Heat Transfer                                                                                                                                                                                                         2/27/2018
                                                                             HEAHEATT TRANSFER TRANSFER
                                                                             In FOOD PROCESSINGIn FOOD PROCESSING
                                                                              Lecture Note                       
                                                                              ITP530
                                                                              Purwiyatno Hariyadi
                                                                              phariyadi.staff.ipb.ac.id
                                                                             Dept of Food Science & Technology
                                                                             Faculty of Agricultural Engineering & Technology
                                                                             Bogor Agricultural University
                                                                             BOGOR
                                                                             2018
                                                                phariyadi.staff.ipb.ac.id                  -- ITP530
                                                                     Heat Transfer
                                                                          ••    Heat Heat ttransfer -ransfer - mmovement of energyovement of energy  due to a  due to a  
                                                                                temperature temperature differencedifference
                                                                          ••    Can onlyCan only  occur if occur if aa temperature difference exists temperature difference exists
                                                                          ••    Occurs through:Occurs through:
                                                                                1.  conduction,1.  conduction,
                                                                                2.  convection, 2.  convection, aandnd
                                                                                3.  radiation, 3.  radiation, oor r 
                                                                                4.  combination 4.  combination of aboveof above
                                                                phariyadi.staff.ipb.ac.id                  -- ITP530
                                  phariyadi.staff.ipb.ac.id                                                                           --
                                  ITP530
                                                                                                                                                                                                                                                                                      1
                            Heat Transfer                                                                                                                                                                                                         2/27/2018
                                                                     Heat Transfer
                                                                           ••     MayMay  be indicated as total be indicated as total transfer transfer 
                                                                           ••     Identified Identified bbyy  total total heat flowheat flow  (Q) (Q) wwiith th units units oof Btuf Btu
                                                                           ••     Identified Identified bbyy  rate of heat flowrate of heat flow  (q) (q) or or Q/Q/t t wwiithth  units units 
                                                                                  of wof watts atts oot Btu/hrt Btu/hr
                                                                           ••     Also, mayAlso, may  be expressed be expressed as as heat transfer per unit heat transfer per unit 
                                                                                  area area = heat flux or q/A= heat flux or q/A
                                                                phariyadi.staff.ipb.ac.id                  -- ITP530
                                                                     Heat Transfer
                                                                    ••     Heat Heat ttransfer mayransfer may  be classified as:be classified as:
                                                                           1.  Steady1.  Steady--state:state:
                                                                                     o  all o  all factors are stabilized factors are stabilized wwitith h respect to timerespect to time
                                                                                     o  temperatures are o  temperatures are constant at all constant at all locationslocations
                                                                                     o  steadyo  steady--state state is sometimes assumed is sometimes assumed if if little little error error 
                                                                                                      resultsresults
                                                                           2. Unsteady2. Unsteady--state state (transient) (transient) heat transfer occurs wheat transfer occurs when:hen:
                                                                                     o  temperature changes wo  temperature changes wiithth  ttime ime             
                                                                                     o  thermal o  thermal processing of fprocessing of foods is an important oods is an important 
                                                                                                      exampleexample
                                                                                     o  must knowo  must know  ttime required ime required for the coldest spot in for the coldest spot in 
                                                                                                      can to reach set temperaturecan to reach set temperature
                                                                phariyadi.staff.ipb.ac.id                  -- ITP530
                                  phariyadi.staff.ipb.ac.id                                                                           --
                                  ITP530
                                                                                                                                                                                                                                                                                      2
                            Heat Transfer                                                                                                                                                                                                         2/27/2018
                                                                        CONDUCTION CONDUCTION HEAHEATT  TTRANSFERRANSFER
                                                                          ••    Occurs wOccurs when hen heat moves heat moves through through a material a material 
                                                                                (usually(usually solid  solid oor viscousr viscous liquid)  liquid) due to molecular due to molecular 
                                                                                action onlyaction only
                                                                                         HEAHEATT
                                                                              ••   Heat/energyHeat/energy  is trasfered at molecular levelis trasfered at molecular level
                                                                              ••   No phyNo physical sical mmovement of materialovement of material
                                                                              ••   Heating/cooling Heating/cooling of solidof solid
                                                                              ••   Heat Heat fflux lux is directlyis directly  pproportional roportional to the temperature to the temperature 
                                                                                   gradient, gradient, and inverselyand inversely  proportional proportional to to ddistance istance 
                                                                                   (thickness of material).(thickness of material).
                                                                phariyadi.staff.ipb.ac.id                  -- ITP530
                                                                        CONDUCTION CONDUCTION HEAHEATT  TTRANSFERRANSFER
                                                                          •• MayMay  occur simultaneouslyoccur simultaneously in one,  in one, or twor twoo, , or three or three 
                                                                               directionsdirections
                                                                          •• ManyMany  ppractical problems invoractical problems involve heat flowlve heat flow  in in onlyonly  one one 
                                                                               or twor twoo  ddirections irections 
                                                                          •• Conduction along Conduction along a rod heated at one end is an a rod heated at one end is an 
                                                                               example of twexample of twoo  ddimensional imensional conductionconduction
                                                                          •• Heat Heat fflowlowss  aalong the length long the length of the rod to the cooler of the rod to the cooler 
                                                                               end (one direction)end (one direction)
                                                                          •• If If rrod is not insulated, od is not insulated, heat is also lost heat is also lost tto o 
                                                                               surroundingssurroundings
                                                                          •• Center Center wwarmer armer than outer surfacethan outer surface
                                                                phariyadi.staff.ipb.ac.id                  -- ITP530
                                  phariyadi.staff.ipb.ac.id                                                                           --
                                  ITP530
                                                                                                                                                                                                                                                                                      3
                             Heat Transfer                                                                                                                                                                                                                    2/27/2018
                                                                            CONDUCTION CONDUCTION HEAHEATT  TTRANSFERRANSFER
                                                                               --   one one ddimensionalimensional
                                                                                 ••    One dimensional One dimensional conduction conduction heat transfer is heat transfer is aa  
                                                                                       function function of:of:
                                                                                       1.  temperature difference,1.  temperature difference,
                                                                                       2.  material 2.  material tthickness,hickness,
                                                                                       3. area 3. area through through wwhhich ich heat flowheat flowss, , andand
                                                                                       4.  resistance of the m4.  resistance of the material to heat flowaterial to heat flow
                                                                   phariyadi.staff.ipb.ac.id                  -- ITP530
                                                                            CONDUCTION CONDUCTION HEAHEATT  TTRANSFER RANSFER -- one one ddimensionalimensional
                                                                        FourierFourier’’s s LawLaw  Of Of HHeat eat CConduction:onduction:
                                                                       QQ== qq                    ==        -- kAkA dTdT
                                                                                               xx                                                                                                                                    qq
                                                                         tt                                                    dxdx                                                                                                   xx
                                                                                                                                                                                       XX                     XX
                                                                     Q Q = T= Tootal heat flowtal heat flow                                                                              11                    22
                                                                     qq = rate of heat flow= rate of heat flow  in in xx direction  direction byby  cconduction, onduction, WW
                                                                        x x 
                                                                     k  = thermal conductivityk  = thermal conductivity, W/mC, W/mC
                                                                                                                                                                                                                                               22
                                                                     A= area (normal to x-direction) A= area (normal to x-direction) tthrough whrough whhich ich heat flowheat flowss, , mm
                                                                     T = temperature, CT = temperature, C
                                                                     x = distance increment, variable, mx = distance increment, variable, m
                                                                   phariyadi.staff.ipb.ac.id                  -- ITP530
                                    phariyadi.staff.ipb.ac.id                                                                                --
                                    ITP530
                                                                                                                                                                                                                                                                                                    4