Forced-circulation, once-through steam generators of large capacity are constructed of several parallel circuits connected between inlet and discharge headers. Unequal flow distribution between these circuits, possibly resulting in tube failure, may be due to unequal frictional resistances, unequal heat absorption, or improper arrangements of circuits. The investigation which is reported in this paper was undertaken in order to determine the effect of unequal heat absorption, unequal circuit resistances, and the location of equalizers upon the flow distribution in the circuits of forced-circulation steam generators.
Pressure-drop calculations were based on the measurement of friction factors of water at temperatures between 57 and 694 F; also, of superheated steam at pressures between 1120 and 3578 lb per sq in abs with a maximum temperature of 806 F. Head-loss measurements were made with a special glass manometer for pressures up to 2200 lb per sq in., and for higher pressures by radiographs of the mercury level in a steel-tube manometer.
An analysis of flow distribution between parallel circuits leads to the following conclusions:
(1) Parallel circuits connected between the feedwater header and the superheater outlet header are unstable, the pressure drop being nearly independent of the flow rate. Consequently, slight variations in frictional resistance or heat-transfer rates may result in decreased flow and in overheated tubes.
(2) The flow in unstable circuits can be controlled by the use of an inlet resistance such as a section of small-diameter tube.
(3) The use of a common header or equalizer on each side of the evaporating zone will stabilize the flow in parallel circuits.
(4) Superheater and economizer circuits are more stable than evaporating-zone circuits.
(5) If only one equalizer is used, it should be placed at the end of the economizer section rather than at the end of the evaporating surface.
(6) The stability of circuits is independent of their capacity.
(7) The stability of superheater circuits is independent of pressure. Combined evaporating zone and superheater circuits are more stable at high pressures.
Curves are presented to show the extent to which the enthalpy of the steam and of the water delivered from parallel circuits will vary as a result of unequal circuit resistances and unequal heat absorption.