OCR Text |
Show Conf1rmation of the computer analys1s has been provided through practical observat1ons. In a 6eries of tests on production sensors a mean calibration figure of 4.91 V/K~.M2 was derived which compared w1th a theoretically estab11shed calibration of 4 . 82 V/K~/K2, Figure 5 shows the long term (2 month) averages of two Fluxdomes and one Fiuxtube arranged in the boller as shown in Figure 6. All three instruments are 4w1th1n 250 rom of each other and their average outputs are seen to glve excellent correlat1on , A number of other methods are available to measure heat flux and 1ndicate slag levels. These have various limitations as follows: 1. Indirect methods. Rely upon theoretical boiler models and general measurements at various po1nts throughout the bo1ler. Slagg1ng patterns however are random and unpred1ctable and in the absence of localised heat flux information indirect methods offer only a partial solution to the slagging problem. 2. Thermocouple Measurements of tube temperatures have been used to deduce the heat flux at selected locations <Figure 7> At best, the thermal circuits of such systems are ill defined rely1ng on commerc1al boiler components. The calibration of each arrangement depends not only on the posltion of the thermocouple in the tube wall but also on the tube dimensions, tube conductivity, the ash/slag distr1bution and internal boiling condltions. None of these are known with any great certainty and some vary with time. No lndependent calibration of the location 1s possible, and in general these methods provide m1slead1ng results. 3. Disc meter. This relles on the rad1al temperature gradient produced in a thin disc (Figure 8), ~hile this concept functions well under clean "blackM radiation conditions, it suffers intr1nsically from (unpredlctable) ca~ibration Changes under ash coatings. Th1s is Evident from the disc temperature distribution shown in Figure 9 . The tnermal resistance of a coatlng of asb present at the centre of the di~c has a ~uch greater effect on the overall radial temperature gradient than B simllar area of ash deposited near the periphery. |