Chemical Engineering October 2013 - 54

Engineering Practice
To stack
Water in
Economizer
Evaporator
Superheater
Gas turbine exhaust in
Superheated
steam
Forced
circulation
heat-recovery
steam generator
(HRSG)
FIGURE 3a (above). In this forced-circulation
boiler (generally seen with horizontal
evaporator tubes), a circulation pump is
used to suck water from the drum and ensure
its fl ow through the evaporator tubes
and back into the drum. The pump capacity
determines the CR, which can vary from 3
to 8, depending on the designer's choice
FIGURE 3b (right).
In this once-through
boiler, there is no circulation
system, so
CR = 1. Water enters at
one end and fl ows out
as steam at the boiler
exit. There are different
considerations
for designing this type of boiler, but that is
beyond the scope of this article
Fluegases
Steam
out
Feedwater
in
Once-through boiler
the boiler drawings from which the various
hydraulic losses in the circuits can
be computed. These include:
* Thermal head available for circulation
(this depends on the location of
the drum)
* Friction loss in the downcomer tubes
(this is due to the single-phase flow)
* Losses in evaporator tubes (this consists
of friction losses, acceleration
loss due to two-phase flow, and gravity
loss due to varying quality along
the evaporator height)
* Losses in the drum internals
* Losses in riser tubes in cases where
external riser pipes are used
Charts for the calculation of various
two-phase losses are available in Ref. 1.
The total losses are matched with the
available thermal head. If they match,
then the CR assumed is correct. If
they do not match, then another iteration
is carried out to obtain the CR at
which the available head matches the
various losses. A computer program is
usually used for these calculations.
Pump
FIGURE 4. In this waste-heat boiler with external downcomers and risers, feed
water enters the drum from the boiler feed pump or from an economizer. The water
mixes with the water from the evaporator tubes and the mixture fl ows down the
downcomer pipes to the bottom of the evaporator tubes. The CR, established by an
iterative process, can vary from 5 to 40, depending on a variety of variables
FIGURE 5. In this fi re-tube boiler confi guration (with a common drum and standalone
downcomers and risers), the feedwater is admitted into the drum, where it mixes
with the hot water from the risers and fl ows through the external downcomer tubes
If the CR is relatively low (say, in the
single digits) and if the steam pressure
is relatively high (say 1,500 psig or
above), then DNB checks1 may be carried
out after obtaining the heat flux in
each region. If the CR is low - for instance,
in the single digits, such as 5-6,
as in the case of high-pressure boilers
operating above 1,500 psig - then efforts
may be taken to revise the tube
sizes of the risers and downcomers.
The actual heat flux in the evaporator
tubes can be calculated from the
thermal calculations results done earlier,
using this relationship:
Actual heat flux = overall heat transfer
coefficient × (the gas temperature
- the saturated steam temperature)
One should ensure that the actual
1. DNB checks ensure that the boiling process
in the evaporator tubes is nucleate and not film
boiling. Charts and correlations are available to
do this evaluation, which is required if it is felt
that the CR is low, as discussed in the text.
54 CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2013
heat flux is far less than the allowable
critical heat flux all along the evaporator
tubes. Hundreds of correlations are
available in the literature to determine
the allowable critical heat flux.
The Macbeth correlation provided
below in Equation (2) shows the relationship
among the several variables
and may give a high allowable flux.
Using this correlation, two heat flux
values are computed. One is the actual
heat flux inside the tubes (determined
from thermal calculations done by the
boiler designer, based on the boiler fluegas
velocity, heat transfer coefficient
and tube geometry used). Then the
allowable heat flux is estimated from
charts or correlations available in the
literature (based on the CR computed
and the tube geometry, flow inside the
evaporator tube and steam pressure.)
One should ensure that the actual
heat flux is far lower - for instance,
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Chemical Engineering October 2013

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