DRY COOLING
From dry to hybrid: which dry cooling technology is
suitable for which application? A variety of fundamentally different dry cooling
technologies is available on the market for cooling down water/glycol mixtures. The decision for or against one of these technologies during the planning stage of a project has an impact not only directly on the investment sum but also on the subsequent operating costs of the plant as a whole – across the entire life cycle. Michael Freiherr, managing director at Güntner explains.
I
t is important to know the applications of dry cooling technologies as well as their specific advantages and disadvantages, and to evaluate them accordingly. There are the two basic dry cooling technologies: Units in dry operation Units with ‘wet operation’ (sprayed, adiabatic, hybrid, cooling towers).
The key factors for evaluation are essentially the approach, the footprint and the investment costs. However, every individual unit has further specific advantages and disadvantages that limit the suitability for certain applications/areas of application to some extent.
Let’s take a closer look at these aspects and evaluate them on the basis of an exemplary design. Within this context though, cooling towers will not be considered.
Practical example of water chillers To demonstrate which effects the specific advantages and disadvantages of the different dry cooling technologies ‘dry’, ‘sprayed’, ‘adiabatic’ and ‘hybrid’ have on a real-life plant, all four technologies will be compared with one another on the basis of a virtual case study. For cooling a continuous process, a refrigerating plant with a refrigerating capacity of Q˙ 0
≈750 kW is to be established. For refrigeration, a water chiller is to be used that provides cold brine with a terminal temperature difference of ‘6/12’ for the air coolers.
28 October 2020
Dissipating heat from the cold rooms requires, of course, a primary energy input P˙ for driving the compressors (and pumps). The sum of the air coolers’ capacity (Q˙ 0
) and the driving power P˙ is to be dissipated via the dry cooler.
In the following analysis, the respective dry cooling technologies – dry, sprayed, adiabatic and hybrid – will be compared on different temperature levels (40/45°C, 35/40°C, 30/35°C and 27/32°C). All data were calculated for an installation site in Berlin (Germany) and for an annual operating duration of the refrigeration system of 8,000 hours at full load for process cooling or similar application. The air inlet temperature for this site has a maximum of 36°C at 33.6% relative humidity, which corresponds to a wet bulb temperature of 23°C. Instead of assessing the energy efficiency of a water chiller only at full load operation via the EER, the efficiency is illustrated using the European Seasonal Energy Efficiency Ratio (ESEER). This value is calculated as the product of four EERs at partial load. These EERs are, in turn, weighted differently on the basis of a standardised temperature distribution for European climate – full load operation thus finds its way into the overall result by only 3%. Within this context the ESEER for water-cooled water chillers refers, per definition, to a temperature level of 30/35 at the dry cooler.
The water chiller chosen for the benchmark of the dry cooling technology has an ESEER of 6.51 according to the data sheet. To ensure a
year-round outlet temperature of 30°C at the dry cooler, it is important to either observe a maximum dry bulb temperature of 25°C at the installation site or to resort to ‘wetted’ dry cooling technologies.
But this implies that an operational cost balance for the different dry cooling technologies has to be created in addition to the energy footprint to take the use of water into account and to look at the comparison in a fair manner. This is why the following description is not simply a comparison of energy footprints and energy efficiencies – it also considers the actual operating costs.
Temperature level and EER
As already recognised by Carnot, lowering the condensing temperature results in a better efficiency of vapour-compression units – and increasing the evaporating temperature also contributes to higher efficiency. The considerations described here are not limited to the lowering of the condensing temperature at water-cooled water chillers. The temperature of the cold side (6/12) is always assumed to be constant.
EER of refrigeration machine and dry cooler
It is well known that the EERs of water chillers improve with decreasing condensing temperatures. If, however, dry coolers with greater driving powers for the fans were to be used to achieve this goal – what would be the
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