Why use heat exchangers?
A heat exchanger is used to transfer energy from one medium to another and to achieve this atemperature gradient is necessary. Energy transfer can take place via direct contact between the two media, or indirectly where the media are separated by a barrier. In indirect exchangers the heat must be transferred through this barrier.
An illustration of the direct method is as follows. Water is fed in at the top of a tower where is it spread on a perforated plate; it falls through as drops which pass through the air into a basin placed below. The water drops have a large surface area exposed to the surrounding air; if this is warm, energy will be transferred from the air into the drops and the water will be heated. The indirect method is illustrated by having water of different temperatures separated by a thin metal plate. Energy will be transferred through the plate from the warm side to the cold side because of the tem-perature gradient until the temperature on both sides is the same. This method is the main one used in fish farming.
Heat exchangers can be used both for heating and chilling. In heat pumps and cold-storage plants, a refrigerant may be used as one of the media . Heat exchangers may also be air to air as in ventilation systems in houses, or air to water as in air coolers.
Heat exchangers can be used to recover energy from the outlet water; for example, to recover the energy from hot industrial effluent water and use it to heat the water supply to a fish farm. Heat exchangers are also commonly used to recover the energy from the outlet water from the fish farm and transfer it to the inlet water. Seawater may also be used as a heat source on one side of the heat exchanger, to supply part of the heat energy to the inlet water supply to a freshwater fish farm.