=New equipment lower section controls and disl~iays the logging feature. Operation of the unit requires little or no training.

The ultrasonics and memory are controlled by state-of-the- art microprocessor technology for stability of readings and ease of use. The upper section can be used with or wi thout the lower datalogging section. The upper section is controlled by five control keys: on/off, inch/mm, 11 pre-programmed material select funct ions and calibration.

To take thickness measurements, the upper section is switched on and the required mode and material programme selected. The thickness is displayed on the larger upper section LCD. Automatic recalibration is simple, requiring only that the sensor is placed on the testpiece supplied and the calibration button pressed.

The lower section is used to log the reading. The measurement is taken as normal and the Enter button pressed. The reading is recorded against the serial number indicated on the lower LCD display. The unit wil l auto on to the next serial number and is ready to record another reading. Browsing through the memory to check readings or to choose start points is controlled by four other buttons.

Erasing and printing the memory are simple, controlled by one switch each and the Enter button. The unit memory can be printed out via a standard computer type nine pin D connector. The company supplies a miniature thermal printer for immediate memory print outs; via the same connector the reading can be down-loaded to a PC.

Range of data loggers for use in thickness gauging from Wylam Hill

The serial number system eases operation by allocating areas of the memory to specific tasks on vessels and industrial construction. The serial numbers can also be allocated with a predetermined survey for pipework and pressure vessels.

Wylam Hill Limited, 2 South Street, Midhurst, West Sussex, GU29 9PD, UK

Analysis of the combustion process

As refuse companies experience increasing pressure from environment groups to make the combustion products of their incinerators more ecologically benign, new techniques for analysing these products are now being evaluated.

In a joint industrial research project conducted by SteinmLiller GmbH in conjunction wi th the Ruhr University Bochum, infrared thermography has been found useful in highl ight ing the possible link between different process condit ions and the emission of pollutants.

The refuse incineration process

depends on three primary components: a substance which can burn, oxygen for the combustion process and an achievable ignit ion temperature required to initiate and maintain the combustion process.

Due to the high temperatures prevailing in the furnace, the refuse is first dried, then heated, ignited and incinerated. Refuse is transported over a grate through which combustion air (primary air) is fed from below. Above the refuse bed, at the front and rear of the incinerator, nozzles inject further combustion air (secondary air) to provide a good mix and complete burn out of the gaseous combustibles.

As the refuse traverses the grate, it passes over five independent zones, each divided into two halves. The transport velocity and quantity of primary combustion air can be adjusted in each of these ten zones. Apart from altering the amount of refuse that can be fed into the incinerator, these variables provide the principal means of inf luencing the combustion process selectively.

As a non-contact technique,

346 NDT&E International 1994 Volume 27, Number 6

-New equipment infrared thermography is able to acquire specific temperature information on each of the ten zones within the incinerator. In particular, it is able to show the location and magnitude of hot spots as well as the intensity of combustion at these points. Using sequential recording of images, it is also possible to study time-related changes and the difference in reactions to various modes of operation.

SteinmL~ller GmbH and Bochum University have been using a high definition thermal imaging system, the Thermovision 870 supplied by Agema Infrared Systems, to provide the initial data for their research. Installed 16 m above the grate, in the boiler ceiling, the camera produces thermal images of the scene below and sends them to an adjacent control room for further analysis. To protect the camera lens from pollution and hot flue gases, a continuous f low of air is passed over the inspection port which provides a view into the combustion chamber. In addition, the lens system is cooled with water.

To obtain the best results from a thermographic survey, certain conditions within the furnace have to be taken into account, in particular the transmission path between the camera and the refuse. This path consists primarily of hot gases (flue gas, oxygen and water vapour) and solid particles such as soot, dust and ash. Of the gases, carbon dioxide and water. vapour have the most influence over the transmission path. However, since such gases absorb or emit infrared radiation only within certain bandwidths, it is possible to neglect their radiation properties by selecting an appropriate spectral filter.

Extract from a recorded sequence of infrared images of a refuse furnace showing the location and behaviour of hot-spots

Solid bodies, however, emit thermal radiation over all wavelengths and their effect on the transmission path cannot therefore be dealt with so easily. However, since the primary objective of this research was to make comparisons in temperature distribution rather than measure absolute values, the effects of solid particle absorption were considered negligible.

Some of the early work included a statistical test program running over several weeks. Volume flows of individual primary and secondary air feeder points were varied along with their composition (fresh air and/or recirculated flue gas). To determine the correlation between these different modes of operation and the level and type of pollutant emissions, 14 000 thermal images providing information on fire position and combustion intensity were recorded.

To create an objective evaluation and reduce the volume of 1 9 600 measurement points per image to a few sets

of critical data. SteinmL~ller and the University of Bochum developed a number of computer programs which allow selective access of images and individual locations within the image. To achieve this, the infrared image was subdivided into areas corresponding to the individual grate zones.

The research has been able to provide information about the location of the main combustion zone (ie which grate). This information can be used to change parameters such as:

• the volume f low and the distribution of the primary air to the separate zones;

• the supply of waste; • the transport velocity in the

separate zones.

The results of these tests wil l prove useful in finding the optimum combustion process which wil l reduce the inhomogeneous composition of refuse to an acceptable number of pollutants.

Agema Infrared Systems Ltd, Arden House, West Street, Leighton Buzzard, Beds, LU7 7DD, UK

NDT&E International 1994 Volume 27, Number 6 347