ASHRAE Journal - March 2022 - 17
LETTERS
safer but requires much higher fl ow
rates than ethylene glycol (EG) solutions
to maintain transitional fl ow.
Designers need to educate owners/
code authorities on risks/rewards
of selecting the proper working
fl uid. Additional safeguards (double
walled pipe in concealed areas and
leak detection) may be required for
safe use of EG.
The design standard of using 50%
PG for freeze protection down to
-40°F may be too conservative. Most
glycol manufacturers' published data
indicate burst protection concentrations
of 36.6% for PG and 31.4% for
EG at -50°F ambient conditions.
Using reverse return piping for
multiple coils is a good provided
that the costs of piping is not too
excessive. Direct return piping can
be used if the system is properly balanced
using balancing valves.
I agree that some selection programs
are better than others. Good
selection programs have options to
vary the construction parameters
as suggested and provide good
selection information such as tube
velocity, heat transfer surface area,
U-factor, log mean temperature difference
and Reynolds numbers.
Sketches 1 and 2 offer an interesting
idea to maintain the lowest possible
fl uid temperatures to avoid
frost and maximize heat transfer in
both supply and exhaust coils. Both
options increase the supply coil's
fl ow and Reynolds number. Don't
use the supply coil to raise the supply
temperature to design setpoint, as
this will increase the fl uid temperature
to the exhaust coil and decrease
the amount of energy recovered. A
heating coil downstream of the supply
energy recovery coil should still
be used for both sketches.
My comment regarding limiting
PG to 30% with a burst protection
of -10°F is appropriate for most climates
but not for -35°F design conditions
as you stated. The comment
was intended to point out that high
concentrations of PG require tube
velocities higher than 6 fps as shown
in Table 2. I would recommend
using the lowest concentration possible
(35% EG or 40% PG) and limit
tube velocities to 6 fps for these
extreme cold conditions.
Gene Nelson, P.E., Life Member ASHRAE, Madison, Wis.
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M A R C H 2 0 2 2 ashrae.org ASHRAE JOURNAL
17
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ASHRAE Journal - March 2022
Table of Contents for the Digital Edition of ASHRAE Journal - March 2022
ASHRAE Journal - March 2022 - Intro
ASHRAE Journal - March 2022 - Cover1
ASHRAE Journal - March 2022 - Cover2
ASHRAE Journal - March 2022 - 1
ASHRAE Journal - March 2022 - 2
ASHRAE Journal - March 2022 - 3
ASHRAE Journal - March 2022 - 4
ASHRAE Journal - March 2022 - 5
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ASHRAE Journal - March 2022 - 13
ASHRAE Journal - March 2022 - 14
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ASHRAE Journal - March 2022 - 17
ASHRAE Journal - March 2022 - 18
ASHRAE Journal - March 2022 - 19
ASHRAE Journal - March 2022 - 20
ASHRAE Journal - March 2022 - 21
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ASHRAE Journal - March 2022 - Cover3
ASHRAE Journal - March 2022 - Cover4
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