Getting Technical
gravity which resulted in sufficient water pressures at lower levels
in the building but reduced pressures at higher levels causing low
flow rates. Even worse were water flows from the holding tanks to
cooling towers and other machinery mounted on the roof at the
same level. Provided that the water level in the tank did not drop
more than about halfway (50% of tank capacity), cooling towers
and humidifiers could run under normal loading conditions.
However, unexpectedly high ambient conditions or even relatively
small leaks in water circulating systems could raise demand from
the tank to greater than the low-pressure supply capability which
often ended up in system failures through trip-outs.
Discomfort caused by air conditioning plant trip-outs tended
to draw attention away from higher expenses and energy losses
inherent in additional plant restarts. In general, electrical power
for refrigerant compressors, water pumps and axial or centrifugal
air moving fans was a minor initial design consideration with little
or no consideration of unexpected energy costs during operation
from commissioning through whatever periods it was expected
(or hoped) that the plants involved would continue to run reliably.
Another of the most common potential problem points in
water circuits, including circuits in HVAC installations, up until
around the end of the 1980s was ‘dead ends’ where the flanged
ends of water pipes were blanked off for future plant expansions.
These dead ends were only small volumes but the water in them
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RACA Journal I April 2020
became rapidly stagnant and de-oxygenated resulting in growth
of acid-producing anaerobic bacteria which spread throughout
the water circuits causing corrosion. Some of the blanked off pipes
were fairly large, between 250 and 350mm in diameter. However,
it was normally relatively easy to weld small valved fittings onto
the flange blanks, drill through the blanks and connect 6mm
tubing from the fitting to the inlet sides of nearby circulating
water pumps – or other lower pressure points of re-entry into the
circuit. The small amount of water flow through the 6mm tube
was sufficient to eliminate the “dead end” situation and had no
measurable impact on design circuit water flow rates. ‘Dead ends’
are seldom seen today but new potential problem points arise
for which mechanical modifications which may seem expensive
initially are often the optimum solution choices.
Water circuits supplied with softened water usually require
little or no bleed-off. Drift and spray losses from evaporative
cooling towers supplemented by low flow rate continuous
trickle drains provided enough bleed-off to prevent cycles of
concentration rising too rapidly between regular draining of
cooling tower sumps which tended to be done as frequently as
needed without any regard to the accompanying increases in
fresh water and treatment chemical usages.
Only very few people seemed to perceive the situation as
being too good to last! It was.
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