Last month’s article covered a 400-series refrigerant blend’s total temperature glide, effective temperature glide, fractionation, superheat, subcooling, and average evaporator and condensing temperatures using a pressure/temperature chart. This month’s article will deal with system pressure drops and net temperature glide in a refrigeration and/or air conditioning system. To get started, let’s review a few terms.

Total temperature glide — The total temperature glide of a refrigerant blend is the temperature difference between the saturated vapor temperature and the saturated liquid temperature at a constant pressure.

Saturated liquid temperature (evaporator) — This is the temperature at which the liquid refrigerant first begins to boil. It’s also referred to as the bubble point temperature.

Saturated vapor temperature (evaporator) — This is the temperature at which the last drop of refrigerant has boiled off. It’s also referred to as the dew point temperature.

Saturated vapor temperature (condenser) — This is the temperature at which the refrigerant vapor first begins to condense. It’s also referred to as the dew point temperature.

Saturated liquid temperature (condenser) — This is the temperature at which all the refrigerant has been condensed to a liquid. It’s also referred to as the bubble point temperature.

Effective temperature glide — The effective temperature glide of a refrigerant is the total temperature glide minus the amount of glide present during the flash gas process in the evaporator at any constant pressure. Effective temperature glide and total temperature are one and the same in the condenser.

Zeotropic refrigerant 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 include R-401A, R-407C, R-409A, and R-438A, to name a few.

Azeotropic refrigerant blends — Refrigerant blends that do not exhibit temperature glide are referred to as azeotropic blends. They are represented by the 500-series blends. Some examples include R-500, R-502, and R-507.

Near-azeotropic refrigerant blends — Often, refrigerant blends exhibit a small amount of temperature glide when they change phase. The HVACR industry refers to these as near-azeotropic refrigerant blends. Examples of these blends are R-410A, R-404A, and many other blends.

Total pressure drop — In the real world, heat-exchange coils, such as evaporators and condensers, will experience pressure drop as the refrigerant travels through their lengths of straight tubing and U-bends. It’s not uncommon in commercial refrigeration and air conditioning for a larger evaporator to experience a 5-psi (or higher) pressure drop and a condenser to experience a 15-psi (or higher) pressure drop from beginning to end. Total pressure drop can be divided into friction and static pressure drops.

Friction pressure losses — Friction pressure drop or losses are caused by the heat exchanger’s resistance to the refrigerant’s flow. The side walls of the heat exchanger’s tubes cause resistance to flow from sidewall friction. U-bends in both the evaporator and condenser add pressure drop as they change the refrigerant’s direction of flow. These same U-bends also cause sidewall friction. In these cases, some of the refrigerant’s fluid energy is used to overcome these resistances. This lost energy in the fluid causes a loss in pressure. This type of pressure loss is referred to as friction pressure losses.

Static pressure losses — Another kind of pressure loss or pressure drop is caused from a liquid flowing uphill or through a vertical lift. A vertical column of liquid naturally has more pressure at its bottom than its top due to the weight of the liquid. This type of pressure drop or pressure loss is referred to as static pressure loss. Static pressure drops or losses occur when liquid refrigerant is flowing uphill, or even when the liquid refrigerant is sitting still or being “static.” The sum of friction pressure losses and static pressure losses is referred to as total pressure losses and can be converted to total pressure drop.

EVAPORATOR NET TEMPERATURE GLIDE

Net temperature glide is the effective temperature glide minus the temperature correlation corresponding to the active pressure drop in the heat exchanger’s coil. For example, if the evaporator has a 6-psi pressure drop through its coil, one can determine, through the use of a pressure/temperature chart, what the corresponding temperature drop will be for the 6-psi pressure drop. The corresponding temperature drop is then subtracted from the effective glide.

Let’s call this corresponding temperature drop delta T for illustrative purposes.

Equation No. 1

Net Glide = (Effective Glide - delta T)

In last month’s article, using the example of R-407C for an evaporating pressure of 70 psig, the effective refrigerant temperature glide in the evaporator was 9°F (75 percent of 12°). If the evaporator had a pressure drop of 6 psi, the corresponding temperature drop or (delta T) would be 4° (46° - 42°). Using the saturated vapor temperature or dew point temperature in Figure No. 1, the 46° corresponded to 70 psig and the 42° corresponded to 64 psig. This would make the net glide 5°.

Net Glide = (Effective Glide - delta T)

Net Glide = (9° - 4°)

Net Glide = 5°

Normally, with a refrigerant that has a temperature glide (zeotropic refrigerant), the saturation temperature increases gradually during the boiling process as refrigerant travels through the evaporator toward its end. However, the pressure drop will act to reduce the evaporation temperature by decreasing the saturation pressure and temperature. In the evaporator, the glide of the refrigerant and the temperature decrease caused by the pressure drop counteract one another. This is the reason for subtracting the delta-T from the effective glide in Equation No. 1.

CONDENSER NET TEMPERATURE GLIDE

In last month’s article, for a condensing pressure of 180 psig, the effective refrigerant temperature glide was 10° (96° - 86°), as shown in Figure No. 1. If the condenser has a pressure drop of 15 psig, the corresponding temperature drop, or delta T, would be 5° (86° - 81°). Using the saturated liquid temperature, or bubble point temperature, in Figure No. 1, the 86° corresponds to 180 psig and the 81° corresponds to 165 psig. This would make the net glide 15°.

Equation No. 2

Net Glide = (Effective Glide + delta T)

Net Glide = (Effective Glide + delta T)

Net Glide = (10° + 5°)

Net Glide = 15°

Normally, with a refrigerant that has a temperature glide (zeotropic refrigerant), the saturation temperature gradually decreases during the condensing process as the refrigerant travels through the condenser to its end. However, the pressure drop will act to reduce the condensing temperature by decreasing the saturation pressure and temperature. In the condenser, however, the temperature glide of the refrigerant and the temperature decrease caused by the pressure drop add to one another. This is the reason for adding the delta T to the effective glide in Equation No. 2. 

Publication date: 11/2/2015

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