ASHRAE Journal - July 2022 - 56

COLUMN ENGINEER'S NOTEBOOK
off from the building riser. I usually use 60 mesh in
these situations (a good size with small rust flakes), and
if there is a practical means to support the weight, it is
much better to provide a duplex (dual) basket strainer. A
duplex strainer allows water flow through both baskets
under normal circumstances (low pressure drop/energy
penalty that way), but if/when blowdown is not enough
to clean the strainer, one basket at a time can be isolated,
disassembled and cleaned while the other basket permits
flow to continue, uninterrupted.
Another approach is to pipe two wye strainers in parallel.
This takes more space, but since larger pipe size
duplex basket strainers are quite expensive, it may make
sense to consider. The two wye strainer approach will
require more frequent blowdown due to much more
limited debris holding capacity, so from an operations
perspective the duplex basket strainer is a better answer
if there will be a lot of debris to catch. Of course, a pressure
gage to read the pressure across the strainer inlet
and outlet gives the operating engineer feedback as to
when cleaning is needed.
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And remember-since frequent blowdown is the way to
minimize the need to disassemble for cleaning, always,
always, always try to locate the basket strainer near a
janitor's closet or other location with a sink.
A Few Other Notes
I've known several people who prefer not to have a suction
strainer on the upstream side of a cooling tower/
condenser water pump. There are pros and cons, and
their feeling has been that the tendency for the strainer
to never be cleaned (seldom-only if there's a problem)
and collect enough junk to cause flow restrictions and
even cavitation in the pump offsets any benefit to having
the strainer.
They feel that the very large mesh, very coarse sheet
metal strainer located at the cooling tower exit is more
than enough to protect the pump, and the accelerated
pump wear due to a semi-plugged suction strainer is not
a good trade-off for the building owner. I can offer that
at this point in my career I pretty much accept that logic.
Certainly, if I were to use a suction strainer, I'd want one
of the oversized, large face area, low pressure drop versions
that are in the market, and I'd want each strainer
permanently piped to discharge above a nearby drain.
This is worth a note, too: when piping blowdown to the
strainer, depending upon the floor sink or other fixture
into which it will discharge, it is sometimes useful during
the life of the system to have enough room to be able
to slip a cup under the discharge so the water can be
visually examined.
Conclusion
The decision whether a strainer is needed comes
down to what you are protecting, how often and in what
manner it will need to be cleaned (importance of downtime),
and what sort of pump energy penalty you will
be paying. Strainer type selection (wye, basket or other)
depends upon whether protection is mostly needed at
start-up or for long-term operation. Strainers needed
for long-term operation need to be close to a sink of
some sort so that blowdown is easy (otherwise it will not
happen). And mesh size selection depends upon the
smallest orifice in the device(s) to be protected; select a
mesh opening size that is less than half the orifice opening
diameter.
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ASHRAE Journal - July 2022

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