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Aviation History
1943
1943 - 1173.PDF
MAY 6TH, 1943 FLIGHT 475 Fig. 5. Section of a Holmes heater with connections for twinmotive units and an auxiliary turbo-generator. pansion range ; the vapour is condensed back to a liquid and the cycle is continuously repeated. A diagrammatic layout of such a thermal plant in an air- craft is shown in Fig. 4. The liquid under pressure is forced along pipe A to the heater B, where the tempera- ture is raised progressively in outer coil C and inner coil D. Pipe E conveys the heated vapour to the throttle valve F controlling admission to the turbine G, where it is expanded to perform useful work. The turbine drives the airscrew through a reduction gear H enclosed in a casing J. On leaving the turbine, the vapour passes through the annular space K around the turbine which .houses a regenerator coil L and enters the condensing chamber M. Here it condenses on the surface of the boundary walls and drains to the lowest point at N. From here the liquid is drawn off by pump P, gear-driven from the airscrew shaft, and forced through the regenerator coil to absorb heat prior to recirculation. fhe heater is shown in more detail in Fig. 5 with connec- tions for a twin-screw installation and a small supplemen- tary turbo-generator set for auxiliary power. Outer and inner coils C and D are of seamless Monel metal tubing, and although only single coils are shown, multiple coils arranged i" parallel may be employed. Heat is supplied by a fuel oil burner Q of conventional type, comprising an electric motor-driven oil pump and air blower and an oil throttle valve. The inner tube coil is wound on at( >re R of copper to prevent any part ol the tube becoming overheated <~o a degree likely to endanger the chemical stability of the working medium. Return pumps P on the turbinel "u'ts deliver the condensed liquid to a receiver S where it absorbs heat from the escaping flue gases. An elec- trically driven pump T, equipped witha pressure relief valve and by-passC11 "cuit to maintain a constant pres- sure, draws the liquid from the receiver and delivers it through the outer coil C and then the inner coil D. The hot combustion gases from the burner travel in a counter direction to the liquid, being directed by a baffle below the receiver and a cylindrical screen between inner and outer coils. Pump T raises the liquid to a pressure of. say, 800 1b./sq. in., that is, above the critical pressure. Consequently, as it travels through the coils of the heater, it can pass from the liquid to the vapour state without boiling as soon as the critical temperature is reached. To maintain the outgoing vapour at a constant temperature, the valve regulating the supply of oil to the burner 's automatically controlled by a thermostatic device compris- ing a bi-metallic strip U embedded in the copper core jf the heater. Wing-mounted Turbine Unit A propulsion unit for wing installation is shown in Fig. 6. The turbine G is'arranged above the airscrew shaft, which it drives through reduction gears H. Vapour from the common heater mounted in the fuselage reaches the turbine by pipe E through valve F, which is under the control of the operator for regulating the power output. After expansion through the turbine, the vapour enters the cen- tral chamber of the regenerator, reverses direction into the annular chamber, and discharges into the condensing chamber M. The outlets from collecting sumps N, pro- vided at the extremities of the lateral extensions of cham- ber M, are controlled by valves V, which are each opened by a float rising on the condensed liquid. This arrange- ment ensures that only liquid is drawn off from the con- densing chamber when the aircraft is being manoeuvred in flight. Pump P, driven from an extension of the air- screw shaft, passes the liquid successively through outer and inner coils, L and L,, of the regenerator and delivers it by pipe A to the receiving tank of the heater unit. The operation of the system can best be understood by following the working medium around the complete circle in the specific example put forward by the sponsor of the scheme. Let us assume that F-11 is employed ; that liquid pressure in the heater is 800 lb./sq. in. ; that heater tem- perature is slightly in excess of 371 deg. C. ; that the air- craft is travelling at a high altitude, and that the ambient temperature will ensure a condensing temperature of -17.7 deg. C. References to temperature are given in deg. C, to pressure in lb/sq. in. absolute, and to heat content in British Thermal Units. One pound of the F-11 liquid in the condenser, having a volume of 0.0192 cu. ft., has temperature —17.7,. pres- sure 2.55, and heat content 7.9. Sump pump P raises the pressure of the liquid and delivers it to the regenerator, where it absorbs heat and is then passed to the receiving tank of the heater at temperature 32.6, pressure 20, and heat content 26.4. Pump T then increases the pressure to 800 and, as a result of the wrork expended, the liquid enters the outer coil C of the heater at temperature 37.7, pressure 800, and heat content 28, an addition of 1.6 B.T.U. In the heater the liquid vaporises as the temperature Fig. 6. Wing-mounted turbine andregenerator units for the Holmes system.
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