Page 308 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining Operational Ef ficiency of a CHP System 281

in turn drives a steam turbine. In CHP applications, the HRSG is generally used to
generate steam to meet facility thermal demands by firing an absorption chiller or run-
ning a steam turbine–driven chiller. An HRSG is similar to an HRU. The main differ-
ence between the HRU and HRSG is that the HRSG generates steam instead of hot
water or hot air.
Effectiveness of Heat Recovery Steam Generator
The effectiveness of a heat recovery steam generator (ε HRSG ) also can be determined from the
general equation for ε , Eq. (17-11). In this case, the actual heat transfer includes the
HRU
heat of vaporization of the water as well as the sensible heat used to increase its tem-
perature. Therefore, when expressed in terms of the change in the water side, the rate of
heat transfer is equal to the difference in enthalpy between the water entering the HRSG
and the steam leaving the HRSG, both of the enthalpies being functions of the fluid
temperatures and pressures, that is,

,
h T P , )
(
Q = v ( ρ ) [ ( − hT P) ] (17-25)
HRSG,actual HRSG, wi , o o HRSG,steaam,o i i HRSG, ,i
w
under the assumption that the mass flow rate of water input to the HRSG is equal to the
mass flow rate of steam output. Here, h is the specific enthalpy of steam leav-
HRSG,steam,o
ing the HRSG at temperature T and pressure P , and h is the specific enthalpy of
o o HRSG,w,i
the water entering the HRSG at temperature T and pressure P . The volumetric flow
i
i
rate ( v ) and density (ρ ) are for water at the inlet to the HRSG.
HRSG, wi , HRSG,wi ,
Alternatively, the rate of heat transfer could be determined for the rate of heat loss
from the hot exhaust gas as it passes through the HRSG (assuming that jacket heat
losses are negligible). In this case, the rate of heat transfer is given by the relation
v c )
Q = ( ρ T ( − T ) (17-26)
HRSG,actual p HRSG,ex, i HRSG,ex, i HRRSG,ex,o
where v and ρ are, respectively, the volumetric flow rate and density of
HRSG,ex, i HRSG,ex,i
exhaust gas coming into the HRSG; c is the specific heat of the exhaust gas mixture;
p,ex
and T and T are the temperatures of the exhaust gas streams coming into
HRSG,ex,i HRSG,ex,o
and leaving the HRSG, respectively.
The maximum possible rate of heat transfer between the two fluids (Q ) is
HRSG, max
given by
v c )ρ
Q = ( T ( − T ) (17-27)
HRSG,max p HRSG,ex, i HRSG,ex, i HRSG,,wi ,
where T HRSG,w,i is the temperature of the saturated liquid water coming into the HRSG.
Therefore, for an HRSG, the effectiveness can be expressed as ∗
( v ρ) [ ( , ) − ( h TTP,) ]
h T P
ε = HRSG, ,wi o o HRSG,steam,o i i HRSG, w i , (17-28)
HRSG ( ρ T ( − − T )
vc )
p HRSG,ex, i HRSG,ex, i HRSG, wi ,
or
T − T
ε = HRSG,ex,i HRSG,ex,o (17-29)
HRSG
T − T
HRSG,ex,i HRSG,,wi,
p ex,i ( vcρ p ) w,i
∗ Assuming that ( v cρ ) <