Networks Europe Issue 13 January/February 2018 - Page 22

22 COOLING compressor. So as you can see, there’s a direct influence. The evaporation temperature should be as high as possible, while the condensing temperature needs to be as low as possible. The difference between the evaporation and condensing temperature is the path, so to speak, that the refrigerant has to take with the aid of the compressor. Every kelvin that can be saved here translates into energy savings of 3-5 % for the compressor. But how do we obtain a low condensing temperature and a high evaporation temperature? Both depend on the design and the material. For both the evaporator and the condenser: Q = A•k •ΔT A is the surface area in m2 k is the heat transfer coefficient ΔT is the difference in temperature between the chilled water inlet and the chilled water outlet temperature. This formula demonstrates that both the material and the size/area are decisive factors. In this case, size matters. Consequently, a glance at the technical drawing of the chiller can already help us to determine whether the data are plausible. Can the chiller with a smaller condenser surface area really have a lower condensing temperature and better energy efficiency? As a rule, the answer is NO. The same rule applies to free cooling. The larger the free cooling surface – i.e. the free cooling coil – the sooner the system can switch to free cooling mode. Here too size matters, and plausibility can be checked by taking a look at the drawing. The pressure drop The data sheets contain yet another value that can often only be compared with difficulty: the pressure drop. The pressure drop determines how large the chilled water pump has to be. If the overall pressure drops are much higher, the chilled water pump may have to be bigger. Even if the pump only accounts for approximately 10% of energy expenditure in a CW system, which when viewed over the life cycle large savings can be achieved. Why are the values often difficult to compare? Not all manufacturers calculate their data on the same basis. For instance, manufacturer A may only state pressure drops over the evaporator, while manufacturer B specifies the total pressure drops over the entire chiller, including chilled water piping. Here, it pays to exercise caution. In addition to energy efficiency, the subject of noise is also becoming ever more relevant. Here, too, a simple comparison can reveal whether the data is correct. Manufacturer A and manufacturer B provide different noise data. Which value are they comparing? The sound pressure, or the sound power level? What’s the difference? Sound pressure depends to a large extent on the acoustic properties of the environment. Furthermore, this begs the question of how and under what conditions these measurements took place? As we can see, a genuine comparison is not possible on this basis. Since the sound power level is not dependent on the acoustic properties of the environment, it is a characteristic specific to the equipment in question and is, therefore, the only admissible value that should be used for a serious comparison. If these values are compared, it’s relatively simple to check whether the data is realistic. For chillers, in most cases, it’s the fan noise that dominates. Therefore, we need to take a closer look at the fan and its associated data. What is the fan diameter? And how many revolutions a minute does it need to produce a given airflow? A smaller fan is hardly likely to deliver the same airflow with lower power consumption and less noise. Is the fan an AC or an EC model? If it’s an EC fan, we have to ask what its speed is at the operating point. If it runs at full load, it doesn’t offer any advantage over an AC fan, as it achieves its greatest savings in partial load mode. Last but not least... The integration of the chiller in a system also has to be considered. As mentioned above, it is the flexibility of the manufacturer that counts here. However, optional extras and operating limits may also play a role. If too many electrical extras are needed, an external switch gear cabinet is frequently required. This is a vital consideration, because this also increases both CapEx and the footprint, for at the end of the day space needs to be found for this external cabinet. If everything fits inside the chiller’s switch gear cabinet, installation is vastly simplified. Another aspect of this connection is ѡ٥ȁѡ)ȁɥѕ䁅ѕȁݕȁЃLѡ)ݽе͔͍ɥȁٕ䁑фɔɅѽȄ!܁)́ѡȁѼɕɸѼɅѥ)!܁᥉́Ёݡɔݥэݕݼ)ݽɭ́́ɹ!܁ե䁍Ёݥэ!)ѡ݅ѕȁѕɅɔȁЁѼɕхЁݥѡЁ)ɽ]ɔɔѡɅѥ $ٔͅՙ)хɔѡɽ՝ɽɅѥ%)ѡЁ͔ѡх͵Ȱ͔ѡȁ)хЁݥѡЁɽє݅ѕȁѕɅɕ̸)MݡЁ͡ձ$+$)U͔Ʌѥɔ͕ѡͅݕѡ)ɽѼɔɝ䁕+$) ѡͥ䁽ѡѕф+$)A͔Ʌݥ́ձЁѡȁȁɥͽ+$)݅́хչЁѡѕ́ݡ)ѡمɥ́Օѽ)ܹݽɭ͕ɽ饹