial pressure:

p = mRT/v

Where R is the gas constant and T is the temperature in Kelvin. You can use the same equation to find the final pressure, but you will need to use the final specific volume.

2) In the second problem, you are given the initial and final temperatures (160 C and 400 C) of water vapor in a closed, rigid tank. To solve this problem, you will need to use the ideal gas law:

p1v1 = p2v2

Where p1 and v1 are the initial pressure and specific volume, and p2 and v2 are the final pressure and specific volume.

To find the initial and final specific volumes, you can use the following equations:

v = RT/p and v = vf + x*vfg

Where v is the specific volume, R is the gas constant, T is the temperature in Kelvin, vf is the specific volume of saturated liquid, x is the quality, and vfg is the difference between the specific volumes of saturated gas and liquid.

I hope these hints help you in solving these thermo problems. Remember to always double check your calculations and units. Best of luck!
 

What is thermodynamics?

Thermodynamics is the branch of science that deals with the relationships between heat, energy, and work. It studies how energy is transferred and transformed between different forms and how it affects the properties of matter.

Why is thermodynamics important?

Thermodynamics is important because it helps us understand and predict the behavior of energy and matter, which are essential components of our world. It is the foundation of many engineering and technological applications, from power plants to refrigerators.

What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern the behavior of energy and matter. They include the first law, which states that energy cannot be created or destroyed, only transferred or converted; the second law, which states that the total entropy of a closed system will always increase over time; and the third law, which states that it is impossible to reach absolute zero temperature.

What are some common thermodynamics problems?

Some common thermodynamics problems include calculating the efficiency of a heat engine, determining the change in internal energy of a system, and solving for the final temperature of a mixture of gases at different temperatures.

How can I improve my understanding of thermodynamics?

To improve your understanding of thermodynamics, it is important to have a strong foundation in physics and mathematics. You can also read textbooks and practice solving problems to gain a better grasp of the concepts. Additionally, conducting experiments and simulations can help you visualize and apply thermodynamic principles in real-world scenarios.

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