The Blend Equation

December 6, 2010
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With wave after wave of new refrigerants coming on the market, there are sure to be more and more blends. This is a back-to-the-basics article on refrigerant blends.

Blends are made up of two or more single component refrigerants. Each single component refrigerant has its own pressure-temperature relationship and unique physical properties.

In order to match the properties of a blend with a single component refrigerant, the blend components must be mixed in the right proportions. This mixing provides an opportunity for the blend to behave as a zeotrope or azeotrope.

Azeotropic blends behave like single-component refrigerants at or near their defined “azeotropic point,” while zeotropic blends do not behave like single component refrigerants and can present some unique behavior during system operation or when leaked.

FRACTIONATION OF BLENDS

When vapor is removed from a cylinder or system containing a zeotropic blend, two things are going to happen:

• The vapor being removed is at the wrong composition - it will have more of the higher- pressure/high-capacity refrigerant component.

• The liquid that is left behind boils more of the higher-pressure component out of the liquid to replace the vapor. Therefore, more of the low-pressure components remain behind.

As a result, refrigerant blends should be removed from the cylinder as liquid to avoid causing fractionation.

FRACTIONATION EFFECTS ON SYSTEM CHARGE

A system at rest will allow the refrigerant to pool and the vapor to come to an equilibrium concentration above the liquid. If the system has been sitting off for a while, then leaks occurring with high-temperature glide blends need to be recovered and recharged with new refrigerant.

In a running system, it has been found that the circulating composition is the bulk blend composition. Therefore, leaks occurring with blends can be topped off.

FRACTIONATION EFFECTS ON SOME SYSTEM COMPONENTS

Flooded evaporators: They are designed to keep a pool of boiling refrigerant surrounding a bundle of tubes. In the case of zeotropic blends, the vapor that boils off this pool of refrigerant will be at the fractionated composition. This will cause the properties to differ from what the compressor expects, causing high head pressures, high amperage draw at the compressor, and reduced cooling capacity in the evaporator. Normally it is not recommended to use refrigerant blends in this type of system.

Suction accumulators: They are placed in the suction line before the compressor to keep liquid from flowing into the compressor. Zeotropic blends will fractionate in the accumulator; however, the temporary shift in composition will only show a short-lived spike of higher pressure at the compressor.

TEMPERATURE GLIDE IN THE EVAPORATOR

As a blend begins to boil in an evaporator, the vapor that forms will contain more higher-pressure (lower-boiling point) components. The remaining liquid composition will shift to more of the high-boiling point components, so the overall boiling point of the blend will go a little higher.

As the process continues throughout the length of the evaporator tube, more composition shift will cause higher and higher boiling temperatures to occur.

The difference between the beginning boiling temperature and the ending temperature is the temperature glide. This glide may show colder or warmer spots on the evaporator that will affect frost formation, temperature sensors, or control settings.

PRESSURE-TEMPERATURE CHARTS

Single-component refrigerants will stay at one temperature as they boil (the boiling point), and will need only one column on a pressure-temperature (PT) chart to show this relationship. Higher-glide blends will boil across a range of temperatures in an evaporator at constant pressure (the temperature glide), and therefore you can’t have just one column to explain the PT relationship. Blends at a given pressure will begin boiling (or finish condensing) at the saturated liquid temperature (bubble point) and finish boiling (or start condensing) at the saturated vapor temperature (dew point). PT charts for higher-glide blends have two columns to show these end points.

SETTING SUPERHEAT AND SUBCOOLING

When a single refrigerant boils, any heat picked up after it reaches the vapor state will cause the temperature to rise (superheat). Similarly, when a single refrigerant condenses, any heat removed after it reaches saturated liquid will cause the temperature to go down (subcooling).

The process is the same for higher-glide blends - the refrigerant will boil until it reaches saturated vapor, then any additional heat will cause it to superheat. The difference is that the blend changes temperature while boiling, so superheat should not be confused with temperature glide.

It is especially important to check the superheat settings for thermostatic expansion valves after a retrofit since the temperature glide of the blend can reduce the original superheat value. The superheat setting should be checked on the PT chart against the saturated vapor column. Subcooling should be checked against the liquid column.

Some PT charts might only show one column, but the data at lower temperatures will be for saturated vapor (for setting superheat after the evaporator) and the data at high temperatures will be saturated liquid (for setting subcooling out of the condenser).

[Editor’s note: A more detailed discussion of blend behavior is available in National Refrigerants Inc.’s 2010 Refrigerant Reference Guide. Go to www.refrigerants.com.]

Publication date: 12/06/2010

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