Temperature and Heat

What are the 3 methods of heat transfer and key points for each?

Conduction
 - Direct contact with molecules
 - Common in solids
 - Vibrations transfer between molecules
 - Heat exchangers transfer heat via conduction
 - Primary cause of heat loss in buildings
 - Silver and copper conduct well; air, wood, water do not
Convection
 - Heat transfer within gases/liquids due to density
 - Seen in hot air balloons, hot water tanks, baseboard heaters, forced air furnaces
Radiation
 - Transmitted via infrared (electromagnetic) rays
 - Anything above absolute zero emits IR
 - Passes through vacuum
 - IR heaters heat objects directly, even in cold
 - Shiny surfaces reflect IR, absorb poorly
 - Emits sensible heat

What does Charles' Law state and how is it applied?

Volume is directly proportional to absolute temperature at constant pressure
 - Uses absolute temperatures (F + 460 = Rankine, C + 273 = Kelvin)
 - Formula: V1/T1 = V2/T2
 - Heat a gas, it expands

What does Boyle's Law state and how is it applied?

Volume of dry gas varies inversely with absolute pressure at constant temperature
 - Formula: P1V1 = P2V2
 - Compressing a gas increases pressure

What is the ideal gas law and what do its variables represent?

Formula: PV = nRT
 - P = pressure (kPa or Pa), absolute not gauge
 - V = volume (L or m³)
 - n = amount of gas (moles)
 - R = ideal gas constant (depends on units)
 - T = temperature (Kelvin)

What is Gay-Lussac’s Law and how is it derived?

Combination of Charles’ and Boyle’s Laws
 - Used when number of moles stays constant
 - Formula: P1V1/T1 = P2V2/T2
 - Cross-multiplied: P1V1T2 = P2V2T1

What is the difference between BTU and kJ, and what are the key specific heat values?
Specific Heat BTU/(lb·°F) - Brackets (kJ/kg*DegC):
 - Water:
 - Ice:
 - Steam:
 - Air:
 - Propane (liquid):

BTU = imperial, kJ = metric
Specific Heat BTU/(lb·°F) - Brackets (kJ/kg*DegC)::
 - Water: 1 (4.2)
 - Ice: 0.5 (2)
 - Steam: 0.5 (2)
 - Air: 0.24 (1.01)
 - Propane (liquid): 0.6 (2.47)

How do you calculate sensible heat and what does it represent?

Sensible heat = specific heat × mass × temperature difference
 - Change in temperature, not state
 - Works in reverse as well

What is latent heat and what terms must you memorize?
- Fusion
 - Solidification
 - Vaporization
 - Condensation

Latent heat = heat required to change state, not temperature
 - Fusion = solid to liquid
 - Solidification = liquid to solid
 - Vaporization = liquid to gas
 - Condensation = gas to liquid
 - 970 BTU to vaporize water = 970 BTU recoverable when condensing steam

What are the key latent heat values to memorize?
Ice:Water
Water:Steam
Propane

Fusion/Solidification BTU/lb (kJ/kg):
 - Ice:Water = 144 (335)
 - Water:Steam = 970 (2257)
 - Propane = 185 (356)

How do you calculate total heat in a phase change example?

Total heat = sensible heat + latent heat
Example: 325L water from 8°C to steam at 115°C
 1. Sensible (8°C to 100°C): 4.2 × 325kg × 92°C = 125,580 kJ
 2. Latent (water to steam): 325kg × 2257 = 733,525 kJ
 3. Sensible (steam 100°C to 115°C): 2 × 325kg × 15°C = 9,750 kJ
 4. Total = 125,580 + 733,525 + 9,750 = 868,855 kJ

What type of heat transfer does insulation on a pipe prevent?

Conduction

Gas with volume of 120 ft³ at 300°F increased to 500°F — what’s the new volume?

11.57 ft³

Gas with 625 ft³ at 127 psia increased to 500 psig — what’s the new volume?

154.2 ft³ (mixed pressure units in question - watch out for this)

How much heat energy is needed to raise 6 lb of steam from 200°F to 300°F?

240 BTU

How much energy to heat 150L water from 5°C to 95°C?

56,700 kJ