1. Field of the Invention
This invention relates broadly to measuring temperatures. More particularly, this invention relates to a method and apparatus for measuring surface temperature of product moving through a spray cooling chamber. Reference will be made herein to measuring the surface temperature of strand moving through a spray cooling chamber in a continuous metal caster, but the invention may be used inside a spray chamber in other manufacturing processes.
2. Description of the Prior Art
A variety of continuous casting machines are used in the basic metals industry to continuously produce semifinished billets, slabs and other types of strands. In each type machine, freezing of molten metal occurs in a one-step solidification process. Successful caster operation in each case involves casting strands without cracks or defects while optimizing throughput. Overall caster operation is based on a number of key parameters, one of which is secondary cooling inside a spray cooling chamber of the caster. Therefore, it is important that good control of secondary cooling be achieved in order that a hot metal skin surrounding a molten core be properly solidified and cooled in this region of the caster.
Proper control of secondary cooling in the caster can only be made by determining the degree of strand cooling occurring inside the spray cooling chamber. Most caster operators would prefer to make this determination by actually measuring strand surface temperature at several locations inside the spray cooling chamber. However, after many attempts there is still no satisfactory method and/or apparatus for measuring strand surface temperature inside the spray cooling chamber of commercially available casting machines.
On numerous occasions, conventional radiation pyrometers have been used to measure strand surface temperature inside the spray cooling chamber. The results of such trials were too inaccurate and unreliable for caster operators to use. A major reason for such poor results is the difficulty of facilitating strand surface temperature measurements under severe environmental conditions constantly present in the spray chamber during caster operation.
The severe environment in the spray chamber is caused by a combination of factors such as liquid water, water mist, steam and foreign particles in cooling water as well as from mill scale flaked away from the metal surface being spray cooled. Each of these environmental factors cause a reduction in radiant energy to the temperature sensor used in the spray cooling chamber. Consequently, this produces corresponding erratic and inaccurate slab surface temperature measurements that vary unpredictably according to the effects of any one or a combination of the environmental factors.
For example, when using a radiation pyrometer that is responsive to only a single color, the measurement error is on the low side of normal. For two-color radiation pyrometers, the measurement error is usually on the high side. The latter instance is due to unequal energy losses of the different wavelengths of radiation passing through the severe environment inside the spray cooling chamber. Known prior art attempts to overcome these measurement difficulties has been limited to three areas. First, air blasts were used to physically blow liquid water, water mist and steam out of the pyrometer sight path. Second, two-color pyrometers were used to cancel out the effect of energy losses. Third, to a limited extent, light pipes and sighting tubes were used for the purpose of minimizing the length of path where optical interference occurs. Each of these three methods have met with only partial success for various reasons.