Page 309 - Sustainable On-Site CHP Systems Design, Construction, and Operations

282 Op erations

HRSGs often have stages, which produce steam at different pressures (e.g., high-
pressure steam, medium-pressure steam, and low-pressure steam). In these cases, the
enthalpy difference of each output stream must be considered separately, so that
HRSG,actual ∑ v ⎡ ρ ⎤ − v ρ
=
Q ⎣ HRSG, so , HRSG, so , h T P ) HRSG,wi , HRSG,wi , h T P) w i , (17-30)
(,
(,
o HRSGG,steam,o ⎦
o
i i HRSG,
i
j j
and
∗
∑ ⎡ ( v ρ) HRSG, ,so h T P o HRSG,steam, ⎦ ⎤ − ( ρ HRSG, wi , h T P ) HRSG, w i ,
( , )
,
(
v )
⎣
o
i
i
o
j j
ε = j (17-31)
HRSG ( ρ T ( − T )
vc )
p HRRSG,ex,i HRSG,ex,i HRSG,w i ,
Here, the summation in the numerator is over all HRSG stages of steam production
with the flow rate, density, and enthalpy for each stage corresponding to the conditions
(e.g., temperature and pressure) of the steam flow exiting the jth stage of the HRSG. If
energy losses from the HRSG are negligible and essentially all of the energy transferred
from the exhaust gas is used to produce steam, the effectiveness of the HRSG can still
be determined from the relation
T − T
ε = HRSG,ex,i HRSG,ex,o (17-32)
HRSG
T − T
HRSG,ex,i HRSG,,wi,
Heat Recovery Steam Generator Calculation
To estimate the HRSG effectiveness (ε ), three temperature measurements are needed
HRSG
[see Eq. (17-32)]. To determine the rate of useful heat output [Q ; see Eq. (17-30)]
HRSG,actual
from the HRSG, additional measurements are needed. These include the flow rate of
the water input to the HRSG, the flow rate, temperature, and pressure for each steam
flow output from the HRSG, along with the corresponding water and steam densities.
In addition, enthalpy tables are needed from which to determine the specific enthalpies
of each steam flow and the water flow from their corresponding measured tempera-
tures and pressures. The measurement of auxiliary input flow is optional and is not
needed to estimate the effectiveness or the rate of useful heat output; however, its mea-
surement will provide information useful to characterizing the fuel use and overall
performance of the CHP system.
Absorption Chiller
As discussed in Chap. 4, absorption chillers are cooling machines that operate similarly
to the mechanically/electrically driven (vapor-compression cycle–based) chillers, except
for the compression process. Like vapor-compression cycle–based chillers, absorption
chillers use a condenser, evaporator, and expansion device. The main difference between
the two types of chillers is how the low-pressure vapor exiting the evaporator is con-
verted to high-pressure vapor that enters the condenser. Instead of a mechanically
driven compressor, absorption chillers use heat to drive the refrigeration cycle. The heat
needed to operate an absorption chiller can be delivered directly or indirectly. In a
∗ Note that because water entering the HRSG is in the liquid state, h is essentially a function of
w,i
temperature only, so that P need not be measured.
HRSG,i