Circulation Pumps

These are some of the larger pumps in any installation and so it is worth ensuring they are as efficient as possible. There are two types that are normally selected. Due to the low NPSH available, they are either vertical turbine type or horizontal with double suction impellers. These pumps operate without control valves against system friction back pressure.

It is difficult to monitor pump condition since, for example, wear ring erosion results in loss of output but the resulting internal recirculation means little or no

Tapered velocity recovery stack

Inlet bell mouth

Tapered velocity recovery stack

Figure 8.21 Cooling tower velocity recovery stack.

No V

R.

With

V.R."

0 2 4 6 8 10 12 Velocity pressure/mm H2O

0 2 4 6 8 10 12 Velocity pressure/mm H2O

Figure 8.22 Effect of velocity recovery stack.

Front edge of impeller vanes

Figure 8.23 Improvement to water pump impeller.

Existing edges

Improved profile

Front edge of impeller vanes

Figure 8.23 Improvement to water pump impeller.

change in input power. The cost of individual pump flowmeters is not justified, but by using a portable ultrasonic strap-on flowmeter, pumps can be compared to identify poor performance.

Double suction impellers are often supplied in rough ' as -cast' condition and simply machined and then ground for balance. It is possible to improve efficiency by improving the blade profile, especially at the suction as shown in Figure 8.23. This also reduces the risk of cavitation which is common for cooling water pumps. The grinding should be done on the convex side of each impeller vane.

These pumps are also much improved by impeller coatings. Application of a suitable coating reduces power requirement of a typical cooling water pump by ~6% - a considerable saving. See also the last paragraph of Section 8.2.2.1.

Double suction pumps should not have a horizontal elbow in the suction line since this can push flow preferentially to one side of the impeller - almost certainly

leading to cavitation damage as well as increasing thrust load and reducing performance.

Surprisingly, there is often significant loss across the pump discharge check valves. Cooling water pumps are generally high flow, low head and so balanced check valves should be used and swing check valves avoided unless they are fitted with an external counter weight as sketched in Figure 8.24. Pressure drops of 0.2 bar have been observed in this service. Another advantage of the external weighted arm is that operators can physically move it to check that the valve is not seized in the open position (and also that it is not stuck part open).

8.3.7.4 Tower Packing

Tower packing (or 'fill') improves evaporative cooling by exposing a large surface area of water to the air that is drawn through the tower. Packing is available in a range of styles and designs, from old style wooden slats to modern structured blocks.

The performance of older towers can often be improved by installing modern packing, though care is required to ensure that the possible increase in packing pressure drop can be handled by the fan(s).

There is a particular concern in countries where winter temperature is very low. In this case, without remedial action, the packing can become part blocked with ice and the extra weight risks mechanical damage. Various strategies have proved successful, including partial blocking of the air intakes, reversing fan direction and isolating and draining a percentage of the tower cells. Hybrid towers are attractive in extreme low temperatures since the packing can be by-passed completely and evaporation loss eliminated along with the cost of chemical dosing.

8.3.7.5 System Tuning

It is common to find the cooling water isolation valves fully open at all water-cooled equipment. This is not good practice since conservative design generally allows for flow rates in excess of what is required.

If the temperature rise between water inlet and outlet of an exchanger is small- say 1 or 2 °C-then almost certainly the water flow rate is higher than necessary. In this case, the water exit valve should be throttled to the minimum safe flow rate, consistent with achieving the desired cooling and avoiding the risk of scale and fouling. Generally, exit water temperature should not exceed 40-45 °C to avoid scaling, and exchanger tube velocity should not be lower than ~0.8ms-1. A portable strap-on ultrasonic flowmeter can give a reasonable guide to water flow rate.

There are a number of benefits from system tuning. In most cases, overall water circulation rate is reduced. The resultant higher supply pressure means that flow is increased through critical service exchangers. In existing plants this can result in significant energy savings where, for example, improved cooling of a steam turbine surface condenser gives better vacuum and reduced turbine steam requirement.

Reduced water circulation rate through a cooling tower increases cooling residence time and also the lower water flow rate through the packing results in increased fan air flow. Both of these factors tend to give a better approach to ambient wet bulb temperature -colder cooling water supply.

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