DRY COOLING
drinking water quality. This offers advantages not only with regard to the annual water costs. Furthermore, this system requires less maintenance compared with sprayed dry coolers and hybrid cooling towers as there is, for example, no need at all for a water treatment plant. Just the regular works need to be performed, such as basic cleaning of the heat exchanger coil following the pollen and flower season in September. This provides the perfect opportunity to remove the pads and to store these somewhere else during the cold season. No pads attached to the unit means the fans’ efficiency increases, resulting in a reduced power consumption of the unit during the winter period. The operating costs of a dry cooler are comparatively low, which gives rise to the question of how high the total refrigeration costs are according to the different dry cooling technologies.
Operating costs for refrigeration The operating costs – generally not including maintenance for all technologies – of the adiabatic units are, in most of the cases, only marginally higher than the operating costs for sprayed units.
For the temperature level of ‘40/45’, dry coolers are the best choice as the fans operate at a fraction of their rated speed for every hour during which temperatures are below the design temperature of the units, thereby lowering the capacity to the power of three. Alternatively, the condenser fans can also be kept in operation at full speed to reduce the condensing temperature for most of the year – making the plant operate even more efficiently.
Lowering the condensing temperature is generally supported in terms of costs. For this, however, ‘wetted’ dry coolers have to be operated exclusively. This means a higher maintenance/ inspection effort for the operator, and other operator obligations. On the other hand, the smooth operation of such units has been tried and tested for decades and proven to be a practicable solution if the boundary conditions are adhered to.
If the operating costs of dry coolers are set as benchmark, adiabatic dry coolers at the immediately lower temperature level ‘35/40’ allow annual operating cost savings of €13,000. The difference in operating costs between dry coolers at ‘40/45’ and hybrid cooling towers at ‘27/32’ is even @€33,000/year in favour of the hybrid dry cooler.
Investment costs
The only question remaining is, how about investment costs?
30 October 2020
The lower the design temperature of the dry cooling plant, the higher the investment costs. When looking at the temperature level ‘27/32’, it becomes obvious now why the adiabatic unit could keep up with the hybrid unit – this is simply due to the fact that three units need to be operated to dissipate waste heat. Such a large installed area of course has its advantages for partial load operation. Of note, it requires only one hybrid unit to dissipate the heat. Speaking of hybrid cooling towers – compared with dry coolers, they pay for themselves after a little more than five years thanks to their lower operating costs. This is because, opposed to the currently required additional investment of €175,000 in addition to the price of a dry cooler, there are operating cost savings of 5 x €33,000 whereas cost increases are not included. A hybrid dry cooler, however, would pay for itself several times over during its service life. For the temperature level ‘40/45’, the acquisition costs of sprayed and adiabatic units are lower than for dry coolers. For the temperature level ‘35/40’, adiabatic dry coolers cost about €7,000 more – however, there are annual savings of €13,000 with regard to operating costs. This means the payback period is only seven months. For sprayed units, investment costs for an appropriate water treatment plant need to be considered though.
Free cooling operation
For a year-round application as described here, it has to be considered if free cooling operation makes sense or not. This operating mode can, in general, be applied at any time if a plate heat exchanger is integrated into the cooling circuit in addition. As from a certain “wintry” limiting temperature (depending on the design), dry coolers can dissipate 750 kWh of heat from the process in free cooling operation without support from other units, and the cold water generator can be switched off. What is left are only energy costs for a circulation pump and for the fans of the dry coolers. The EER increases considerably here.
All of the coolers mentioned here were not optimised for free cooling operation in their original design. Instead, we recalculated their potential afterwards. Regarding dry coolers (‘40/45’), the EER increases from 4.1 to 31 in free cooling operation at external temperatures of 1.5°C and upwards. This amounts to 1,260 hours with siginificantly reduced operating costs. Hybrid dry coolers (‘27/32’) can reach an EER of 31.2 for 1,044 hours in dry free cooling operation. In wet operation, the free cooling duration can further be extended. The EER does not change during wet operation.
Summary and outlook
This article shows and compares the specific advantages and disadvantages of different dry cooling technologies. Furthermore, a specific example demonstrates the impact of these different technologies on the plant as a whole, with a focus on energy consumption and water requirement. Looking at the figures, it becomes obvious that there is no ‘ideal’ dry cooler. It rather depends on selecting the most promising dry cooling technology for a given project and to use its advantages for the benefit of our environment. Future comparisons of this kind for different capacity categories would propably provide more information as some effects may become apparent only when a certain capacity level is reached.
As a matter of fact, however, the decision for or against a specific dry cooling technology determines not only the investment costs but also subsequent operating costs of the plant as a whole. This is why it is crucial to carefully choose the operating mode of the plant and then opt for the most appropriate dry cooling technology already during the planning stage instead of blindly resorting to established plant concepts for every project over and over again.
As a rule, every design benefits from free cooling operation. However, dry cooling plants with large installed heat exchanger surfaces naturally benefit most as their switch points are clearly higher than those of the rather small units. This becomes particularly apparent when looking at dry coolers (savings of €27,000) and adiabatic units at temperature level ‘27/32’ (savings of €48,000).
Footprint
Studying theoretical papers on which plant concept would be ideal – under laboratory conditions so to speak – we all end up with another design in practice, given certain aspects that need to be observed on the spot. Sound specifications are one of those requirements that come into play. Another important aspect that often determines the design is the space made available by the operator for setting up the dry cooling plants. In many cases, we go for the optimum – meaning the smallest possible unit – during the design stage.
As can easily be seen, ‘wetted’ coolers are smaller than dry coolers. Hybrid units are almost always the first choice when space is at a premium. At a given set-up space, two of the plants that are discussed here could be installed using hybrid units. And the temperature level would be much more efficient at ‘27/32’ instead of ‘40/45’.
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