Fluid FlowNATURAL CIRCULATIONNATURAL CIRCULATIONNatural circulation is the circulation of fluid within piping systems oropen pools that is due to the density changes caused by temperaturedifferences. Natural circulation does not require any mechanicaldevices to maintain flow.EO 1.25 DEFINE natural circulation and forced circulation.EO 1.26 DEFINE thermal driving head.EO 1.27 DESCRIBE the conditions necessary for naturalcirculation to exist.EO 1.28 EXPLAIN the relationship between flow rate andtemperature difference in natural circulation flow.EO 1.29 DESCRIBE how the operator can determine whethernatural circulation exists in the reactor coolant systemand other heat removal systems.EO 1.30 DESCRIBE how to enhance natural circulation flow.ForcedandNaturalCirculationIn the previous chapters on fluid flow, it was explained that any time that fluid flows there issome friction associated with the movement, which will cause head loss. It was pointed out thatthis head loss is commonly compensated for in piping systems by pumps that do work on thefluid, compensating for the head loss due to friction. Circulation of fluid in systems by pumpsis referred to as forced circulation.It is possible to design some fluid systems in a manner that does not require the presence ofpumps to provide circulation. The head required to compensate for the head losses is created bydensity gradients and elevation changes. Flow that occurs under these circumstances is callednatural circulation.ThermalDrivingHeadThermal driving head is the force that causes natural circulation to take place. It is caused bythe difference in density between two bodies or areas of fluid.Rev. 0 Page 37 HT-03
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