Chapter 2: Input Devices and Sensors
Part 2: Analog Devices

Flow Measurements

Flow measuring devices are widely used in DDC control systems for HVAC to monitor and control various air and liquid flows. Typically, airflow-measuring devices are used to monitor and control the output of fans, dampers, and associated equipment used to control outside airflow, VAV box airflow, and building and space pressures. Liquid flow is commonly measured to maintain required flows in boilers, chillers and heat exchangers, and to control and monitor energy production and use (requires temperature measurement also).

Numerous reliable technologies are available for use with DDC systems. Some technologies have been applied to both air and liquid flow measurements as their principles of operation hold true in either application. Other technologies lend themselves to being airflow or liquid flow specific.

Methods for Measuring Flow

Flow rate is typically obtained by measuring a velocity of a fluid in a duct or pipe and multiplying the by the known cross sectional area (at the point of measurement) of that duct or pipe. Common methods for measuring airflow include hot wire anemometers, differential pressure measurement systems, and vortex shedding sensors. Common methods used to measure liquid flow include differential pressure measurement systems, vortex shedding sensors, positive displacement flow sensors, turbine based flow sensors, magnetic flow sensors, ultrasonic flow sensors and ‘target’ flow sensors.

Hot Wire Anemometers
'Hot Wire" or thermal anemometers operate on the principle that the amount of heat removed from a heated temperature sensor by a flowing fluid can be related to the velocity of that fluid. Most sensors of this type are constructed with a second, unheated temperature sensor to compensate the instrument for variations in the temperature of the air. Hot wire sensors are available as single point instruments for test purposes, or in multi-point arrays for fixed installation. Hot wire type sensors are better at low airflow measurements than differential pressure types, and are commonly applied to air velocities from 50 to 12,000 feet per minute.

Differential Pressure Measurement Systems
Differential pressure measurement technologies can be applied to both airflow and liquid flow measurements. Sensor manufacturers offer a wide variety of application specific sensors used for airflow and pressure measurements, as well as ‘wet-to-wet’ differential pressure sensors used for liquid measurements. Both lines offer a wide variety of ranges.

For airflow measurements, differential pressure flow devices in common use in HVAC systems include Pitot tubes (Figure 2.15) and various types of proprietary velocity pressure sensing tubes, grids, and other arrays. All of these sensing elements are combined with a low differential pressure transmitter to produce a signal that is proportional to the square root of the fluid velocity. For example, when using a Pitot-static tube, this signal can be related to the flow according to the following equations (source iii):

Where:

Velocity = Velocity (ft/min)
VP = velocity pressure (in w.c.)
p = density of air (lbm/ft2)
gc = gravitational constant (32.174 lbm× ft/lbf×s2)
C = unit conversion factor (136.8)

Figure 2.16 depicts an example of a velocity pressure measurement with a U tube manometer and Figure 2.17 depicts an example of the relationship between velocity pressure (VP), static pressure, and total pressure.

As a permanently mounted sensor, the Pitot tube is limited to small ducts and applications with low accuracy requirements due to the need to sense the velocity at more than one point to achieve reliable measurements in larger ducts. The need to sense multiple points in the cross section of a duct gave rise to averaging type sensors with arrays of pressure sensing points. This type is most commonly used in HVAC applications.

Some differential pressure based flow stations include transmitters that have the capability to electronically extract the square root of the measured pressure and provide an analog signal that is linear with respect to velocity, whereas others provide an analog signal that is proportional to measured pressure and depend upon the DDC system to calculate the square root and therefore, resulting (averaged) velocity. Once the velocity is obtained, flow can be calculated by multiplying by the cross sectional area of the duct. Velocity range is limited by the range and resolution of the pressure transmitter used. Most differential pressure type stations are limited to a minimum velocity in the range of 400 to 600 feet per minute. Maximum velocity is only limited by the durability of the sensor.

For water flow measurements, differential pressure flow devices in common use in HVAC systems operate either by measuring velocity pressure (insertion tube type), or by measuring the drop in pressure across a restriction of known characteristic (orifice, flow nozzle, Venturi).

Insertion tube type flow sensors are usually constructed of a round or proprietary shape tube with multiple openings across the width of the flow stream to provide an average of the velocity differential across the tube and an internal baffle between upstream and downstream openings to obtain a differential pressure. Insertion tube type meters have a low permanent pressure loss, and with proper installation and associated pressure instruments are satisfactory for many common applications. Insertion tube flow sensors are available that can be installed and removed through a full port valve so that installation and service are possible without draining the section of piping in which they are installed.

A concentric orifice plate is the simplest and least expensive of the differential pressure type meters. The orifice plate constricts the flow of a fluid to produce a differential pressure across the plate (see Figure 2.18). The result is a high pressure upstream and a low pressure downstream that is proportional to the square of the flow velocity. An orifice plate usually produces a greater overall pressure loss than other flow elements. An advantage of this device is that cost does not increase significantly with pipe size.

Venturi tubes exhibit a very low pressure loss compared to other differential pressure meters, but they are also the largest and most costly. They operate by gradually narrowing the diameter of the pipe, and measuring the resultant drop in pressure (see Figure 2.19). An expanding section of the meter then returns the flow to very near its original pressure. As with the orifice plate, the differential pressure measurement is converted into a corresponding flow rate. Venturi tube applications are generally restricted to those requiring a low pressure drop and a high accuracy reading. They are widely used in large diameter pipes.

Flow nozzles may be thought of as a variation on the Venturi tube. The nozzle opening is an elliptical restriction in the flow but with no outlet area for pressure recovery (Figure 2.20). Pressure taps are located approximately 1/2 pipe diameter downstream and 1 pipe diameter upstream. The flow nozzle is a high velocity flow meter used where turbulence is high (Reynolds numbers above 50,000) such as in steam flow at high temperatures. The pressure drop of a flow nozzle falls between that of the Venturi tube and the orifice plate (30 to 95 percent).

The turndown (ratio of the full range of the instrument to the minimum measurable flow) of differential pressure devices is generally limited to 4:1. With the use of a low range transmitter in addition to a high range transmitter or a high turndown transmitter and appropriate signal processing, this can sometimes be extended to as great as 16:1 or more. Permanent pressure loss and associated energy cost is often a major concern in the selection of orifices, flow nozzles, and venturis. In general, for a given installation, the permanent pressure loss will be highest with an orifice type device, and lowest with a Venturi. Benefits of differential pressure instruments are their relatively low cost, simplicity, and proven performance.

Vortex Shedding Sensors
Vortex shedding flow meters operate on the principle (Von Karman) that when a fluid flows around an obstruction in the flow stream, vortices are shed from alternating sides of the obstruction in a repeating and continuous fashion. The frequency at which the shedding alternates is proportional to the velocity of the flowing fluid. Single sensors are applied to small ducts, and arrays of vortex shedding sensors are applied to larger ducts, similar to the other types of airflow measuring instruments. Vortex shedding airflow sensors are commonly applied to air velocities in the range of 350 to 6000 feet per minute.

Vortex flow meters provide a highly accurate flow measurement when operated within the appropriate range of flow. Vortex meters are commonly applied where high quality water, gas and steam flow measurement is desired. Performance of up to 30:1 turndown on liquids and 20:1 on gases and steam with 1-2 percent accuracy is available. Turndowns are based on liquid velocities through the meter of up to 25 feet per second for liquids, 15,000 feet per minute for steam and gases. Actual turndown may be less depending on design velocity limitations.

Positive Displacement Flow Sensors
Positive displacement meters are used where high accuracy at high turndown is required and reasonable to high permanent pressure loss will not result in excessive energy consumption. Applications include water metering such as for potable water service, cooling tower and boiler make-up, and hydronic system make-up. Positive displacement meters are also used for fuel metering for both liquid and gaseous fuels. Common types of positive displacement flow meters include lobed and gear type meters, nutating disk meters, and oscillating piston type meters. These meters are typically constructed of metals such as brass, bronze, cast and ductile iron, but may be constructed of engineered plastic, depending on service.

Due to the close tolerance required between moving parts of positive displacement flow meters, they are sometimes subject to mechanical problems resulting from debris or suspended solids in the measured flow stream. Positive displacement meters are available with flow indicators and totalizers that can be read manually. When used with DDC systems, the basic meter output is usually a pulse that occurs at whatever time interval is required for a fixed volume of fluid to pass through the meter. Pulses may be accepted directly by the DDC controller and converted to flow rate, or total volume points, or a separate pulse to analog transducer may be used. Positive displacement flow meters are one of the more costly meter types available.

Turbine Based Flow Sensors
Turbine and propeller type meters operate on the principle that fluid flowing through the turbine or propeller will induce a rotational speed that can be related to the fluid velocity. Turbine and propeller type flow meters are available in full bore, line mounted versions and insertion types where only a portion of the flow being measured passes over the rotating element. Full bore turbine and propeller meters generally offer medium to high accuracy and turndown capability at reasonable permanent pressure loss. With electronic linearization, turndowns to 100:1 with 0.1% linearity are available. Insertion types of turbine and propeller meters represent a compromise in performance to reduce cost. Typical performance is 1 percent accuracy at 30:1 turndown. Turbine flow meters are commonly used where good accuracy is required for critical flow control or measurement for energy computations. Insertion types are used for less critical applications. Insertion types are often easier to maintain and inspect because they can be removed for inspection and repair without disturbing the main piping. Some types can be installed through hot tapping equipment and do not require draining of the associated piping for removal and inspection.

Magnetic Flow Sensors
Magnetic flow meters operate based upon Faraday's Law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field.

Faraday's Law: E=kBDV

The magnitude of the induced voltage E is directly proportional to the velocity of the conductor V, conductor width D, and the strength of the magnetic field B. As shown in Figure 2.21, magnetic field coils are placed on opposite sides a pipe to generate a magnetic field. As the conductive process liquid moves through the field with average velocity V, electrodes sense the induced voltage. The distance between electrodes represents the width of the conductor. An insulating liner prevents the signal from shorting to the pipe wall. The only variable in this application of Faraday's law is the velocity of the conductive liquid V because field strength is controlled constant and electrode spacing is fixed. Therefore, the output voltage E is directly proportional to liquid velocity, resulting in the linear output of a magnetic flow meter.

Magnetic flow meters are used to measure the flow rate of conducting liquids (including water) where a high quality low maintenance measurement system is desired. The cost of magnetic flow meters is high relative to many other meter types. Typical performance is 30:1 turndown at 0.5% accuracy.

Ultrasonic Flow Sensors
Ultrasonic flow sensors measure the velocity of sound waves propagating through a fluid between to points on the length of a pipe. The velocity of the sound wave is dependant upon the velocity of the fluid such that a sound wave traveling upstream from one point to the other is slower than the velocity of the of the same wave in the fluid at rest. The downstream velocity of the sound wave between the points is greater than that of the same wave in a fluid at rest. This is due to the Doppler effect. The flow of the fluid can be measured as a function of the difference in time travel between the upstream wave and the downstream wave.

Ultrasonic flow sensors are non-intrusive and are available at moderate cost. Many models are designed to clamp on to existing pipe. Ultrasonic Doppler flow meters have accuracies of 1 to 5% to the flow rate (source iv).

Target Flow Sensors
A target meter consists of a disc or a "target" which is centered in a pipe (see Figure 2.22). The target surface is positioned at a right angle to the fluid flow. A direct measurement of the fluid flow rate results from the force of the fluid acting against the target. Useful for dirty or corrosive fluids, target meters require no external connections, seals, or purge systems.

Target flow meters are commonly used to for liquid flow measurement and less commonly applied to steam and gas flow. Target Meters offer turndowns up to 20:1 with accuracy around 1%.

Installation

All airflow sensors work best in sections of ducts that have uniform, fully developed flow. All airflow sensing devices should be installed in accordance with the manufacturers recommended straight runs of upstream and downstream duct in order to provide reliable measurement. A number of manufacturers offer flow straightening elements that can be installed upstream of the sensing array to improve undesirable flow conditions. These should be considered when conditions do not permit installation with the required straight runs of duct upstream and downstream from the sensor.

As with airflow, all liquid flow sensors work best when fully developed, uniform flow is measured. To attain fully developed, uniform flow sensors should be installed in accordance with the manufacturers recommended straight runs of upstream and downstream pipe in order to provide the most reliable measurements.

With most liquid flows measured for HVAC applications, density changes with pressure and temperature are relatively small and most often ignored due to their insignificant effect on flow measurements. When measuring the flow of steam or fuel gases, unless temperature and pressure are constant, ignoring the effect density changes with varying temperature and pressure will often result in significant or gross errors. For this reason, it is common to measure the temperature and pressure, in addition to the flow, and electronically correct the result for the fluid density. This correction may be done using an integral or remote microprocessor based "flow computer" or it may be made in the DDC controller with suitable programming.

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