Question

Methods for measuring mass flow rates of gases and liquids flowing in pipes and ducts include: rotameters, turbine flowmeters, orifice-type flowmeters, thermal flowmeters, and Coriolis-type flowmeters. Determine the principles of operation of each of these flow-measuring devices. Consider the suitability of each for measuring liquid or gas flows. Can any be used for two-phase liquid- vapor mixtures? Which measure volumetric flow rate and require separate measurements of pressure and temperature to determine the state of the substance? Summarize your findings in a brief report.

Short answer

Rotameters, orifice, and turbine flowmeters measure volumetric flow; Thermal and Coriolis flowmeters measure mass flow. Two-phase suitability: Coriolis only in some cases.

Step-by-step solution

- Rotameters

Rotameters operate based on the principle of variable area flow measurement. A float rises in a tapered tube as flow increases, causing the annular area between the float and the tube to increase until the drag force equals the weight of the float. Suitable for both gases and liquids but not two-phase mixtures. Measures volumetric flow rate.

- Turbine Flowmeters

Turbine flowmeters measure flow by counting the rotating blades of a turbine placed in the flow stream. The rotational speed is proportional to the flow velocity. Suitable for both gases and liquids but not for two-phase mixtures. Measures volumetric flow rate, independent of pressure and temperature.

- Orifice-Type Flowmeters

Orifice-type flowmeters measure the differential pressure drop across an orifice plate inserted in the flow stream. The drop in pressure is proportional to the flow rate. Suitable for gases and liquids, not for two-phase mixtures. Measures volumetric flow rate; additional data for pressure and temperature may be needed.

- Thermal Flowmeters

Thermal flowmeters operate based on heat transfer to the fluid flowing through the meter. They detect flow rate by measuring the cooling effect of the fluid. Suitable primarily for gases and not for two-phase mixtures. Typically measures mass flow rate directly.

- Coriolis-Type Flowmeters

Coriolis flowmeters use the Coriolis effect to measure the mass flow rate directly. Oscillating tubes through which the fluid passes experience a phase shift proportional to the mass flow rate. Suitable for both gases and liquids and can be used for two-phase mixtures in some cases. Measures mass flow rate directly.

- Summary and Suitability

Rotameters and turbine flowmeters are best for liquid and gas, not suitable for two-phase. Orifice meters need pressure and temperature data for volumetric flow, not for two-phase. Thermal flowmeters measure mass, ideal for gases. Coriolis meters handle liquids, gases, some two-phase, measure mass flow.

Key concepts

Rotameters

Rotameters operate on the principle of variable area flow measurement. They consist of a tapered tube and a float inside it. When the fluid flows through the tube, it forces the float to rise. As the float rises, the area between it and the tube widens, allowing more fluid to pass. The float moves up until the force from the flowing fluid equals the weight of the float.

This device is very simple and easy to use. Rotameters are suitable for both gases and liquids but are not appropriate for measuring two-phase liquid-vapor mixtures. They measure the volumetric flow rate, meaning the volume of fluid per unit of time. However, they do not directly measure mass flow rate, so if mass flow is needed, separate measurements of the fluid's density, pressure, and temperature are required.

Turbine Flowmeters

Turbine flowmeters measure flow by placing a turbine with blades in the fluid stream. As the fluid moves, it spins the turbine blades. The speed of the turbine's rotation is directly proportional to the flow velocity. Sensors count the number of rotations to determine the flow rate.

These flowmeters are prized for their accuracy and reliability. They are suitable for measuring the flow of both gases and liquids but can't be used for two-phase liquid-vapor mixtures. Turbine flowmeters measure volumetric flow rate and are typically independent of the fluid’s pressure and temperature. This independence makes them useful for a wide range of applications.

Orifice-Type Flowmeters

Orifice-type flowmeters work by inserting an orifice plate into the flow stream. This plate creates a pressure drop that is proportional to the flow rate. By measuring the pressure difference across the plate, the volumetric flow rate can be determined.

These flowmeters are suitable for both gases and liquids but not for two-phase mixtures. To get accurate results, additional measurements of the fluid’s pressure and temperature might be necessary. They are often used in process industries where they are favored for their simplicity and robustness. However, the presence of the orifice plate can cause a significant pressure drop, which may be a disadvantage in some applications.

Thermal Flowmeters

Thermal flowmeters operate based on the heat transfer to the fluid. These meters usually contain heated elements that lose heat as the fluid flows over them. By measuring the rate of heat loss, the fluid's flow rate can be determined.

Typically, these flowmeters are used for gases rather than liquids. They provide a direct measurement of mass flow rate, without requiring additional pressure and temperature data. However, thermal flowmeters are not suitable for two-phase liquid-vapor mixtures because the different phases can have significantly different heat transfer properties, leading to inaccurate measurements.

Coriolis-Type Flowmeters

Coriolis flowmeters measure mass flow rate by utilizing the Coriolis effect. These meters have tubes that oscillate and as the fluid flows through these tubes, the movement coincides with the mass flow, causing a phase shift.

Coriolis meters are highly versatile and can measure both gases and liquids accurately. They can even handle certain two-phase mixtures, although results may vary with the complexity of the mixture. One key advantage of Coriolis flowmeters is that they measure mass flow rate directly. This eliminates the need for separate measurements of pressure and temperature, simplifying the process and reducing potential errors.