ASHRAE Journal - June 2022 - 28

COLUMN ENGINEER'S NOTEBOOK
resulting ionic purity depends on the incoming water
quality. In general the ionic quality is not quite as good as
for distillation, but you can put two RO filters in series,
and the energy use will still be lower than for distillation.
RO filters are also fine enough that unless there is a
breach in the membrane, no microorganisms will pass
through the RO membrane. RO membranes are two layers
of the membrane with a permeable foam between them
wound tightly like a toilet paper roll. Every other row is for
inlet water and the opposite ones are for outlet (permeate)
water. This provides a huge amount of surface area
for filtration. There are three types of RO membranes.
1. Cellulose acetate is what movie film is made from
and is used in many residential RO filters that you might
put under your kitchen sink. It is used in those applications
because cellulose acetate is resistant to chlorine,
which is in nearly all U.S. domestic water. The disadvantage
of cellulose acetate is it requires much more pressure
to produce a given amount of pure water from the
same size filter, so either energy or first cost is higher.
2. Thin film membranes are the dominant RO technology.
They have the best throughput for a given size and
DP (i.e., the most energy efficient option). However, the
membranes are degraded by chlorine, so de-chlorination
is required, either via chemical anti-oxidant or by physical
adsorption with equipment such as a carbon filter.
3. Hollow fiber membranes are tubes of polymer
wrapped in a spiral fashion. The disadvantage is that if
a particle gets lodged in the entry to a tube, that tube is
out of service, just like plugging a boiler or chiller tube.
Deionization uses ionized " beads " to remove cations
and anions from water. Some are single- or double-bed
systems. These do a decent job of removing ions, but do
not remove microorganisms. In small systems they are
usually portable exchange cylinders. They are regenerated
back at a plant with a strong acid (such as hydrochloric
or sulfuric) regenerating the cation cylinders and
sodium hydroxide for the anion cylinders.
For the maximum in ionic purity, a mixed bed is
required. A mixed bed is commonly used as a polishing
unit following individual cation and anion beds. The
mixed bed tank contains both anion and cation beads,
so the water bounces from one to the next. These are
more expensive to regenerate because back at the plant
the beads must be separated with a sieve into anion and
cation beads, regenerate them, and then remix them.
The resin beads are different diameters so they can be
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ASHRAE JOURNAL ashrae.org J U N E 2022
separated. You can pay extra to have your beads not
mixed with other client's beads. For facilities with a very
large need for deionized water, an on-site deionization
system can be installed, eliminating the complexity of
exchangeable beds. The downside is in capital cost, floor
space, possible large backwash flow rates, and handling
of hazardous chemicals.
Pure Water Piping
Choices vary depending on the needs of the end user.
If only moderate purity is needed, PVC is often used.
For most lab systems polypropylene without additives
is common but is subject to degradation near ultraviolet
(UV) lights and does not comply with the 25/50
fire/smoke rating that is required for use in return air
plenums. Fortunately, stainless steel does not reflect
254 nm UV light, so putting a little stainless steel pipe
and an elbow on either side of the UV light solves the
problem. For higher-grade water and where use in a plenum
is required, polyvinylidene fluoride (PVDF) is often
used. 316L stainless steel is also often used when trace
amounts of nickel or chromium added to the water are
not an issue.
Consider Your Application
Dialysis requires high microbiological purity. RO is
great for this. Labs often have RO for the water that is
circulated throughout the building and that is normally
used for glassware rinsing and applications that require
microbiological purity, but not extremely high ionic
purity. For applications that require both, a small " polisher "
is added in some individual labs. These usually
include circulation, filtration, and mixed-bed deionization
to produce 18 Mohm (nearly completely ionically
pure) water that is free of microorganisms.
Summary
This month we concentrated on domestic (service
water) system water treatment (including sprinklers) and
pure water systems. Unless you have strong knowledge in
these areas, we recommend involving a water treatment
professional, especially for pure water systems.
References
1. 2019 ASHRAE Handbook-HVAC Applications, Chap. 50.
2. EPA. 2009. " National Primary Drinking Water Regulations. "
U.S. Environmental Protection Agency. " https://tinyurl.
com/56m55wta
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ASHRAE Journal - June 2022

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