Combining the best features of evaporative and dry cooling for heat rejection
Hybrid dry coolers combine the best features of dry coolers and evaporative cooling according to ambient conditions.
The choice for heat rejection from an air-conditioning system should not just be between dry coolers and evaporative cooling. Both techniques can be used to advantage, as Stephen Fairgrieve and Jaeggi Guentner explain.When planning and designing any cooling and chilled water systems, different types of closed-circuit cooling equipment are used depending on the required temperature of the circulating water. With coolant temperatures below about 20°C, chillers are used. These can be air- or water-cooled depending upon design with COPs (coefficient of performance, which is cooling capacity divided by electrical input) between three and 10. With dry coolers, which cool finned radiators using ambient air, coolant temperatures of around 37°C can be reached with a typical design ambient temperature of 32°C. As a result of climate change, many planners are now designing for 35°C ambient temperature, and the minimum attainable coolant temperature thus increases to about. 40°C. Dry coolers require large surface areas and have relatively high energy consumption, even with higher water temperatures. In comparison, evaporative cooling provides better energy efficiency with significantly smaller surface areas. Evaporative coolers, can achieve cooling water return temperatures only 4 to 5 K above the design wet-bulb temperature (typically 19 to 22°C for the UK). And remember, each 1 K reduction in chiller condensing temperature equals 3% energy saving! Many types of traditional evaporative coolers are available, such as open and closed wet cooling towers or evaporative condensers. However, these traditional systems also have some big disadvantages. Depending on operating condition and design a visible vapour plume is emitted, which is often considered visually undesirable. In addition, relatively large quantities of fresh water quantities are needed to reconcile the evaporation and draining losses.
Hybrid coolers Hybrid dry coolers offer the benefits of traditional systems such as energy efficiency while saving water and being plume-free. Jaeggi’s hybrid dry cooler for example unites the advantages of conventional dry and wet cooling systems, without any of their disadvantages. Efficient cooling is achieved with low energy and water consumption. In addition the coolers are extremely quiet and guaranteed to operate without vapour plumes. Integrated ‘plug-and-play’ control, compact cooler dimensions and low weight enable them installed easily into modern building systems –—whether new-build projects or retrofits. Hybrid dry coolers are always optimised for water-saving operation. At lower ambient temperatures or part load, hybrid coolers operate as pure dry coolers, i.e. with convective transfer of heat to the ambient air. The wetting cycle intelligently activates only if the chilled-water demand cannot be achieved in the dry mode. The coolers then make use of the principle of natural evaporation energy efficiently, to save 70 to 90% of the water consumption and reducing associated costs compared with wet cooling towers. The energy efficiency of hybrid dry coolers can be impressive. The hybrid cooler shown in Fig. 2 is used for building cooling. It has three fans, each rated at 4.8 kW and two wetting pumps of 700 W each. The total power consumption is 15.8 kW for a cooling performance of 1900 kW using 230 m3/h of water to reduce incoming water temperature from the system from 38°C to 28°C with a wet-bulb temperature of 21.5°C. The ‘COP’ is 120. In partial load operation or with lower wet and dry bulb temperatures the fan speed is reduced using frequency inverters. Since the absorbed fan power falls by the cube of the fan speed, the ‘COP’ value then rises rapidly. In the above example, the cooler achieves a ‘COP’ value of 590 at half fan speed.
Hybrid dry coolers can operate dray at lower ambient temperatures and wet at higher temperatures to boost cooling performance.
As soon as the changeover point for dry operation is reached, the cooler intelligently switches the wetting off and then operates in dry mode without using any water. The changeover point for dry operation is usually at an ambient temperature between 10 and 20°C. The operating characteristics of such a hybrid cooler for ambient temperatures ranging from -5 to +32°C are show in Table 1. The change from dry to wet cooling begins at 18°C,and the cooling load is constant al year round at 1000 kW. A typical operational characteristic for a hybrid dry cooler in the UK is represented in Table 1. The point of switching for dry mode, in this example, is 18°C, with a constant cooling load of 1000 kW all year round. Only when the ambient temperature rises above 23.7°C does evaporative cooling take over entirely — and then only for a hundred hours of the year. In the example summarised in the table, the hybrid dry cooler has an annual water consumption (evaporation and draining) of 888 m3 and a waste water discharge of 297 m3. This consumption is to be compared with a wet cooling tower using 14 534 m3 a year of fresh water, with a resulting waste water discharge of 4845 m3.
Energy-saving with ‘free cooling’ With chilled-water installations, substantial energy savings are possible with ‘free cooling’. A chilled-water installation with a water-cooled chiller can be planned in such a way that the energy-intensive chiller can be switched off at low ambient temperatures and the heat exchanger (cooling tower, dry cooler or hybrid dry cooler) takes over the cooling process instead of the condenser. Often due to short term business strategies, purchasing decisions are based only on capital outlays and so allegedly ‘cheaper’ cooling systems are installed, but which have high operating costs and are detrimental to the environment by higher energy consumption. However, with well planned installations and modern hybrid heat exchangers, businesses can save energy and reduce costs — as well as preserve the environment. In particular, hybrid dry coolers which save energy and water, and, at the same time, reduce operating costs, can make an effective and valuable contribution to the conservation of resources and to environmental protection. Stephen Fairgrieve is with the UK office of Jaeggi Guentner (Schweiz), and Joachim Weber is based in Switzerland.
Table 1: For much of the year, a hybrid cooler operates in dry mode and can even deliver free cooling. Only when the ambient temperature rises does evaporative cooling come into play.
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