Tube Type Heat Flux Sensor

The design objective with this device was to develop a boiler heat flux measuring instrument that was as close to the ideal as possible. This integral sensor has an external profile that is identical to the boiler tube, a surface temperature elevation that is minimal and is able to determine absorbed heat flux, tube surface and internal fluid temperatures. All thermocouples are duplicated to provide an instrument with good life expectancy (5 years +). The instrument is constructed from a length of (customer supplied) boiler tube (Min 300mm, Max 2 metres) that contains a locally thickened wall section . This construction is achieved by indenting the tube and then filling the depression using an automatic spiral welding process. The measuring cylinder is then accurately machined into the weld fill by 'electro discharge machining'. This construction ensures that the walls of the parent tube are untouched and the mechanical integrity of the tube is unimpaired. It is an essential feature of this device that the surface profile is identical to that of an undisturbed boiler tube and thermal modelling has shown that the surface temperature elevation is not large enough to cause any emissivity errors.

THERMALLY GUARDED CYLINDER

The sensor is designed to measure heat transfer into the boiler tube, therefore it is essential that only the radial component of the heat transfer is measured. In order to achieve this the measuring cylinder, where the thermocouples are located, is surrounded by an air gap which acts as a thermal barrier to longitudinal heat flow. The air gap is weld sealed on the surface to stop corrosive furnace gases entering the sensor body. This seal is very important as sensors that are open to the furnace gases will suffer rapid erosion of thermocouple cables and consequent rapid failure.

LOCATION CONSIDERATIONS

TT heat flux sensors can be fitted in most boiler areas. For areas where a high temperature space needs to be traversed (wind-box etc.) the sensor has extended thermocouples fitted which run through a gas tight flexible conduit.

BOILER TUBE DRY-OUT

When tube dry-out occurs the high heat transfer coefficients that are normally associated with boiling are replaced by significantly lower coefficients associated with dry steam. This condition rapidly leads to a very high localised tube metal temperatures as the heat transfer capacity is insufficient to provide adequate cooling. In extreme cases the tube temperature will rise to near flame temperature and cause rapid tube failure. However, dry-out is often intermittent and only occurs for short time periods. The tube damage is then seen as concentrations of soluble salts, from the feed water, accumulating in defects or porous oxides at the inner surface of the tube. This chemical erosion will, over a period of time, lead to tube failure. TT Heat Flux Sensors are ideally suited to identifying this problem.

HIGH HEAT FLUX AND TUBE METAL TEMPERATURES

Premature boiler tube failures are often associated with high heat fluxes and tube metal temperatures. Heat flux sensors can accurately measure absorbed heat flux and the undisturbed tube surface temperature. Therefore areas that are prone to tube failures can be analysed, using directly measured data, and either; have high heat flux/temperature eliminated as a problem cause or, have a positive identification of the problem made.

Localised high heat fluxes and tube metal temperatures can be caused by direct flame impingement, slag blanketing, etc.

OUTPUTS

Outputs from both types of sensor are in mV (or optional 4-20mA). An individual calibration constant is applied to each sensor to provide a heat flux output in kW/m²(or BTU/ft²h).

Both types of sensor can provide very accurate measurements of 'undisturbed tube surface temperature'. This is an extrapolation of heat flux and inner thermocouple temperature. This measurement is calculated using a linear equation. No additional sensor modification is required for this measurement.

Typical output :	0-500 kw/m2= 0-5mV
Accuracy:	0-100 kw/m2 2% of value
	101-600 kw/m2 1% of value
Thermocouple type:	Type K
Sheath material	Inconel

Surface Type Heat Flux Sensor

The BMS (International) Surface Mounted Sensor provides a cheap, accurate method of determining heat transfer in most boiler areas. The sensor can be fitted directly onto boiler tubes therefore ensuring minimal plant disturbance.

The SS Sensor can be used for:

• Observing and solving slagging problems
• Determining sootblower problems
• Heat transfer mapping in new and existing boilers
• Identifying areas that suffer from direct flame impingement, boiler tube dry-out
• (departure from nucleate boiling), high localised heat fluxes ,etc.

The SS sensor was originally designed by the former Central Electricity Research Laboratories and extensive modelling was undertaken to provide a design that was both accurate and robust. The sensor consists of a 12mm 'stud' which is attached to the boiler tube either in situ or to a length of tube that has been removed from the boiler. Measurement is made using a dual thermocouple array within a thermally guarded cylinder in the sensor. Each sensor is individually calibrated. Attachment to the tube is done using a special welding technique. Direct installation of the SS Sensor is only possible in membrane wall boilers.

The expected life of a SS Sensor can (depending on the boiler conditions) be from 1 to 3 years.

Location Considerations

If required, the SS Sensor can be manufactured for fitting into wind boxes or high temperature dead spaces, etc. A high temperature extension is used in this instance and the thermocouples leave the hot interspace (up to 800°C) via stainless steel or Inconel extension cable.

Signal Output Typical Output:	0-500 kW/m2 = 0 - 5mV
Accuracy:	0-100kW/m2 - 3.5% of value
	101-900kW/m2 - 2% of value
Thermocouple type:	Type K

Outputs from the sensor are in mV (or optional 4-20mA).

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