Procedure for Energy Balance Calculations

1
Procedure for Energy Balance Calculations

• Draw and completely label a process flowchart
• Perform all material balance calculations
• Write the appropriate form of the energy balance
  equation and remove any negligible terms
• Choose a reference state (phase, T, P) for each
  species involved
• If using enthalpy tables, use reference state
  used to generate table
• If no tables are available, choose one inlet or
  outlet condition as the reference state for the
  species
• Construct an inlet-outlet enthalpy table
• Columns for inlet and outlet amounts of each
  species along with their corresponding Ûi or Hi
  values
• Use a separate row for each phase of a species
• Identify unknowns with variables (e.g., H1, H2,
  etc.)

2
Procedure for Energy Balance Calculations

• Calculate all required values of Ûi or Hi and
  insert values into table
• Calculate ?U or ?H (e.g., ?H?miHi- ?miHi)
• Calculate any other terms in the energy balance
  equation (i.e., W, ?Ek, ?Ep)
• Solve for the unknown quantity in the energy
  balance equation
• Typically solve for Q but sometimes asked to
  solve for a mass (mole) flow or occasionally a T!

3
Example 1

• A stream containing 10 CH4 and 90 air by volume
  is to be heated from 20?C to 300?C. Calculate
  the required rate of heat input in kilowatts if
  the flow rate of the gas is 2.00 x 103 liters
  (STP)/min.

4
Example 2

• Acetone (denoted as Ac) is partially condensed
  out of a gas stream containing 66.9 mole acetone
  vapour and the balance nitrogen. Process
  specifications and material balance calculations
  lead to the flowchart shown below.
• The process operates at steady-state. Calculate
  the required cooling rate.

5
Interactive Tutorial

• Now is a good time to try Interactive Tutorial 5