![]() Because the difference is quite small between the two values some manufacturers' PT charts will only list one column for these blends. For these blends, the vapor and liquid pressures are only separated by 1 or 2 psi. Some of the zeotropic blends have very low glide (from 1*F to 2.5*F). PT charts for the zeotropic blends list two columns next to each temperature: one for the saturated liquid (bubble point) and the other for the saturated vapor (dew point). Imagine a room full of vapor and dew drops forming on the furniture. The saturated vapor condition is referred to as the dew point. ![]() Imagine a pot of liquid sitting on a stove as it begins to boil it forms bubbles in the liquid. The saturated liquid condition is often referred to as the bubble point. The most important values for checking superheat and subcool are the end points of the glide or the pressure-temperature relationship for saturated liquid and saturated vapor. The temperature glide will cause different values for temperature at a given pressure, depending on how much refrigerant is liquid and how much is vapor. Zeotropic blends cannot be defined by a single pressure-temperature relationship. The overall shift of temperature from one side of the heat exchanger to the other is called the temperature glide. The changing composition of the liquid causes the boiling point temperature to shift as well. As the blend changes phase, more of one component will transfer to the other phase faster than the rest. Zeotropic blends shift in composition during the boiling or condensing process (see Fig. The properties of the new zeotropic blends are somewhat different than the traditional refrigerants. Therefore, only one column is needed to show the pressure-temperature relationship for any phase-change process in a system (see Fig. Single-component refrigerants and azeotropes boil and condense at one temperature for a given pressure. Traditional PT charts list the saturated refrigerant pressure, in psig, with a column for temperature down the left side. PT charts are most often used for three purposes: to set a coil pressure so that the refrigerant produces the desired temperature to check the amount of superheat above the saturated vapor condition at the outlet of the evaporator and to check the amount of subcooling below the saturated liquid condition at the end of the condenser. The pressure-temperature (PT) chart is a valuable tool that service technicians use to check proper system operation.
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