Chillers and Heat Pumps
Circulating pump selection
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 .
HEAD
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 .
HEAD
Design operating point
Both pumps operating
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 .
One pump on system operating point
FLOW
FLOW
Series operation pump curve
Design system curve
Actual system curve moved to right due to circuit imbalance
Actual operating point
Both pumps on system operating point
One pump on system operating point
Both pumps on system operating point
System pressurisation
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
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 .
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 .
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 . 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 .
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 .
HEAD
Single pump curve
FLOW
Each pump operating point both pumps on
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