Energy Balance Procedure
Typically, solving energy balance problems is only marginally
more involved than mass balances alone. In addition to whatever
else we might be solving in the mass balance, we will now be
asked to calculate the amount of heat or work going to/from the
system or possibly determine something about the state of one of
the streams (by finding an unknown enthalpy or energy).
The procedure to solve mass/energy balances...
- (TRY TO) Perform all material balances (doing degrees of
freedom and all the rest of that procedure)...it's possible
that you will have to count the energy balance as an additional
equation in your degrees of freedom analysis (of course, you
must then also count any unknowns that arise in the energy
equation in this analysis)
- Write down the appropriate energy balance equation (closed
- Choose a reference state for all the species in the process
(T, P, phase)
- Simplify the energy balance equation (see previous lectures
for hints here)
- Obtain values for all the specific energies (U) or
enthalpies (H) and calculate the changes in these values
(remember that you are adding all the ins (initials) for each
species and all the outs (finals) for each species, you are NOT
doing a separate energy balance for each material).
- Calculate any (other) terms that didn't drop out (kinetic
- Solve the problem!
Simplify and solve the General Energy Balance
for non-reactive systems
Try this simple example!
You want to make do a mass balance on the following evaporator
system, yet you do not have enough material data. Instead you
need to rely on an energy balance to help you along. Determine
the split of liquid and vapor coming out of the evaporator.
(NOTE: check the units, the H's listed here are actually specific