CHILLERS FANS


condenses it to a liquid from a vapour, before being passed through a small hole reducing its pressure and temperature so that the same refrigerant can be used in the cycle as it re- enters the evaporator. Water chillers come in a very wide range of cooling capacities and confi gurations to suit many varied applications.


Some water chillers are capable of reverse cycle heating (air-to-water or water-to-water heat pumps) and free cooling. The fi rst named medium is that which absorbs from or rejects heat to atmosphere. The second named medium is that from which heat is extracted, or added to, and circulated to the conditioned space. There are many types of packaged water chillers, diff ering in the type of compressor used (screw, reciprocating, scroll or centrifugal) and are off ered with either air-cooled or water- cooled condensers. Water chillers are also available in a split format where one part of the system contains the compressors, evaporator/ heat exchanger, metering device and controls and this is connected via refrigeration pipework to a remote air-cooled or water-cooled condenser. Modular systems are now available, these can be operated in parallel with each other. Chilled water temperatures are normally around 6°C


fl ow temperature to such units with 12°C return water temperature to the water chiller(s) at peak thermal load. If necessary, a two-pipe system can change over to heating, simply by sending hot water through the fan coil or air handling units, a heat pump executing the heating. However, where simultaneous cooling and heating are required within the same system, a 4-pipe system is used, the hot water being supplied through a separate low-pressure hot water (LPHW) pipe work system supplying separate heating coils.


Chilled water indoor fan coil units (FCUs) are similar in


appearance to split air conditioning models. These are available in the same format, including ceiling cassettes.


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Chilled water calculation Compared to other secondary cooling fl uids, water has the


highest heat absorbing capacity. In other words, for every kilogram of water circulated, more heat energy is absorbed for every °C temperature rise than most other fl uids. Air has only ¼ of this capacity, thus requiring much larger ducts to transport heat energy as compared to small diameter pipes for chilled water applications. The specifi c heat of water is 4.19 kJ/kg K. Thus, for every °C rise (1K) in temperature, a 1 kg mass of water will absorb 4190 J. (1) Q = m × c (T2


- T1 )


Where: q = the quantity of heat energy in kilojoules (kJ) m = the mass in kilograms (kg) c = the specifi c heat in kilojoules per kilogram per kelvin (kJ/kg K)


T1 T2


= the initial temperature in degrees Celsius or Kelvin


= the fi nal temperature in degrees Celsius or Kelvin*


* Must be the same units as T1 While the above equation (1) tells us how much heat is


removed (or added) per kg mass of a substance, it does not tell us what size of cooling plant is required. Since the density of water at 4°C is equal to 1000kg/m³, it can be assumed for a water chiller plant that 1 litre of water will have a mass / weight of 1 kg. Using the equation (1), the required fl ow rate to absorb / cool a given heat load can be calculated. Example: A small offi ce has a cooling load of 5.8 kW. What


mass fl ow of water must be recirculated through a fan coil unit, based on a water temperature rise of 5K? By transposing the equation (1), we can calculate the water


mass fl ow rate: (2) m = Q / (c × (T2-T1 ) ) 0.27685 l/s = 5800 / (4190 × (10 - 5) )


Note that the water can be measured in kg/s or l/s as they are very nearly exactly the same at these temperatures.


www.acr-news.com • August 2024 19


Sometimes pure water cannot be used


because the temperature of the


application will be close to or below 0°C (the


temperature at which


pure water at atmospheric pressure will freeze).


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