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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 the behaviour of the chiller during and immediately after a power blackout – the worst-case scenario for every data centre operator ! How long does the chiller need to return to generating 100 % cooling capacity ? How flexible is it where switching between two networks is concerned ? How quickly can it be switched ? How high can the water temperature be for it to restart without a problem ? Where are the operating limits ? Can I save buffer tank expenditure through a broad range of application ? In the best case , the tank can be smaller , because the chiller can start without a problem despite high water temperatures .
So what should I do ?
• Use operating expenditure based on the same weather profile to compare energy efficiency
• Check the plausibility of the technical data
• Peruse drawings and consult them for your comparison
• Always take account of the system as a whole , and all the various influencing factors
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