The Professor: The Correlation Between Refrigerant Blends and Temperature Glide
Composition of the liquid and vapor phases differ due to phase changes
The total temperature glide of a refrigerant blend is defined as the temperature difference between the saturated vapor temperature and the saturated liquid temperature at a constant pressure. Another definition is the temperature difference between the starting and ending temperature of a refrigerant phase change within a system at a constant pressure.
Many refrigerant blends have temperature glide when they change phase in both the condenser and evaporator. In the evaporator, the refrigerant changes phase (evaporates) from a liquid to a vapor. In the condenser, the refrigerant changes phase (condenses) from a vapor to a liquid. Refrigerant blends exhibit temperature glide because there’s more than one molecule present in their compositional makeup. As these refrigerant blends change phase (evaporate and condense), there’s a change in their composition by preferential evaporation or condensation of the more or less volatile component in the blend. This process is referred to as blend fractionation.
When the liquid refrigerant boils in the evaporator, the composition of the liquid and vapor phases are different. The liquid phase becomes richer in the higher-boiling-point component as the low-boiling-point components boil off into the vapor phase. In the condenser, as the refrigerant vapor continues to condense, the vapor phase becomes richer in the low boiling-point components as the higher boiling-point components of the refrigerant blend condense to a liquid. So, the composition of the liquid and vapor phases are different as condensation takes place.
Refrigerant blends can have two, three, four, five, and even more refrigerants making up the composition of the blend.
• R-410A consists of two refrigerants: R-32 and R-125.
• R-404A consists of three refrigerants: R-125, R-143a, and R-134a.
• R-427A consists of four refrigerants: R-32, R-125, R-143a, and R-134a.
• R-438A consists of five refrigerants: R-32, R-125, R-134a, R-600, and R-601a.
ZEOTROPIC, AZEOTROPIC, AND NEAR-AZEOTROPIC BLENDS
Refrigerant blends that exhibit temperature glide are often referred to as zeotropic refrigerant blends or zeotropes. They are represented by the 400 series blends. Examples of these are R-401A, R-407C, R-409A, and R-438A, to name a few.
Refrigerant blends that do not exhibit temperature glide are referred to as azeotropic blends and are represented by the 500 series blends. Some examples are R-500, R-502, and R-507.
Often, refrigerant blends exhibit a small amount of temperature glide when they phase change, and the HVACR industry refers to them as near-azeotropic refrigerant blends. Examples of these blends are R-410A, R-404A, and many other refrigerant blends.
As zeotropic and near-azeotropic refrigerant blends’ phase changes through the length of the evaporator’s and/or condenser’s heat exchanger, there will be a change in their composition by preferential evaporation or condensation of the more- or less-volatile component in the blend. The amount of temperature glide depends on the refrigerant blend’s makeup and the system’s design. By all means, system performance, design, and service conditions must be evaluated when retrofitting with a zeotropic or near-azeotropic refrigerant blend. Some examples of “total” temperature glide magnitudes are:
On the other hand, single-component refrigerants, such as R-22, R-32, R-123, R-125, R-134a, and R-143a, to name a few, have only one molecule in their structures, so they evaporate and condense at one constant temperature for a given pressure. This means their saturated liquid temperatures and saturated vapor temperatures are the same for one given pressure. In both the evaporator and the condenser, both liquid and vapor exist in equilibrium. As a result, the temperature glide of a single-component refrigerant is zero.
R-407C is a zeotropic refrigerant blend consisting of three hydrofluorocarbon (HFC) refrigerants. It is composed of 23 percent R-32, 25 percent R-125, and 52 percent R-134a. R-407C is commercially available for retrofit of existing equipment and as a long-term replacement option for R-22 in new equipment.
Using retrofit guidelines, many R-22 systems can be retrofitted for use with R-407C in air conditioning, heat pump, and refrigeration applications to allow existing equipment to continue to operate safely and efficiently, even after R-22 is no longer available. All retrofit refrigerants for R-22 have retrofit guidelines, which must be followed.
R-407C was arbitrarily chosen for this article to illustrate an example of refrigerant total temperature glide, effective temperature glide, fractionation, superheat, subcooling, and average evaporator and condenser temperatures using a refrigerant blend’s pressure/temperature chart.
Here are the given conditions for our example:
Summer air conditioning application;
• R-407C refrigerant;
• Suction (evaporating) pressure = 70 psig;
• Head (condensing) pressure = 180 psig;
• Evaporator outlet temperature = 56°; and
• Condenser outlet temperature = 74°
EVAPORATOR TEMPERATURE GLIDE AND SUPERHEAT
As shown in the pressure-temperature chart in Figure 1, for a pressure of 70 psig, there is a saturated liquid temperature and a saturated vapor temperature, and these temperatures are different from one another. For a suction (evaporating) pressure of 70 psig, the saturated liquid temperature is 34° and the saturated vapor temperature is 46°. This would give the refrigerant a total temperature glide of 12° (46 - 34).
The saturated vapor temperature of 46° is referred to as the dew point temperature. The saturated liquid temperature of 34° is referred to as the bubble point temperature. Figure 2 illustrates both the bubble point and the dew point temperatures on a pressure/enthalpy diagram with a constant evaporator pressure of 70 psig. However, as liquid refrigerant starts to vaporize or is expanded through the metering device, some of the liquid vaporizes inside the metering device and not in the evaporator. The refrigerant actually enters the evaporator as a mixture of liquid and vapor, not as a saturated liquid. This means the effective temperature glide would be less than the total temperature glide.
For a typical zeotropic refrigerant blend entering the evaporator with a quality or percent vapor of 25-35 percent, a typical effective temperature glide would be about 70-75 percent of the total temperature glide. In our example, this would mean the effective temperature glide would be about 9° (75 percent of 12°).
Since superheat is defined as any sensible heat gained after the saturated vapor point in the evaporator, the evaporator superheat would be 10°F, (56 - 46), where 56° is the evaporator outlet temperature and 46° is the saturated vapor (dew point) temperature. When calculating superheat values, HVAC service technicians must always use the dew point values from the chart. Figure 3 is a pressure/temperature chart that instructs the technician to use dew point values when determining superheat and to use bubble point values when determining subcooling.
The average evaporator temperature occurs at the midpoint of the effective temperature glide. Since the effective temperature glide is 9°, the average evaporator temperature would be 41.5° (46 - 4.5). The 4.5° comes from dividing 9° in half, and the 46° is the saturated vapor (dew point) temperature in the evaporator. It is this average evaporator temperature that must be used for system design and service considerations.
CONDENSER TEMPERATURE GLIDE AND SUBCOOLING
As shown in Figure 1, for a head (condensing) pressure of 180 psig, there is a saturated liquid temperature of 86°, and a saturated vapor temperature of 96°. This would give the refrigerant a total temperature glide of 10° (96 - 86).
Since all the refrigerant is condensed from a saturated vapor to a saturated liquid inside the condenser, the condenser temperature glide is always the difference between the saturated vapor and saturated liquid temperatures. This means total temperature glide and effective temperature glide in the condenser are equal and one of the same.
Since subcooling refers to a liquid and is defined as any sensible heat lost after the saturated liquid point in the condenser, the condenser subcooling would be 12° (86 - 74), where 74° is the condenser outlet temperature and 86° is the saturated liquid (bubble point) temperature. Again, when calculating subcooling values, HVAC service technicians must use the bubble point values from the chart.
Since the condensing temperature ranged 96° to 86° while condensing at 180 psig pressure, the average condensing temperature would be 91°, ([96 + 86] divided by 2). It’s this average condenser temperature that must be used for system design, performance, and service considerations.
The bottom line is that system design, performance, and service conditions must be evaluated when retrofitting with a zeotropic or near-azeotropic refrigerant blend. Always follow the refrigerant manufacturer’s retrofit guidelines before retrofitting with any refrigerant, or system performance could be affected and/or injury may occur.
Publication date: 10/05/2015