Airflow Measurements

Operating Principles of FE and PE Series Sensing Elements

The FE-1000 Series units are aerodynamic head devices which generate a differential (velocity) pressure output signal. The FE-1000 Series output is similar to that of an orifice plate, venturi, flow nozzle, or other head-producing primary elements, in that the differential pressure produced by these devices is related to the air quantity passing through the measurement plane.

The basic design of the flow sensing element is a "cylindrical tube within a cylindrical tube" that permits the simultaneous measurement of both impact (total) pressure and static pressure. The total and static sensing port design eliminates the need for air straightening based on a known pressure distribution over a cylindrical surface and the incorporation of multiple impact (total) and static pressure sensing ports strategically located along its length provide comprehensive averaging of the velocity variations within the duct.

The inner tube is an isolated chamber serving as a manifold for the impact (total) pressure sensing ports. The pressure within this manifold is influenced by the internal movement of air between the multiple ports that occurs because of their pressure differences.

Similarly, the outer tube is an isolated chamber serving as a manifold for the static pressure sensing ports. The offset static sensing ports are located at the critical angle of the cylinder's surface where there is no influence of dynamic (velocity) pressure when the element is inserted perpendicular to flow.

The static port design of the FE-1000 Series permits flow-angle variations of up to �20 degrees to occur without affecting the output value due to the linear characteristic of this portion of the pressure distribution curve. As the flow-angle changes from nominal, one static port is exposed to a higher pressure and the other static port is exposed to a lower pressure of equal value.

The pressure within each manifold is sent through signal ports to become the output source of FE-1000 Series units. Total and static pressure outputs are transmitted through interconnecting tubing as primary input signals to a differential pressure measurement device (gauge, transmitter, etc.) for flow indication, alarming, data logging and/or feedback control purposes. The PE-5000 Series ductmounted Primary Pressure Sensing Elements are similar to FE elements but are furnished with static pressure ports only.

The cross-sectional views shown in the adjacent figures identify the distribution of pressure around the cylindrical surface of the FE-1000 Series flow sensors in a moving stream of air or gas. These figures illustrate the innovative "Double-Hollow" FE-1000 Series design, the implementation of existing pressure measurement fundamentals and the ability of the flow sensing elements to measure true air velocity in moderately turbulent flow conditions.

Multiple FE-1000 Series elements are required for most applications. Application and configuration tables are found in the Data Sheets of the individual FE-1000 Series airflow sensing products, and were developed to comply with the guidelines for Pitot-tube traversing of air ducts as published in the ASHRAE Fundamentals Handbook.

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Pressure distribution on the surface of a FE- 1000 Series unit when inserted nominal to the direction of flow The impact (total) pressure sensing port is located in the center of the positive pressure portion of the cylinders surface. The static pressure sensing ports are located in the some plane and separated by about twice the critical angle, and one is centered in each of the two neutral pressure portions of the cylinder's surface.

Pressure distribution on the surface of a FE- 1000 Series unit when inserted at a 20 degrees flow angle. The impact (total) pressure port remains within the positive pressure portion of the cylinders surface. The effect of one static sensing port entering into the positive portion of the cylinders surface is offset by the other static sensing port entering into an equally negative portion of the cylinders surface.

FE-1050 units are provided as complete assemblies for each inlet of a specific fan model, and each assembly has two airflow sensing elements, pivot mounting hardware and signal connection fittings. Standard construction is anodized aluminum that is suitable for most applications. The units are also available in 316 stainless steel for corrosive or high temperature duty.

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FE-1500 Airflow Measurement Stations

Model FE-1500 airflow measuring stations consist of single or multiple FE-1000 air flow sensing element(s) factory-mounted and externallymanifolded in an 8 inch (200mm) deep, 16 gauge galvanized sheet metal casing. FE-1500 units are connected to new or existing ductwork by 90 degrees flanges on the air entering and leaving sides of the casing. They are available in all sizes and shapes to match the duct, and casings are optionally available in aluminum or stainless steel or with corrosion resistant coatings. Units are also available with an aluminum honeycomb airflow straightening section for exceptionally high turbulent flow conditions.

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FE-2000 Pitot Tube Airflow Measuring Stations

Model FE-2000 units are airflow measuring stations consisting of a flanged casing as described for FE-1500 models plus an integral aluminum honeycomb airflow directionalizer/, straightener section and bullet-nose shaped static pressure sensors and total pressure impact sensors. The multiple total and static pressure sensors are internally manifolded for signal self-averaging, and the manifold pressures are carried in tubing to ports on the casing wall for external connection to indicating and/or control devices.

The Model FE- 1000
air flow sensor is
available in (from left
to right) stainless steel,
aluminum, or PVC

FE-1000 Airflow
Sensing Elements

Model FE-1000 units are primary airflow sensing elements designed for quick and easy installation through small cutouts in new or existing ductwork. They are true value-added products ideal for retrofitting (upgrading) existing systems with flow measurement capability. They are cost-effective replacements of failed or unreliable existing airflow sensing devices, and a design consideration for new systems and for applications requiring frequent sensor inspection and cleaning.

Multiple FE-1000 elements are required for proper duct traversing in most applications, and their external total (high) and static (low) output ports are then field-manifolded for a single high and low connection. Each element is furnished with mounting hardware, seating gaskets and signal connection fittings. Application information for circular, rectangular, and oval duct configurations is included in the FE-1000 Data Sheets.

Standard construction is anodized aluminum that is suitable for most applications. FE-1000 units are optionally available in Type I PVC plastic or Type 316 stainless steel for corrosive atmospheres and Type 316 stainless steel for high temperature air or gas.

FE-1050 Fan Inlet Airflow Sensing Elements

The FE-1050 units are airflow sensor assemblies specifically designed for installation in the inlet cone(s) of centrifugal fans or inlet bell of vane-axial rans. The high velocities and predictable flow pattern of the air entering the fan assure measurement accuracy and excellent response during system turn-down conditions. Application of this unique method of fan capacity measurement is simple and eliminates the costly, often frustrating, engineering design time required to meet the minimum inlet and outlet requirements of a traditional airflow measuring station.

FE-1050 units also provide a most economical means of total fan air volume measurement. Only two flow sensing elements are required per fan inlet compared to duct units that may require as many as six elements for a proper traverse, or as a worst case, a system requiring summing of multiple branch ducts.