Armored Medical Research Laboratory Fort Knox, Kentucky PROJECT NO. 2 - OPERATIONS AT HIGH TEMPERATURES First Partial Report On Sub-Project No. 2-24, Study of Methods of Reducing the Heat Load in Tanks Project No. 2-24 11 May 1943 ARMORED FORCE MEDICAL RESEARCH LABORATORY Fort Knox, Kentucky Project.NOo 2-24 UI06.GNOML 11 May 1943 lo PROJECT: N00 2 - Operations at High Temperatures0 First partial report on: Sub-Project No, 2-24 - Study of Methods of Re- ducing the Heat Load in Tanks0 a„ Authority - Letter Commanding General, Headquarters Armored Force, Fort Knox, Kentucky, 400o112/6 GNOHD, dated September 24, 19420 bo Purpose - To investigate the value of heat-reflecting paint on the exterior surfaces of tanks as a means of reducing the solar heat loado 20 DISCUSSION: ac General - (1) Absorption of solar heat by a tank raises the inside air and wall temperatures appreciably and thus increases the heat load to which the crew is exposed,, (2) The degree to which the air temperature within the tank increases depends upon several factors, including the following; (a) Percentage of solar energy absorbed by the vehicle0 a (b) Rate of heat transmission through the tank walls,, » i (c) Rate of ventilatione (3) These factors are subject to some control; the rate of heat transfer through the walls can be reduced by insulation; the rate of heat removal can be increased by increasing the ventilation and the per- centage of solar radiation which is absorbed can be reduced by proper selection of paint for the exterior surface of the vehicle,. 3* CONCLUSIONS; a<, Solar radiation constitutes an important source of heat in tanks and other closed vehicles when exposed to the sun„ b6 The amount of heat absorbed is sufficient to increase the air temperature 20°F or more above outside temperature within a buttoned-up tank with minimum ventilation (engine idling)«, - 1 - £o The standard O.D0 paint now employed absorbs approximately 90% of the solar radiation received,, do Absorption of solar heat can be reduced by painting the ex- terior surfaces with an infra-red reflecting paint, which will approximate present paint so far as color and visibility to the eye are concerned but which, at the same time, will reflect a high percentage of solar energy in the infra-red region* e, Even greater reduction in solar heat absorption can be secured by using a paint with the highest permissible reflection in the zone of visible light, as well as maximum infra-red reflectance0 4o RECOMMENDATION: That a paint for exterior surfaces of tanks be developed to give the maximum possible reflection of solar energy, consistent with the require ments for concealment0 Submitted by: Lt. Col* Theodore F* Hatch, Sn C 1st Lto Robert H. Valpole, Sn C APPROVED WILLARD MACHLE Colonel, Medical Corps Commanding 6 Inclosures #1 - Appendix with tables 1 thru 3 #2 - Fig. 1 #3 - Fig. 2 #4 - Fig. 3 #5 - Fig. 4 §6 - Fig. 5 APPENDIX SOLAR HEAT ABSORPTION BY TANKS Radiant energy from the sun varies in wave length from the ultra-violet through the region of visible light into the infra-red zoneQ All of the solar energy absorbed by a receiving body, however, is converted to heat0 On a clear day in the tropics with the sun directly overhead the solar heat received at sea level reaches a maximum of 332 Btu per hour per sq. ft® of surface normal to the sun® The intensity of radiation varies with the distance of atmospheric penetration and therefore with elevation of the sun and with altitude0 The effect of the latter is to add approximately 10 Btu per sq. ft. per hour to the sea level value for every 1000 fW elevation® The variation with height of the sun is in direct proportion to the sine of the angle of elevation® In round numbers, the maximum daily intensity of solar radiation may be taken as 300 Btu per sq® ft. per hour® Lesser values will be found on cloudy or hazy days0 The diurnal variation for a midsummer day in the latitude of Washington is shown in Fig® I0 Total energy received by a horizontal surface during the entire day amounts to approximately 2200 Btu per sq0. ft0 of horizontal surface. Thus, the heat received by a medium tank would be sufficient, if none were lost, to raise the temperature of the entire vehicle more than 40°F® At midday, the solar heat received by the shadow area of the crew compartmeht* amounts to about 24,000 fitu per hour® Assuming a rate of ventilation through the tank of 500 cfm and also that one- half of the absorbed heat is transmitted to the inside, the temperature rise of the air flowing through the tank would be 20°F® Some actual tank temperatures are given in Table 1® It will be noted that the observed values for buttoned-up tanks, with the engine idling, are in close agreement with the foregoing calculated temperature rise. One also notes a marked increase in wall temperatures with corresponding increase in the radiation heat load inside the tank® Accompanying the rise in temperature within the tank there is an increase in the moisture content of the air from the evaporation of sweat, which adds to the discomfort® In Egypt the increase in water vapor in tne air of a fully ioanned tank was found to average 40 grains per pound of dry air. This corresponds approximately to an hourly water loss per man of 1 liter, which is in agreement with the findings on water loss in extreme desert heat® The combined effect of these factors (increase in air and wail temperatures and moisture content) is to raise the effective temperature** to undesirable levels® In Egypt, effective temperatures * Assuming 80 sq0 ftc of shadow area and intensity of solar radiation of 300 Btu per hour per sqc ft„ Effective temperature is an arbitrary scale for comparing various combinations of temperature, humidity and air movement in relation to man’s relative sensar- tion of warmtho An EoT0 of 90° is equivalent to a still saturated atmosphere at 90°Fo - 1 - Table 1 Turret Temperatures in Buttoned-up Tanks Exposed to Sun (Headlined, Engine Idling and Driving) Place Outside Air Temperature i Model lank Operation Excess Turret Air Temperature Inside Turret Wall Temperature Remarks Ft0 Knox 85° i M3 Medo Deadlined 10-18° 113-130° 5 hrs, exp0 on mode clear day Fto Knox 92-98° M3 Medo Deadlined 7-27° 128-141° 5 hrso expo on cloudless day Camp Young, Calfornia 104° (Max) M3 Medo Deadlined• 25° 119° Max, cloudless day Tt„ Knox 85° M3 Med# Eng, Idling 2-25° 102-122° 3 hrs, exp0 on mod. clear day Egypt 88° M3 light Eng, Idling 32°* —* — Ft. Knox 85° M3 Medo Driving Aver, 6° 85-97° 100 miles driv, mode clear day Camp Young, California ' 110° M4A1 Driving 5-13° 130P cloudless day Egypt 87-103° Various Driving Aver0 10°* — — * Temperatures taken at the various crew positions. Table 1 above 90° wereorecorded and for moist climates such as in Rangoon, values as high as 100 E,T0 have been predicted,. This is equivalent to a saturated atmosphere at 100°F, which is intolerable0 The effect of increasing ventilation with high engine speeds and the beneficial influence of motion of the tank are also seen in the data presented in Table 1, The action of the latter is to increase the outward loss of heat absorbed by the tank walls in proportion to £he amount transmitted to the in- side; this, together with the greater air.flow through the tank results in a lower internal temperature rise, In contrast to a 20° rise in a stationary tank with engine idling, an increase of only 5° is experienced under these more favorable conditions0 For a gas-proof tank, the present indications are that a considerably lower rate of ventilation will have to be provided than is now suppliedo This situation, together with the fact that tanks must, at times, remain stationary in the open makes it desirable, in the interest of crew efficiency, to reduce the amount of solar heat absorbed by tanks0 Among the various , means for accomplishing this is to paint the exterior surface of the tank so as to provide the maximum possible reflection of the sun’s rays which strike it0 Thus, a paint having an overall reflection factor of 50$ would reduce the amount of heat absorbed to one-half of that taken up by a com- pletely black surface, with comparable improvement in interior temperatures« The 0»Do tank paint now employed has been found to be very little better than black paint in this respect. Compare, for example, the temperature- rise curves in Fig, 2, Curve A show's the temperature rise of a black test panel when exposed to a constant source of radiant energy. Curve B is for a similar panel painted with the standard 0,Do paint0 The final temperature f reached by the latter was only 10$ below that of the black plate. In strik- / ing contrast, a plate painted with titanima oxide white and exposed under exactly the same conditions exhibited a temperature rise of only 33$ of the black platte (Curve C)o The difference was due to the high coefficient of reflection of the white surface,. In the same way, an olive drab paint having high reflectance in v the infra-red was found to reduce heat absorption about 2/3 (Curve D)0 The source of radiation in these tests was a tungsten lamp which was relatively richer in infra-red than the sunG Hence, the results with the IH olive drab paint are more favorable than would be obtained under solar radiation. The comparative benefits to be obtained by various types and colors of paints are also shown in Table 2, which gives the equilibrium temperatures for a series of cans of gasoline painted in different colors and exposed to sun- light in Florida, One notes a marked difference between the temperatures with- in the can painted flat white and the one painted medium gray. Other colors / gave intermediate readings. The lower temperature obtained with flat white / paint resulted from its high-reflectance. This very feature, however, makes white an unacceptable color for a tank because of its high visibility. To be acceptable, the paint used must possess a sufficiently high reflectance of total solar energy to give useful results without exceeding the maximum per- missible reflectance of light in the visible region. Paints designed with this 2 Table 2 Effect of Paint Reflectance Upon the Temperature of Gasoline in Cans Exposed to the Sun* Paint Gasoline Temperature °F Flat White 82 Gloss 'White 84 Aluminum 87 Chrome Yellow 90 Lab0 Olive I0R« 95 Chrome Oxide No. 1 96 Cora. Brown I0R0 96 Com0 Green I.R. 96 Com. Green I0Ro 97 Com. Gray IcRa 98 Med0 Gray 99 * Infra-red Reflectance versus Evaporation, H0 Ao Gardner, Circular No. 636, Sci0 Sec0, Nat'l, Paint, Varnish and Lacquer Assn., Inc0, Washington, Mar., 1942, Table 2 objective in view are now available0 They are formulated to meet the necessary specifications as to color brightness in the zone of visible light but at the ' same tine to have the highest possible coefficient of reflection throughout the region of* invisible infra-red rays. They are generally termed IH or infray paints a In the development of these paints advantage is taken of the fact that a considerable proportion of the energy received froia the sun lies beyond the invisible region, i.e, wave lengths longer than 097&jKo The degree of reflection or absorption of this invisible energy does not affect the ap- pearance of the painted surface in visible light and from the standpoint of camouflage it is said to be desirable to reflect a high proportion of the infra-red energy in order more nearly to simulate the background of vegetation„ The spectral distribution of solar energy as received at sea level on an average midsummer day at Washington, D, C., is shown in Fig. Maximum intensity of radiation occurs in the visible region. It rises very sharply from the minute percentage in the ultra violet and decreases more gradually in the infra-red. The distribution of energy between the visible and infra- red is, according to this curve, in the ratio of 55:45; that is, rather more than half of the total lie? within the region of visible light0 In Fig. 4, the reflectance curve for a high I0Ro reflecting paint has been plotted in conjunction with the approximate solar energy distribution curveQ The products of the reflectance and the corresponding solar energy values for various wave lengths are also plotted in the Figure., The area under this curve (cross-hatched) represents the total amount of’ solar energy reflected by the paint while the area between the Rs and I curves represents the amount absorbed, For the paint in question, the heat absorption, as compared with that taken up by a perfect black surface, is only 56$, A reduction in solar heat gain of the order exhibited by the IR paint in Fig. 4 will result in a definite improvement in tank tamperatures, as shown in Fig. 5o The three sets of temperatures were obtained under the same con- dition with respect to outside air temperature and wind velocity. The radiant heat load differed in the tests, however, by factors of 1„0, 0o8, and 0o6; that is, in test 2 the amount of radiant heat absorbed was reduced 2C$ as compared with test 1 while in test 3 the reduction was 4Ofr* Thei?was a relative improvement in turret temperatures after 1000 hours, resulting in maximum temperatures 9°F and 18°F lower in the two tests with reduced radiant heat than when the tank was exposed to the full radiation load, These were laboratory tests in which the source of radiant energy was directly controlled.. They do not represent actual results obtained with heat-reflecting paints but were conducted to show the magnitude of the benefit to be derived from paints giving the same reduction in solar, radiation. The results are in close agree- ment with the comparative findings in two test chambers exposed to sunlight, * Measurement and application of visible and near-visible ftadiation, F. S0 Brackett, Biological Effects of Radiation, McGraw-Hill Book Co., Inc0, New York. 3 the roof of one being coated with an IR paint somewhat similar to that shown in Fig, 4 and the other with an ordinary paint of the same color and visibility. The effect of the HI paint was to reduce the total accumulation of heat during a day by approximately 33$. The difference in maximum temperatures in the two chambers was 12°F, The characteristic feature of an IR paint is that it has relatively high reflectance in the infra-red region. Its in the zone of visible light is determined by requirements for concealments For low visibility, low reflectance in the visible region is essential. The IR paint shown in Fig, 4 possesses this characteristic, with a rapid rise in reflectivity with increasing wave length above 0,76yi. So long as low visibility must be maintained, the maximum percentage reduction in solar heat absorption which can be secured by a paint is limited since so large a proportion of the solar energy lies in the visible region— up to 60$. It is clear that the heat reflecting properties of a paint can be greatly improved if a higher reflectance is permitted in the region of visible light. The benefits to be gained thereby are shown in a striking manner by the following temperature records taken in gasoline storage tanks over a period of three (3) months. Table 3 Temperatures of Stored Gasoline in Relation to Reflectance in Visible and Infra-Red Zones^ Paint Aver* Excess Temp*. over Air Temp.°F White 1,0 Aluminum 6.7 I.R. Olive Drab, Specification #9 9.0 StcU Clive Drab, Specification #9 13.7 The use of the I.R0 olive drab paint reduced the excess temperature to a significant degree as compared with the standard 0,D, but a much greater effect v/as noted with the aluminum and white paints which also had high re- flectance in the visible region. It is not suggested that white paint be employed on tanks. It is recommended, however, that, in the selection of paint for tanks, consideration be given to the heat-reflectihg properties as well as the camouflage requirements and that, so far as possible, max- imum use be made of reflection in the zone of visible as well as the infra- red radiation. It is not within the scope of this report to consider camou- flage requirements but the question is raised as to the actual need for a paint of extremely low visible light reflection. To what extent can visible reflection be increased without destroying essential hiding j5ower, especially in arid open country? * Courtesy Capt, l/etz, Ord. Dept, 4 Fig. I SOLAR RADIATION RECEIVED ON A HORIZONTAL SURFACE DURING AVERAGE MIDSUMMER DAY, WASHINGTON, D. C. SOLAR RADIATION, B.T.U. PER SO. FT PER HR. (SCALE APPROXIMATE) SUN TIME - HOURS Fig. I Fig. 2 EFFECT OF PAINT UPON TEMPERATURE RISE OF TEST PLATES EXPOSED TO CONSTANT RADIATION T - TEMP RISE R = REFLECTIVITY RELATIVE TEMPERATURE RISE AND REFLECTIVITY TIME -MINUTES Fig. 2 Fig. 3 SPECTRAL DISTRIBUTION OF SOLAR RADIATION AVERAGE MIDSUMMER DAY AT NOON, WASHINGTON, D. C MAX. DAILY SOLAR RADIATION BTU, PER SO FT, PER HR. (APPROX.) VISIBLE INFRA RED WAVE LENGTH-MICRONS Fig. 3 Fig. 4 DISTRIBUTION OF SOLAR ENERGY, I (O.U INCREMENTS) REFLECTIVITY OF AN I. R. PAINT, Rp, PERCENT SOLAR ENERGY REFLECTED (PRODUCT IxRp),Rs Q LU I— O LU -I iij o § < < cr Q X LU > cr LU £ 0 CL LU cr R- u_ B a 1 < * § a- o: P < R~ _i m o co TOTAL SOLAR ENERGY ABSORBED TOTAL SOLAR ENLhGY REFLECTED Q LU \- O LU _J U_ LU cr §