CHILLERS
Cooler than you think A
Martin Cook of Business Edge discusses chilled water systems.
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.
chilled water system or water chiller is the common name for a refrigeration system that cools water. A pump circulates water around a closed loop where it
is used to remove unwanted heat from a space, process or substance. The term ‘chillers’ therefore normally refers to machines
that extract heat energy from water or brine. They are extensively used in refrigeration and process cooling, as well as air conditioning applications. As an example, a building that requires air conditioning could have a water chiller. This water is pumped through pipework to fan coil units and / or air handling units incorporating cooling coils, over which the recirculating room air is passed. The heat from the conditioned space is transferred into the chilled water, which rises in temperature before returning to the water chiller. 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). So, when pure water is no good an anti-freeze is added, often this is referred to as brine. Brine within refrigeration terminology refers to any mixture of water with an additive that reduces the freezing point of the water. This can be, as is suggested by its name ‘brine’, a salt
(calcium chloride or sodium chloride), but ethylene glycol and propylene glycol are more commonly used. A chilled water / brine mix can be used to chill or even
freeze poultry. A low temperature chiller would cool brine to 0°C or less and the poultry would then be immersed in the brine solution. The brine could also be passed through an air handling unit (AHU), which would produce very cold air, which is blown directly over the product at high velocity for effi cient chilling / freezing, as used in a blast freezer.
How they work A vapour compression refrigeration system is used to remove
heat energy from a closed loop of pumped water / brine mix. The evaporator is usually a plate heat exchanger where the cold refrigerant is in contact with a metal plate on one side and water on the other. Within a modern plate heat exchanger there are many parallel channels in each plate heat exchanger for the fl uids to fl ow, this increases the heat conducting surface area which increases the effi ciency. The heat from the water evaporates the refrigerant from a liquid to a vapour then it is compressed raising its pressure and temperature so that it can reject heat energy to a space where the heat does not aff ect the temperature of the conditioned space. The rejection of heat from the refrigerant
18 August 2024 •
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