Lesson 3 ENERGY, WORK, AND HEAT DEFINE the following: – Heat – Latent heat – Sensible heat – Units used to measure heat DEFINE the following thermodynamic properties: – Specific enthalpy – Entropy

Energy The capacity of a system to perform work or produce heat Categories – Potential Energy – Kinetic Energy – Specific Internal Energy – Specific P-V Energy – Enthalpy – Work – Heat – Entropy

Potential Energy Energy of position PE = mgz/g c where PE = potential energy (ft-lbf) m = mass (lbm) z = height above some reference level (ft) g = acceleration due to gravity (32.17 ft/sec 2 ) g c = gravitational constant = ft-lbm/lbf-sec 2

Kinetic Energy Energy of motion KE = mv 2 /2g c where: KE = kinetic energy (ft-lbf) m = mass (lbm) v = velocity (ft/sec) g c = gravitational constant = ft-lbm/lbf-sec 2

Specific Internal Energy Internal energy per unit mass Equal to total internal energy (U) divided by the total mass (m). u = U/m where: u = specific internal energy (Btu/lbm) U = internal energy (Btu) m = mass (lbm) Internal energy includes – Energy due to rotation, vibration, translation, and interactions among molecules – Can not be measured or evaluated directly

Specific P-V Energy Energy per unit mass. Equals the total Pressure times the Volume of a liquid divided by the total mass m Also equals the product of the pressure P and the specific volume v, and is written as Pv Pv = PV/m where: P = pressure (lbf/ft 2 ) V = volume (ft 3 ) n = specific volume (ft 3 /lbm) V m m = mass (lbm)

Enthalpy A property of a substance, like pressure, temperature, and volume, Cannot be measured directly Normally given with respect to some reference value. Usually used in connection with an "open" system problem in thermodynamics Specific enthalpy (h) h = u + Pv where u is the specific internal energy (Btu/lbm) P is the pressure of the system (lbf/ft 2 ) v is the specific volume (ft 3 /lbm) of the system.

Work Energy in transit Not a property of a system – it is a process done by or on a system Defined as the action of a force on an object through a distance W = Fd where: W = work (ft-lbf) F = force (lbf) d = displacement (ft)

Heat Like work, is energy in transit Occurs at the molecular level as a result of a temperature difference Denoted by the letter Q Heat transferred per unit mass denoted by q q = Q/m where: q = heat transferred per unit mass (Btu/lbm) Q = heat transferred (Btu) m = mass (lbm)

Heat (continued) Sensible Heat - The heat added to or removed from a substance to produce a change in its temperature Latent Heat - The amount of heat added to or removed from a substance to produce a change in phase. – Latent Heat of Fusion - the amount of heat added or removed to change phase between solid and liquid. – Latent Heat of Vaporization - the amount of heat added or removed to change phase between liquid and vapor

Heat Capacity The ratio of the heat (Q) added to or removed from a substance to the change in temperature (ΔT) produced Denoted by C p Specific heat (c p ) is heat capacity of a substance per unit mass Applies when the heat is added or removed at constant pressure

Heat Capacity (continued) C p = Q/ΔT c p = Q/mΔT c p = q/ΔT where: C p = heat capacity at constant pressure (Btu/°F) c p = specific heat at constant pressure (Btu/lbm-°F) Q = heat transferred (Btu) q = heat transferred per unit mass (Btu/lbm) m = mass (lbm) ΔT = temperature change (°F)

Entropy (S) A property of a substance Quantifies the energy of a substance that is no longer available to perform useful work ΔS = ΔQ/T abs Δ s = Δq/T abs where: Δ S = the change in entropy of a system during some process (Btu/°R) ΔQ = the amount of heat transferred to or from the system during the process (Btu) T abs = the absolute temperature at which the heat was transferred (°R) Δs = the change in specific entropy of a system during some process (Btu/lbm -oR) Δq = the amount of heat transferred to or from the system during the process (Btu/lbm)

Power Power - The time rate of doing work. Units are energy per unit time – English system - ft-lbf/sec or ft-lbf/hr – Horse Power - hp – British thermal units per hour - Btu/hr – Electrical units - watts (W) or kilowatts (kW)

Energy and Power Equivalencies 1 ft-lbf = x Btu = x kW-hr 1 Btu = ft-lbf = x kW-hr 1 kW-hr = x 10 3 Btu = x 10 6 ft-lbf 1 hp-hr = x 10 6 ft-lbf 1 Joule = 778 ft lbf/Btu 1 ft-lbf/sec = Btu/hr = x kW 1 Btu/hr = ft-lbf/sec = x kW 1 kW = x 10 3 Btu/hr = ft-lbf/sec 1 hp = ft-lbf/sec