CE 10167 - CE Principles (Energy Balances)

Question 1

What is heat capacity / specific heat?

Answer 1

The energy required to raise the temperature of a unit (mass or mole) of a substance by one degree.

This suggests how well various substances can store energy.

Question 2

What are the two types of heat capacity?

Answer 2

Cv - heat capacity at constant volume.
It’s used for closed systems.

Cp - heat capacity at constant pressure.
It’s used for one systems.

Question 3

How can internal energy be calculated using specific heat capacity?

Answer 3

By integrating Cv (heat capacity at constant volume)

Question 4

How can specific enthalpy be calculated using specific heat capacity?

Answer 4

By integrating Cp (heat capacity at constant pressure)

Question 5

What’s the relationship between Cp and Cv (heat capacity at constant pressure and constant volume) for solids or liquids?

Answer 5

Cp is approx equal to Cv

Question 6

What’s the relationship between Cp and Cv (heat capacity at constant pressure and constant volume) for ideal gases?

Answer 6

Cp = Cv + R

Question 7

What does the symbol Q represent?

Answer 7

Heat energy

Energy transferred as a result of temperature difference between a system and its surroundings

Question 8

What does Ws represent?

Answer 8

Work done on the process fluid (stream) by a moving part within the system (e.g. pump rotor)

Shaft work

Question 9

What’s a steady and unsteady state system?

Answer 9

Unsteady - If the quantities of the variables or operating conditions within the system are changing with time
(also known as ‘transient state’ or ‘dynamic’ system).

Steady - If the quantities of the variables or operating conditions within the system are not changing with time. There is no accumulation.

Question 10

What kinds of energy transfer can occur in closed systems (between system and surroundings)?

Answer 10

Heat
Work

No mass can cross

Question 11

What’s a saturated liquid?

Answer 11

A liquid that’s about to vaporise (e.g. water at 1 atm and 100’C)

Question 12

What’s a subcooled liquid?

Answer 12

A liquid that’s NOT about to vaporise (e.g. water at 1 atm and 25’C)

Question 13

What’s saturated vapour?

Answer 13

Vapour which is about to condense (e.g. vapour at 1 atm and 25’C)

Question 14

What’s a superheated vapour?

Answer 14

A vapour which is not about to condense (e.g. vapour at 1 atm and 200’C)

Question 15

What are steam tables?

Answer 15

Tables which can be used to determine the volume, internal energy, enthalpy and entropy of water or steam and different pressures or temperatures.

Question 16

How is heat (enthalpy) of reaction, ΔĤr calculated?

Answer 16

ΔĤr(T,P) = (Ĥproducts – Ĥreactants)

Where:
Stoichiometric quantities of reactants are fed
The reaction reaches completion
Reactants are fed and products leave at same temp’ T and pressure P

Question 17

What does the sign, +/-, of ΔĤr indicate?

Answer 17

Negative - exothermic and heat given out

Positive - endothermic and heat is taken in/must be supplied

Question 18

What is standard heat (enthalpy) of reaction, ΔĤro?

Answer 18

The heat (enthalpy) of reaction under standard conditions, T = 25°C and P = 1 atm

Question 19

What are standard T and P?

Answer 19

T = 25°C and P = 1 atm

Question 20

What is standard heat (enthalpy) of formation, ΔĤfo?

Answer 20

The heat of reaction in which the compound is formed from its atomic constituents as they normally occur in nature, under standard conditions (e.g. O2 rather than O).

The standard heat of formation of any element, as it occurs in nature, is 0.

Question 21

What is standard heat (enthalpy) of combustion, ΔĤco?

Answer 21

The heat (enthalpy) to produce completely oxidised products at standard conditions (25C and 1 atm).

Question 22

What is a WGS reaction?

Answer 22

Water Gas Shift reaction.

[CO + H2O -> CO2 + H2]

Question 23

What is latent heat of vaporisation, ΔĤv

?

Answer 23

Heat energy required to vaporise a unit amount of liquid (at constant P and T)

Liquid to Gas

Question 24

What is latent heat of fusion, ΔĤm?

Answer 24

Heat energy needed to melt a unit amount of solid (at constant T and P)

Solid to Liquid

Question 25

What is latent heat of sublimation, ΔĤs

?

Answer 25

Heat energy needed to vaporise a unit amount of solid (constant T and P)

Solid to Gas

Question 26

What is humidification and dehumidification?

Answer 26

Humidification - addition of moisture

Dehumidification - removal of moisture

Question 27

What is absolute humidity, hₐ?

Moisture content

Answer 27

It’s the measure of water vapor (moisture) in the air, regardless of temperature.

= mass of water (W) / mass of dry air (DA)

= hₐ

Question 28

What is relative humidity, hᵣ?

Answer 28

It’s a measure of water vapor/moisture RELATIVE to the temperature of the air.

It is expressed as amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature.

= 100 * absolute humidity /saturated humidity

= ( hₐ / hₛ ) * 100

Question 29

How is absolute humidity, hₐ, affected by heating?

Answer 29

hₐ does not change with temperature.

Question 30

How is relative humidity, hᵣ, affected by heating?

Answer 30

Relative humidity decreases

This is because saturated humidity, hs increases (and hᵣ = hₐ/hs)

Question 31

What is saturated humidity, hs?

Answer 31

= mass of water when air is saturated / mass of dry air

Question 32

What do T(db) and T(wb) represent?

Answer 32

T(db) is dry bulb temperature. This shows actual air temperature.

T(wb) is wet bulb temperature. This shows steady state temperature of the wet bulb.

Question 33

How do dry bulb temperature (Tdb) and wet bulb temperature (Twb) compare for unsaturated and saturated air?

Answer 33

For unsaturated air (hᵣ < 100%):
Wet bulb temperature [T(wb)] < dry bulb temperature [T(db)]

For saturated air (hᵣ = 100%):
Wet bulb temperature [T(wb)] = dry bulb temperature [T(db)]

Question 34

What is dew point temperature, T dew?

Answer 34

The temperature to which air must be cooled for it to reach saturation, at a given pressure and water-vapor content.

The temperature at which air with humidity hₐ becomes saturated at the same pressure when cooled.

Question 35

What is a psychometric chart?

Answer 35

A humidity chart.
Psychrometric processes include physical and thermodynamic properties such as dry bulb temperature, wet bulb temperature, humidity, enthalpy, and air density.

Question 36

What is the heat of solution (aka integral heat of solution), ΔĤs(T, n) and ΔĤs(T, ∞) ?

Answer 36

ΔĤs(T, n):
It’s the change in enthalpy for a process in which 1 mole of a solute (gas or solid) is dissolved in n moles of a liquid solvent at constant temperature, T.

ΔĤs(T, ∞) is the heat of solution at infinite dilution.