ASHRAE Journal - July 2022 - 53

COLUMN ENGINEER'S NOTEBOOK
PHOTO 1 Basket strainer basket.
PHOTO 2 Installed basket strainer.
the mesh sizes common in HVAC,
the total free area of the openings in
the strainer surface is about 60% to
65%, leaving the rest as material to
hold the basket together.
Fun fact: if you multiply the mesh
size by the nominal hole diameter
in inches, you will get about
0.6, once the opening size is small
enough to divide evenly into an inch
(though this can vary somewhat
depending upon how the mesh is
constructed, and-if woven wire is
used-t he wire diameter). Twenty
mesh openings are about 0.033
in. (0.838 mm) in diameter, so 20
openings/in. × 0.033 in. = 0.66. One
hundred mesh openings are about
0.006 in. (0.152 mm); so doing
the math yields 100 openings/in. ×
0.006 in. = 0.6.
Back in the dark ages, I was taught
by a rather practical professor that
the mesh size number was actually
the number of openings in a lineal
inch of strainer surface; that is
pretty accurate, so long as the openings
are not so large that they don't
divide evenly across an inch.
Again, the fi ner the mesh, the
more signifi cant the diameter of the
wire used to construct the strainer
basket is in the actual net opening
size. No local supply house stocks
strainers with all mesh sizes, so the
stuff on the shelf typically has a " one
size fi ts all " strainer basket mesh
size. I've ordered strainer baskets
with mesh sizes different from what
was inventoried many times; usually
the supply house gives you the
" off the shelf " housing and basket so
the piping can be installed, and they
have to separately ship you a basket
from the factory with the specifi c
non-stock mesh size you want.
This practical aspect of warehouse
inventory stocking has driven the
industry toward common mesh
sizes because that is what is locally
stocked and available. In any situation
where you ask yourself if you
need a strainer and are willing to
pay the pumping energy penalty
inherent in having a strainer, you
should also ask yourself what mesh
should be specifi ed. Then when the
project goes ahead, the submittals
should be checked, because you
may not get what you wanted if the
contractor's project manager just
tells his piping foreman to pick up a
strainer at the supply house.
It's been my experience that
common wye strainers are usually
stocked in 40 mesh at supply houses
that primarily serve the HVAC
industry, and the strainers used by
chiller manufacturers to protect
brazed plate heat exchangers are
also typically 40 mesh. Strainers that
come with control component packages
vary in mesh size.
Blowdown. Most strainers of
any size for HVAC use will have a
threaded plug in the bottom that
can be removed and a valve installed
so the upstream side of the strainer
basket can be fl ushed. In some cases
it makes sense to hard pipe that
fl ush valve to a drain; in other cases
a hose connection makes sense. The
process of opening the valve and
fl ushing is referred to as blowdown.
It is critical for the system's
sustainability that the installation of
strainers that need periodic cleaning
(open loop condenser water
pump inlet strainers, as an example)
make it easy to perform blowdown,
because a failure to adequately blow
down a suction strainer leads to
excessive bearing and seal wear (a
plugged strainer kills the available
net positive suction head, causing
unbalanced loading, and in extreme
cases cavitation) as well as higher
energy consumption.
Some practical aspects should be
considered. In a perfect world, the
size of the threaded opening should
dictate the valve and drain line size,
since necking down the size will cut
back on fl ow rate, thereby diminishing
the procedure's cleaning effectiveness.
The drain capability/holding
capacity of the fl oor sink or other
drain location is also important; if
the fl oor sink has a small drain line,
a larger (1 in. to 1.5 in. [25 mm to
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ASHRAE Journal - July 2022

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