ASHRAE Journal - May 2023 - 10

LETTERS
choice. It is, however, vital that the
architect, owner, general contractor
and engineer control the dialogue
and manage expectations when
considering VRF.
George Orff, Life Member ASHRAE,
Laguna Beach, Calif.
GALLAGHER RESPONDS
Thank you for your comments,
George. As we have both indicated,
VRF is not the sort of system that
lends itself well to the design professional
losing contact with the
project once the initial design is
complete. It is particularly important
for the engineer to review an
accurate " as-built " piping drawing
when the piping is complete, and to
at least spot-check a system or two to
make sure the as-built drawings are
accurate-as well as verify that the
final result was actually run through
the manufacturer's software. This
as-built then needs to be suitably
memorialized so subsequent TI
changes and service work are based
upon correct information and a
proper set of drawings.
VRF also rewards getting the
entire design and construction team
selected and together early in the
process. Most VRF system issues
can be avoided if the entire design
and construction team is involved
throughout the project.
Michael Gallagher, P.E.,
Fellow/Life Member ASHRAE,
Santa Fe Springs, Calif.
Under Pressure:
Steam Drainage
Systems
The article " Understanding
Steam Drainage Systems " by Gene
Nelson, Life Member ASHRAE,
10
in the February 2023 issue of
ASHRAE Journal highlights several
important issues pertaining to
steam condensate recovery piping
systems. I commend the author for
a thorough and well-researched
review of two-phase condensate
flow characterization, air venting
and pressure gradients within steam
condensate piping systems.
However, on the topic of dryclosed
condensate return pipe sizing,
the author conflates gravity flow
and steam pressure motivated sizing
practices. Of particular concern
is the recommendation to use " ...
pressure drop rates consistent with
gravity flow... " for dry-closed returns
and the advised caution against pipe
sizing based on flash steam flow and
maximum velocity, without consideration
for slope.
Dry-closed condensate return
pipe sizing is based on a continuous
pressure differential from the
condensate inlet to the discharge
point, not pipe slope. Although it
is common practice to slope this
piping in the direction of flow for
sediment removal, steam serves
as the motive force for conveying
condensate through the system
rather than gravity.
Pressure drop rates listed in
ASHRAE Handbook-Fundamentals
Table 37 (determined from the
Darcy-Weisbach equation) are
appropriate for this closed flow
condition. The author's proposed
revision of Table 37 pressure drop
rates to align with corresponding
gravity drainage slopes is not
applicable to dry-closed condensate
return piping.
Additionally, the author's suggested
modification of conventional
dry-closed return pipe capacity
ASHRAE JOURNAL ashrae.o rg M AY 2023
tables to account for slip velocity
(and reduce the risk of slug flow)
using the provided curve fit equation
should be considered with caution.
According to the research project
cited in the article, this method
is subject to appreciable error and
further study was recommended.
Alternative slip velocity correlations
offering accuracy improvements
have been developed and warrant
further investigation.
Traditional pipe sizing methods
offer a practical and empirically
proven means of preventing water
hammer in steam condensate systems.
For example, flash steam
velocity limitations for condensate
piping are kept significantly lower
than that of steam supply piping,
tacitly acknowledging the increased
risk of water hammer with lower
quality steam. Of course, accounting
for all variables in our computations
is often desirable, but inherent
errors and complexity trade-offs
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ASHRAE Journal - May 2023

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