ASHRAE Journal - March 2022 - 80

COLUMN ENGINEER'S NOTEBOOK
to replace the filter twice weekly at
first, then he changed to 10 micron,
and eventually to 2 micron. Since
then, he has replaced zero control
valves. This is an extremely cheap
option to add to your master specs.
A good starting point for sufficient
filtration in closed loops is to direct
2% to 5% of the circulation rate for
the main pumps through a filter
system. These bypass filters can
be installed across the supply and
return sides of a pump, or directly
at the end of main legs/risers in the
system, to ensure dirt and debris are
removed quickly from the system.
Corrosion Rate Monitoring
The inclusion of a corrosion coupon
rack for all water systems (even
specially designed racks for condensate
return loops) should be part of
every large or open hydronic system.
Coupon racks are inexpensive and
when installed correctly at the start
of a project, they provide real-time
performance data for the health of
the water loop before failures occur.
Figure 3 is a typical four-port corrosion
coupon rack with particulate
filter and flow regulator to ensure
the environment the coupons are
exposed to is consistent each time
the coupons are checked. Coupons
for each metal used in the system
(i.e., black steel, copper, etc.) should
be included in the rack. Corrosion of
the coupons is indicative of the corrosion
of these metals elsewhere in
the system. Coupon studies can then
be performed in 90 to 360 day intervals
to quantify the corrosion rates of
each metal. Each coupon is cleaned
and weighed to determine the loss
of material, so changes to the system
chemistry can be made as necessary.
80
Makeup Metering
Aside from glycol loops, which
should only makeup premixed glycol
from a tank, the inclusion of a
makeup meter for all water systems
is a staple of modern design. Many
of the problems encountered in the
water systems discussed in this column
can be traced back to the fact
that the system loses water, either by
design, failure, or error. Tracking the
volume, frequency, and severity of
makeup water is critical for correct
adjustments to treatment programs,
and for identifying leaks. Makeup
water meters as shown in Figure 4 are
low cost and easily installed on any
water system. These meters display
an " odometer " locally and can send a
digital pulse based on volume to any
building automation or water treatment
control system.
One author had a project where
the owner complained that he had to
bleed air out of his compression tank
every week. After thinking about it,
he realized that the only possible
source of air was makeup water,
so we went to look for the leak. He
had a preheat coil that had partially
frozen and was leaking. The cooling
coil drain pan in his AHU extended
under the heating coil also, so the
leak wasn't noticed. If this had continued,
the continued addition of
oxygen to the system could have
greatly accelerated system corrosion.
Summary
Water treatment is complex.
This column summarizes the most
important items for HVAC designers
to know and improve their master
specifications for water treatment.
Hydronic systems offer significant
energy benefits compared to alternatives,
and proper water treatment
ASHRAE JOURNAL ashrae.o rg M A R CH 2022
FIGURE 2 Bypass filter-feeders and filter housing.
FIGURE 3 Typical corrosion coupon rack. (Image
courtesy of DuBois Chemicals.)
FIGURE 4 Contacting makeup water meter.
(Courtesy of DuBois Chemicals.)
helps ensure these benefits are fully
realized. Owners and operators
should always consult a qualified
water treatment professional. We
also direct you to the reorganized,
improved water treatment chapter
in the 2019 ASHRAE Handbook-HVAC
Applications, Chapter 50.
http://ashrae.org
ASHRAE Journal - March 2022

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