POWER May 2021 - 23

EMISSIONS CONTROL
Are Model and Field Liquid
Collection Efficiency the Same? No.
" Because droplet trajectories and deposition patterns cannot be modeled simultaneously
with the liquid-film motion, physical-flow models cannot be used to accurately
determine the collection efficiency of the final recommended liquid-collection
system, " says the Electric Power Research Institute's Revised Wet Stack Design Guide.
This article offers a comparison of droplet deposition and entrainment in full-scale
wet stack systems versus subscale flow models.
Lewis A. Maroti, David K. Anderson,
and James M. Daniel
M
odeling wet stacks started in the
1970s to help power plants prepare
their existing dry stacks for
wet operation, spurred by changes in environmental
regulations on sulfur dioxide
emissions. Three key Electric Power Research
Institute (EPRI) reports were developed
to educate the industry on what
it means to operate a wet stack and how
to properly design a liquid management
system for it. They are:
■ Entrainment in Wet Stacks (1982)
■ Wet Stacks Design Guide (1997)
■ Revised Wet Stack Design Guide
(2012)
Two fundamental components of a
wet stack study are the condensation
calculations and the physical wet stack
model. These components are used to
analytically calculate the amount of liquid
condensation and mist eliminator carryover
that can be expected within the wet
duct and stack system, and to perform
scaled physical cold-flow analysis to aid
in the design of a liquid collection sysmodels
can be used to provide the liquid
collection efficiency of the field installation.
Simply put, this is not possible because
there is no way to simultaneously
simulate both the airborne droplets and
the liquid films in a scaled model because
we cannot scale gravity. Furthering
the issue is the fact that a cold-flow
model does not simulate the formation
of condensation on internal surfaces.
The main objective of this paper is
to demonstrate that it is not possible
to model droplet trajectories and droplet
re-entrainment simultaneously in a
scaled physical-flow model, thus proving
that the liquid collector efficiency
results obtained from a scaled model
have no correlation to the liquid collector
efficiency of the field installation. A
liquid collector efficiency value obtained
from a scaled model is only a measurement
of the collection efficiency of the
model and does not represent the realworld
stack performance.
The different sets of physics associated
with droplet deposition and re-entrainment
will be presented. A comparison
will be made of the differences between
the physics for droplets and liquid film
A common misconception is that the models
can be used to provide the liquid collection
efficiency of the field installation. Simply
put, this is not possible.
tem to collect and remove liquid from
the system. These two components together
allow one to make engineering
decisions on liner material and ductwork
insulation, design of liquid collection devices
in the ductwork and liner, and sizing
of system drainage.
A common misconception is that the
May 2021 | POWER
layers in the full-scale field unit versus a
scaled flow model. The implications with
regard to liquid collector collection efficiency
will be discussed.
Model Constraints
People sometimes ask for the liquid collection
efficiency of the recommended
www.powermag.com
1. Forces on a suspended droplet, where CL
is centerline of the duct, VG is gas velocity, Rc
is curvature radius, Fg is gravitational force,
FC is centrifugal force, and VF is field velocity.
Courtesy: Alden Research Laboratory
liquid collection system for their wet
stack application, where the collection
efficiency would be defined as the ratio
of the amount of collected liquid divided
by the amount of incoming liquid. However,
due to the constraints of physics
when using a scaled cold-flow model,
this is simply not possible.
Scaled cold-flow models are used to
design the recommended liquid collection
system for a wet stack installation.
The flow models typically begin at the
discharge face of the absorber mist eliminators
and continue through the ducting
to a point approximately four liner diameters
above the top of the stack breaching
duct. The model ducting typically
includes at least two 90-degree turns
(absorber outlet and stack inlet) along
with other changes in flow direction such
as bends and transition sections where
the duct shape is changing.
23
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POWER May 2021

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