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COOLING TOWER

Identifying Energy Efficiency

ENERGY EFFICIENCY OPPORTUNITIES

This section includes main areas for improving energy efficiency of cooling towers. The main areas for energy conservation include: 

  • Selecting the right cooling tower (because the structural aspects of the cooling tower cannot be changed after it is installed)
  • Fills
  • Fans and motors

Selecting the right cooling towers

Once a cooling tower is in place it is very difficult to significantly improve its energy performance. A number of factors are of influence on the cooling tower’s performance and should be considered when choosing a cooling tower: capacity, range, approach, heat load, wet bulb temperature, and the relationship between these factors. This is described below.

Capacity

Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are an indication of the capacity of cooling towers. However, these design parameters are not sufficient to understand the cooling tower performance. For example, a cooling tower sized to cool 4540 m3/hr through a 13.9 0C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5 0C range. Therefore other design parameters are also needed.

Range

Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger is determined entirely by the heat load and the water circulation rate through the exchanger and going to the cooling water. The range is a function of the heat load and the flow circulated through the system:

Range 0C = Heat load (in kCal/hour) / Water circulation rate (l/hour)

Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 4540 m3/hr from 48.9oC to 32.2oC at 26.7oC wet bulb temperature.

Approach

As a general rule, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8oC approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. When the size of the tower has to be chosen, then the approach is most important, closely followed by the flow rate, and the range and wet bulb would be of lesser importance.

Approach (5.50C) = Cold-water temperature 32.2 0C – Wet bulb temperature (26.7 0C)

Heat load

The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. However, in some applications (e.g. internal combustion engines) high operating temperatures are desirable. The size and cost of the cooling tower is increases with increasing heat load. Purchasing undersized equipment (if the calculated heat load is too low) and oversized equipment (if the calculated heat load is too high) is something to be aware of.

Process heat loads may vary considerably depending upon the process involved and are therefore difficult to determine accurately. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy.

Wet bulb temperature

Wet bulb temperature is an important factor in performance of evaporative water cooling equipment, because it is the lowest temperature to which water can be cooled. For this reason, the wet bulb temperature of the air entering the cooling tower determines the minimum operating temperature level throughout the plant, process, or system. The following should be considered when pre-selecting a cooling tower based on the wet bulb temperature:

Theoretically, a cooling tower will cool water to the entering wet bulb temperature. In practice, however, water is cooled to a temperature higher than the wet bulb temperature because heat needs to be rejected from the cooling tower.

  • A pre-selection of towers based on the design wet bulb temperature must consider conditions at the tower site. The design wet bulb temperature also should not be exceeded for more than 5 percent of the time. In general, the design temperature selected is close to the average maximum wet bulb temperature in summer.
  • Confirm whether the wet bulb temperature is specified as ambient (the temperature in the cooling tower area) or inlet (the temperature of the air entering the tower, which is often affected by discharge vapors recirculated into the tower). As the impact of recirculation cannot be known in advance, the ambient wet bulb temperature is preferred.
  • Confirm with the supplier if the cooling tower is able to deal with the effects of increased wet bulb temperatures.
  • The cold-water temperature must be low enough to exchange heat or to condense vapors at the optimum temperature level. The quantity and temperature of heat exchanged can be considered when choosing the right size cooling tower and heat exchangers at the lowest costs.

Relationship between range, flow and heat load

The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a larger tower. There are two possible causes for the increased range:

  • The inlet water temperature is increased (and the cold-water temperature at the exit remains the same). In this case it is economical to invest in removing the additional heat.
  • The exit water temperature is decreased (and the hot water temperature at the inlet remains the same). In this case the tower size would have to be increased considerably because the approach is also reduced, and this is not always economical.

Relationship between approach and wet bulb temperature

The design wet bulb temperature is determined by the geographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540 m3/hr cooling tower selected for a 16.67oC range and a 4.45oC approach to 21.11oC wet bulb would be larger than the same tower to a 26.67oC wet bulb. The reason is that air at the higher wet bulb temperature is capable of picking up more heat. This is explained for the two different wet bulb temperatures:

Each kg of air entering the tower at a wet bulb temperature of 21.1oC contains 18.86 kCal. If the air leaves the tower at 32.2oC wet bulb temperature, each kg of air contains

24.17 kCal. At an increase of 11.1oC, the air picks up 12.1 kCal per kg of air.

• Each kg of air entering the tower at a wet bulb temperature of 26.67oC contains 24.17 kCals. If the air leaves at 37.8oC wet bulb temperature, each kg of air contains 39.67 kCal. At an increase of 11.1oC, the air picks up 15.5 kCal per kg of air, which is much more than the first scenario.

Fill media effects

In a cooling tower, hot water is distributed above fill media and is cooled down through evaporation as it flows down the tower and gets in contact with air. The fill media impacts energy consumption in two ways:

  • Electricity is used for pumping above the fill and for fans that create the air draft. An efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore lead to lower electricity consumption.
  • Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. The fill media determines all of these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower becomes.
  • Splash fill media. Splash fill media generates the required heat exchange area by splashing water over the fill media into smaller water droplets. The surface area of the water droplets is the surface area for heat exchange with the air.
  • Film fill media. In a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill sheets is the area for heat exchange with the surrounding air. Film fill can result in significant electricity savings due to fewer air and pumping head requirements.
  • Low-clog film fills. Low-clog film fills with higher flute sizes were recently developed to handle high turbid waters. Low clog film fills are considered as the best choice for sea water in terms of power savings and performance compared to conventional splash type fills.

Optimize cooling water treatment

Cooling water treatment (e.g. to control suspended solids, algae growth) is mandatory for any cooling tower independent of what fill media is used. With increasing costs of water, efforts to increase Cycles of Concentration (COC), by cooling water treatment would help to reduce make up water requirements significantly. In large industries and power plants improving the COC is often considered a key area for water conservation.

Install drift eliminators

It is very difficult to ignore drift problems in cooling towers. Nowadays most of the end user specifications assume a 0.02% drift loss.

Cooling tower fans

The purpose of a cooling tower fan is to move a specified quantity of air through the system. The fan has to overcome the system resistance, which is defined as the pressure loss, to move the air. The fan output or work done by the fan is the product of air flow and the pressure loss. The fan output and kW input determines the fan efficiency.

The fan efficiency in turn is greatly dependent on the profile of the blade. Blades include:

  • Metallic blades, which are manufactured by extrusion or casting processes and therefore it is difficult to produce ideal aerodynamic profiles
  • Aerotech Fiber reinforced plastic (FRP) blades are normally hand molded which makes it easier to produce an optimum aerodynamic profile tailored to specific duty conditions. Because FRP fans are light, they need a low starting torque requiring a lower HP motor, the lives of the gear box, motor and bearing is increased, and maintenance is easier.

A 85-92% efficiency can be achieved with blades with an aerodynamic profile, optimum twist, taper and a high coefficient of lift to coefficient of drop ratio. However, this efficiency is drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc.

Cases reported where metallic or glass fiber reinforced plastic fan blades have been replaced by efficient hollow FRP blades. The resulting fan energy savings were in the order of 20-30% and with simple pay back period of 6 to 7 months.


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