Log pressure–enthalpy (p-H)
The heat pump process is often illustrated in a log pressure (p)–enthalpy (H) diagram (log p–H diagram) (Fig. 7.13). Enthalpy is a parameter that is a measure of the energy content of refrigerant; units are kJ/kg. The log p–H diagram illustrates clearly the changes of phase in the refrigerant. The pressure of the refrigerant is constant through the evaporator, but its energy content (H) increases because it gradually changes phase from liquid to gas and in doing so takes up energy from its surroundings. When the refrigerant enters the compressor it is in the gas phase. Electric energy is supplied to the compressor, most of which is transferred to the refrigerant, so further increasing its enthalpy. In the compressor the pressure of the gas increases and it is then fed into the condenser. Here energy is released because the gas changes phase to liquid and the enthalpy drops, but the pressure remains stable: all the energy that was stored when the liquid changed phase into gas is now released. The refrigerant exits the condenser as liquid and then enters the expansion valve where its pressure drops, but no energy is removed or added (assum-ing ideal conditions). Therefore the enthalpy is the same, as shown by a vertical line.
The p–H diagram is specific for each medium, and shows the phase of the medium in relation to its pressure and enthalpy content. It is also known as a nose diagram, depending on the presentation. On one side of the ‘nose’ the phase of the working medium is liquid; on the other side it is gas, and in between both gas and liquid. The heat pump processes therefore happen inside the ‘nose’, because it is here that the phase transfers occur.
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