Mass and energy balances must be completed for each piece of equipment before detailed design can begin. A mass balance is done for each unit shown on the process flowsheet, while an energy balance is done over groups of units where energy is transferred between process fluids. Even at the design stage, some flowrates and temperatures may only be estimates. The mass and energy balances will need updating after detailed design is complete. Calculations are included as an appendix and provide details like mass flowrates, compositions, temperatures, pressures, and enthalpy contents of all streams.

1. 3. MASS AND ENERGY
BALANCES
3.1 PREPARATION OF MASS AND
ENERGY BALANCES
Mass and energy balances must be
completed for each item, of equipment
before the detailed design of these items
can begin.
A mass balance is performed for each
individual unit shown on the process
flowsheet and included in the,
equipment list.
An energy balance is performed over a
group of units (or a section of the plant)
where energy is being transferred
between various process fluids, e.g. the
feed-preheating heat exchangers or the,

2. reactor and a subsequent
cooler/condenser. After the energy
balances have been completed it is
possible to consider the efficient
utilisation of energy within the plant,
and hence prepare an energy
conservation scheme.
Even at this stage in the design work, it
may not be possible to finalise all
details of the mass and, energy
balances. A specification of
performance should be available for
each unit, but only estimates of certain
flowrates and temperatures may be
available.
These mass and energy balances will
need amendment and checking
following the detailed and final design

3. of each item, of equipment. For
example, it may be specified that a
liquid-washing absorption tower is to,
remove at least a certain percentage of a
gaseous product stream. It is usually
possible to, estimate the flowrate of
liquid required, but this quantity will
not be known accurately until the
detailed design of the unit is completed.
The design will consider the, type and
height of packing materials required,
and the liquid flowrate necessary for
efficient operation of the tower.
The calculations are detailed separately,
and often included as an appendix to
the design report. The mass and energy
balance for each unit should include the
following details:

4. (a) Mass flow of all streams into and
out of the equipment per unit time.
Molar or volume flowrates must not be
used. A convenient and appropriate unit
of time is chosen, e.g. per minute, hour,
day, etc., so that the numbers being
handled can he easily appreciated, e.g.
between 0 and 1000, and as far as
possible avoiding the use of 10 or 10-',
etc. All balances must be performed for
the same time period.
(b) Composition (mass %) of all
streams.
(c) Sometimes the molar flowrates (and
mole %) of gas streams are also
included.
(d) Temperature of each stream; use o
C
or K consistently (for temperatures
below Oo
C, absolute temperatures may
be preferred).

5. (e) Pressure of each stream, or state the
pressure once if no significant pressure
drop occurs.
(f) Enthalpy content of each stream (J,
MJ, GJ, etc.).