Regulation components and electronic control – expansion valves, pressure switches and PLC
From mechanical thermostatic valve to programmable control: how a modern cooling system is regulated
A cooling system running without regulation would quickly either freeze up or run away. The control components are the system's senses and reflexes: they measure pressure and temperature and adjust the flow of refrigerant so the system operates within safe and efficient limits. The education's competence objectives emphasise that you can both select mechanical control components for smaller systems and later programme electronic regulators and PLC controllers for large commercial systems.
§Thermostatic expansion valve (TXV)
The thermostatic expansion valve is a mechanical component that controls how much liquid refrigerant is allowed into the evaporator. It uses a sensor bulb filled with a substance that reacts to temperature to measure superheat at the evaporator outlet. As superheat increases the valve opens more; as it decreases the valve closes slightly. The valve thus regulates mechanically according to superheat without the use of power or electronics which makes it robust and widespread particularly on smaller and medium-sized systems.
§Electronic expansion valve (EEV).
The electronic expansion valve is instead controlled by a microprocessor based on signals from pressure and temperature sensors and adjusts the flow with a small stepper motor. It provides more precise and faster regulation than the thermostatic valve, especially with changing load, and makes it possible to regulate superheating closer to an optimum — which can improve both capacity and energy consumption. On the other hand, it is more expensive and complex, and troubleshooting requires that you understand both the mechanics and the associated electronics and software.
§Pressure switches and thermostats — the system's guard posts
Pressure switches monitor pressure and switch in or out if pressure becomes too high or too low — a high-pressure pressure switch, for example, stops the compressor if the condensing pressure becomes dangerously high, while a low-pressure pressure switch protects against too low suction pressure. Thermostats control whether the system should run or stop based on the desired temperature in the cooling room or on the product. Together they form a layer of safety and operation control that works independently of the more advanced electronic control — an important detail if the electronic part were to fail.
§PLC SCADA and DCS: how the digital systems that let you see and control the entire system from one screen fit together
On commercial and industrial systems, regulation is often collected in a PLC (programmable logic controller) that can control many compressors, valves, and circuits at once, log operational data, and communicate with a monitoring system. The competency goals for the last part of the education specifically mention that you must be able to use PLC controls and use a PC to program both PLCs and electronic controllers. This is where the profession really meets IT: you must be able to read a program, understand the logic behind it, and troubleshoot when a signal does not behave as it should.
- 01Thermostatic expansion valve: mechanical, robust, controls by superheat.
- 02Electronic expansion valve: precise, fast, requires electronic troubleshooting.
- 03Pressure switches: protect against dangerously high or low pressures.
- 04Thermostats: control temperature and operation as needed.
- 05The PLC (the programmable controller) is what determines what the system does. It reads signals in runs a program and sends signals out. Understand the PLC logic and you understand the system behaviour.
§Why it must all work together
A system with the best compressor in the world performs no better than the regulation allows. Incorrectly set superheat a pressure switch with the wrong limit or an incorrectly programmed PLC can make an otherwise well-functioning system both inefficient and unsafe. That is why the regulation components are not a side occupation next to the 'real' refrigeration work — they are just as central a part of the trade as the pipework and refrigerant.
“A combination where something runs privately and other in the public cloud”