Close Temperature Control of a Process Fluid Flow

Temperature sensor is one component
of a successful temperature control
system
Courtesy Smart Sensors, Inc.

Temperature control is a common operation in the industrial arena. Its application can range across solids, liquids, and gases. The dynamics of a particular operation will influence the selection of instruments and equipment to meet the project requirements. In addition to general performance requirements, safety should always be a consideration in the design of a temperature control system involving enough energy to damage the system or create a hazardous condition.

Let's narrow the application range to non-flammable flowing fluids that require elevated temperatures. In the interest of clarity, this illustration is presented without any complicating factors that may be encountered in actual practice. Much of what is presented here, however, will apply universally to other scenarios.

What are the considerations for specifying the right equipment?

KNOW YOUR FLOW

First and foremost, you must have complete understanding of certain characteristics of the fluid.

• Specific Heat - The amount of heat input required to increase the temperature of a mass unit of the media by one degree.
• Minimum Inlet Temperature - The lowest media temperature entering the process and requiring heating to a setpoint. Use the worst (coldest) case anticipated.
• Mass Flow Rate - An element in the calculation for total heat requirement. If the flow rate will vary, use the maximum anticipated flow.
• Maximum Required Outlet Temperature - Used with minimum inlet temperature in the calculation of the maximum heat input required.

SELECT SYSTEM COMPONENTS WITH PERFORMANCE TO MATCH THE PROJECT

• Heat Source - If temperature control with little deviation from a setpoint is your goal, electric heat will likely be your heating source of choice. It responds quickly to changes in a control signal and the output can be adjusted in very small increments to achieve a close balance between process heat requirement and actual heat input. 
• Sensor - Sensor selection is critical to attaining close temperature control. There are many factors to consider, well beyond the scope of this article, but the ability of the sensor to rapidly detect small changes in media temperature is a key element of a successful project. Attention should be given to the sensor containment, or sheath, the mass of the materials surrounding the sensor that are part of the assembly, along with the accuracy of the sensor.
• Sensor Location - The location of the temperature sensor will be a key factor in control system performance. The sensing element should be placed where it will be exposed to the genuine process condition, avoiding effects of recently heated fluid that may have not completely mixed with the balance of the media. Locate too close to the heater and there may be anomalies caused by the heater. A sensor installed too distant from the heater may respond too slowly. Remember that the heating assembly, in whatever form it may take, is a source of disturbance to the process. It is important to detect the impact of the disturbance as early and accurately as possible.
• Controller - The controller should provide an output that is compatible with the heater power controller and have the capability to provide a continuously varying signal or one that can be very rapidly cycled. There are many other features that can be incorporated into the controller for alarms, display, and other useful functions. These have little bearing on the actual control of the process, but can provide useful information to the opeartor. 
• Power Controller - A great advantage of electric heaters is their compatibility with very rapid cycling or other adjustments to their input power. A power controller that varies the total power to the heater in very small increments will allow for fine tuning the heat input to the process.
• Performance Monitoring - Depending upon the critical nature of the heating activity to overall process performance, it may be useful to monitor not only the media temperature, but aspects of heater or controller performance that indicate the devices are working. Knowing something is not working sooner, rather than later, is generally beneficial. Controllers usually have some sort of sensor failure notification built in. Heater operation can be monitored my measurement of the circuit current.
  
  

SAFETY CONSIDERATIONS

Any industrial heater assembly is capable of producing surface temperatures hot enough to cause trouble. Monitoring process and heater performance and operation, providing backup safety controls, is necessary to reduce the probability of damage or catastrophe.

• High Fluid Temperature - An independent sensor can monitor process fluid temperature, with instrumentation providing an alert and limit controllers taking action if unexpected limits are reached.
• Heater Temperature - Monitoring the heater sheath temperature can provide warning of a number of failure conditions, such as low fluid flow, no fluid present, or power controller failure. A proper response activity should be automatically executed when unsafe or unanticipated conditions occur.
• Media Present - There are a number of ways to directly or indirectly determine whether media is present. The media, whether gaseous or liquid, is necessary to maintain an operational connection between the heater assembly and the sensor. 
• Flow Present - Whether gaseous or liquid media, flow is necessary to keep most industrial heaters from burning out. Understand the limitations and operating requirements of the heating assembly employed and make sure those conditions are maintained. 
• Heater Immersion - Heaters intended for immersion in liquid may have watt density ratings that will produce excessive or damaging element temperatures if operated in air. Strategic location of a temperature sensor may be sufficient to detect whether a portion of the heater assembly is operating in air. An automatic protective response should be provided in the control scheme for this condition.

Each of the items mentioned above is due careful consideration for an industrial fluid heating application. Your particular process will present its own set of specific temperature sensing challenges with respect to performance and safety. Share your requirements with temperature sensing experts, combining your process knowledge with their expertise to develop safe and effective solutions.

Direct Insertion Gaseous Oxygen Sensor - No Sampling

Ingold 6800G insertion sensor for gaseous oxygen
Courtesy Mettler Toledo

Gas phase oxygen measurement is used industrially for process safety and the prevention of oxidation. The ability to obtain oxygen level readings in real time can be advantageous in both application classes. While numerous measurement methods are available, Ingold (Mettler Toledo) provides a rugged and robust solution in their direct insertion sensors. Some measurement systems require gas sampling or conditioning, but the Ingold sensor is inserted directly into the process to provide fast and accurate oxygen readings. Coupling the direct insertion feature with Ingold's Intelligent Sensor Management delivers fast and direct O2 measurements with minimized maintenance, no sampling or conditioning, and limited downtime.

There is more to be learned. Reach out to a product application specialist and share your O2 measurement challenges. Combine your process knowledge with their product application expertise to develop the best solutions.

Gas phase oxygen measurement systems from Alliance Technical Sales, Inc.

New Laser Level Transmitter Brings Application Advantages for Process Measurement

Jogler, manufacturer of level measurement instruments, has released the new Model LLT 1000 Laser Level Transmitter for industrial process measurement and control applications. The newly designed instrument provides continuous non-contact level measurement for process automation and inventory management across an array of industrial applications.

The laser level transmitter provides reliable measurement of solids or liquids, even clear liquids. Long measuring range, narrow beam, and high accuracy make the LLT 1000 suitable for application in silos, tanks, hoppers, chutes, and bunkers.

The data sheet below provides more detail on the new level measurement instrument. Share your level measurement requirements and challenges with a product application specialist, combing your process knowledge with their product expertise to develop effective solutions.

Laser based level transmitter for industrial process measurement from Alliance Technical Sales, Inc.

New Website Design for Alliance Technical Sales

Alliance Technical Sales has a redesigned website

Alliance Technical Sales, manufacturer's representative and distributor of industrial process measurement and control equipment, has released a newly designed website which brings new functionality and easy navigation for visitors.

The newly designed site provides simple and rapid access to technical information regarding an array of process measurement and control products. Direct links to represented manufacturers is provided, as well as simple contact avenues to get visitors information quickly. The site is mobile friendly, adjusting its layout to accommodate whatever device the visitor is using and provide easily readable page views.

ATS now has an ongoing social media presence, with weekly blog posts about products and technical issues. All of the social media channels are directly accessible from the website.

Alliance Technical Sales is fully committed to providing top flight service to its customers. Visit the new website and reach out to ATS with your process measurement and control challenges.

Optical Sensor for Dissolved Oxygen Monitoring in Breweries

Dissolved oxygen measurements are an essential part of the brewing process. Keeping oxygen out of beer is a key production goal of brewers on any scale, since oxygen noticeably diminishes the taste of beer.

The measurement levels for oxygen are very low, so a sensor with high accuracy and response is needed. Hygienic construction and the ability to accommodate CIP operations is also a plus. Any features that reduce maintenance time budget enhance the value of the instrument.

The video, provided by Mettler Toledo, provides useful insight into how their optical DO sensor can benefit a brewing operation.

More detail, as well as application assistance, is available from product specialists. Combining your process knowledge and experience with their product application expertise will deliver effective solutions.

Transformer Monitoring for Hydrogen, Moisture, and Temperature

Specialized transmitter for monitoring moisture,
hydrogen, and temperature levels in electrical
transformers. (Vaisala)

Electrical transformers are an intricate part of the power transmission and distribution system. Many transformers are insulated with mineral oil that also facilitates heat transfer for unit cooling. Abnormal thermal and electrical stresses, such as local overheating and electrical discharge ocurring in the transformer, cause decomposition of insulating oil and papers, resulting in production of a range of gases, one of which is hydrogen.

Routine dissolved gas analysis, or DGA, is performed on oil filled transformers to provide a measure of unit health. The process involves the extraction of an oil sample from the transformer, which is then subjected to laboratory analysis. The process is manual, time consuming, and only performed infrequently. It is known that the presence or increasing levels of certain gases is an indicator of internal faults or general wear and tear in a transformer. A reliable stream of data can provide value as a predictive indicator of overall transformer health and maintenance requirements. The endgame is to prevent unit failure and the resultant shutdown of connected customer equipment.

General recommendations call for trend monitoring of hydrogen. The rate of change in the concentration can be more indicative than the overall level in some cases. Any rapid change is a harbinger of potential problems. Adopting a proactive approach, based on transformer oil condition, can extend transformer useful life.

Moisture content of transformer oil is another concern. The presence of water boosts oil and paper insulation deterioration. Water presence in transformers generally has two potential sources, atmospheric and internal. The sources of moisture in the oil are not a subject of this article, but water reduces the dielectric strength of insulating oil and can facilitate corrosion or other material degradation within the transformer case.

Internal temperature is an indicator of the current operating condition of a transformer and is useful in evaluating its performance.

All three of the parameters mentioned, moisture, hydrogen, and temperature, are continuously measured by the new MHT-410 from Vaisala. The transmitter provides three isolated loop powered 4-20 mA outputs that provide the user with continuous data on moisture content, hydrogen concentration, and oil temperature. The compact unit installs easily in less than 30 minutes to provide online monitoring of insulating oil without any need for pumps, membranes, or sampling.

The new MHT410 provides real time transformer health monitoring, as well as information on transformer fault situations. You can find out more with a review of the data sheet included below. Reach out to a product specialist with your measurement challenges.

Moisture, Hydrogen, and Temperature Measurement in Electrical Transformers from Alliance Technical Sales, Inc.

Magnetostrictive Level Transmitter Retrofit to Magnetic Level Gauge

This magnetic level gauge has been fitted
with a magnetostrictive transmitter to
provide a continuous process level signal
Jogler

A magnetic liquid level indicator provides a reliable and highly visible reading of its indirect level measurement in tanks and vessels. While useful for local visual indication of tank level, the instrument does not provide an electrical signal output that can be utilized for process control or other monitoring functions. Several magnetic level gauge manufacturers offer combination units that incorporate another measurement technology to provide an analog signal corresponding to liquid level.

What if your installation has an existing magnetic level indicator and you need an analog level signal?

One manufacturer, Jogler, offers a magnetostrictive level measurement instrument that can be retrofitted to an existing magnetic level gauge, even those from other manufacturers. The low profile waveguide mounts slightly away from the existing magnetic level unit to isolate it from temperature and vibration. It is specifically designed for use with an existing magnetic level indicator and provides an analog output signal that is accurate, stable, and responsive.

A magnetostrictive level measurement instrument employs a magnetic float that moves along a sensing wire as the tank level changes. Fixed interval current pulses are sent down the wire, with the resulting electromagnetic field being interrupted by the magnetized float. This interruption causes magnetostriction in the sensing wire, which is manifested as a torsional stress wave along the wire. A piezoceramic sensing element converts the torsional stress into an electrical signal and the transmitter produces an output signal based upon the time interval between the initial pulse and the return pulse produced as a function of the float position.

There is more to be learned, so share your liquid level measurement requirements and challenges with instrumentation specialists. The combination of your process knowledge and their product application expertise will produce effective solutions.

Magnetostrictive level transmitter added to magnetic level gauge from Alliance Technical Sales, Inc.