Lecture 7 Example Problems
1. Carbon dioxide is throttled from 20C, 2 MPa to 800 kPa. Assuming ideal
gas behavior, what is the exit temperature?
2. A piston-cylinder with a 5 cm ID contains 1.4 grams of nitrogen. The mass
of the piston itself is 5.00 kg a
Review of the 2nd Law
We have discussed the first law and ways of calculating heat and work. Overall, the first
law concludes that energy is conserved.
Certain observations lead to the formulation of the 2nd law.
1.
Work is readily transformed to other fo
Gas turbines
Note: this is not a closed cycle!
1 2
air is compressed with an increase in P and T
2 3 air and fuel are mixed and combusted at constant P
3 4 the combustion products are expanded, driving the turbine
@ position (4), the turbine exhaust is at
Entropy balance
Consider heat flow from a hot reservoir to a colder reservoir
hot reservoir, TH
Q
cold reservoir, Tc
SHR =
Q
TH
SCR =
S total = SHR + SCR =
Q
Tc
Q Q
+
TH TC
1
1
= Q
T
TH
c
With TH > Tc
S total > 0
If the process approaches reversibil
Example
Water at 93.3C is pumped from a storage tank at a rate of 189 L/min. The motor of
the pump supplies work at a rate of 1.5 kW. The water goes through a heat
exchanger, giving up heat at a rate of 42,201 kJ/min and is delivered to a second
tank at a
Example
Water at 93.3C is pumped from a storage tank at a rate of 189 L/min. The motor of
the pump supplies work at a rate of 1.5 kW. The water goes through a heat
exchanger, giving up heat at a rate of 42,201 kJ/min and is delivered to a second
tank at a
Refrigeration
Refrigeration maintaining a temperature below that of the surroundings.
With the heat engine, we produced work by supplying heat.
For refrigeration, we supply work to absorb heat.
As the basis for a heat engine was one operating on a Carnot
Extra Problems
1. A compressor receives 0.1 kg/s R-134a at 150 kPa, -10C and delivers it at 1000 kPa, 40C.
The power input is measured to be 3 kW. The compressor has heat transfer to air at 100
kPa coming in at 20C and leaving at 30C (i.e. like a heat exc
Chapter 1 Introduction
Why Thermodynamics
Thermodynamics is the science of energy
o Applications in power generation
o Refrigeration and liquefaction
o Separations
o Chemical processes involving solution chemistry
There are two modes of thermodynamics
o C
Extra Problems
1. A compressor receives 0.1 kg/s R-134a at 150 kPa, -10C and delivers it at 1000 kPa, 40C.
The power input is measured to be 3 kW. The compressor has heat transfer to air at 100
kPa coming in at 20C and leaving at 30C (i.e. like a heat exc
Efficiency Losses
The T across the boiler represents an irreversibility.
Fluid friction in the piping, boiler and condenser leads to small pressure drops.
This means the pump must supply more work to maintain the target pressure.
The temperature differenc
Review of First Law
For closed systems
only heat & work (Q and W) no mass transfer
E
sys
=Q-W
In closed systems commonly
E
n =Q-W
U
or
sys
= (internal energy)
U
ndU = dQ - dW
Aside: U is path independent, then Q - W must be also
Q may vary by path, W may
ENGI 2334
INTRODUCTION TO ENGINEERING THERMODYNAMICS
Fall 2014
Monday, August 25, 2014 Friday, December 12, 2014
01-LEC(11780) Mondays and Wednesdays, 5:30PM - 7:00PM AAA Aud1
Instructor
Dr. William Epling
Office Hours (by appointment only)
Dr. Konstantin
University of Houston
ENGI 2334: Introduction to Thermodynamics
Teaching Assistant Hours
ADVICE:
Homework problems are a teaching tool that is meant to help you better learn the course
material. All of you can follow the examples in the text (or in any te
More random extras
1. A rigid, insulated vessel contains superheated vapor steam at 3 MPa, 400C. A valve on
the vessel is opened, allowing steam to escape. The overall process is irreversible, but the
steam remaining inside the vessel goes through a rever
More random extras
1. A rigid, insulated vessel contains superheated vapor steam at 3 MPa, 400C. A valve on
the vessel is opened, allowing steam to escape. The overall process is irreversible, but the
steam remaining inside the vessel goes through a rever
Rankine Cycle Examples
A steam power plant following the Rankine cycle has the following conditions: the pump outlet
pressure is 10 MPa, the boiler outlet temperature is 600C, the turbine outlet pressure is 10 kPa,
the turbine efficiency is 80%, the pump
We have discussed the energy balance and applied it to ideal gases and water. A great
source of data for water, specifically, is the steam tables we used. Practice!
In using them, as you have seen, we have seen two sets of tables, or used two sets of
tabl
Polytropic Processes and Equations
With the 2 characteristics of an ideal gas, many useful relationships can be derived. The
equations derived depend on given properties versus those you are trying to find.
One example is an adiabatic process: dQ = 0
dU =
Ideal Gas Equation of State Review
Empirical gas laws:
Boyles Law:
PV = constant
Charless Law:
V/T = constant
Avogadros Law
Combining the above
Combine Boyles and Charless laws:
V = T*constant
V = constant/P
V = T/P * constant
Expressed in terms of molar
October 13 Lecture Problems
1. An inventor claims to have devised a cyclic engine which exchanges heat with reservoirs
at 25C and 250C, and which produces 0.45 kJ of work for each kJ of heat extracted
from the hot reservoir. Is this claim believable?
2. A
October 20 Practice Problems
1. A piston/cylinder device contains refrigerant 134A at 20C and 1600 kPa. The refrigerant
2.
3.
4.
5.
is heated to 60C by an external heat source that is always at 70C. The refrigerant
heating process is internally reversible
1. A car engine running at 65 horsepower has a thermal efficiency of 24%. What is the fuel
consumption for this car, if the fuel has a heating value of 19,000 BTU/lbm?
2. An air conditioner is rated for 5000 BTU/hr of cooling and requires 500 W of power.
Application of 1st and 2nd laws to unit processes
1.
Turbine (or expander)
Expansion of a gas in a nozzle to produce a high velocity stream
Using steam is historically associated with turbines
Using a high-pressure gas is typically associated with an expa
October 22 Practice Problems
1. A reversible steady state device receives a flow of 1 kg/s air at 400 K, 450 kPa and the air
leaves at 600 K, 100 kPa. Heat transfer of 900 kW is added from a 1000 K reservoir, 50
kW rejected at 350 K and some heat transfer
1. One kg of water at 25C:
a. Experiences a temperature rise of 1K. What is Ut, in kJ? Ans: 4.18 kJ
b. Experiences a change in elevation of z. The change in potential energy is the same as Ut
for part a. What is z, in meters? Ans: 427m
c. Is accelerated f
Examples
1. A pipe contains steam at 1.4 MPa and 300C. An initially evacuated tank is attached to
this pipe and the valve is opened until the tank pressure reaches 1.4 MPa, adiabatically.
What is the final temperature of the steam?
2. In a well-insulated