The effectiveness of a recuperator drops if the heat capacitance flows of
primary and secondary media are not equal. The effectiveness of a
counter flow recuperator in unbalance is described by:
cpmin smallest heat capacity flow [WK-1]
cpmax largest heat capacity flow [WK-1]
kA heat exchanging capacitance [WK-1]
If the heat capacity flows are equal, (1) degrades to a simpler form:
ε= kA / (kA + C) (2)
In fig.1 the effectiveness’s of the primary and the secondary flows are
shown as function of the ratio between the flows (at 1 they are balanced).
Figure 1 effectiveness at unbalanced flow
If the primary flow (blue curve) is smaller, the apparent effectiveness
of the primary flow is high, but the real effectiveness is that of the
secondary flow, which is lower than in balanced flow. As follows from (1)
only the lower branches of the curves have to be used. It may seem that
at a smaller primary flow the effectiveness is higher, but one must
consider that the potential to recuperate heat from the secondary flow is
not used to its full potential.
Balancing the mass (heat capacity) flows is crucial for a good system
performance; 1% unbalance leads to 1% effectiveness decrease.