# VENTURI NOZZLE WITH CRITICAL FLOW

Principle of the Venturi nozzle with critical flow is based on a fact, that under supercritical pressure ratio “downstream” and “upstream” the nozzle, gas flow is accelerated in the throat to critical velocity is equal to local speed of sound provided that there is a single-phase gas flow. Mass flow is the highest possible flow under conditions (pressure, temperature) that the gas has on the nozzle inlet. If we compare the Venturi nozzle with critical flow to subsonic meters based on differential pressure, it is valid that in case of the Venturi nozzle with critical flow, the flow is proportional to steady pressure downstream the nozzle, not square root of measured differential pressure as it is valid in the case of subsonic meters.

ISO 9300 standard defines possibility to use the Venturi nozzle with critical flow within specified limits of Reynolds number in the throat Re_{nt} and diameter ratio of the nozzle throat to inlet diameter β. There are the Venturi nozzles with toroid throat and with cylindrical throat. Precisely machined Venturi nozzles have to be constructed in accordance with toroid solution.

**The Venturi nozzle with critical flow is a meter with very high measurement accuracy and it is used for testing, calibration and flow control for the mass production of compressors and other air conditioning equipment.** There are two possible layouts: There is either straight piping of circular cross section completely filled with flow (downstream the Venturi nozzle), or there is a large space. The Venturi nozzles can be installed in the set next to each other. This configuration offers possibility to achieve large range of flow rates. There is only one requirement for the outlet piping: The flow may not be restricted enough to prevent critical flow in the Venturi nozzle.

To avoid influence of stream turbulence it is recommended to incorporate the flow straightener (e.g. “Étoile” type), impact of incorporation is minimal except for impact of current torsion. Accuracy of flow measurement depends especially on precise determination of the throttle cross section.

Only measurement of gas pressure and temperature is required. In some cases it may be desirable to measure density of gas directly on the nozzle inlet, e.g. if the molecular weight of gas is not known with sufficient accuracy. Measurement of density with hydrometer can be replaced by calculation performed with chromatographic determination of gas composition and equation of state.

The Venturi nozzle with critical flow is embedded in the flanged joint of the piping. Sealing surfaces of the orifice plates can be produced except for CSN standard also in compliance with ISO, DIN, GOST standards, American ASME and ANSI standards or, on specification and request of the customer. The Venturi nozzle with critical flow can be installed into piping through various technical designs of detachable joints.

For the production of the Venturi nozzles with critical flow there are mostly used stainless steels.

The Venturi nozzle with critical flow is manufactured as a working meter.

The Venturi nozzle with critical flow can be incorporated both in vertical and horizontal piping.

Calculation of the Venturi nozzle with critical flow is ensured by our own software.