23 July 2020
Andrea Dalan, CAREL Valves Drivers & Flow Control Platform Manager, explains the advantages of electronic valves.
An electronic expansion valve is best defined as a valve that combines the strength of mechanics with the intelligence of electronics.
I was recently embarrassed to find myself having to discuss this subject with a major international player, convinced that the transition from mechanical to electronic thermostatic valves was by now an obsolete topic. I therefore had to search around for some documents that we had published about 10 years ago.
The role of the expansion valve - the antagonist of the compressor in traditional systems - is to guarantee the correct flow of refrigerant to the evaporator, so as to obtain the best heat exchange performance with the highest possible efficiency. Among other things, the valve also has the function of protecting the system against the return of liquid.
Using an electronic valve instead of a mechanical valve brings numerous advantages:
- Much more stable and precise superheat control (i.e. measurement of the complete vaporisation of the liquid in the evaporator) as a result of the intelligent control system comprising probes, the controller and its algorithms.
- Wide range of control, from 0 to 100% of flow-rate
- Compatibility with a wide range of refrigerants without the need to select a specific electronic valve based on the refrigerant used. The type of refrigerant is selected by setting a parameter on the controller.
- Unlike thermostatic valves, electronic valves do not require periodic calibration.
- Fast response to changes in the surrounding conditions (on, off, ...).
- Hermetic seal ensured by the use of high-performance gaskets and motors. Thermostatic valves, on the other hand, need to be used in combination with a solenoid valve in order to prevent refrigerant from migrating to the compressor.
The result is:
- Energy savings for the end user. The lower condensing pressure means less work for the compressor. Comparative studies between electronic and thermostatic valves carried out together with customers and universities have shown savings in annual energy consumption of 15 to 35% when using electronic valves.
- Lower maintenance costs. Better compressor operating conditions (lower pressures, lower discharge temperatures), no liquid return and therefore less deterioration of the mechanical parts. No maintenance needed for valve calibration.
- Less components for manufacturers to handle as the same solution covers a wide range of refrigerants.
All with a view to safeguarding our planet, ensuring the most efficient solutions to achieve what we need (heating or cooling).
The advent of variable-speed compressors has made the use of electronic valves even more essential, by virtue of their ability to respond quickly to changes in operating conditions and the wide control range they allow.
One interesting example that helps understand the direction we need to take comes from the automotive sector.
The development of electric cars has led to the need to maximise the performance of all auxiliary vehicle systems. As the energy stored in the batteries is precious and needs to be used primarily to move the car, high efficiency of the passenger compartment air conditioning and battery cooling systems make electronic expansion valves a must.
The obstacles that prevented electronic valves from completely replacing thermostatic valves in the past were:
- Upfront cost
- Apparent complexity of the system
- Distrust in something “new”
I can today say the electronic valves have become the standard, due to:
- Upfront costs that are comparable to thermostatic solutions. Simple product packages are available (electronic valve + controller + 2 probes) to replace the thermostatic valve + solenoid.
- Setting the parameters on the controller is now child’s play. All the user needs to do is set three parameters: refrigerant, type of control and superheat set point - the system is ready to go.
- The use of electronic valves for many years now and their proven reliability have overcome the obstacle of distrust among users.
Regulations and standards
Being pressure devices, electronic valves need to be tested in accordance with the PED (CE).
The UL 429 standard describes various tests that certify valve reliability and safety.
Clearly, the valves and the electronics they contain also need to comply with the RoHs and Reach directives and the Conflict Mineral Regulation, which limit the use of materials such as lead, mercury, cadmium...
Last but not least, the WEEE directive, which regulates the handling and recycling of waste electronic equipment.
The diffusion of flammable refrigerants has made it necessary to carry out safety assessments in flammable environments, and therefore it is important to comply with the reference legislation according to the type of unit (these include: EN 378-2, EN 60335-2-40, EN 60335-2-89, Atex: EN 60079)
In reality, these are the same as for thermostatic valves, with the addition of electromagnetic compatibility and electrical safety, for which the relevant tests need to be carried out in accredited laboratories.
As components of the refrigerating unit, they need to meet all of the machinery requirements based on the final application (household, residential, industrial).
The evolution according to the use of low GWP refrigerants and CO2
The continuous search for the refrigerant with the lowest environmental impact has led to the development of a series of new fluids that have created a lot of confusion on the market.
Electronic valves in this sense have an advantage compared to mechanical valves, as their construction principle ensures compatibility with most refrigerants.
For each new fluorinated refrigerant or HFO, only chemical compatibility tests and the related fluid dynamics/thermodynamics analyses are required.
The greater diffusion of flammable refrigerants (above all R290) has resulted in manufacturers being more sensitive to the tightness the product and to preventing the generation of sparks (it can be easily overcome by applying protection to the contacts).
The adoption of CO2 has had a much greater impact, requiring the creation of valves with specific high pressure ranges.
Indeed, to obtain UL certification of an expansion valve, a burst pressure that is five times higher than the PS (maximum operating pressure) needs to be guaranteed. For transcritical systems operating at up to 140 bar, the valve must be able to withstand pressures above 700 bar!
This requires significant design effort to ensure product thicknesses and sizing that are suitable for these operating standards.
Further complexity has been introduced by flash gas valves. The control of high-pressure gas does not necessarily follow the design criteria of expansion systems for two-phase fluids in the same way as in traditional systems. Specific control profiles have therefore been identified to limit turbulence in these applications.
And it doesn’t end here... the constant search for system optimisation and the desire to extend the areas where transcritical systems can operate to even warmer climates, has led to the introduction of ejectors.
Few manufacturers have managed to try their hand with these products, and CAREL is one of them. When developing an ejector, the greatest cost involves the fluid dynamics simulation, which requires the use of very powerful software and computers.
An ejector is a device that uses the energy of a high-pressure fluid flowing through a nozzle to carry and compress another fluid at much lower pressure.
In this way, exploiting the Venturi effect, compressor work can be reduced.
The market is moving towards increasingly complex developments (as a result of the race for energy efficiency) that, while keeping the focus on reliability, bring requirements that at a first glance may seem contradictory.
- Shorter supply lead times. For this reason, CAREL and other companies operating in this sector have had to adopt a local production solution in order to be closer to the end customer.
- Lower product costs. Modularity has made it possible to optimise raw material purchases, thus guaranteeing maximum competitiveness with thermostatic valves.
- Extensive product customisation. Each player in our market needs to differentiate from the others in order to sell value to the end customer. The combination of flexibility of the electronic valve and customisation of the control algorithms has made it possible to satisfy this need.
Above all the demand for compatibility with new refrigerants, as addressed previously. Even though the strategy regarding green refrigerants tends to be based on the use of flammables or CO2, we periodically receive requests for use with new fluid formulations.
In reality, all areas of use are interesting: there is always a way to improve system operation!
CO2 and flammable gases are certainly giving particular impetus to innovative ideas and particular uses of these valves.
A lot will depend on the development of new refrigeration and air conditioning systems. Significant international investments devoted to searching for more efficient systems could put us in the position of having to invent new temperature control systems.
In the medium-term, I would expect three types of evolution.
The first change will involve the actuators. Continuous experiments on smart materials will lead to the replacement of traditional stepper motor systems with new actuators that incorporate elements that change shape according to an electrical signal.
The second change will involve control and measurement. The development of new sensor technologies (above all driven by IoT) and the miniaturisation of electronics will lead to the development of smart objects capable of measuring and acting immediately and independently of the most complex system.
The last evolutionary step involves systems that are self-powered and do not require cable communication, but rather send data via wireless signals. This progress will greatly simplify installation and maintenance.
CAREL provides a vast portfolio of solutions for flow control, from the simplest to the most complete configuration.
When designing valves, CAREL has adopted a modular approach. The common elements are similar, yet by combining them together we can satisfy any customer need.
This has allowed us to strengthen our skills in core components, ensuring maximum reliability for each type of construction and faster time to market for new products.
CAREL electronic valves stand out for the following pillars:
- Reliability - each product is tested for one million opening and closing cycles under various operating conditions.
- High control precision, thanks to the equipercentile flow profile, meaning a variation in flow-rate in proportion to the change in steps. This ensures constant flow control with high precision at low flow-rates and a rapid response at high flow-rates.
- Hermetic seal of refrigerant flow to prevent damage to the compressor.
The complete CAREL solution comprises integration with electronic controllers, probes (temperature, pressure) and a possible backup module to manage power failures.
The strength of our system indeed lies in this extensive integration (every product is perfectly integrated into the system) and the intelligence of the algorithm, which harmonises the operation of the components to achieve the performance that the customer demands.
For those who still need to replace thermostatic valves with more intelligent systems, we provide various options:
A simple parametric controller (EVD mini) connected to the electronic valve and two probes (two temperature or temperature + pressure). Simply power it on, set the three basic parameters and you’re done.
A completely sealed version is also available that allows the controller to be installed on the evaporator (EVD ICE).
More complex and complete solutions are also possible, using CAREL programmable controllers that have built-in valve drivers.