The use of L-valves or other non-mechanical means for controlling the flow rate of particulate material in circulating fluidized bed boilers is well known. However, when the particulate material reaches or exceeds a temperature of about 900 degrees, serious problems occur due to the thermal gradients involved. During periods of sporadic use, the material in the horizontal transport conduit will have had time to cool down. Then, when flow is again required, the new hot material entering the horizontal conduit causes extreme cases of longitudinal and circumferential temperature gradients to occur within the conduit. If frequently repeated, the conduit will fail due to buckling, twisting or tearing as a result of its expansion and contraction. Welds have been known to fail due to the tremendous forces involved that rapidly build-up. Temperature swings on the order of several hundred degrees over a period of only a few minutes have been recorded. Also, vastly different temperature readings oftentimes occur on opposite sides of the conduit. Such patterns of distortion will lead to failure of the conduit itself.
Attempts to insulate the horizontal conduit have been to no avail as this only protects the outer casing surrounding the conduit (if at all) and not the conduit itself which is in direct contact with the hot particulate material. Additionally, attempts to secure this conduit to its outer casing have also failed due to the buckling and/or required movement of the conduit under such conditions. Oftentimes, the temperature gradients in the conduit from top to bottom cause the conduit to bend into an arc or a `banana` shape thereby tearing itself loose inside the outer casing. As a result, the gas and solid matter being recirculated is no longer contained but instead begins to leak from the conduit and potentially can eventually escape from within the outer casing.
It is thus an object of this invention to provide a transport conduit that can accommodate extreme temperature gradients without releasing any contained material. Another object of this invention is to provide a transport conduit that can `flex` as needed while still remaining intact within an outer casing. Still another object of this invention is to provide a transport conduit having low thermal mass which allows for relatively rapid heat-up and cool down for more even temperature distribution. Yet another object of this invention is to provide a means that will buffer the conduit from the extreme temperature levels, thermal transients, and differentials that arise during the transport of the hot particulate material. These and other features of this invention will become obvious upon further investigation.