Solenoid Valves and Their Importance in Refrigeration Systems February 2007
Overview Introduction Solenoid valves play an important role within refrigeration and air conditioning systems, controlling the flow of refrigerants. Though their base function – turning the refrigerant flow on and off – is quite simple, this function is key to ensuring system performance. Understanding how solenoid valves work increases the likelihood that contractors will install, remove and reinstall valves correctly, ensuring optimum system performance and protection.
Basic operation of solenoid valves Definitions A solenoid is a simple form of an electromagnet consisting of a coil of insulated copper wire. A solenoid valve is an electromechanical valve frequently used to control the flow of liquid or gas. Function Solenoid valves are found in many applications and are commonly used in refrigeration and air conditioning systems. Their function is simply to turn refrigerant flow on and off.
Solenoid-valve types Introduction Solenoid valves can generally be divided into two types: • Direct acting • Pilot operated Direct-acting solenoid-valve operation In a direct-acting solenoid valve, the pull of the coil opens the valve port directly, by lifting the pin off the valve seat. Diagram of direct-acting solenoid valve Figure 2 shows the parts of a direct-acting solenoid valve.
Diagram of pilot-operated solenoid valve Figure 3 shows the parts of a pilot-operated solenoid valve. Figure 3 Pilot Operated Top plug Enclosing tube Plunger Piston Pilot Main port Risk of oversized pilot-operated valve With a pilot-operated valve, there is a minimum operating-pressure differential (minOPD) required for the piston to stay open. If the valve is oversized for the application, then the minOPD may not be achieved, resulting in a valve that will not open or will fail to stay open.
Mechanical construction Introduction The basic principles of operation hold true for all refrigerant solenoid valves, although certain mechanical variations in construction can be found. Common variations Common examples of mechanical variations include: • Short-stroke plungers: The plunger is rigidly connected to the valve needle or poppet. • Long-stroke/lost-motion plungers: During opening the plunger imparts a “hammer blow” to the valve stem.
Coils Description Coils used in refrigeration applications are usually encapsulated, providing better protection against environmental extremes and longer life. In this coil type, the magnet wire is wound in layers over a nylon bobbin. This assembly is then completely molded or cast over all exposed portions. The most common encapsulants used for coils are epoxy formulations.
Temperature ratings Introduction Solenoid coils are classified by their insulating materials with regard to coil-temperature rise. The maximum permissible temperature that can be tolerated is limited by the type of insulation and materials used in the construction of the coil assembly. Temperature limitations of insulation classes There are three classes of insulation commonly used for refrigeration coils. These are referred to as Class F, H and N.
Effect of temperature Figure 5 shows the effect of temperature on solenoid-coil temperature. Figure 5 Effect of Temperature 450°F Temperature 200°F 100°F Time Figure 6 shows the effect of frequency on solenoid-coil temperature.
Elastomers Definition An elastomer, by American Society for Testing and Materials (ASTM) standards, is defined as “a substance that can be stretched at room temperature to at least twice its original length and, after having been stretched and the stress removed, returns with force to approximately its original length in a short period of time.
Solenoid-valve applications Introduction Solenoid valves are used for controlling refrigerants in liquid or suction lines or in hot-gas lines.
Solenoid-valve applications (continued) Suction applications In a suction application, the solenoid valve provides complete isolation for temperature control and defrosting, or operates as a suction bypass on installations with two or more evaporators in a series fed by one expansion valve (see Figure 9). Figure 9 Suction-Stop Solenoid Valves Note: When used in suction applications, a valve with a low pressure drop should be used (less than two pounds-per-square-inch differential [psid]).
Hot-gas bypass The compressor capacity can be reduced by bypassing the hot discharge gas through a solenoid directly to the suction line. A desuperheating valve must be used in this type of system, to prevent the compressor from overheating and tripping its internal protector. As shown in Figure 10, an alternate approach is to feed the discharge gas to the inlet of the evaporator.
Maximum operating-pressure differential Introduction Solenoid valves are rated in terms of maximum operating-pressure differential (MOPD) against which the valves will open. Example With the solenoid valve closed against an inlet pressure of 250 poundsper-square-inch gauge (psig) and outlet pressure of 50 psig, the pressure differential is 250 minus 50, or 200 pounds per square inch (psi).
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