ASHRAE Journal - August 2023 - 9

LETTERS
regime, especially when lifting
condensate. Keeping the pipe size
the same as the trap outlet for a
short distance will assist in maintaining
slug fl ow in a vertical riser.
In sizing the common dry-closed
steam condensate return lines, I
agree that applying the pressure
drop limit of 0.125 psi/100 ft listed
in Table 3 is appropriate when
using slopes between 0.36 in. and
0.5 in. per 10 ft. I do not agree the
maximum fl ash steam velocities
should apply to dry-closed steam
condensate pipe sizing. This is a
departure from Table 37 in the 2021
ASHRAE Handbook-Fundamentals,
Chapter 22 and from my discussion
in my February 2023 ASHRAE
Journal article.
I disagree that dry-closed steam
condensate lines can be characterized
by the upstream pressure. I
contend that they should be characterized
by the backpressure of
the receiver. I contend that this
is a better method based on the
fact that average pressures in the
common drain line are closer to
the receiver pressure than to the
upstream supply steam pressure.
Regarding steam condensate
return pipe design, I encourage
the reader to review my February
2023 article along with my
response to Mr. LaPlante's letter
to editor in the April 2023 issue of
ASHRAE Journal.
Gene Nelson, P.E.
Life Member ASHRAE
Madison, Wis.
LAPLANTE RESPONDS
I thank Mr. Nelson for his interest
in the article and the opportunity
to respond. We agree on the main
points of steam supply pipe sizing.
As the article describes, proper sizing
will ensure both pressure drop
and velocity limitations are not
exceeded; one of these factors will
govern for a given mass fl ow rate
and supply pressure. The article also
acknowledges that average specifi c
volume (and consequently, pressure)
should be used in calculations
for pressure drops ranging from 10%
to 40% of upstream absolute pressure
to achieve accurate results.
High pressure condensate (HPC)
piping is pressurized by fl ash steam
at drip trap outlets. A positive pressure
is maintained due to lack of
vents to the atmosphere at condensate
entry points in the piping.
This pressure allows condensate to
be lifted above the steam trap discharge
elevation without the aid of
a condensate return pump. If it is
not practical to directly return HPC
to the plant, it would be prudent
to consider routing this valuable
condensate to a fl ash steam recovery
vessel to generate low pressure
steam for other needs, rather than
discharging it directly to vented
pump receiver sets. This approach
offers several benefi ts, including
lower makeup water use, reduced
boiler input energy and decreased
chemical treatment requirements.
Mr. Nelson accurately explains
the process of obtaining kinematic
viscosity from dynamic viscosity
and specifi c volume. Alternatively,
kinematic viscosity values can be
obtained from a variety of online
sources. One may elect to calculate
this value or retrieve the desired
data points for use with computational
software that can leverage
lookup tables to automate this part
of the sizing process.
A minimum elevation difference
from equipment condensate outlets
to steam trap inlets, known as
a " liquid leg, " is indeed necessary
for proper system operation. Slope,
velocity and pressure drop per unit
length guidelines provided in the
article are applicable to pipe sizing
practices; they do not preclude the
need for a liquid leg at steam trap
inlets or other necessary provisions.
Mr. Nelson's comment on dryclosed
condensate return pipe sizing
reveals a disagreement concerning
the defi nition of these piping
networks. The article supports the
2021 ASHRAE Handbook-Fundamentals
description, which includes:
* Piping is not completely fi lled
with liquid ( " dry " );
* Piping is non-vented ( " closed " );
* Piping is motivated by steam
pressure;
* Sizing is based on condensate
capacity without slope, although it is
" ...common practice to slope the line
in the direction of fl ow to a collection
point to clear the lines of sediment
or solids. "
Based on this information, it
would be inadvisable to use pipe
slope as a sizing basis for dry-closed
condensate return piping in lieu of
the fl ash steam velocity and pressure
drop guidelines outlined.
The article also notes that normal
operating pressures of high,
medium and low-pressure condensate
returns are much lower
than their associated steam supply
piping. It is neither expressed nor
implied that the operating pressure
of dry-closed condensate piping can
be solely characterized by upstream
pressure.
Kevin R. LaPlante, P.E.,
Member ASHRAE
Concord, N.H.
A U G U S T 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
9
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ASHRAE Journal - August 2023

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