GB697288A - Improvements in or relating to apparatus for finding the dryness fraction of a fluid - Google Patents

Abstract

697,288. Electric control systems. BABCOCK & WILCOX, Ltd. April 24, 1951 [April 24, 1950], No. 10079/50. Class 40 (i) [Also in Group XX] The dryness fraction of a stream of fluid is renewed by imparting heat to the stream at a rate sufficient to produce a dry saturated state and determining the rate of fluid flow, the rate of heat input to the stream and the pressure or temperature of the fluid. In finding the dryness fraction of steam flowing through a pipe the end of a duct, which may be formed as a Pitot tube, is adjusted transversely with respect to the flow path so that a sample stream of steam, the quantity of which is controlled by a valve, is led through the duct, part of which is provided by a vertical silica tube 20, Fig. 2, coaxially arranged within a calorimeter 21. The steam entering through the central nozzle 29 is heated by an electric heater 7, comprising a helix of platinum wire, the lead to the upper end thereof running vertically within a small diameter silica tube 33, each lead being connected to a bolt 34. A silica tube 37 projects through the upper end wall 26, its upper portion passing through stuffing box 39 and is held against axial thrust by a plug 40 screwed into an extension 41 of the stuffing box. The rounded lower end of rod 37 has the ends of the platinum wires 42 flush therewith, the surface being given a fine ground finish and immersed in concentrated sulphuric acid to ensure that a water film formed thereon will maintain a conductive path between the wires. The steam after passing the heater 7 and rod 37, which constitutes an electrical detecting element 11, passes through the metal sleeve 43 projecting downwardly from the end wall 26, and leaves the calorimeter 21 by the outlet nozzle 24. The flow rate is measured by means of two pressure gauges arranged on either side of an orifice in the stream. The power supplied to heater 7 is adjusted to maintain the steam dry saturated, this condition being detected by constancy of the conductivity of element 11. To effect this automatically the grid voltage of a thyratron 52, Fig. 3, is regulated in phase by a phase control circuit controlled by the conductivity of element 11. A condenser 54 is connected in series with element 11 across the secondary 51 of a transformer 50, the junction of condenser 54 and element 11 being connected through safety resistance 55, and coupling condenser 57 to the thyratron grid to which latter a leak 58 is connected. The heater 7 is connected in the thyratron anode circuit so that the power supplied thereto, as measured by wattmeter 10, varies according to the time in each alternate halfcycle when the thyratron fires, which in turn depends on the conductivity of the element 11. In determining the dryness fraction of partially condensed vapour in another fluid such as water vapour in air, the flow rate of vapour is found from readings of the pressure gauges provided the vapour temperature is known so as to determine the partial pressure of the vapour. According to the Provisional Specification a duct is mounted through contractile means on a flanged nozzle projecting sidewardly from a steam pipe to enable the inlet to the duct to be moved thereacross. The duct is supported within a jacket, a part of which telescopically engages a tube secured to the flanged nozzle and provided with gland means for preventing escape of steam between the jacket and the tube, an arrangement of threaded spindles and nuts allowing sliding movement of the jacket within the tube. At its opposite end the duct is double walled, forming an inner part of smaller diameter and an outer part of larger diameter, the inner part containing the electric heater and terminating within the outer part in a sidewardly directed part immediately opposite the electric detecting element, a portion of which, including the current path between electrodes, which may be across a surface of heat resistant glass, being within the outer part of the duct. A reverse flow path within the outer part of the duct leads to an outlet branch of the duct which outside the jacket has a valve for controlling flow, a pressure measuring means and an orifice or venturi nozzle for determining rate of flow. The jacket, which carries a stream of the vapour, is of tubular form open at one end, through which the duct projects, closed at the other and provided with a valved outlet, the closed end being of increased diameter and provided for the passage therethrough of the electrical detecting elements, heater conductors, and the outlet branch of the duct.