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Thread: Does it get harder to chill water as it gets colder?

  1. #1
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    Does it get harder to chill water as it gets colder?

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    Assume the ambient temperature does not change.

    For a given body of water, is it harder to drop it's temperature by X degrees from a lower starting temperature than from a higher temperature?

    In more practical terms, is it harder for a chiller to remove 5 degrees C from water at 30 degrees C, than for the same chiller to remove 5 degrees C from water at 20 degrees C?
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    My take is it will be the latter, ie. it is harder for a chiller to remove 5 degC from water at 20 degC based on the assumption that ambient temperature does not change.

    My explanations are such - assuming the water was at room temp of 30 degC. The first few degrees drop will be quite easy as it is still near to the ambient temp. However, as the chiller works to bring the temp down further, the differential will be greater. We have to TIC that as the chiller is trying to bring the temp down, the ambient will be doing the exact opposite, ie. warming the water up.

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    Another example is IPPT.
    To improve from 12min to 11min may be easy.
    But to improve further down, you have to work harder.

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    it takes the same amount of "work done" (remember this term from physics?) in order to reduce the temperature of a given volume of water by a given amount. this is dictated by the "specific heat" of that particular water (remember we are talking about aquarium water, which is quite different from pure water).

    in practice, it takes less effort ("work done") to chill water from 30 to 25 than from 20 to 15, if assuming the ambient temperature is fixed at 30. the reason is due to heat loss from the water to the surrounding atmosphere, which is proportional to the difference between ambient temperature and water temperature.

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    Cool topic

    It is definitely harder to reduce (cool) the temperature by the same absolute amount as the temperature decreases. Why? There is a tendency to move towards the ambient temperature, the greater the temperature difference the higher the tendency. This is one main reason why it is so hard (impossible) to achieve absolute zero degree Kelvin (-273.14 degrees Celcius).

    In fact, for airconditioning, you can save very significantly in electricity cost if you maintain the desired temperature at a higher temperature. As you cool the enclosed space, there will be "leakage" to the outside ambient. One way to cut down on such "leakage" is to insulate/isolate the two environments; this way, two (conduction and convection) of the three methods (the third is radiative) of heat exhcange are minimise.

    In addition, by the nature of cooling/airconditioning, heat removed from the inside enviroment needs to be dumped to the outside ambient. This causes the external environment to be "hotter" and thereby making heat exchange harder when the compressed refrigerant and the ambient temperatures are closer. To achieve very high levels of cooling, the refrigerant needs to be compressed to very high pressures (to "force" the heat out); this translates to extremely costly cooling system that can withstand such pressures. Furthermore, this type of system will be expensive to maintain and prone to mechanical damage/wear and tear.

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