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Need help with Thermodynamics homework! Number 4 is giving me trouble.

CHEG 2313

Thermo I HW#5

Spring 2016

Due Monday, February 29th at the beginning of class.

1. A plant hopes to cool a steam line by sending it through a throttling valve to expand it to

atmospheric pressure. The steam enters the valve at 550oC and 250 bar. The expansion

in the valve happens so quickly that it can be assumed that any heat loss across the valve

is negligible.

a) Define the system. State whether it is open or closed, steady-state or not, adiabatic or

not. (1 pt)

b) Develop the reduced energy balance for this process by starting with the full (correct

form) EB, reducing the terms, and justifying any terms that can be neglected. (2 pt)

2. For Problem 1, determine the temperature (oC) of the steam after the expansion:

a) using the steam tables. (3 pts)

b) using the Mollier diagram (Fig 3.3-1a in book and on next page). You must submit a

hardcopy of the Mollier diagram with the initial and final states marked. (3 pts)

c) assuming the steam behaves as an Ideal Gas with a Cp=1cal/goC. (2 pts)

3. Water in an open metal drum is to be heated from room temperature (25oC) to 80oC by

adding steam slowly enough that all the steam condenses. The drum initially contains

100 kg of water, and steam is supplied at 3 bar and 300oC. How many kilograms of

steam should be added so that the final temperature of the water in the tank is exactly

80oC? Neglect all heat losses from the water to the room in this calculation. (4 pts)

4. Refrigerant-134a (Mollier diagram given in Fig 3.3-4) enters the capillary tube of a

refrigerator as saturated liquid at 0.8 MPa and is throttled to a pressure of 0.12 MPa.

Determine the quality of the refrigerant at the final state and the temperature drop during

the process. (3 pts)

5. There are 78 students in our room, which can be assumed to be 30m x 25m x 10m. An

average person?s basal metabolic rate is 84 W when resting. This means that, on average,

a person dissipates 84W of heat to their surroundings. If the air handling system breaks,

what temperature rise would be expected in the room over the 50 minute class period?

Assume the doors are closed and approximate it as closed to air flow.

a) Assume air behaves as an ideal gas with a constant Cp=1.005 kJ/kgK. (3 pt)

b) Assume the air behaves as an ideal gas with a temperature dependence as prescribed

in App A.II. (4 pt)

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