ASHRAE Journal - March 2022 - 75

COLUMN ENGINEER'S NOTEBOOK
will need to consider microbiological issues. For chilled
water, nitrite is not a preferred corrosion inhibitor
because of the risk of " bloom " where nitrite provides
food for nitrogen-reducing microbes. Maintaining corrosion
inhibitor levels within recommended limits is
important. Careful control of the nitrite level is required
to prevent metabolism of the nitrogen and can be diffi
cult to maintain at all points in large systems. We normally
only allow molybdate as a corrosion inhibitor.
Copper pipe and/or coils are often used in chilled
water systems, and a yellow metal corrosion inhibitor is
included. And, chemicals such as methylbenzotriazole
(MBT) or tolyltriazole (TT) can protect copper and brass
and minimize dielectric pitting of steel. This applies to
heating, cooling and glycol water systems.
Glycol Systems
Systems fi lled with water-glycol-based heat transfer
fl uids require less ongoing treatment but are not
immune to design and maintenance problems. Glycol
is an additive to water that alters the freezing and boiling
points of the fl uid. However, glycol on its own is
not enough for a healthy hydronic loop. Glycol must
be combined with buffering (alkalinity†) agents, corrosion
inhibitors, alkalinity boosters, and various other
chemistries to become a good heat transfer fl uid. Heat
transfer fl uids should be designed for the metallurgy of
the loop, the intended temperature range, and sensitivity
for environmental discharge or human/animal consumption.
Many manufacturers provide a heat transfer
solution ready for use in a hydronic loop that contains
the necessary constituents to ensure the fl uid performs
as designed.
Glycol belongs to the alcohol chemical family, but
should not be confused with ethanol-based antifreeze
(i.e., RV antifreeze). Typical use concentrations are 25%
to 50%. Concentrations over 50% raise the freeze and
burst temperatures. (Freezing and bursting are different
things. Freezing means that liquid stops fl owing.
Bursting means that you have a mess to clean up, a coil
to patch or replace, and possibly expensive building
damage.) When the concentration gets below approximately
22%, glycol can " ferment like bad wine " as
bacteria feast on the glycol. You will then have a wonderful
green, brown, black substance to clean out of your
piping system. To avoid concentration dilution, use a
premixed tank for maintaining system pressure and
never connect domestic water to a glycol system like you
would for a water hydronic system. Otherwise, every
time a strainer is cleaned or a pump is serviced, the
glycol percentage will decrease and you will slowly lose
both corrosion and freeze protection.
Ethylene glycol has better heat transfer and pumping
characteristics than propylene glycol, but propylene
glycol and in some cases food-grade propylene glycol
is needed because ethylene glycol is toxic. Geothermal
systems in many states require propylene glycol or even
food-grade propylene glycol. Do not use automotive
antifreeze in non-automotive applications. It does not
contain the same level or quality of corrosion inhibitors,
is incompatible with most industrial heat transfer fl uids,
and is not designed for commercial and industrial use.
In no case should propylene and ethylene glycol be
mixed in the same system. Although generally compatible,
system effi ciency is reduced, the fl uid is still toxic,
and freeze/burst point identifi cation becomes extremely
diffi cult. Additionally, each manufacturer uses different
inhibitor, buffer, and stabilizer packages, which may not
be compatible with one another and can lead to system
damage. Clearly mark the system for operators as to
which aqueous glycol solution is used.
Good glycol-based heat transfer fl uids will contain
yellow metal inhibitors initially but will still need occasional
testing and addition of inhibitors.
Condenser Water Systems and Other Open Loop Systems
Cooling towers, evaporative condensers and fl uid
coolers use less energy than dry cooling systems. But
they present unique challenges because the recirculating
spray systems are open to the environment and
susceptible to contamination, often operate within
temperature ranges suitable for microbiological growth,
and can consume large volumes of water, which can
introduce large quantities of deposit-forming material,
such as mineral hardness.‡ These systems are also
designed to evaporate water for heat rejection, which
†Alkalinity: a measure of water's ability to resist pH changes due to addition of acids and bases. Alkalinity is unaffected by changes in temperature,
pressure, or pH and is easily experimentally determined. I (Jeff) tend to think of alkalinity as buffering ability, like buffered aspirin.
‡Hardness: The concentration of cations in water (cations carry a positive charge) , usually expressed as CaCO3 (calcium carbonate)
equivalent hardness in grains per gallon (gpg) or parts per million (ppm). 1 gpg = 17.1 ppm.
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