DRY COOLING
point with regard to energy efficiency of the plant as a whole?
The overall EER increases with decreasing condensing temperatures. Lowering the temperature level at the dry cooler always pays off from an energy point of view. It is worth noting for the temperature level 40/45 that the dry cooler performs worst regarding the EER when compared with the sprayed and adiabatic versions. The reason for this is that both ‘wetted’ versions operate with water which means that the required primary heat exchanger surface can be reduced dramatically. This generally leads to units with less fans and thus to a reduced energy consumption at the design point.
Comparison of water requirements This gives rise to the following question: “How about the annual water requirement of the individual ‘wetted’ technologies?”
Dry coolers have the best values here as they are the only units requiring no water at all. This does not only save valuable resources but also results in the fact that no one has to deal with any of the topics related to water with respect to planning, installation and operation.
It is apparent that better EERs go along with increased water requirements. It first seems as if the additional costs for water have no relation to the marginal improvement of the EER of the complete system. The big difference between the wetted cooler types are also remarkable. From all ‘wetted’ technologies, sprayed dry coolers require the least amount of water in the course of a year. This is due to the intelligence of the technology ensuring that water is supplied with an excess of 1.2 - 1.4 as an annual average. The ‘section cycling’ function also allows for wetting only individual fan chambers at partial load operation. This further reduces the consumption values significantly – without sacrificing capacity losses.
By contrast, adiabatic dry coolers are operated with much more excess water. This technology’s water requirement – four to five times higher – is also due to a lower switch point that results in a larger number of wetted operating hours per year.
However, the water requirement rather takes a back seat when it comes to evaluating the operating costs as there are huge differences regarding the required water qualities and hence the costs for water. What also comes into play
here is that operators are mainly interested in saving operating costs and not costs for water and/or energy.
Operating costs of dry coolers To evaluate this aspect, the respective water requirement was converted into costs that were added, along with the energy costs, to the operating costs of the dry cooling technologies. Dry coolers have the lowest operating costs as these units are designed for a maximum air inlet temperature of 36°C. The very large heat exchanger surface allows for reducing the fan speed in case the unit operates at partial load below the design temperature or below the maximum load. In this regard, it is well known that the energy consumption of the fans reduces to the power of three with the fan speed reduction. This means, for instance, the energy consumption reduces by 87.5% when halving the speed. For dry coolers, there are of course no costs for water.
In terms of operating costs, the differences in water consumption between adiabatic and sprayed cooling cancel each other out. The major strength of adiabatic dry coolers is that they can virtually always be operated with water of
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