Chilling of water - Aquaculture Engineering

Chilling of water

When water is to be chilled, energy has to be removed; the amount is the same that has to be added when heating for a given temperature dif-ference, so of course the same equation can be used.

P =mc_pdt

where:

m =water flow (kg/s)

c_p=specific heat capacity (kJ/(kg°C))

dt= temperature decrease for the water (°C),

i.e. the difference between inlet and outlet temperatures.

Example

A water flow of 60 l/min is to be chilled from 4°C to 2°C. Find the rate of energy removal.

P =1 kg/s×4.18 kJ/(kg °C)×(4°C−2°C)

= 8.36 kJ/s

= 8.4 kW

Heat exchange can be used if there is an avail-able water source that could be used as a chilling medium. For instance, freshwater can be chilled using bottom water from the sea during the summer when freshwater is warmer. Direct mixing of colder water may also be used to avoid the use of a heat exchanger, but requires satisfactory water quality.

Alternatively, ice may be added directly to the water, or one circuit of a heat exchanger can pass through a basin to which ice is added. If this method is used, it is necessary to have an ice machine on the farm; however, this is not very satisfactory because it is a costly method involving two stages: first production of ice and then chilling of the water. Only when small amounts of water are to be chilled at specific times of the year, is it a viable solution, for example in small slaughter houses, and where ice is available for chilling the fish.

A cooling plant is therefore used to chill the water. This is basically the same as a heat pump, but is optimized in another way (Fig. 7.18). The evapo-rator is placed in the water or liquid to be chilled. When the working medium or refrigerant evapo-rates, energy is taken from the water or liquid, the temperature of which falls. This is made possible by choosing a suitable refrigerant (working medium). After this the working medium goes into the com-pressor and then on to the condenser where it condenses, releasing heat to the surroundings. A cooling plant often contains an air–liquid heat exchanger (the condenser) so the heat is released directly to the air. This is the same principle that is used in a refrigerator where the energy is also released to the air. As seen from the set-up, the main component in a cooling plant is the evapora-

tor that ‘removes’ energy. When talking about the efficiency or COP when a heat pump is used as a cooler, it is the relation between the energy (Q) that is added to the compressor and the energy removed from the water in the evaporator that is critical. The equation to use is therefore:

e=Q_{removed  in  the  evaporator}/Q_{added  to  compressor}

The same circulation circuits that are used on heat pump installations to prevent refrigerant from con-taminating the inlet water and avoid freezing are also used in cooler units.

If both chilling and heating need to be performed on a fish farm, it might be possible to utilize the same heat pump for both purposes. Energy can be taken from the inlet water to be chilled and added to the inlet water to be heated. This means placing the evaporator in the water to be chilled and the condenser in the water to be heated. It will, however, normally be a rather unsatisfactory solution because it is difficult to optimize the heat pump with pressure conditions throughout the evaporator and condenser, and to find good refrigerant.