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 or open)
• 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 or potential)
• Solve the problem!

OUTCOME:

Simplify and solve the General Energy Balance for non-reactive systems

TEST YOURSELF

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 enthalpies!)