Pyrometric Gas Temperature Measurement System

The present invention relates to a system for remotely measuring the average temperature of hot gases. In particular, the invention relates to hot gases in fired furnaces.

The flue gas temperature in the transition zone between radiant and convection section of a fired furnace (frequently called the Bridgewall temperature) is a relative measure of the heat release in the radiant section. In order to maintain a proper balance of heat release between radiant and convection section and also to protect the metal tubes in the convection section from overheating, one must monitor the flue gas temperature in the transition zone. Generally, this is done by thermocouples at the refractory wall near the transition zone. While this measurement gives some indication of gas temperature, a direct measurement of the flue gas temperature in the transition zone, averaged over the length of the furnace, is preferable.

The present invention is a system and a method for remotely measuring the temperature of hot gases. In a preferred embodiment, the hot gases are the flue gases in fired furnaces. The system includes a black body placed on one side of the furnace and a spectral pyrometer viewing the black body from the opposite wall across the hot flue gas. A means for heating the black body and measuring its temperature independent of the pyrometer are included in the system. The temperature of the black body is increased until the physical temperature of the black body as measured by the independent means is the same as the radiation temperature read by the pyrometer through the flue gas. At this point, the temperature of the gas is necessarily the same as the temperature measured by the pyrometer.

FIG. 1 illustrates a typical arrangement for this invention.

FIG. 2 illustrates the components of a typical system in detail.

FIG. 3 shows a plot of the spectral emissivity of flue gas from a pyrolysis furnace.

For the purposes of description, the present invention shall be illustrated and described with respect to the monitoring of the bridgewall temperature of fired furnaces (the temperature of the flue gas at the transition zone between radiant and convection section). The invention described here involves a pyrometer which reads the apparent temperature of a black body across the furnace, i.e., through the hot flue gas. The radiation received by the pyrometer consists of two components, the black body radiation, attenuated by absorption in the flue gas and radiative emission by the hot flue gas itself.

Gas absorptivity equals emissivity and the measured radiation temperature, T.sub.2, can be related to the physical temperature of the black body T.sub.1, the gas temperature T.sub.g and the gas emissivity (which equals absorptivity) .alpha., as

where h is Planck's constant, k is Boltzmann's constant and .nu. is the optical frequency of the spectral pyrometer. To obtain the gas temperature, T.sub.g, one has to adjust the physical temperature of the black body T.sub.1, until it equals the measured radiation temperature, T.sub.2. It then follows that if T.sub.1 =T.sub.2, then T.sub.1 is also equal to T.sub.g.

The equation above applies strictly to the case of uniform gas temperature and emissivity along the line connecting pyrometer and black body. In actual installations, the flue gas temperature varies across the furnace. In that case, the measured gas temperature represents a weighted average value. The gas temperature typically rises from some value near the refractory wall to a maximum in the center and falls off again toward the opposite wall. The weighted average that results from the above measurement deviates somewhat from the (calculated) unweighted average. This deviation can be negative or positive depending on the value of .alpha.. By selecting a proper wavelength band for the pyrometer, one can choose a value of .alpha. that minimizes the averaging error.

If the gas is optically very dense, (.alpha..about.1), the black body radiation is essentially blocked, and the measured temperature is that of a portion of the flue gas closest to the pyrometer. At the other extreme of a very thin optical density (.alpha.<<1) the radiative contribution of the flue gas is negligible and the observed black body radiation is essentially uneffected by the flue gas. The spectral emissivity of flue gas is dominated by the absorption bands of hot CO.sub.2 and H.sub.2 O vapor, and exhibits strong variations with wavelength. The operating wavelength for the spectral pyrometer therefore should be chosen according to the absorption spectra of hot CO.sub.2 and H.sub.2 O vapor to yield a flue gas emissivity within the range of 0.1 and 0.9 at the chosen wavelength (wave number). A preferred value is about 1/.sub.e =0.37. Relevant spectra are available for instance from NASA publication SP-3080.

FIG. 3 shows a plot of flue gas emissivity as a function of wave number for the particular example of a pyrolysis furnace. Such furnaces are used throughout the petroleum industry. Preferably the narrow spectral receiving band of the pyrometers should be substantially centered at any one of the wavelengths corresponding to the intersects of the emissivity curve 20 with the 1/.sub.e emissivity line 21 as indicated by the open circles 24. Lines 22 and 23 correspond to the 0.1 and 0.9 emissivity values representing the limits for the useful range of operating wavelength.

The black body as well as the spectral pyrometer can be constructed in a variety of different ways. FIG. 1 illustrates a typical installation using the present invention. A typical furnace 1 is shown whose flames 2 produce flue gases whose temperature is to be measured along a line 3 through the furnace. A black body 4 is viewed along line 3 through the flue gases by a spectral pyrometer 5. The furnace wall interior is a refractory material such as brick 10 bounded by a steel wall 19.

A preferred design for the black body 4 is a hollow conical or cylindrical structure made from heavily oxidized and rough-surfaced furnace steel. This device is fixed to the furnace wall 10. Being fixed to the interior furnace wall 10 allows the black body to be heated radiatively and convectively by the hot flue gas and conductively by the refractory wall up to a temperature to within about 100 degrees below the gas temperature. In many practical cases, the black body may even approach about 10.degree. C. below the gas temperature. By providing additional heating, say via electrical heater elements, one can bring the blackbody temperature up to that of the gas as required. The temperature of the black body is measured independently of the pyrometer by, for example, a thermocouple or a resistance thermometer.

FIG. 2 shows the components of a typical system in detail. The black body 4 consists of an inner shell of furnace steel 6 formed as a cone with cylindrical extension surrounded by a heating blanket, a Nichrome winding 7 embedded in heat-resistant cement 8. The black body is surrounded by a packing of kao-wool 9 for thermal insulation and placed into the refractory wall 10. The Nichrome windings 7 are connected to a controlled power supply 11. The spectral pyrometer 5 is positioned at the opposite furnace wall 12 viewing the black body through a hole 13 in the refractory wall 12 across the hot flue gas 14. The temperature of the black body 4, T.sub.1, is measured by thermocouples 15 which are welded to the inner shell 6. The radiation received by the spectral pyrometer 5 is converted via the Planck formula into an equivalent temperature T.sub.2. Digital signals proportional to T.sub.1 and T.sub.2 are generated in electronic units 16 and 17, respectively and sent to the digital comparator 18, which sends a control signal (proportional to the temperature difference (T.sub.2 -T.sub.1) to the power supply 11. As long as T.sub.1 <T.sub.2 the power supply 11 is activated such that current runs through the Nichrome windings 7 heating the black body 4 until T.sub.1 =T.sub.2. At that point T.sub.1 is also equal to the gas temperature, T.sub.g.