11. Technical terms
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Knowledge of some of the terms associated with heat transfer will promote a better understanding of the elements which contribute to the effective cooling and storage of chilled fish.
Hot and cold
Hot and cold are only relative terms and therefore do not give a quantitative expression of either the heat content or temperature of a body.
Heat is a form of energy; it is the addition or removal of heat which results in a temperature change or a change of phase. It is therefore easier to understand what is happening during cooling if this is correctly thought of as a transfer of heat and not as the addition of something called "cold". Heat can be measured, "cold" cannot. Heat transfer occurs in the direction of decreasing temperature. In simple terms, this means that fish cannot be cooled without using something colder to act as a recipient for the heat to be removed from the fish. For instance, if ice is colder than the fish, heat can be transferred from the fish to the ice, and the fish are thereby reduced in temperature.
Specific heat is heat which results in a temperature change. The specific heat of a substance is a measure of the quantity of heat required to raise a unit mass of substance one degree in temperature provided that no phase change occurs. The specific heat of pure water is one calorie per gram under specified conditions. Therefore, if the specific heat of fish is given as 0.8, it is both an absolute value of 0.8 calories per gram and the ratio of the specific heat of the fish to that of water. Specific heat may not be a constant value, but can vary with temperature, for example. Specific heat values may also change with a change of state. For instance, the specific heat of frozen fish is about 0.4, which is approximately half the value of the specific heat of unfrozen fish.
The three phases in which a material can exist are solid, liquid and vapour/gas. Thus, if water is frozen to form ice, it has experienced a phase change. Similarly, there is a phase change when water is evaporated to form a vapour. Ice melts to give water and vapour condenses to give water.
It is possible for a material to experience two phase changes at the same time or, more correctly, to miss out the intermediate phase, for instance, by changing from a solid to a vapour. If ice changes directly into a vapour without first becoming a liquid, this double phase change is known as sublimation. Sublimation also occurs when frozen fish dehydrate during low temperature cold storage.
Latent heat is the amount of heat absorbed or evolved by a unit mass of material during a phase change. Thus, there is a latent heat of liquification (when ice turns into water), a latent heat of evaporation (when water turns into vapour) and, a latent heat of sublimation (when ice turns to vapour). In each of these phase changes heat is added but, if the changes are reversed with a change of vapour to liquid, liquid to solid or vapour to solid, heat is removed or lost during the phase change.
If a substance experiences a temperature change or a change of phase then a transfer of heat has occurred. Heat is transferred in three basic ways: by conduction, convection and radiation. In most practical situations where a heat transfer takes place, two, or even all, of these heat transfer methods may apply.
Conduction is heat transfer achieved by direct contact. Fish being cooled by direct contact with ice will experience heat transfer by conduction.
Convection is heat transfer by natural or forced movement of a fluid (liquid or gas). Fish in a chillroom can be cooled by convective heat transfer due to natural circulation or fan circulation of the air. Similarly, fish in refrigerated seawater are cooled by convection resulting from the pumped circulation of the chilled water.
Radiation heat transfer from a heat source to a body is achieved without heating the intermediate space and without the need of an intermediate material. Fish will be exposed to radiated heat from the sun if they are left uncovered outdoors. Fish exposed to a light source indoors will also experience a radiant heat transfer.
Newton's Law of Cooling
The rate of cooling of a hot body which is losing heat both by radiation and natural convection is proportional to the difference in temperature between it and its surroundings. In practical terms, this means that when fish is cooled in ice, the initial cooling rate, when the temperature difference is greater, will be higher than the cooling rate later, when the temperature of the fish has been reduced.
Factors affecting heat transfer rates
Whether heat transfer is in a steady state, for example between outside air and a chilled container, or in an unsteady state, for example between ice and a fish being cooled, the factors affecting it are similar.
The rate increases with increasing temperature difference, with larger heat transfer coefficients, and with larger surface areas.
Factors affecting rates of temperature change
It is important to distinguish between the rate of heat transfer and the rate of temperature change. For example, if we have two fish of similar shape, one 40 cm long and the other 30 cm long, the surface areas would be in the ratio of about 16:9. Thus, if both were cooled by melting ice, the heat transfer from the larger fish would be almost twice as much as from the smaller. However, the mass of the two fish is likely to be in the ratio of about 64:27. The temperature change depends on specific heat, and on mass, so although the larger fish has a higher heat transfer rate, it has a lower rate of temperature change. For small thicknesses of material, the ratio of mass to surface area is an indicator of the rate of temperature change. When considering fish in ice, the other factors are fixed, so that mass to surface area is the only variable (since mass is essentially proportional to volume, this ratio can be taken as volume to surface area). Small fish can be chilled more quickly than large fish; flat fish or fillets more quickly than round fish of the same thickness (but generally more slowly than round fish of the same weight or length).
When very large thicknesses of material are to be cooled, heat transfer through the material itself becomes significant. No matter how good the surface heat transfer is the actual cooling rate is approximately proportional to the square of the material thickness.
Heat transfers through substances at different rates. The property which indicates these rates is thermal conductivity. It is the rate of heat transfer through a section of material of 1 m² in area and of thickness one metre when there is a temperature difference of 1°C. The units are kcal/m/m²h C or simplified as kcal/mh C.
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