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By Dilip Datey
Managing Director, K.D. Agencies
Mumbai
Dilip Datey has 30 years of professional experience including 20 years with Crompton Greaves where he was Chief Engineer (Design). He has worked extensively on pumps for circulation purposes and carried out many energy conservation exercises. He is a member of ISHRAE.
Pumps are used to produce water flow in pipe lines. The prime mover is either an electric motor or an air motor (run by compressed air) or an oil/diesel engine, most common being the electric motor. Pressure energy added by the pump is used to:
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Pumps are classified either by their construction like Monobloc, Coupled, Split casing, Back Pull-out or by the axis like Horizontal or Vertical or by their motion like Reciprocating or Rotary or by the number of stages like Single Stage or Multistage. In addition there can be classifications on the basis of liquids to be handled like clear water or slurry or chemicals.
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In most of the pumps the casing is made of Cast Iron. This material gives the designer flexibility with adequate strength and hardness. For special applications like chemicals, the material can be Stainless Steel or even Polymeric. For very small circulators, die-cast Aluminium is used for the casing. Impeller material can be Cast Iron, Bronze, Stainless Steel or Polymer. Bronze is the most common material for the impeller for a long, trouble free life. The shaft is another member where material plays an important part. Popular choices are EN-8(standard Shaft Steel) or Stainless Steel 410, the second being obviously better.
Sealing (to prevent water flowing towards the motor) is done either by a Gland Packing or by a Mechanical Seal. A gland packing requires some water leakage to cool the glands. Mechanical seal pumps are expensive but they do not allow water to drip. In HVAC application, the use of mechanical seal pumps is recommended for chilled water lines because dripping of chilled water means loss of energy. Also use of pressurised expansion tank necessitates the use of mechanical seal pumps because leakage will mean loss of pressure. The seal should be thermally stable up to at least 0°C so that the lapped surfaces of the seal do not warp at the chilled water temperature and allow water to drip. For cooling tower water circulation gland packing can be used.
Pump laws relating to flow rate, head and energy are as follows :
Thus if a pump is selected for 25% excess flow, it would be encountering 56% excess head (square of 1.25 ) and consume 95% excess energy(cube of 1.25 ).
Curve Head v/s Discharge flow curve for the pump and the system curve is as shown in Figure 6. The intersection point is the operating point.
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In HVAC applications, pumps are used for circulation of chilled water and condenser water and hence the pumps have to take care of only frictional losses of the pipes, valves and coils. However in the case of cooling tower water circulation, the cooling tower height is the static head, in addition to the frictional losses. So for a cooling tower, the system curve will start at about 3m (height of cooling tower) on the Y axis.
Generally, frictional losses of terminal units/ chiller/condenser are around 3 m (of water column ) at the rated flow. Frictional losses in the pipes depend on water velocity and are controllable (by choosing a lower velocity these can be reduced). Normally these are around 5% of the pipe length. Thus the total head which the pumps have to encounter is fairly low, requiring low head/high discharge pumps.
For large installations, Horizontal Split Casing pumps running at 1450 RPM and for medium and small installations Circulators (Vertical Inline pumps) are an ideal choice.
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Pumps can be used in :
In a chilled water system, water is circulated through a chiller, pipes, valves and terminal units (FCU or AHU) by pumps. The flow rate for chilled water is derived by considering a fixed temperature differential across the coil/chiller. The design figure for this differential is normally 5°C (9°F ) to 5.5° C (10°F). Corresponding to this differential, the flow through the coil/ chiller comes to 10 lit/min/TR (2.6 USgpm/TR) to 9 lit/min/TR (2.4USgpm/TR). For cooling tower water circulation, the flow is about 25% more as the energy (motor power) spent in cooling also gets rejected in the cooling tower. So flow for a cooling tower would come to 12.5 lit/min/TR (3.3 US gpm/TR) to 11.25 lit/min/TR (3 US gpm/TR).
The head encountered in chilled water lines ranges from 20 m to 30 m and for cooling towers 16m to 20 m depending on piping lengths and water velocities.
The following steps are recommended for selection of pumps for HVAC application :
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Example
Cooling tower water circulation.
Option1– 15 hp
(1 Working +1 Standby).
Option 2 – 5 hp
(2 Working +1 Standby).
The second option gives the benefit of lower connected hp, lower power consumption, use of DOL starters (because of lower hp) and possibility of running one pump at the time of low wet bulb temperature or partial load.
It has been observed that many of the old installations have oversized pumps. There are many possible reasons for this, such as :
With high electricity tariffs prevailing today, it is worthwhile undertaking this activity of optimisation of pumps for energy conservation. This has been successfully carried out at :
In all cases the payback period was very attractive. For optimisation or reselection the following steps are recommended :
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