Needle valve, also known as throttle. The structure of the needle valve is similar to that of the ball valve. The difference is that the valve core of the needle valve has a slender cone and is streamlined. When the spool leaves the seat at different distances, there is a different annular gap between the spool and the seat, ie the fluid has a different passage area so that the fluid flow can be changed by changing the distance between the spool and the seat or By its throttling effect and change the fluid pressure. Needle valves have many uses in thermodynamic systems, such as control valves for temperature reduction and pressure reducer; some boilers serve as control valves for temperature reduction water; and in secondary doors for pressure transmission in thermal inspection systems.