The purpose of a thermowell is to allow removal of the temperature sensing element without disrupting the process.  With the temperature sensing element installed however, it is important that process temperature is transferred through the thermowell efficiently and with minimal effect on response time.

Thermowell Wake Frequency calculations are generally conducted prior to thermowell manufacture.  They ensure that the thermowell design is robust enough to cope with varying stresses and strains produced by the process media.  Fluidic are able to provide Wake Frequency Calculations where the following process data is available:

1. Pressure
2. Temperature
3. Velocity (or flowrate with linesize)
4. Viscosity
5. Density

Any thermowell failure will occur at the highest stress point, ie between the flange/shaft join.  With the exception of the use of velocity collars (which cause their own problems, and are not recommended within current ASME PTC 19.3 TW-2010 standards), this has traditionally meant shortening of, or bulking up the thickness of, the thermowell.  Both of these techniques will significantly increase response time and are detrimental to temperature measurement performance.

In applications where the Thermowell Wake frequency calculation fails due to Strouhal frequency (such as in the example calculation shown below) Fluidic’s partner, Okazaki, have developed the VortexWell thermowell.  This helical design sheds vortexes normally caused by the thermowell itself and therefore cancels out the resonance failure issue apparent in traditional thermowell design.  Using an Okazaki Vortexwell in the example shown would therefore be deemed a technically sound install for this application.

 Download   Sample Thermowell Wake Frequency Calculation

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Fluidic represent Kurz Instruments, suppliers of high accuracy thermal mass flow meters. Kurz have developed a wet gas flowmeter for accurately measuring mass flow in biogas, condensing gas and wet-stack environments.

In condensing gas environments such as landfill or digester gas measurement, moisture can cause problems with thermal mass flowmeters. Water droplets forming on the sensors heated element result in an increase in energy to the element.  This causes the meter to report much higher flowrates than are actually occurring. Large water droplets can cause significant flow spikes for up to 30 minutes.

To overcome this problem, Kurz have developed the 454FTB-WGF, wet gas flowmeter. Kurz have adapted their successful 454FTB design by heating the heated sensor to 300°C above the ambient temperature sensor. As the water droplets impact the heated sensor they immediately vaporize and create a layer, caused by the Leidenfrost effect, which diverts the remaining water around the sensor leaving it unaffected by the moisture in the gas.

The Kurz 454FTB-WGF allows for reliable and accurate mass flow measurement in wet gas applications.