ACR Journal

CMYK / .ai CMYK / .ai CMYK / .ai acrjournal.uk 29 REFRIGERATION 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 diusion 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 CO 2 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 diusion 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 CO 2 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 eort 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 eect, compressor work can be reduced.