Circulating pump selection System pressurisation
Pumps should have a flat characteristic and should operate near to the
left of the maximum point of efficiency on the curve to allow for any
deviation in the position of the actual system curve from that estimated
in the design process. This will ensure satisfactory pump operation with
no overloading of water volume or reduction in available head. A diaphragm expansion tank, pre-charged to the system fill pressure
and sized to accept the expansion of the water, is normally used in
larger water circulating systems. The air charge and the water are
permanently separated by a diaphragm that eliminates corrosion and
noise caused by air in the system.
Water treatment
Design system curve
Actual system curve
moved to right due to
circuit imbalance
Design
operating
point
Water system problems can be recognised from the following
symptoms:
1. A reduction in heat transfer, which is a sign of insulating deposits on
heat transfer surfaces reducing the cooling or heating efficiency of
the equipment. This can be caused by scale or biological growths.
Actual operating
point
2. A reduction in water flow which is generally caused by a restriction
in pipework, condenser or evaporator tubes or other components
due to a build-up of scale. Bacterial and algae accumulate in Cooling
Towers and can radically reduce water flow and are a major cause of
corrosion. As the water evaporates in a Cooling Tower the dissolved
solids originally present in the water remain in the system and
suspended solids scrubbed from the air can also contribute to the
blockage and corrosion of condenser water pipework and the Chillers
condenser tubes.
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Parallel pumps
When two pumps, of equal flow, are used in parallel they operate at the
same head and share the total system flow. When only one of the two
pumps is in operation the flow volume can be as much as 80% of total
flow resulting in a very good performance for stand-by in case of pump
failure.
3. Corrosion of materials or undue wear to pumps ,shafts ,seals etc.
Unless a Cooling Tower is regularly cleaned and protected with
adequate water treatment biological contaminants, including
Legionella, may be introduced into the re-circulating water.
A water treatment program must be employed to control all possible
contaminants. It must be compatible with all the materials of
construction and the pH of the circulating water must be maintained
between 7 and 9. Biological contamination can be controlled by the
use of biocides.
Both pumps on system
operating point
Both
pumps
operating
The proper control of water treatment is dependent on the proportional
addition of the relevant chemicals to maintain the correct concentration
at all times. The relevant chemical treatment of water systems is a
complicated matter and it is therefore important that a specialist water
treatment company is involved early in the design stage of the project.
One pump on
system operating point
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Mechanical filtration
A Filter, with a 40 mesh screen, must be installed as close as possible
to the water inlet of both the cooler of air cooled and water cooled
Chillers and Heat Pumps and the condenser of water cooled equipment.
A means of local isolation should be provided. The Filters will also
protect the circulating pumps in the system.
Series pumps
When two pumps, of equal flow, are used in series they operate at the
same flow and share the total system head. When only one of the two
pumps is in operation the flow volume can be as much as 80% of total flow
resulting in a very good performance for stand-by in case of pump failure.
Series operation pump curve
Both pumps on system
operating point
One pump on system
operating point
Single pump curve
Each pump operating point
both pumps on
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