ASHRAE Journal - March 2022 - 16

LETTERS
Optimizing Coil
Loop Energy
Recovery Systems
" Optimizing Coil Loop
Energy Recovery Systems " by
Gene Nelson, P.E., Life Member
ASHRAE, in November 2021's
ASHRAE Journal provides comprehensive
design requirements for
an energy recovery system. A few
thoughts:
1. In our area propylene glycol is
demanded by the owners to provide
a level of personnel safety and
allow disposal through conventional
drains; ethylene glycol is not
allowed in conventional drains. This
use of propylene glycol will probably
be universally required in all HVAC
systems in the future.
2. The design standard commonly
used in our area for air-handling
(AHU) units, whether roof mounted
or indoors, is to use a 50% propylene
glycol solution for all heating coils,
whether energy recovery or traditional
heating. The common design
outside air temperature used is
-35°F and farther north is -40°F.
3. Where multiple coil sections are
used in a coil configuration, it needs
to be emphasized the coil sections
need to piped in reverse return
to minimize the flow differences
between coil sections, particularly
when a variable frequency drive
(VFD) is applied to the pump.
4. A quality coil manufacturer's
selection program should be used;
some very good programs are available
that allow selections based on
varying the tube size, tube thickness,
circuiting, spacing of fins, fin
size, fin type, pressure drop, etc.,
to maximize the effectiveness and
minimize the overall pressure drop
16
and flow requirements
on the air
and fluid side of the
coils.
5. In Figure 3 of
the article, a connection
to the heating
glycol system
can be provided,
as per Sketch 1, to
assist in frost control
and maximize the
energy transfer at
any time. Since glySketch
1
OA
EA
OA Coil
EA Coil
Glycol Heating System
Sketch 2
OA
EA
OA Coil
EA Coil
Glycol Heating System
col solutions are used in AHU units
in this area, it is not a major issue to
provide this additional connection.
The essence of control is to have the
three-way valve operate to at least
at 95% i.e., full flow through both
coils; if it drops lower than 95%,
inject heat from the heating system
to keep the valve to at least 95%. This
ensures a consistent flow in both
coil circuits.
In VAV systems the dynamics of
performance will change, and this
additional source of heat will assist
in effective frost control and overall
heat transfer at all volumes and
outside air temperatures. The additional
logic in the digital controller
is not complex.
6. Per Sketch 2 an option is to add
a second pump so each coil can be
optimally selected. The coils may
be of different physical sizes and
the selected coils may have different
pressure drop requirements
to maximize the heat transfer
and design airflow temperatures.
Keeping constant flow in each
coil eliminates laminar flow and
optimizes the coil performance. As
well there will not be an issue with
variable flow in the individual coil
ASHRAE JOURNAL ashrae.o rg M A R CH 2022
selections in a coil with multiple
sections.
The addition of a second pump
is not a major additional cost compared
to the overall total cost of the
energy recovery system. With digital
controls the addition of a heating
injection valve only requires simple
additional basic logic. The logic for
control of the valves is the same as
in point 5 of this letter. In addition,
during unoccupied modes when
there is no airflow, the coil in the
outside air intake can be kept at a
nominal temperature by running
the pump in the outside air coil and
managing the two-way valve.
The author states the glycol solution
should not be more than 30%;
based on a common supplier's
chart, this only provides a nominal
freezing temperature of about 10°F
and a burst temperature of about
-10°F. In our climate with a design
of -35°F, this level of protection is
insufficient.
Cliff Badger, P.Eng., Life Member ASHRAE;
Calgary, AB, Canada
The Author Responds
Thank you for your additional
insights on practical applications
in extreme cold weather conditions.
Using propylene glycol (PG) is
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ASHRAE Journal - March 2022

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