Download Saturation Pressure Lab v3c PDF

TitleSaturation Pressure Lab v3c
File Size810.3 KB
Total Pages17
Table of Contents
                            6 Saturation Pressure & Vapor Quality
	6.1 Background
		6.1.1 Quality of Vapor
		6.1.2 Pressure, Temperature and Density of a Saturated Mixture
		6.1.3 Thermodynamic Property Data
		6.1.4 Measuring Quality
		6.1.5 Empirical Correlation for Saturation Pressure and Temperature
		6.1.6 Atmospheric Pressure
	6.2 Objectives
	6.3 Experiment
		6.3.1 Procedures
	6.4 Measuring Atmospheric Pressure
		6.4.1 Data Log for Atmospheric Pressure Measurement
References
                        
Document Text Contents
Page 1

Chapter 6

Saturation Pressure & Vapor Quality

Liquid-vapor phase change (evaporation and condensation) are extremely important to many, many
industries. Processes such as distillation and separation in petroleum refineries, electrical power
generation in steam power plants, and refrigeration cycles all depend upon control of evaporation
and condensation.

6.1 Background

Evaporation (and boiling) is the process in which liquid becomes vapor and in doing so absorbs a
measure of thermal energy known as latent heat. As an example, to maintain a constant temper-
ature the human body requires cooling to offset the thermal energy released during the metabolic
process. Perspiration, consisting primarily of salt water, evaporates thereby cooling the surface of
the skin. The process of evaporation occurs at a constant temperature. The cooling effect arises
from the loss of thermal energy; that is, the transfer of latent heat. The temperature at which evap-
oration and condensation occurs is known as the saturation temperature. The corresponding
pressure is known as the saturation pressure. The temperature at which evaporation or boiling
occurs varies with pressure. It is a common observation that water boils at a temperature less than
100 XC at a high altitude, such as encountered on mountains, because the atmospheric pressure is
less at these elevations.

t e m p e r a t u r e

pre
ssu

re

s o l i d
l i q u i d

v a p o r
t r i p l e
p o i n t

c r i t i c a l
p o i n t

Figure 6.1. General pressure-temperature relationship.

Figure 6.1 illustrates the relationship between pressure and temperature for the solid, liquid,
and vapor phases of a substance. The triple point is the temperature and pressure at which all three
phases can coexist. The line separating the solid-liquid regions represents a set of temperatures

Page 2

quality.eps


38 CHAPTER 6. SATURATION PRESSURE & VAPOR QUALITY

and pressures at which the solid and liquid phases (ice and water) may coexist. Similarly, the
line separating the liquid-vapor regions represents a set of temperatures and pressures at which
the liquid and vapor phases (water and steam) may coexist. The critical point is the pressure-
temperature state beyond which there is no distinction between liquid and vapor phases.

6.1.1 Quality of Vapor

The thermodynamic state of a single phase fluid (gas or liquid) can be determined if two properties
are known. So, if the pressure and temperature are measured and the system is in thermal equilib-
rium, then all of the other properties at this state can be determined. If two phases are present (va-
por and liquid), then three thermodynamic states must be known. For

l i q u i d l i q u i d

v a p o r v a p o r

v a p o r

x 1 x 2 > x 1 x 3 = 1

T s a t T s a t
T s a t

example, consider a liquid is in
equilibrium with its vapor in a
closed system at some tempera-
ture and pressure as illustrated.
Since the two phases coexist in
equilibrium, the temperature is
the saturation temperature, Tsat.
The exact same saturation tem-
perature and pressure can be ob-
tained with less liquid in the sys-

tem. In fact, the exact same temperature and pressure can be obtained without any liquid in the
system. Just knowing the temperature and pressure is insufficient to determine the system’s state
because the mass, density, and specific volume are not a unique to this temperature and pressure.

Any three properties may be used in specifying the thermodynamic state of a two-phase mixture.
One property typically used, in addition to temperature and pressure, is quality. Quality, x, is
the ratio of vapor mass, mg, to mixture mass, mg +mf :1

x =
mg

mg +mf
(6.1)

Therefore, x1 < x2 < x3 = 1. The quality of saturated liquid is 0 an the quality of saturated
vapor is 1. The thermodynamic properties of the mixture which are dependent upon mass can be
expressed using quality. The specific volume (volume per mass) of the systems in the illustration is
v = (1 − x)vf + xvg. Other properties dependent upon mass such as internal energy, enthalpy, and
entropy can be determined in a similar manner.

6.1.2 Pressure, Temperature and Density of a Saturated Mixture

Three properties are required to specify the thermodynamic state of a two-phase mixture. Of
the numerous fluid properties, there are three which are relatively easy to determine; pressure,
temperature, and density. The specific volume, v, is an intensive property which is the inverse of
density, v = 1/ρ.

Figure 6.2 illustrates the relationship between pressure, temperature and specific volume for a
liquid-vapor system. The diagram is of pressure versus specific volume (P -v diagram) and lines of
constant temperature (isotherms) are shown. The saturated state, that is the state at which vapor
and liquid coexist, is defined by the saturation curve. The region to the right of the saturation
curve is superheated vapor and the region to the left of the saturation curve is subcooled liquid. In

1 By convention, a subscript f is used to denote the liquid phase and a subscript g to denote the vapor phase.
The subscript fg denotes the difference between the vapor and liquid, i.e. hfg � hf − hg.

Page 8

44 CHAPTER 6. SATURATION PRESSURE & VAPOR QUALITY

6.3 Experiment

The saturation and throttling experiments will be conducted on the Armfield TH3 Saturation
Pressure Units. Figure 6.5 is a schematic of the basic system. Refer to the Fig. 6.6, 6.7, and 6.8
and Table 6.1 for location and description of the part numbers.

Figure 6.5. Schematic of Saturation Pressure Rig.

The saturation pressure apparatus consists of a fluid loop with an insulated cylindrical boiler (2)
in one of the vertical lines. Distilled water in the boiler is heated to the boiling point using a pair of
cartridge heaters (11) that are located near the bottom of the boiler. A sight glass (10) on the front
of the boiler allows the internal processes to be observed, namely boiling patterns at the surface
of the water while heating or reducing the system pressure and cessation of boiling/condensation
during cooling. The sight glass also allows the water level in the boiler to be monitored. Saturated
steam leaving the top of the boiler passes around the loop before condensing and returning to the
base of the boiler for reheating. The operating range of the boiler and loop is 0 to 8 bar gauge.
A pressure relief valve (5) is set to open at 8 bar. NEVER lean over or place your hand
above the pressure relief valve! The top line of the loop incorporates an platinum RTD (3)
and a pressure transducer (9) to measure the properties of the saturated steam. A Bourdon tube
pressure gauge allows for monitoring of the boiler pressure even when there is no power to the unit.
A fill/vent tube (38) connected to the fill/vent valve (4) on the line allows the loop to be filled
with distilled water and allows all air to be vented safely before sealing the loop for pressurized
measurements. The bottom of the fluid loop has a drain valve (39).

A throttling valve (6) and a throttling calorimeter (7) are attached to the vapor line, the
purpose of which is to demonstrate the measurement of steam quality, x. The steam expands
to atmospheric pressure as it passes through the throttling calorimeter. A platinum RTD (14)
measures the temperature of the superheated vapor. A container (15) below the calorimeter collects
condensing vapor and allows it to be drained safely from the apparatus.

6.3.1 Procedures

Review all of the experiment procedures prior to starting this experiment. Refer to the
Fig. 6.6, 6.7, and 6.8 and Table 6.1 for location and description of the part numbers.

Page 9

6.3. EXPERIMENT 45

Startup

1. Verify proper water level in the sight glass (10) of the boiler (2).

2. Verify that the fill/vent valve (4), the throttling valve (6), and the drain valve (39) are closed. The
throttling valve is closed when the valve handle is perpendicular to the tube; for this apparatus, the
valve is closed when the handle is vertical.

Saturation Pressure Experiment

3. Switch the heaters (36) ON and turn the heater power control (37) to MAXIMUM. Verify that the
throttling valve closed (6).

4. Observe the appearance of the fluid in the boiler (2) through the sight glass (10) as the temperature
increases.

5. Record in the saturation curve data table the pressure and temperature at approximately every 1 bar
(100 kPa) increment until the boiler reaches the maximum working pressure of 7 bars gauge. The
pressure can be read from the sensor readout display (26) on the console (20). The sensor selector
switch (27) on the console may be used to toggle the readout between the platinum RTD sensor,
PT100(1), and the pressure transducer. The pressure reading is gauge pressure so the atmospheric
pressure will have to be measured in order to convert the transducer reading to absolute pressure. The
temperature reading is the resistance of the RTD. The resistance can be converted to temperature
using Table 6.2.

Throttling Experiment

6. When a pressure of 7 bar gauge has been reached, turn off the heaters (36) and reset the heater power
control (37) to zero.

7. OPEN the throttling valve (6).

8. As the pressure decreases, record in the throttling process data table the pressure and both RTD
readouts at every 100 kPa decrement until the boiler reaches zero pressure. The pressure decreases
rapidly so plan in advance who will be switching the display and who will be recording each of the
sensor readouts. Note that since the throttling calorimeter insulation has been removed, the first
few seconds of throttling is not adiabatic. Heat is being transferred from the fluid to the throttling
calorimeter (7). Therefore, the assumption that the process is adiabatic is incorrect and equation (6.3)
is invalid. The temperature of the throttling calorimeter will increase quickly and after a few seconds
the process becomes adiabatic and equation (6.3) will be valid.

Shutdown

9. After the last set of readings SWITCH OFF the unit. LEAVE THE THROTTLING VALVE
OPEN to bleed some steam. Leaving the valve closed when the system is at high temperature may
produce a partial vacuum upon cooling which could damage the apparatus.

Page 16

Bibliography

[1] Yunus A. Çengel and Michael A. Boles. Thermodynamics: An Engineering Approach. McGraw-
Hill, 5th edition, 2006. ISBN 0-07-288495-9.

[2] Michael J. Moran, Howard N. Shapiro, Bruce R. Munson, and David P. DeWitt. Introduction to
Thermal Systems Engineering: Thermodynamics, Fluid Mechanics, and Heat Transfer. John
Wiley & Sons, Inc., 2003. ISBN 0-471-20490-0.

[3] Agilent Technologies. Agilent 54621A/22A/24A/41A/42A User’s Guide. Publication Number
54622-97036, September 2002.

[4] L. Solnik manag. ed. M. Kromida D. Irizarry L. C. Forier, ed. and assoc. ed. W. Schildknecht,
editors. MOTOR Auto Engines and Electrical Systems. Motor, New York, NY, 1977. ISBN
0-910992-73-8.

[5] Edward F. Obert. Internal Combustion Engines and Air Pollution. Harper & Row, Publishers,
Inc., New York, NY, 1973.

[6] Bruce R. Munson, Donald F. Young, and Theodore H. Okiishi. Fundamentals of Fluid Me-
chanics. John Wiley & Sons, Inc., 5th edition, 2006. ISBN 0-471-67582-2.

[7] Frank M. White. Viscous Fluid Flow. McGraw-Hill Co., 1974. ISBN 0-07-069710-8.

[8] Frank M. White. Fluid Mechanics. McGraw-Hill, 2003. ISBN 0-07-283180-4.

[9] F-Chart Software. EES Manual. www.fchart.com, v7.663 edition, 2006.

[10] Stephen R. Turns. Thermodynamics: Concepts and Applications. Cambridge University Press,
40 West 20th Street, New York, NY 10011-4211, USA, 2006. ISBN 0-521-85042-8.

[11] J.P. Holman. Experimental Methods for Engineers. McGraw-Hill, 7th edition, 2001. ISBN
0-07-366055-9.

[12] Duane Abata. ME223 Thermodynamics Laboratory Manual. Mechanical Engineering - En-
gineering Mechanics Department, Michigan Technological University, Houghton, Michigan
49931, 2nd edition, 1983.

[13] N.V. Suryanarayana and Öner Arici. Design and Simulation of Thermal Systems. McGraw-
Hill, Inc., 2003. ISBN 0-07-249798-X.

[14] Faye C. McQuiston and Jerald D. Parker. Heating, Ventilating, and Air Conditioning. John
Wiley & Sons, Inc., 2nd edition, 1982. ISBN 0-471-08259-7.

157

www.fchart.com

Page 17

158 BIBLIOGRAPHY

[15] Michael J. Moran and Howard N. Shapiro. Fundamentals of Engineering Thermodynamics.
John Wiley & Sons, Inc., 5th edition, 2004. ISBN 0-471-27471-2.

[16] J.A. Goff. Standardization of thermodynamic properties of moist air. Transactions ASHVE,
55, 1949.

[17] Richard E. Sonntag, Claus Borgnakke, and Gordon J. Van Wylen. Fundamentals of Thermo-
dynamics. John Wiley & Sons, Inc., 5th edition, 1998. ISBN 0-471-18361-X.

[18] Jim Lally and Dan Cummiskey. Dynamic Pressure Calibration. TN-15-0205, PCB Piezotronics,
Inc., Depew, NY 14043 USA, 2005.

[19] Robert Fox and Alan McDonald. Introduction to Fluid Mechanics . John Wiley & Sons, Inc.,
1985. ISBN 0-471-88598-3.

[20] C.C. Heald, editor. Cameron Hydraulic Data. FlowServe Corporation, Canada, 19th edition,
2002.

[21] T. Baumeister, E. A. Avallone, and T. Baumeister III, editors. Marks’ Standard Handbook for
Mechanical Engineers. McGraw-Hill Book Company, 8th edition, 1978. ISBN 0-07-04123-7.

Similer Documents