15/05/1994, Pirelli came out with an ad that made history. The slogan, both simple and effective, was “Power is nothing without control”. With a testimonial by an exceptional athlete: Carl Lewis. The irony in portraying him in the starting blocks of a 100-metre race on the track where he usually trained, in Houston, yet with one unique detail, i.e. high heels, sent a very clear message: you might have the most powerful car in the world, but with the wrong tires it will always be slow and unsafe. It was an immediate success.
We could also apply the same slogan to electronic expansion valves: power (the mechanical part) is nothing without control (the electronic part).
So do you really know how an electronic expansion valve works? Why is this solution increasingly preferred over the classic thermostatic expansion valve? These and other questions will be answered in this article, examining the operating principle, foundations of the control system and reasons why EEVs (electronic expansion valves) are increasingly used in HVAC/R, replacing the less efficient and less precise thermostatic valves (TEVs).
EEV operating principle
A refrigeration system comprises several components, which are traditionally the evaporator, the compressor, the condenser, and the expansion valve, as shown in the following diagram.
The functions of these components are described below:
- Evaporator: transfers heat from the environment to the refrigerant, making it evaporate and change state from liquid to gas. Low-pressure, low-temperature refrigerant thus flows into the compressor.
- Compressor: creates the pressure difference that is the basis for the thermodynamic cycle; it takes in the refrigerant leaving the evaporator, compresses it and delivers it to the condenser.
- Condenser: the fluid at the inlet is at high pressure and temperature. In the condenser, the refrigerant releases heat and condenses at high pressure. The fluid thus changes state, from a gas at the inlet to a liquid at the outlet, still at high pressure yet at a lower temperature than at the inlet.
- Expansion valve: after the condenser, the fluid flows through the expansion valve, where it returns to being a mixture of fluid in the liquid and gaseous states, at a lower pressure and temperature, depending on the request from the evaporator. Subsequently the fluid enters the evaporator again and the cycle restarts.
The refrigerant flow-rate inside a thermodynamic circuit plays a fundamental role in the correct functioning of the system, considerably influencing its efficiency. The analogy of water flow in a river can be used to describe the two risks of operating at the extremes. If the fluid flow-rate is too low, the system will be below capacity and the performance required by the system will not be guaranteed, such as in a drought. On the other hand, when a river floods, the water flow-rate is too high and overflows its banks: in a thermodynamic circuit, a fluid flow-rate that is too high means both wasted work by the compressor and potential damage due to liquid intake.
Unlike the water flow-rate in a river, where the upper and lower limits most often depend on the conditions imposed by Mother Nature, refrigerant flow can be controlled using the expansion valve, like a sort of sluice gate. Control is however implemented not based on the flow-rate, but rather on the temperature.
So let’s dust off our school books. Downstream of the evaporator there are usually a pressure sensor and a temperature sensor: the pressure is measured and converted into temperature by the controller, based on the data relating to the specific refrigerant, so as to calculate the saturation temperature (point 1’). This “optimal” temperature is compared against the “nominal” value, measured by the temperature sensor (point 1): the difference between these two temperatures is known as superheat. The controller adjusts the opening of the valve, and consequently the refrigerant flow-rate, so as to maintain the desired superheat value.
Final steps
Electronic expansion valves represent both a technological evolution compared to traditional systems, and a way to reduce energy consumption and maintenance costs. The most important benefits, in both technical and economic terms, are described below:
Technical Aspects:
- Very fast regulation and control, in an extended range;
- Precise and stable superheat control;
- Compatibility with multiple refrigerants;
- Hermetic seal.
Economical Aspects:
- Considerable electricity savings (15% -35%);
- Reduces maintenance costs (no periodic calibration);
- Reduces maintenance costs (reduces the risk of liquid to the compressor);
- Less variety of components for the builder.
In conclusion, the operation of electronic expansion valves has a direct effect on the other devices, controlling and managing their behavior as efficiently as possible, based on the signals sent by the sensors. I hope this article has clarified the role that electronic expansion valves play as the heart of the thermodynamic circuit, controlling and influencing the functioning of each individual component.
Climate Systems of Sioux Falls is an expert in commercial HVAC design, installation and management. Call 605.334.2164, email info@climatesystemsinc.com or visit climatesystemsinc.com.
Source: “Power is nothing without control: the role of the expansion valve,” Carel
