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Aviation History
1942
1942 - 0552.PDF
226 FLIGHT MARCH 12TH, 1942 NTER-COOLER DESIGN diate cooling, which approximates to isothermal compression with the lowest power requirement. In order to explain these questions clearly, the power requirement of a supercharger, i.e., had and !«*, will be compared for the two cases by means of an example. For an engine of ap proximately 1,000 h.p. the super charger air at about 6,000 m. altitude will be cooled by means of an external temperature of —24 deg. C. from 140 deg. C. to 90 deg. C. in a final cooler, the quantity of heat to be carried away being 40,000 kcal/hr. For the intermediate cooler, the corresponding temperatures will be 50 deg. and o deg. respectively. If the power require ment of the supercharger is calculated in each case, then for the interme diate cooler there is a power economy for the supercharger amounting to about 18 h.p., the efficiency of the supercharger being taken as 75 per cent. In this case an end cooler was taken as having a frontal surface of 6 dm2 and a depth of 450 mm. at a weight of 27 kg. A rough calculation gives, for the low velocity of 70 m/sec. dealt with in this case, a power requirement of Lk = i3 h.p., whereby in the calculation the additional power requirement, L/, due to the extra wing surface, the increased surface drag and the increased surface weight have not been taken into account. The inter mediate cooler in the same case gives Li-=24.3 h.p., for a frontal surface of 14 dm2 and a weight of 70 kg. The weights are total weights in cluding the fixture, and therefore not the weights of the pure block systems. In calculating the power, a cB-value of 0.3 was taken for the cooler; this value had been obtained previously from measurements carried out at Aachen with a cowled radiator in stalled below the engine. The pro peller efficiency was taken to be 70 per cent. Therefore, for the end cooler, in this case there would be an economy of 10.7 h.p. in the power re quired to tow the cooler, Li, by com parison with that for the intermediate cooler, but there is an increased ex penditure of power to drive the super charger, LL, of 18 h.p. In this case, in spite of the larger frontal surface, the preference would be given to the intermediate cooler. However, at a higher velocity of flight the individual values can alter greatly with respect to one another. The small velocities of flight which have been used here have arisen be cause the air coolers which have been previously designed by my firm were determined at this velocity. At higher velocities the end cooler will probably possess the smallest power require- 4 6 8 lOdn* IZ FRONTAL AREA Fig. 5. Power requirements of super charger coolers. Fig. 6. This type of cooler was found to give high values of pressure loss. ment; however, in any case, a subse quent calculation is necessary since the supercharger power also arises. Al though from the points of view of power the intermediate cooler should not prove necessary for high-velocity machines, it will certainly prove neces sary for high altitudes above 12 km. Cooler for High Altitudes With regard to coolers for high alti tudes, details have already been given by Sabel at the conference on coolers at Stuttgart; according to him the frontal surfaces for high altitudes are still relatively small. Owing to the bad heat transfer conditions prevalent at great heights the cooler would have to be*increased, but this is partly counterbalanced by the low tempera ture. Since it is required to have a constant outlet temperature from the cooler, the quantity of heat to be led away is increased, which results in an increase in the frontal surface of the cooler. The values given by Sabel for a 600 h.p. engine at 8,000 m. alti tude were about 8 dm2 for end coolers ; at 12,000 m. altitude, 10 dm2 for end coolers and 5 dm2 for intermediate coolers. Materials The weight of modern supercharger- air coolers lies almost exactly at 1 kg. per dm" of cooler-block. Since these copper coolers still have a relatively high weight, under certain circum stances a light-metal construction will represent an improvement from the point of view of economy in weight. Since the surfaces can hardly be joined by soldering for reason of corrosion, it will be necessary to weld the joints, but only if a strong material is used. It is then questionable as to whether a great advantage in weight will re sult, particularly since present-day coolers are already constructed with the smallest possible wall-thicknesses of 0.07 mm. The following table gives a comparison of the weights of a copper and a light-metal cooler: — Weight of Block ... Weight of Frame ... Weight of Tin Copper Cooler—kg. dm" of Cooier Block 0.6 0.2 0.2 1.0 Ligh'-Metal Cooler—kg. do- of Cooler Block 0.5 0.2 0.7 This is reckoning that the wall- thickness of the light-metal cooler is twice that of the copper cooler. - An Actual Cooler I should now like to speak briefly about the supercharger cooler which has already been constructed by the Suddeutschen Kiihlerfabrik J. F. Behr, Stuttgart-Feuerbach. The coolers shown in Fig. 6 were constructed for a quantity of air of 1.2 kg/sec, to be cooled from 140 de grees to 90 degrees at an altitude of 6,000 m. Each cooler has a frontal surface of 2.6 dm2. Since for certain reasons the test flights were first of all made at low altitudes, the cooling ob tained was from 105 to 85 degrees at an external temperature of 25 degrees C. Since the frontal surfaces were kept very small, when used as cross current coolers with a relatively un favourable supply of blower air, they gave high values for the pressure loss, while from the point of view of cool ing efficiency they were sufficient at the corresponding altitudes. In this case it would be possible, for example, to increase the frontal surface, at the expense of the resistance power, in , order to reduce the pressure loss. The cooler shown in Fig. 7 was built** as a substitute for this cooler and gives values for the pressure loss which lie within the permissible limits of the requirements laid down by the firm. (Continued at foot, of next page.) Fig. 7. An improved type of cooler in which pressure loss was kept within permissible limits.
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