1098
INDUSTRIAL A N D ENGINEERING CHEMISTRY
u s m IS radiation alurie 01 iadiatiun plus conveLtion l'lit heating times computed for plates niay be appro\imatcli contrrted to heating times for bars bv dividing by 2. To help understand in x h a t region of tenipeiatuie- L U U ( ' i t x u l s over radiation, the time required to heat a 2-irich lhicA d a t e to nithin 400" of thc Euinace tempeiature has 1)e+i1( ( I ' I I puted for fuinacc temperatures iioiii 3000" to 1000" F. Figriit $1 At the highest tenipeiatuic (3000 F., the center Of tlLt plnrr ,s heated t o 2600" f , and at tht lone-t temperatuiL (1000 F I che center point is heated to 600" F. The heating t i r l , t * liaii 'een computed for convertion alone, iudiation alone, aid both nechanisins together. I t niay hr seen fioin Figure 8 that tlir onvection effect becomes mole and nioic important d" thr urnace tpmperaturr is reduced. At 3000" E'. the con\cctioIi w a t transfer reduces the heatlng time about I 1 minutes o x i t -adlation alone. K h e n the furnace temperature is 2000" L he time is reduced 5 5 minutes by convection and 42 miiiii[+ at 1000" F. It is also apparent that convective heat tinii-r8 I Decomes larger than radiative heat trnnqfer belon a t u i n ~ c t 'pmperature of 1350' F. Although the problem of heating a plate to a teriipeiatuit a00 O less than the furnace temperature has been arbitiaiilv >elected for illustrative purposes and further designed to make :he computations less laborious, the relations revealed are generally valid under most conditions pi actically encountered Alhough not calculated, i f the plates n ere heated to n ithin 50' or the furnace temperatwe, all the heating times nould be incieaaed bv roughly the same factor. Under the latter conditions the cemperature a t 13 hich convection approximates radiation R o d d h t slightly loizer.
Voi. 40. No. 6
,calls. The furnace \Talk are uniformly heated iri this nianner, a n d the combustion gases leaving the wall are a t the wall ternperature. The extremely high velocity rotary motion of the, gases gives an excellent convection cffect, for heat trnnsfer to tlic
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