The Sonic Nozzle (Critical Flow Nozzle, Critical Flow Venturi, Sonic Venturi) is a converging-diverging flowmeter that has become the standard for air flow measurement in the aerospace industry. It consists of a smooth rounded inlet section converging to a minimum throat area and diverging along a pressure recovery section or exit cone. The Sonic Nozzle is operated by either pressurizing the inlet (P1) or evacuating the exit (P3), to achieve a pressure ratio of 1.2 to 1 or greater, inlet to outlet. When used with Air, for example, this ratio maintains the Nozzle in a "choked" or "sonic" state. In this state, only the upstream pressure (P1), and temperature are needed to calculate the flowrate through the Nozzle. The flowrate through the Nozzle becomes primarily a linear function of the inlet pressure, doubling the inlet pressure doubles the flowrate. The simplest flow system would use an inlet pressure regulator to control air pressure and a thermocouple to measure temperature. Adjusting the pressure regulator will change and maintain the flow through the Nozzle.
As a gas accelerates through the Nozzle, its velocity increases and its density decreases. The maximum velocity is achieved at the throat, the minimum area, where it just breaks Mach 1 or sonic. Pressure differences within a piping system travel at the speed of sound and generate flow. Downstream differences or disturbances in pressure, traveling at the speed of sound, cannot move upstream past the throat of the Nozzle because the throat velocity is higher and in the opposite direction. These are relative velocities and they add algebraically. Since the pressure disturbances cannot move past the throat, they cannot affect the velocity or the density of the flow through the Nozzle. This is what is referred to as a choked or sonic state of operation. Normally in a sub-sonic flowmeter (Venturi, ASME Flow Nozzle, or Orifice Plate), any change in downstream pressure will affect the differential pressure across the flowmeter, which in turn, affects the flow. This is not the case in sonic flow and is one of the strongest advantages of using a Sonic Nozzle. If you have a system with pulsating or varying gas consumption downstream and you want to feed it a constant or locked flowrate, a Sonic Nozzle is an excellent way to achieve this. You won't need a complicated PID control loop system. Adjusting the inlet pressure with a pressure regulator will change the flow to any point within the gas pressure supply available.
FlowMaxx Sonic Nozzles are designed and manufactured in accordance with ASME MFC-7M (American Society of Mechanical Engineers) standards. With these specifications, the ASME predicts the uncalibrated flow accuracy to be at best, ±1% of flow reading. If higher accuracy or certified accuracy is needed, NIST traceable certified calibrations are available. Standard calibration accuracy levels include: ±0.5%, ±0.25%, and ±0.1% of reading.
Typical Installation Configurations
Pressurized System This is the most common installation arrangement. A pressurized air source, shop air for example, is used with a pressure regulator upstream of the Sonic Nozzle. The pressure upstream of the Nozzle must be 1.2 times or greater than the downstream pressure. The manufacturing process may be at atmospheric pressure or greater as long as the 1.2 pressure ratio across the Nozzle is maintained at all flowrates. A set of interchangeable Sonic Nozzles can be used to extend the flow range beyond a single Nozzle. In this arrangement, flow pulsations or fluctuations from the manufacturing process will not influence the flow through the Sonic Nozzle. The airflow range is only limited by the maximum air supply available.
Vacuum System If the manufacturing process operates at or near atmospheric pressure, a pull-thru or vacuum system can be used to measure the airflow. This arrangement is good for maintaining airflow to a steady process, but is limited in the flow range available.
Multiple Nozzle System To extend the flow range beyond that of a single Nozzle, multiple Nozzles can be installed in parallel. In this arrangement, three Nozzles can be operated individually, in pairs, or all at once. This system allows the use of binary operation. Each Nozzle is sized to flow approximately twice that of the next smaller one. It's not exactly twice because a Sonic Nozzle cannot be reduced to zero flow, there is a minimum flow necessary to keep the Nozzle choked. For example, if these three were sized for 1, 2, and 4 SCFM, the total covered flow range is now 0.2 to 7 SCFM, including all flows in between. 2 SCFM would be obtained by opening Nozzle #2. 5 SCFM would be obtained by opening Nozzles #1 and #3. The flows in between are achieved by adjusting the pressure regulator. Although this is a simplified example, the application is valid and can be extended to any number of Nozzles and flow ranges.
Free Standing Inlet System This system provides a constant air flow rate to the manufacturing process. The flow rate is determined by the size of the Sonic Nozzle and the current ambient conditions. Although atmospheric pressure and temperature do vary from day to day, the measured airflow can be corrected for the current pressure and temperature. The Nozzle is sized with the average atmospheric pressure and temperature and flow requirements.