1. -Direct operated
（2w series; HK02; HK11; HKKB; ZCM; HK12; HKFP; HK2231005x series；
Direct operated (direct acting) solenoid valves have the most simple working principle. The medium flows through a small orifice which can be closed off by a plunger with a rubber gasket on the bottom. A small spring holds the plunger down to close the valve. The plunger is made of a ferromagnetic material. An electric coil is positioned around the plunger. As soon as the coil is electrical energized, a magnetic field is created which pulls the plunger up towards the centre of the coil. This opens the orifice so that the medium can flow through. This is called a Normally Closed (NC) valve.
A Normally Open (NO) valve works the opposite way: it has a different construction so that the orifice is open when the solenoid is not powered. When the solenoid is actuated, the orifice will be closed. The maximum operating pressure and the flow rate are directly related to the orifice diameter and the magnetic force of the solenoid valve. This principle is therefore used for relatively small flow rates. Direct operated solenoid valves require no minimum operating pressure or pressure difference, so they can be used from 0 bar up to the maximum allowable pressure.
2. -Direct operated 3/2 way solenoid valves
(HKG11）
A 3/2 way solenoid valve has three ports and two switching states. In each switching state, two of the three ports are connected. By activating the solenoid, the valve switches state and a different connection between the valve ports is established. The drawing below shows a direct operated 3/2 way valve. The port on the left side is the input port (P). In the de-energized state, the medium can flow from the port on the left side to the top port (exhaust port). In the energized state, the medium can flow from the right port to the left port. This is a called a normally closed 3/2-way valve.
2. -Semi-direct operated
Semi-direct operated solenoid valves combine the properties of direct and indirect valves. This allows them to work from zero bar, but still they can handle a high flow rate. They look somewhat like indirect valves and also feature a movable membrane with a small orifice and pressure chambers on both sides. The difference is that the solenoid plunger is directly connected to the membrane. When the plunger is lifted, it directly lifts the membrane to open the valve. At the same time, a second orifice is opened by the plunger that has a slightly larger diameter than the first orifice in the membrane. This causes the pressure in the chamber above the membrane to drop. As a result, the membrane is lifted not only by the plunger, but also by the pressure difference.
This combination results in a valve that operates from zero bar, and can control relatively large flow rates. Often, semi-direct operated valves have more powerful coils than indirect operated valves. Semi-direct operated valves are sometimes called assisted-lift solenoid valves.
