Document wqKg8jMvq8e24dwj0mn7gwk14

800 T. F. HATCH C = 2V200(95 -- 95) = 0 = 0% total heat load M = 800 B.t.u./hr. Er,,,. = 1040 B.t.u./hr. Emal. = 10.3 (200) 4 (42 - 28) = 1100 B.t.u./hr. Heat stress index = Eroq./Emol. X 100 = 95 Thermal balance will be maintained in this environment. The exposure is jus? acceptable for a young acclimatized man. In practical mining operations, ho^j ever, the stress should be lowered. This is a warm-moist environment in whicl evaporative cooling capacity is limited. An increase in the wet-bulb temperatufi to 89 F. would reduce Eroox. to 700 B.t.u./hr., thus making the environmenti wholig unacceptable even to young men in the best physical condition. Example S. The following measurements were made on a hot summer in a glass factory: tg = 123 F.; tn = 103 F.; twl, = 80 F.; V ='300 f.p.m* outside air temperature = 87. F.; calculated tw = 162 F.; M assumed 960 B.t.u./hr. Determine: (a) magnitude of stress and (b) magnitude of change required": each of the environmental factors to establish thermal balance. Solution: R = 22(162 - 95) = 1480 B.t.u./hr. = 54% total heat load C,= 2^300(103 -- 95) = 280 B.t.u./hr. = 10% total heat load M = 960 EreQ. = 2720 B.t.u./hr. Emax. -- 103 (300) ^(42 --^'20)-=1930 B:t.u./hr. .......... Heat stress index = 140 (excessive heat exposure) EreQ. must-be reduced by 790 B.t.u./hr. to attain thermal balance. (7) Radiant heat has to be reduced as follows: (1480 -- 790) = 22 (t,, -- 95); =127 F. The mean radiant temperature, tw, must be reduced by 35 F. (2) To obtain some improvement by reducing the air temperature, the vective load must be reduced to: (280 -- 790) = --510 B.t.u./hr. Entering , value of C in the equation for convection exchange: _a=510..=3.v^300.(.ta_^..95X;.iaJ=.-8Q--.F... A reduction of 23 F. is therefore required in air temperature. (S) Air velocity might be increased to bring about thermal balance, indie by the following equations: ^E"'^SW+nTgCTT``2VFTT03-"95I Emal. = 10.3 F0 < (42 - 20) Equating Ereq. and E,n,,. and solving for V, we get V = 900 f.p.m. Increasi: _ velocity to 900 f.p.m. raises C to 480 B.t.u./hr. and Er,. goes up to 2920 B.t.u., HEAT CONTROL IN THE HOT INDUSTRIES 801 -?Heat stress index = 2920/2400 X 100 = 120, which is excessive, even for a fit -5young man. This illustrates the fallacy in using "man-cooling" fans to combat ..extreme heat exposures. (4) The reduction in humidity necessary to establish balance would be: Ereq. = Enal. = 2720 = 10,3 (300) O " (42 - VP,,); VP0 = 10 mm. This corresponds to = 70 F. and RH (relative humidity) = 20%, compared with original -value of twi = 80 F. and RH = 37%. Since Ereq. exceeds 2400 |.:;I3.t.u./hr., this is not a good solution because of the excessive sweating needed to ^secure thermal balance. The calculations in Example 3 are of practical interest because they reveal l^the nature and magnitude of the corrective problem. The logical first step in T control is obviously to reduce the radiant heat by more than 50 per cent, which has been shown to be easily attainable (see Table 3). Cooling the air is not economically feasible. Increasing the air velocity to 900 f.p.m, appears to be an easy solution. It is not an acceptable solution, however, sinoe thermal balance can be attained only with an actual increase in heat load and a demand for excessive sweating.'Reducing the humidity would be costly and also improper physiologi cally since Ere,. is excessive. IV. Heat Control Heat escaping into the environment from hot processes should be thought of ra-contanrinantp'like'duBfarva'pory-OT'd'm'easuresTorThe'control of heat fall into same classes as do the different procedures for control of other atmospheric contaminants. A. CONTROL AT SOURCE In the application of local control measures, distinctionmust be made between !,,the heat losses from hot bodies by radiation and those bylcbriyectiqn, Since radia'lotion varies with the fourth power of absolute temperature,, it is . clear that the larger share of heat loss from furnaces with high surfaceit'emperatures will be in the form of radiant heat. With a surface temperature..of,, say.,.-500 F., the Eradiation coefficient has a value of Kr = 3.2 B.t.u./ft.2/hr./F., compai:ed with Kc ^^:6rH'en'<re/67_pe|7fff^c(K'H%'ilre^ibs^i,lTbe''b'y-ra'di'a;tibtrr*'0n-ly:``when-the-hot Ifb'bdy is located in a strong forced' air stream will the convection exchange equal or exceed the radiation output. - li'-Ihsulation..... - ...... Insulation of hot surfaces prevents the escape of sensible,,and.radiant heat to ithe environment. Similarly, enclosing hot ,water tanks, coverifig-dfains carrying Iphot water, and continuous maintenance of joints and valves im piping to prevent |Ioss of steam constitute direct measures for.the control of latent.heat^n.yfarna-