8 replies to this topic

[lactobacillus]

How do you interpret and solve this problem?

A vessel of 2 m3 capacity contains .02 m3 of liquid water and 1.98 m3 of water vapor @ 101.33 kPa. How much heat must be added to to the contents of the vessel so that the liquid water is just evaporated?

[Art Montemayor]

Lacto:

Your problem is interpreted as follows:

1. It is an elementary exercise in basic phase equilibria and thermodynamics;
2. It is probably a University Freshman problem;
3. It is structured to find out if you can figure out the difference in applying sensible heat and latent heat of vaporization.

Your problem is solved by:

1. Carefully reading and understanding what is presented;
2. Figuring out what your vessel contains as contents and the contents needs in order to convert the liquid portion into a vapor.

General comment: You should be able to resolve this problem quickly - in the time that it takes to find the LHV of water.

Now for more a important question: How do YOU interpret and intend to solve your problem?

[lactobacillus]

Now for more a important question: How do YOU interpret and intend to solve your problem

I had a solution before but I was not sure about it so I posted it here. So here it goes:

1. The mass of the water can be computed by multiplying the volume of each phases to their specific volumes found at the steam table.
2. I assumed that the pressure remained constant so dQ = dH
3. H1 was solved by multiplying the individual masses of the vapor and the liquid to their enthalpies (sat'd liquid & vapor)
4. H2 was assumed to be equal to the enthalpy of sat'd vapor

Are my assumptions corrrect?

[Art Montemayor]

Lacto:

I make my remarks to your post in RED as follows:

had a solution before but I was not sure about it so I posted it here. So here it goes:
If you had a prior solution, you never posted it here. All you have posted up to now is the problem, but not any solution.

1. The mass of the water can be computed by multiplying the volume of each phases to their specific volumes found at the steam table.
Wrong! You are given VOLUMES of steam and water, therefore to find the corresponding masses you should multiply by the DENSITY (mass/volume) – which is the inverse of Specific Volume.
2. I assumed that the pressure remained constant so dQ = dH;
True.

3. H1 was solved by multiplying the individual masses of the vapor and the liquid to their enthalpies (sat'd liquid & vapor)
True.

4. H2 was assumed to be equal to the enthalpy of sat'd vapor.
Can be done, but how do you define the saturation state if you don’t know the resultant PRESSURE?

Are my assumptions correct?
Your assumptions are not “assumptions”; they are logical reasonings – and some of them are not correct.

You must establish the temperature and pressure of the resulting system when all the water is vaporized. You have been told that the resulting system is saturated steam, so you have another important input value in your calculations. Once you figure out what the resulting temperature and pressure of your saturated system is, the rest should be a piece of cake. The answer is in your steam tables or on a Mollier Diagram – whichever you want to use.

[lactobacillus]

I did another way of solving:

I did steps 1-2 but at step 3 pressure was not assumed to be constant.

By dividing the final volume of the pure sat'd vapor (which is known to be 2.0 m3 ) by the mass of the water gives the specific volume of the sat'd vapor. (True or False?)

The computed specific volume of the sat'd vapor can be used to find the enthalpy at state 2 (H2).

[Art Montemayor]

Lacto:

You seem to have a failure in communicating what you are proposing or what you are doing. Note the comments in red to your post.

did another way of solving:
(This is not correct. What you are describing seems to be exactly what I prescribed. Therefore, it is not another way.)

I did steps 1-2 but at step 3 pressure was not assumed to be constant.
(The pressure cannot stay constant in the system; you are vaporizing liquid water in a fixed volume. The pressure must increase due to the heat input and accumulation)

By dividing the final volume of the pure sat'd vapor (which is known to be 2.0 m3 ) by the mass of the water gives the specific volume of the sat'd vapor. (True or False?)
(This statement is totally FALSE. Please focus and concentrate on what you state. The mass of the water is only 0.02 m³ times its density. That is NOT all of the mass in the system!)

The computed specific volume of the sat'd vapor can be used to find the enthalpy at state 2 (H2).
(True, but only because of the very special condition of saturation. This last property ties down the values of two variables: the density and the state. With these two known, you have identified the enthalpy of the vapor – which lies on the saturated vapor line of the Mollier Diagram.)

If you have been doing all this “solving”, why is it that you don’t show us your calculations and your calculated results? Be fair and honest. If you don’t show your work, I can’t have confidence that you are indeed doing your own work. If you want further help or advice, please present your calculations much as you are expected to present them in a formal classroom atmosphere. That way we can see your algorithm, logic, calculations, and answer.

[lactobacillus]

Mr. Montemayor:

I appreciate your effort in helping me. Being in different time zones, I had no luxury of waiting for your last reply. I did the best thing which was to consult with my professor. His interpretation was same as yours. I don't have any scanner with me right so i can't show my actual solutions. So I can only send the processes of my calculations. We've just finished the term and right now we're starting our summer break. Maybe there's lots of time to key in the actual solutions or I might secure a scanner.

I was wondering if you are a professor coz you write comments like one?

I would want the workbook. It would be of great use to me. Actually, I bought mathlab recently but i don't know how to use it. My professor says its useful for calculations and modelling applications.

Lactobacillus
University of San Carlos- Cebu City, Philippines

[Art Montemayor]

Lacto:

Attached find a workbook that illustrates how professional engineers usually produce their calculations in spreadsheets.

This is the standard, general practice and it is supported because it clearly expresses an engineer's legible calculations and allows the use of graphics, graphs, and charts as well.

These type of calculations can also be checked by peers, or others, very easily and corrected as well. If you follow this type of presentation you can't go wrong in communicating your calculations.

Note that this method forces the engineer to express his calculations in a methodical, logical, and sequential algorithm - which is the natural, and expected method of proving your calculations.

I hope this is of help in guiding you and other students as well.

Art Montemayor
 Chemical_Process_Principles.zip   254.77KB   115 downloads

[lactobacillus]

Thank you for the workbook..