Forum Posts

Bboy88
Jul 12, 2022
In Entropy
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 Entropy
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 General Energy Analysis
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 General Energy Analysis
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 General Energy Analysis
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 Vapor Power Cycles
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 General Energy Analysis
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 General Energy Analysis
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 Refrigeration and AC
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 General Energy Analysis
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 Gas Power Cycles
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 General Energy Analysis
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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Bboy88
Feb 27, 2022
In Entropy
Hello everybody, would somebody please put me on the right track to answering this question? 'Consider water undergoes a heat transfer at constant pressure of 10 MPa and changes from liquid to steam. Find the entropy of the system (sfg) as well as the heat transfer per unit mass in this process?' Is this just looking at steam tables and working per unit mass? I'd appreciate any help Thanks
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