Page 307 - Sustainable On-Site CHP Systems Design, Construction, and Operations

280 Op erations

)
)
when (ρ vc p HRU,min = (ρ vc p HRU,ex
Also from Eq. (17-15),

(T − T )
ε = HRU,,ao HRU, ,a i (17-22)
HRU (T − T ) )
HRU,ex,i HRU, ,i
a
when (ρ vc ) = (ρ vc )
p HRU,min p HRU, a
Determination of which fluid provides (ρ vc ) and, therefore, whether to use
p HRU,min
Eqs. (17-21) and (17-22), for an HRU heating air, can be accomplished using the following
relations obtained by rearranging Eq. (17-19) (with conditions for water replaced with
conditions for air):

)
(ρ vc ) = (ρ vc p HRU,ex for (T HRU,ex, i − T HRU,ex, o ) > (T HRU, , ao − T HRU, , a i ) (17-23)
p HRU,min
and
)
(ρ vc ) = (ρ vc p HRU, a for (T HRU, , ao − T HRU, , a i ) > (T HRU,ex, i − T HRU,ex, o ) (17-24)
p HRU,min
By using Eqs. (17-20) through (17-24), the effectiveness of an HRU using exhaust
gases to produce hot air or hot water can be determined from temperature measure-
ments alone, without the need for any flow rate measurement. However, to determine
the useful heat output of the HRU, one flow rate must be measured.

Heat Recovery Unit Effectiveness Calculation
To estimate the HRU effectiveness (one of the outputs shown), three temperature mea-
surements are needed [see Eq. (17-20)]. Furthermore, to determine the rate of useful
heat output (Q ) from the HRU, the flow rate of the water and one additional
HRU,actual
temperature (T ) must be measured [see Eq. (17-12)]. The measurement of auxil-
HRU,w,o
iary input flow is optional and is not needed to estimate the effectiveness or the rate
of useful heat output. In addition to the five measured inputs, the specific heat and the
density of water are also needed.
For an HRU producing hot air from exhaust gases, four temperature measurements
are required to determine which fluid establishes (ρ vc ) using Eqs. (17-23) and
p HRU,min
(17-24), and then three of those measurements are used to calculate the HRU effective-
ness from Eq. (17-21) or (17-22). No flow rate measurements are required to determine
the HRU effectiveness; however, as with the HRU that produces hot water, determina-
tion of the rate of useful heat output requires measurement of one flow rate, preferably
the flow rate of air, and values for the specific heat and density of the air (only the specific
heat if the mass flow rate is measured directly). The accuracy of results can be increased
by evaluating the specific heat of gases at the average of inlet and outlet conditions.

Heat Recovery Steam Generator
A heat recovery steam generator (HRSG) is a heat exchanger that recovers heat from a
hot gas stream and produces steam that can be used in a thermal process or used to
drive a steam turbine. A common application for a HRSG is in a combined cycle power
plant, where hot exhaust from a gas turbine is fed to an HRSG to generate steam, which