CE 10167 - CE Principles (Energy Balances) Flashcards
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.
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.
How can internal energy be calculated using specific heat capacity?
By integrating Cv (heat capacity at constant volume)
How can specific enthalpy be calculated using specific heat capacity?
By integrating Cp (heat capacity at constant pressure)
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
What’s the relationship between Cp and Cv (heat capacity at constant pressure and constant volume) for ideal gases?
Cp = Cv + R
What does the symbol Q represent?
Heat energy
Energy transferred as a result of temperature difference between a system and its surroundings
What does Ws represent?
Work done on the process fluid (stream) by a moving part within the system (e.g. pump rotor)
Shaft work
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.
What kinds of energy transfer can occur in closed systems (between system and surroundings)?
Heat
Work
No mass can cross
What’s a saturated liquid?
A liquid that’s about to vaporise (e.g. water at 1 atm and 100’C)
What’s a subcooled liquid?
A liquid that’s NOT about to vaporise (e.g. water at 1 atm and 25’C)
What’s saturated vapour?
Vapour which is about to condense (e.g. vapour at 1 atm and 25’C)
What’s a superheated vapour?
A vapour which is not about to condense (e.g. vapour at 1 atm and 200’C)
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.
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
What does the sign, +/-, of ΔĤr indicate?
Negative - exothermic and heat given out
Positive - endothermic and heat is taken in/must be supplied
What is standard heat (enthalpy) of reaction, ΔĤro?
The heat (enthalpy) of reaction under standard conditions, T = 25°C and P = 1 atm
What are standard T and P?
T = 25°C and P = 1 atm
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.
What is standard heat (enthalpy) of combustion, ΔĤco?
The heat (enthalpy) to produce completely oxidised products at standard conditions (25C and 1 atm).
What is a WGS reaction?
Water Gas Shift reaction.
[CO + H2O -> CO2 + H2]
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
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
What is latent heat of sublimation, ΔĤs
?
Heat energy needed to vaporise a unit amount of solid (constant T and P)
Solid to Gas
What is humidification and dehumidification?
Humidification - addition of moisture
Dehumidification - removal of moisture
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
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
How is absolute humidity, hₐ, affected by heating?
hₐ does not change with temperature.
How is relative humidity, hᵣ, affected by heating?
Relative humidity decreases
This is because saturated humidity, hs increases (and hᵣ = hₐ/hs)
What is saturated humidity, hs?
= mass of water when air is saturated / mass of dry air
What do T(db) and T(wb) represent?
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.
How do dry bulb temperature (Tdb) and wet bulb temperature (Twb) compare for unsaturated and saturated air?
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)]
What is dew point temperature, T dew?
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.
What is a psychometric chart?
A humidity chart.
Psychrometric processes include physical and thermodynamic properties such as dry bulb temperature, wet bulb temperature, humidity, enthalpy, and air density.
What is the heat of solution (aka integral heat of solution), ΔĤs(T, n) and ΔĤs(T, ∞) ?
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.
