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Fundamentals of EEV’s & Refrigerant Flow Control

When the development of mechanical refrigeration systems was in its infancy, the fundamental system design required the basic components we still use today:

 

1. A compressor that is capable of accepting a low pressure vapor and delivering a high pressure vapor.

 

2. A coil that allows for gathering heat when the refrigerant changes in state from a liquid to a vapor (Evaporator).

 

3. A coil that allows for dispensing heat when the refrigerant changes in state from a vapor to a liquid (Condenser).

 

4. A metering device that controls the flow of refrigerant into the evaporator coil.

 

In regard to the metering device, when we consider it from the perspective of simply creating a controlled restriction in the flow of refrigerant so as to allow expansion to take place (liquid entering into the larger volume are of an evaporator), the fundamental construction of the device is simple. It could be a hand operated valve that is manually adjusted to the point where it almost totally closed. Or, it can be a fitting in the liquid line with a small opening drilled in it, which is commonly referred to as a fixed bore metering device.

 

Going beyond the simplest of all restrictors, a modulating type metering device, such as a TXV (Thermostatic Expansion Valve, sometimes referred to as a TEV), allows for a more efficient operation because it employs a system that senses a change in the heat load, then adjusts accordingly, enlarging or restricting a needle valve assembly opening to allow more or less flow of refrigerant into the evaporator.

 

The TXV is strictly a mechanically operated device, which means that while it is more efficient that a fixed bore device, a given amount of time lapse has to occur in its operation while its sensing bulb responds to temperature change and adjusts the pressure in the valve assembly that ultimately changes the position of the needle valve.

 

Which brings us to the EEV. (See Figure One)

 

Figure One

 

Sometimes referred to as an EXV, it also adjusts in accordance with the varying heat load to regulate refrigerant flow into the evaporator coil. But its feedback loop system employs a controller circuit board that receives inputs from electronic sensors. This can be a thermistor that senses tubing temperature at an evaporator outlet. In some cases, multiple thermistors may be employed to sense tubing temperature at an evaporator inlet and outlet. Depending on the design of the system, other sensors may also be positioned to sense compressor discharge temperature, compressor inlet temperature, or to sense the temperature of compressor motor windings.

 

Going beyond the aspect of temperature, some sensing systems are related to pressure. The controller may also receive information from a device known as a pressure transducer, adding that input to the temperature information it receives, then directing the EEV to allow more or less refrigerant into the evaporator coil.

 

When it comes to the EEV assembly, we can refer to it as “Electric Expansion Valve” because its fundamental design is not “Electronic”. It’s simply an electric motor that is of the same fundamental design of all electric motors that have stator windings that employ the electromagnetic field of a stator winding along with a permanent magnet mounted to a rotor to accomplish a mechanical motion. As Figure Two shows, it’s still about N and S (North and South), a power supply, and the push/pull of magnetism.

 

Figure Two

 

The device becomes more sophisticated when the power supply is a controller that causes what is referred to as a direct current stepper motor to operate on one direction, and then reverse in order to allow a threaded shaft needle assembly to move closer to or away from a seat, which, in the end, allows more or less refrigerant flow from the valve into the evaporator. (See Figure Three)

 

Figure Three

 

The bottom line here is a refined control of superheat. The electronic controller modulates the valve needle and seat assembly open and closed in tiny increments, which results in keeping the superheat within a ± range of 1° F.

 

This is far beyond the flow control that a mechanically operated valve can achieve, and, as technicians, we  welcome this technology that allows a system to perform at maximum efficiency.

 

Learn From Yesterday, Live For Today, Look Forward to tommorow

Jim