Applying thermal conductivity flow switches in industrial settings

Thermal conductivity (TC) flow switches offer plant engineers many advantages over traditional mechanical flow switches. A TC flow switch (Fig. 1) uses the cooling effect produced by a flowing fluid to monitor the flow velocity of that fluid.

05/01/2000


Thermal conductivity (TC) flow switches offer plant engineers many advantages over traditional mechanical flow switches. A TC flow switch (Fig. 1) uses the cooling effect produced by a flowing fluid to monitor the flow velocity of that fluid. It has no moving parts that can wear or break, and creates very little obstruction to the flow of the liquid or gas. A rugged switch with a one-piece corrosion resistant probe, the device is easy to install and set up for use.


This type of switch is a setpoint device. It provides no direct readout of the flow rate. Its function primarily is to monitor loss of flow or confirm that flow has been established.

How a TC flow switch works


The tip of the probe of a TC flow switch houses two thermistors and a heater element (Fig. 2). Thermistors are solid state devices that exhibit a large, predictable change in their resistance in response to changes in temperature. One thermistor is bonded to the center of the flat tip of the flow switch probe. The other is bonded to the cylindrical wall of the probe. When the two thermistors are at the same temperature, their resistance values are equal. And they experience the same change in resistance in response to a changing temperature.

Applying power heats the probe tip. When the sensor is immersed in a fluid that is not flowing, heat is carried away from the tip primarily by conduction through the cylinder walls and through the fluid surrounding the probe. Under these conditions, the temperature of the thermistor on the tip of the probe is several degrees hotter than the fluid. A temperature gradient is established in the cylinder wall and a temperature differential exists between the two thermistors.

A flowing fluid carries heat away from the sensor tip by the forced convection of the flowing fluid. The heater cannot maintain the tip at the same temperature as it did under the no-flow condition and the differential between the two thermistors decreases. As the velocity increases, more heat is carried away and the temperature differential decreases further.

The two thermistors are incorporated into a circuit that produces an electrical signal that depends on temperature difference. Figure 3 (below) shows signal voltage versus velocity of flowing water on a nonlinear graph. At low velocities, the unit is highly sensitive to changes in velocity. At medium velocities, it is less sensitive to changes and at high velocities it is almost insensitive to changes.


The changes in sensitivity occur because at low velocities, the temperature of the tip is at its maximum. A small increase in velocity removes a large amount of heat from the tip. At higher velocities, the tip temperature is lower and changes in velocity remove a proportionally smaller amount of heat from the tip.

Plotting signal voltage versus fluid velocity on a logarithmic scale (Fig. 4 below) shows the response is linear roughly in the range between 3 and 60 cm/sec. This "sweet zone," as it is called, represents the most desirable operating range for the sensor. At velocities between 60 and 300 cm/sec, the unit still provides a flow monitoring function, but with greatly reduced sensitivity to changes in flow.


Impact of temperature changes

A TC flow switch monitors velocity of a fluid by measuring the ability of the heater to maintain tip temperature above fluid temperature. The thermistor on the side of the probe monitors fluid temperature. The temperature difference between the two thermistors is the measure of fluid velocity.

To provide useful flow monitoring, the sensor must be reasonably independent of the fluid temperature. When fluid temperature rises, the whole tip temperature tracks the elevation. Because heater power is constant, the temperature gradient along the tip remains the same as it was before the fluid temperature increased. If the two thermistors exhibit a constant change of resistance with temperature changes, they can provide perfect compensation for changes in fluid temperature.

At one time, rapid fluid temperature changes caused difficulties with TC flow switches. The thermistor on the side of the probe was unable to track fluid temperature changes accurately. As a result, rapidly increasing fluid temperatures could produce a false loss-of-flow output. However, redesigning the geometry of the sensor tip to track temperature changes more rapidly has essentially solved the problem.

Response to flow rate changes

Many TC flow switch applications involve protecting equipment against the loss of flow. Therefore, it is important to understand how fast the sensor will respond. Response time is not fixed. Rather, it varies with the relationship of the setpoint to the initial and final flow rates, the velocity of the flow, and the characteristics of the flow medium.


Figure 5 shows the change in signal over time when flow increases from rate A to rate B. The Y-axis is the percent of signal change. The X-axis represents time. For the sake of illustration, the time to reach 98% of the final value covers four equal intervals called time constants. The response is not linear. The signal changes rapidly at first, then slows considerably as it approaches its final value. The closer the setpoint is to the final value, the longer it will take to reach it.

The table below shows the percentage change in signal that occurs after each time constant:

Time constant Change, %

1 63

2 86

3 95

4 98

If the setpoint is established at 63% of the final value, the sensor recognizes the change twice as fast as it would if the setpoint were at 86%, three times faster than if the setpoint were at 95%, and four times faster than 98%.

Figure 5 illustrates a case in which flow suddenly increases. Figure 6 (below) shows a case in which flow suddenly decreases. Here again, the closer the setpoint is to the final value, the longer it takes for the sensor to recognize the change. For a response time of one time constant, the setpoint should be positioned at 37% of the starting value (representing a 63% change in signal). The time constant for loss of flow is much longer than that for increased flow .


 

C
ompared to other liquids, water has greater thermal capacity and higher conductivity. Its low viscosity causes turbulence to homogenize velocity and temperature variations across the cross section of a pipe. These factors combine to provide the best performance of a TC flow sensor in water applications.

-Edited by Jeanine Katzel, Senior Editor, 630-320-7142, jkatzel@cahners.com

&HEADLINE>Key concepts&/HEADLINE>

TC flow switches are commonly used in systems that protect equipment against the loss of flow.

The location of a TC flow switch depends upon many considerations, including the application.

This type of device has no moving parts and creates little obstruction to the flow of liquid or gas.

&HEADLINE>More info&/HEADLINE>

Technical questions about this article may be directed to the author by phone at 610-524-2000 or by e-mail at karl_klinger@efector.com.

A guide to the anticipated linear velocity range and relative response time for various fluids as well as pipe size/flow charts for water and gases are included in the web site version of this article at www.plantengineering.com.

For additional articles on related subjects, see the Instrumentation and controls channel at www.plantengineering.com.

&HEADLINE>Common applications&/HEADLINE>

TC flow switches find many applications in industrial settings.

Providing dry-run protection for pumps. A TC flow switch mounted in a pump input line guards against pump damage. If flow drops below the setpoint, the switch provides an output signal to alert an operator or shut down the pump. These switches have corrosion resistant probes to make them compatible with a wide variety of process fluids. Because they are insensitive to any buildup of fats, greases, or other solids, they are particularly well suited for wastewater and other dirty liquid streams.

Protecting against weld water loss. TC flow switches are used extensively to monitor the flow of cooling water to electrodes, electrode holders, transformers, and ignition tubes in welding equipment. The device frequently replaces a flow sensor that uses springs and gears to detect a differential pressure between supply and return lines.

Over time, scale and dirt build up on moving components, making the sensor unreliable. The problem is solved by using a flow switch and a check valve in the return line. The switch detects any loss of cooling water and the check valve prevents the switch from being fooled by any backflow.

Guarding against pump seal failures. Shaft seals often depend on the flow of a lubricant or sealant to prevent leaking of process fluids or product contamination. A TC flow switch mounted in a low-flow adapter meets the needs of this application. An adapter provides for piping connections to 1/8-in. dia piping and lets the switch sense flow rates as low as 2 ml/min.

Guarding against cutting fluid losses. Interrupting the cutting fluid supply during machining operations can damage the workpiece and the tooling. A TC flow switch reliably detects any supply interruptions and its stainless steel housing is rugged enough to withstand most manufacturing environments. The wide range of the device allows the setpoint to be adjusted for the optimum cutting fluid rate in a variety of situations.

Monitoring nozzles, applicators, and dispensers. TC flow switches are also found in a variety of applications involving glues, inks, paint, or chemical injection. A flow switch mounted in the supply lines of liquids that must be mixed helps ensure the proper proportion of each material is used.

&HEADLINE>Mounting considerations&/HEADLINE>

The location of a TC flow switch is an important consideration that depends both on the application and on some common concerns. Consider these six points when placing a TC flow switch.

  • A flow switch probe must be inserted in the fluid stream. The internal diameter of the upstream pipe must equal that of the tee for at least the length of five pipe diameters. A smaller pipe diameter may lead to poor results. Similarly, the outlet of the tee must be the same diameter and have a straight run of at least three pipe diameters. For optimum thermistor orientation, the flow switch must be tightened until the connector cable is directed downstream along the axis of the pipe.

      • Do not mount a sensor in the bottom of a horizontal pipe because sediment sometimes collects there. Side mounting typically works best.

          • Avoid mounting a sensor at the top of the pipe, because a horizontally mounted pipe might not be filled with liquid.

              • If a sensor must be mounted in a vertical pipe, choose one in which the liquid is moving upward and fills the pipe.

                  • Liquid moving through a pipe creates turbulence that results in local flow velocities that are different from the average flow velocity. Elbows, valves, and other obstructions increase turbulence, as do high average flow velocities. In general, sensors should be mounted at least 4%%MDASSML%%5pipe diameters before or after an obstruction. However, if flow velocity and accuracy requirements are both low, closer mounting may give satisfactory results.

                      • When the sensor is larger than the line to which it is to be mounted, an adapter should be used. Reducing the inside diameter of the line increases the fluid velocity.


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