Refrigeration Temperature and Refrigeration heat | Explained
The general temperature scale currently in use is the Celsius scale, which is based nominally on ice melting point at 0 ° C and water boiling point at 100 ° C at at atmospheric pressure. (Strictly defined, the triple point of ice at a pressure of 6.1 mbar is 0.01 ° C.) On a Celsius scale, the absolute zero is – 273.15 ° C.
The Kelvin or absolute temperature scale is also used in the study of refrigeration. It starts at absolute zero and has the same degree periods as the Celsius scale, and ice melts at + 273.16 K and water boils at + 373.15 K at at the atmospheric pressure.
Refrigeration is the heat removal process, and the practical application is to produce or maintain temperatures below the ambient. Heat is one of the many types of energy which originates mainly from chemical sources. A body’s heat is its thermal or internal energy, and a change in that energy can be seen as a temperature shift or a transition between the solid, liquid, and gaseous states.
Matter can also have other sources of energy, whether potentially or kinetically, based on heat, position and movement. Enthalpy is the sum of the internal energy and function of the flow, and is given by:
H = u+Pv
The element Pv does not change appreciably in the phase where there is steady flow and the difference in enthalpy is the amount of heat obtained or lost. Enthalpy can be expressed in terms of total above absolute zero, or some other suitable basis. Tabulated enthalpies used in reference works are sometimes seen about a base temperature of – 40 ° C, since that is – 40 ° C on the old Fahrenheit scale. This fundamental assumption will also be verified in every equation to eliminate the mistakes that will occur if two different bases are used.
When an enthalpy change can be detected as temperature change it is called sensitive heat. This is expressed in kJ/(kg K) as specific heat capacity, i.e. the increase in enthalpy per degree of temperature change. If there is no temperature difference just a change in state (solid to liquid, liquid to gas or vice versa) it is called latent heat. This is expressed as kJ / kg but varies with the boiling temperature, and so is usually qualified by this condition.