Energy and Energy Balances
NOTE: Superscripts are placed before their respective symbol to indicate time
derivatives (m = mass/time) or specific quantities (^U = internal energy/mol or internal
The First Law of Thermodynamics.
Example 2 (A Transient-State, Reactive Material Balance Problem with Step Input)
A beaker with a sidearm is continuously fed with a clear water stream (inflow) at a constant
volumetric rate of V (volume/time). The beaker is placed on a magnetic stirrer to
Energy Balances on Reactive Processes
Heats of Reaction.
AA + BB CC + DD: ^Hr(T, P)
(1) states that A moles of A react with B moles of B to form C moles of C and D moles
of D at the temperature of T and the pressure P. (1) also implie
Spray Cooling (Humidification/Dehumidification)
8.80 (Felder and Rousseau)
Base: 1 kg dry air (DA).
The intersection of the 18oC (Tas = Twb) adiabatic saturation curve line and the vertical
40oC (Figure 8.4-1) yields (ha)inlet air = 0.0039 kg H2O(v)/k
Phase Changes (Latent Heats)
^H associated with the transition of a substance from one phase to another at
constant temperature and pressure is the latent heat of the phase change.
^Hm is the latent heat of fusion (or melting) (between solid and liquid ph
The condtions of a gas change from 20oC and 1,000 kPa to 60oC and 100 kPa. Devise a process
with any number of hypopthetical paths for accomplishing this change of state, and calculate
^U and ^H for the process on the basis of 1 mole of gas. Ass
Energy Balances on Non-Reactive Processes
Four Important Energy Balance Equations:
Ek + Ep + U = Q W
H + Ek + Ep = Q - Ws
U = (ni^Ui)final (ni^Ui)initial
U = (mi^Ui)final (mi^Ui)init
H = (ni^Hi)output (ni^Hi)input
H = (mi^Hi)ou
In the system shown below, water is discharged from the water tank through a pipe (pipe
diameter: 0.15 m) at point 2 at a rate of 60 kg/s. Is the machine a pump or a turbine?
Assumptions: (1) P1 = P2 = Patm, (2) the surface area of the tank is v
Changes in Temperature at Constant Volume (a Constant-V Process). Since the specific internal
energy of a substance depends strongly on temperature, the ratio ^U/T approaches to a
limiting value when the system volume is kept constant. This