.1 Summary

 Improvement on decreasing dust clog in ANTARES has been made by modifying the probe design and reducing the ejector flow rate. Dust clogging is suppressed by preventing temperature drop of piping, because it was caused by the low temperature at the tip of probe due to the ejector air. The tip of modified probe is cut into 45-degree angle instead of perpendicular angle, so that gas flow is naturally created at the tip of probe. It decreases the required flow rate of ejector air, and reduces the temperature drop of piping and dust clog.

　

.2 Concept

 In case the cut end of probe tip is perpendicular, negative pressure, -Pa, is generated at the probe tip (A) due to the exhaust gas flow, and negative pressure, -Pc, is also generated at the ejector outlet (C). The pressure is lower at A than at C (-Pa<-Pc<0) because flow at the center of stack is faster than that at the neighbor of stack wall. Thus, in case of without ejector air, a reverse flow from B to A will be created. Ejector air generates the negative pressure, -Pb, at B and creates normal sample gas flow from A to B. Here, flow rate of ejector air must be set large enough to make the pressure -Pb lower then -Pa (-Pb<-Pa<0). Flow rate of ejector air is determined according to the exhaust flow rate of stack, because the pressure -Pa and -Pc depends on the flow rates. For probe with perpendicular cut, flow rate of ejector is typically set as 9 L/min to the flow rate 20 L/min of stack, based on the past experiences.

　

 In case tip of probe is cut into 45-degree angle (cut surface is facing windward), positive pressure, +Pa', is generated when gas flows in the stack, and there is negative pressure, -Pc′, at C. Normal flow from A to B is created regardless of the flow rate of the ejector air, because the pressure is always +Pa >0>-Pc′. It is possible to decrease the flow rate of ejector as small as that can maintain equivalent flow rate to that of probe with perpendicular-angle cut (conventional cut) requires.

　

.3 Confirmation by Wind-tunnel Test

 For two kinds of probe shapes (45-degree cut and perpendicular cut), flow rate of ejector is determined by observing the presence of reverse flow, by varying the flow rate of ejector. As a stack model, flow speed of exhaust gas was set to 20 m/s and temperature of exhaust gas set to 200 degrees Celsius.

　

 In the experiment, presence of reserve flow is confirmed by monitoring the pressure difference of two points, i.e. calibration gas inlet at the sensor connection and purge air inlet, by varying the flow rate of ejector air in 3 L/min to 9 L/min range. The required flow rate can be determined according to the difference pressure, without confirming directly the flow rate inside the piping.