Flow Meter basics

Major types, how they work and what they do.

Flow Meter basicsMore

Flow Meter basics

The key basics of flow meters in HVAC system

There are a number of designs for flow meters, suitable for HVAC applications, each with distinct principles of operation.

One commonly used type is the differential pressure flow meter, which includes orifice plates, venturi meters, and pitot tubes. Orifice plates create a pressure drop across a constriction, and the differential pressure is correlated with flow rate. Venturi meters, with a tapered constriction, also measure pressure differences to determine flow. Pitot tubes utilize the dynamic pressure of moving air to estimate flow rates.

Thermal mass flow meters are another option, relying on the heat transfer between a heated sensor and the fluid. The amount of heat required correlates with the mass flow rate, providing an accurate measurement for gases in HVAC systems.

Electromagnetic flow meters operate on Faraday's law, where a conductive fluid passing through a magnetic field induces a voltage proportional to the flow rate. This type is suitable for water-based HVAC systems due to its non-intrusive nature.

Ultrasonic flow meters use ultrasonic waves to measure the velocity of the fluid, and their transit time or frequency shift helps determine the flow rate. They are non-intrusive and suitable for various fluids, making them versatile in HVAC applications.

Vortex shedding flow meters detect the vortices formed by a bluff body placed in the flow. The frequency of vortex shedding is directly proportional to the flow rate, providing a reliable measurement method for gases and liquids in HVAC systems.

The choice of flow meter depends on factors such as the type of fluid, application requirements, and system specifications. As always, HVAC professionals must choose the most appropriate solution for efficient and precise flow measurement.

In a heating system, flow meters are used to measure the flow rate of hot water or steam that is circulated through the heating system to maintain the desired indoor temperature. In a cooling system, flow meters measure the flow rate of chilled water or refrigerant that is circulated through the cooling system to remove heat from the indoor environment.
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Common configuratons of flow meters include the insert style and inline style.

Turbine flow meters have a small turbine that rotates with the flow in the pipe and provide a flow reading based on the velocity of the fluid.
Insert flow meters often require a straight run pipe both before and after the device to ensure the correct readings are recorded. Most also require calibration, which is typically done annually for installations where flow accuracy is critical, or it may be every two to three years. Flow meters generally require maintenance, and it is recommended that also be completed annually.
Electromagnetic, or simply “mag” flow meters are used on projects where highly accurate flow is required. These clamp-on flow meters use ultrasonic (either transit time or Doppler) technology to measure the velocity of a fluid flowing through a pipe. With no moving parts, maintenance requirements are low.