CE 10167 - CE Principles (Energy Balances) Flashcards Preview

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Flashcards in CE 10167 - CE Principles (Energy Balances) Deck (36)
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1

What is heat capacity / specific heat?

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.

2

What are the two types of heat capacity?

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

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

3

How can internal energy be calculated using specific heat capacity?

By integrating Cv (heat capacity at constant volume)

4

How can specific enthalpy be calculated using specific heat capacity?

By integrating Cp (heat capacity at constant pressure)

5

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

Cp is approx equal to Cv

6

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

Cp = Cv + R

7

What does the symbol Q represent?

Heat energy

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

8

What does Ws represent?

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

Shaft work

9

What's a steady and unsteady state system?

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.

10

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

Heat
Work

No mass can cross

11

What's a saturated liquid?

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

12

What's a subcooled liquid?

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

13

What's saturated vapour?

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

14

What's a superheated vapour?

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

15

What are steam tables?

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

16

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

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

17

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

Negative - endothermic and heat given out

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

18

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

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

19

What are standard T and P?

T = 25°C and P = 1 atm

20

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

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.

21

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

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

22

What is a WGS reaction?

Water Gas Shift reaction.

[CO + H2O -> CO2 + H2]

23

What is latent heat of vaporisation, ΔĤv
?

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

Liquid to Gas

24

What is latent heat of fusion, ΔĤm?

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

Solid to Liquid

25

What is latent heat of sublimation, ΔĤs
?

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

Solid to Gas

26

What is humidification and dehumidification?

Humidification - addition of moisture

Dehumidification - removal of moisture

27

What is absolute humidity, hₐ?

(Moisture content)

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

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

= hₐ

28

What is relative humidity, hᵣ?

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

29

How is absolute humidity, hₐ, affected by heating?

hₐ does not change with temperature.

30

How is relative humidity, hᵣ, affected by heating?

Relative humidity decreases

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