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Examen2 2020 I

This document contains three multi-part thermofluid problems related to Brayton cycles, steam power plants, and air conditioning systems. Problem 1 involves determining the efficiency of an ideal Brayton cycle with one reheater and expressing it as a function of temperature ratios and the isentropic temperature ratio across the high-pressure turbine. It also involves deducing the efficiency expression for a simple Brayton cycle without reheating and demonstrating that reheating always reduces efficiency. Problem 2 involves calculating the fraction of steam extracted for feeding a closed feedwater heater and the total condenser heat transfer rate, given specifications of a steam power plant including pressures, temperatures, turbine output power, and pump requirements.

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62 views1 page

Examen2 2020 I

This document contains three multi-part thermofluid problems related to Brayton cycles, steam power plants, and air conditioning systems. Problem 1 involves determining the efficiency of an ideal Brayton cycle with one reheater and expressing it as a function of temperature ratios and the isentropic temperature ratio across the high-pressure turbine. It also involves deducing the efficiency expression for a simple Brayton cycle without reheating and demonstrating that reheating always reduces efficiency. Problem 2 involves calculating the fraction of steam extracted for feeding a closed feedwater heater and the total condenser heat transfer rate, given specifications of a steam power plant including pressures, temperatures, turbine output power, and pump requirements.

Uploaded by

Deimer Ortiz
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Universidad de Pamplona

Facultad de Ingeniería y Arquitectura


Programa Ingeniería Mecánica
Termofluidos III
Examen 2. 13 de mayo de 2020
Profesor: Juan Carlos Serrano Rico
Nombre:________________________________________________ Código: ____________________

1. [18 POINTS] The ideal Brayton cycle


shown in Fig. P1 employs one reheater, which is
positioned between states 2′ and 2′′. The
compressor 4–1, the high-pressure turbine 2–2′,
and the low-pressure turbine 2′′–3 function
reversibly and adiabatically. The heater 1–2 and
the reheater and the cooler 3–4 cause negligible
pressure drops along the fluid circuit. The heater,
reheater, and cooler pressures are known (PH, PR,
PC, respectively). The working fluid is an ideal
gas with constant specific heat.
a. Determine the heat engine efficiency of the
cycle and express your result as
Figure P1 𝜂𝑅 = function(𝜏, 𝜏𝑚𝑎𝑥 , 𝑥)
where  is the isentropic temperature ratio T1/T4, max is the overall temperature ratio spanned by the
cycle (T2/T4), and x is the inverse of the isentropic temperature ratio across the high-pressure turbine,
x = T2′/T2.
b. From the R formula derived above, deduce the heat engine efficiency expression for a simple
Brayton cycle in which the reheater is absent. Label this new result B.
c. Demonstrate analytically that R < B, in other words, that the addition of one reheater to a closed
Brayton cycle always diminishes the efficiency.

2. [15 POINTS] A steam power plant has high and low pressures of 20 MPa and 10 kPa, and one
closed feedwater heater heater and a drip pump to add the extraction flow to the feedwater line at 20
MPa. Assume the temperature is 175◦C after the drip pump flow is added to the line. The maximum
temperature is 800C, and the turbine has a total power output of 5 MW. One main pump brings the
water to 20 MPa from the condenser. Find the fraction of the flow for extraction to the feedwater and the
total condenser heat transfer rate.

3. [17 POINTS] A laboratory has a volume of 470 m3, and is to be maintained at 20ºC, 52.5% RH.
The air in the room is to be completely changed once every hour and is drawn from the atmosphere at
1.05 bar, 32ºC, 86% RH, by a fan absorbing 0.45 kW. This air passes through a cooler which reduces its
temperature and causes condensation, the condensate being drained off at 8ºC. The resulting saturated
air is heated to room condition. The total pressure is constant throughout. Determine
a. The temperature of the air leaving the cooler,
b. The rate of condensation,
c. The heat transfer in the cooler, and
d. The heat transfer in the heater.

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