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## Forum Posts

Bboy88

Aug 01, 2023

In Thermodynamics Forum

A rigid tank contains 2.0 kg of superheated steam initially at temperature of 250 °C and pressure 0.5 MPa (state 1). Heat is added to the steam leading to a rise in temperature and pressure. The final pressure at state 2 is 1.0 MPa. Consider that the specific volume remains constant throughout the process as the tank is rigid.
Calculate final temperature and heat added.
I can find enthalpy at state 1 from tables, but then I am stuck.

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Bboy88

Jul 26, 2023

In Thermodynamics Forum

Why can we use specific heat capacity at constant pressure to calculate work into the compressor? Eg Brayton cycle:
W_1-2 = mCp(T2 - T1)
This always confuses me as it is not a constant pressure process!
Help :(

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Bboy88

Jul 25, 2023

In Thermodynamics Forum

I have used the two efficiency formulae to solve the following problem, but I get a different answer from each:
If I use the 1 - (1/PR^1.4-1/1) I get approx. 40%
If I use Work_nett/Heat_in I get approx. 27%
I can't figure why there is a discrepancy, is it do with the isentropic efficiencies of the compressor and turbine?
Many thanks

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Bboy88

Jul 16, 2023

In Thermodynamics Forum

How do I start this problem please?
A closed cylinder contains 40 ml of gas at 800 kPa. The gas expands to a volume of
400 ml according to the relationship pV1.1 = constant. If the heat transfer from the gas to the
surroundings is 300 J, calculate the change in internal energy of the gas during the expansion
process.
Do I use W = P(V2-V1)^1.1 ?
Thanks

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Bboy88

Apr 24, 2023

In Thermodynamics Forum

Here's a question: A gas flows in a pipe at a rate of 1 kg/s. The pressure and temperature at one section of the pipe are 0.6 MPa and 527K. At the second section of the pipe, the pressure is 0.5MPa and temperature is 320K. The specific volume of the inlet gas is 0.249 m3/kg and at the second section, it is 0.24 m3/kg. Assume that the specific heat at constant volume is 1.33 kJ/kg ºC. If no shaft work is done by the gas and the velocities are small, determine the magnitude and direction of the heat transfer. Assume the pipe to be horizontal, and neglect velocity terms. Could you please tell me which of the following answers is correct and why? 1) 2) h = u + Pv and u = Cv.T h1 = (1.33 x 527) + (0.6 x 103 x 0.249) = 850.31 kJ h2 = (1.33 x 320) + (0.5 x 103 x 0.24) = 545.6 kJ W = m(h2 – h1) -304.71 kW (Loss) Many thanks

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Bboy88

Mar 28, 2023

In Thermodynamics Forum

Hi all, Any idea about how to solve this problem? Consider a cylinder fitted with a piston which contains saturated Steam at 30°C with 80% quality. Let this vapor be compresses in a reversible adiabatic process until the pressure is 10 bars in superheated conditions .Determine the work per kilogram of the steam for this process. Assume the system is open system I would start by looking at steam tables I think. Many thanks

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Bboy88

Mar 17, 2023

In Thermodynamics Forum

Hi, I'm confused by this question and don't know where to start: Do I use the ideal gas law pV= mRT and take it from there? A method would be useful. Many thanks

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Bboy88

Feb 08, 2023

In Thermodynamics Forum

Hi all, the problem below is confusing me. It states the process as a constant pressure process, so in part (b), why are they using Cv ?

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Bboy88

Jul 12, 2022

In Thermodynamics Forum

What's the method to solve this problem please? Air is throttled through a porous plug. The inlet conditions are 30 bar and 85°C and the outlet is 15 bar. The process is adiabatic and kinetic energies are negligible. Calculate the change in entropy per mass. Many thanks

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Bboy88

Jul 07, 2022

In Thermodynamics Forum

Hi again, just wondering about this: "Air is compressed in a reversible polytrophic process (pv^n=constant) from an initial state of 15 bar and 100 ° to a final pressure of 50 bar. The polytropic exponent is n=1.3. Calculate the change in entropy per mass of air." Would we use perfect gas relationships - specific - pv=RT at state 1. Looking at entropy in tables though, they show a change in entropy. Not sure about this one.

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Bboy88

Jul 05, 2022

In Thermodynamics Forum

Any thoughts on the method to calculate this? Consider a cylinder fitted with a piston which contains saturated Steam at 30°C with 80% quality. Let this vapor be compressed in a reversible adiabatic process until the pressure is 10 bars in superheated conditions .Determine the work per kilogram of the steam for this process. Assume the system is open

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Bboy88

Jun 30, 2022

In Thermodynamics Forum

Hi, I asked this question before, but wanted to check my solution is correct: Question: A cylinder fitted with a piston has a volume of 0.1 m3 and contains 0.5 kg of steam at 0.4 MPa. Heat is transferred to the steam until the temperature is 300 °C, as a result the volume increases to three times and the change in internal energy is 680.1 kJ. Determine the work and heat transfer for this process. Neglect the change in kinetic and potential energies. Solution: I don't think we're interested in tables, what do you think? Work = P.dV = P(V2 – V1) Also, V2 = V1 x 3 - So V2 = 0.3m3 Work = 400 kPa x (0.3 – 0.1) W = 80 kJ dQ = dU + dW dQ = 680.1 + 80 dQ = 760.1 kJ

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Bboy88

Jun 28, 2022

In Thermodynamics Forum

Hi, seems simple, but I'm struggling with this: A gas in a closed system expands from an initial state of P1 = 5 bar and V1 = 0.1 m3 to state 2 where P2 = 1 bar. The pressure and volume are related during the expansion by PV = constant. Calculate: Work done during expansion I know that W = integral from state 1 to 2 of P.dV but am I integrating v2 - v1 and also p2 - p1, I'm not sure what I'm doing here.

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Bboy88

Jun 16, 2022

In Thermodynamics Forum

Hi, here is a problem which I've started to solve: A cylinder fitted with a piston has a volume of 0.1 m3 and contains 0.5 kg of steam at 0.4 MPa. Heat is transferred to the steam until the temperature is 300 °C, while the pressure remains constant. Determine the heat transfer and work done for this process. Neglect the change in kinetic and potential energies. I have found x1 by solving for specific volume and using steam tables for hf and hfg. x1 = 0.4311 Now I used initial conditions to find h1; = 1524.6kJ/kg How do I find h2? I use the same formula - h2 = x2.hg + (1-x2).hf but not sure which x2 value to use or which hf & hg values? Thanks

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Bboy88

May 25, 2022

In Thermodynamics Forum

Why do we change the use of the heat formula in the following problem from using Cp to then using Cv: Problem: A gas is initially at 38 ̊C and having Cp= 0.92 KJ/kg.K and Cv=0.71KJ/kg.K is placed within a cylinder. If 8.5 KJ of heat is added to 5 kg of the gas in a non- work flow, constant pressure process. Determine: (a) The final gas temperature (b) Work done on or by the gas Solution: We use Q = mCp (T2 - T1) to find T2 because it is a constant pressure process; but then use Q = mCv (T2 - T1) to find work. Shouldn't we still use Q = mCp (T2 - T1) to find Work ? Thanks

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Bboy88

Apr 06, 2022

In Thermodynamics Forum

Hi, using the SSSF equation for this problem: A large steam turbine operates with a steam flow rate of 72,000 kg/hr. The conditions at the inlet and outlet are shown in the table below. Determine the power produced by the turbine a. If heat losses are negligible, and b. If heat losses are equal to 50,000 KJ/hr. Inlet Conditions / Exit Conditions Pressure inlet 1 MPa - outlet - 0.1 MPa Temperature inlet 540°C - outlet - 69°C Velocity inlet 40 m/s - outlet 130 m/s Elevation above reference plane 3 m - outlet 0 m Enthalpy ( kJ/kg) inlet 3505.4 - outlet 2189.3 g=9.81m/s2 I got a) = 26169.58 kW b) = 26155.69 kW Am I way off, or is that correct? Thanks in advance.

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Bboy88

Apr 01, 2022

In Thermodynamics Forum

Hi, how do I attempt to answer this: At the commencement of compression in the reciprocating compressor of a refrigeration plant the refrigerant is dry saturated at 1 bar. The compression process follows the law pV^1.1 = C, until the pressure is 10 bar. •Calculate the work done on the refrigerant during the process? Is it W = m(h2 - h1) ? I don't know if the pV^1.1 is relevant. Thanks for any help.

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Bboy88

Mar 30, 2022

In Thermodynamics Forum

With regards the following question: A cylinder fitted with a piston has a volume of 0.1 m3 and contains 0.5 kg of steam at 0.4 MPa. Heat is transferred to the steam until the temperature is 300 °C, as a result the volume increases to three times and the change in internal energy is 680.1 kJ. Determine the work and heat transfer for this process. Neglect the change in kinetic and potential energies. Is work found by using W = P x Delta V = 400 x (0.3-0.1) = 80kJ or is it more involved than this?

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Bboy88

Mar 17, 2022

In Thermodynamics Forum

Hi, working through gas turbine problems, when calculating work through the compressor, = Cp(T2 - T1), sometimes Cp is 1.005, sometimes is 1.0035. Is this due to initial temperature?

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Bboy88

Mar 08, 2022

In Thermodynamics Forum

Hi, which is the correct/best formula to use for calculating the final pressure when adding heat in kJ to gas in a closed container assuming ideal gas and constant specific heat? Much appreciated.

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