Thermodynamic Units and Properties

Question 1

What are the two systems of measurement in the United States?

Answer 1

American Engineering System, also known as English Engineers System; and International System of Units (SI), also known as Metric System

Question 2

What is a conversion factor?

Answer 2

A conversion factor is ratio of two equivalent physical quantities that are expressed in different units

Question 3

What is a system?

Answer 3

A system is general term that describes quantity of matter or space that is separated from its surroundings by system boundaries

Question 4

What is a working fluid?

Answer 4

Fluid in which energy can be stored and removed

Question 5

What are properties?

Answer 5

Properties of substance are measurable characteristics used to describe substance

Question 6

Explain the difference between intensive and extensive properties.

Answer 6

An intensive property is independent of mass and does not depend on how much of substance is present. 4. An extensive property depends on mass (or how much of substance is present).

Question 7

State the definition of Mass.

Answer 7

The mass of a body is a measure of the amount of material present in that body.

Question 8

State the definition of Specific Volume.

Answer 8

The specific volume of a substance is the volume occupied by a unit of mass of the substance.

Question 9

State the definition of Density.

Answer 9

The density of substance is its total mass divided by its total volume. Density is reciprocal of specific volume

Question 10

State the definition of Specific Weight.

Answer 10

Specific weight is weight per unit volume of substance.

Question 11

State the definition of Specific Gravity.

Answer 11

The specific gravity of fluid is ratio of fluid density to density of water at 40°F (62.4 lbm/ft3).

Question 12

State the definition of Temperature.

Answer 12

Temperature is measure of random motion of molecules of system

Question 13

State the definition of Pressure.

Answer 13

Pressure is force per unit of area that substance exerts on its surroundings

Question 14

Explain the difference between absolute and relative temperature scales.

Answer 14

Relative scales (F & C) are based on boiling and freezing points of water at standard atmospheric pressure. The scales provide only relative thermal energy content. Analysis of system thermodynamic properties requires absolute temperature scale with reference point corresponding to zero energy content. Both common temperature scales (F & C) have corresponding absolute temperature scales (Rankine and Kelvin)

Question 15

Convert from Fahrenheit to Rankine.

Answer 15

Add 460 to F

Question 16

Convert from Celsius to Kelvin.

Answer 16

Add 273 to C

Question 17

Describe how a manometer works.

Answer 17

A u-tube manometer is usually filled with water or mercury. With both ends of tube subjected to same pressure, fluid levels on each side will be equal. If the pressure applied to one of the tube ends is increased, fluid column in opposite tube end will increase. The difference in height between two levels is equal to differential pressure

Question 18

Describe how Bourdon tubes work.

Answer 18

The most common type is “C” type. It consists of a flattened, thin walled, hollow, metal tube with oval cross section bent into an arc of circle. One end is connected to pressure source and is fixed. The other end is connected, via linkage, to gauge indicator, and has some freedom of movement. When pressure is applied within tube, force exerted on outer tube wall is greater than force exerted on inner tube wall because area of outer tube arc is greater than area of inner tube arc.

Question 19

Describe how Diaphragm pressure gauges work.

Answer 19

Pressure to be sensed is applied to one side of diaphragm and atmospheric pressure is applied to other. The diaphragm moves in response to changes in pressure in chamber. This motion can be used to change resistance in electrical circuit, vary pressure in pneumatic system, or actuate a switch. A disadvantage of is that large differential pressures across diaphragm can damage diaphragm, including rupture.

Question 20

Describe how Bellows pressure gauges work.

Answer 20

Usually built as one-piece, collapsible, seamless metallic unit with deep folds formed from very thin-walled tubing. . The opposing spring of bellows gauge exerts a compressive force on bellows. The movement or of bellows must overcome this spring pressure to move linkage. In practice, bellows is always opposed by spring, and movement characteristics of bellows are result of spring and bellows. The stroke of bellows usually encompasses from fifty to ninety percent of bellows length.

Question 21

Describe how Hydrostatic head level detectors operate.

Answer 21

A vertical column of liquid in vessel exerts hydrostatic pressure equal to product of density of liquid times height of liquid column. A hydrostatic level detector is simply pressure gauge at bottom of vessel that uses this principle of operation. Must compensate for changes in density via temperature.

Question 22

Describe how open vessel differential pressure level detectors operate.

Answer 22

A differential pressure transmitter measures vessel hydrostatic pressure in reference to atmospheric pressure. As level rises, hydrostatic pressure increases.The differential pressure felt by transmitter increases since atmospheric pressure remains relatively constant. This increase in differential pressure is processed by level indication circuitry.

Question 23

Describe how closed vessel differential pressure level detectors operate.

Answer 23

Instead of referencing hydrostatic head developed by fluid to known reference such as atmospheric, static pressure above fluid is applied to reference (low pressure, LP) side of detector by pipe and isolation valve. The high pressure side of detector senses both hydrostatic pressure due to fluid level and static pressure due to volume of vapor or gas. In this manner, static pressure in tank is sensed on both sides of detector, and therefore cancelled. Reference leg can by dry or wet.

Question 24

Describe Work.

Answer 24

In physics and thermodynamics, work is defined as force (F) used to move mass, multiplied by distance (d) mass is moved

Question 25

Describe Power.

Answer 25

Power is rate of doing work, or work done per unit of time

Question 26

Describe Energy.

Answer 26

Energy is capacity of system to perform work or produce heat

Question 27

State the difference between mechanical work and flow work.

Answer 27

Mechanical work is mechanical energy in transition. The energy transfer is effected and evidenced by a force acting through a distance. Flow work is the work required to maintain a continuous, steady flow of fluid.

Question 28

Identify the four forms of energy possessed by a working fluid in an energy transfer system.

Answer 28

Potential energy, kinetic energy, internal energy, and PV energy

Question 29

Explain the difference between absolute and relative pressure scales.

Answer 29

Abs = Atm plus guage. Abs = Atm - Vacuum

Question 30

If a main steam line pressure gauge reads 900 psig, what is the absolute pressure (Assume an atmospheric pressure of 15 psia

Answer 30

915 psia

Question 31

Convert Celsius to Farenheit

Answer 31

(9/5)C + 32

Question 32

Convert Farenheit to Celsius

Answer 32

(5/9)(F-32)