initial commit / add all books

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diff --git a/2510/CH21/EX21.1/Ex21_1.sce b/2510/CH21/EX21.1/Ex21_1.sce
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+//Variable declaration:
+m = 1                           //Mass flowrate (lb)
+cP = 1                          //Heat capacity (Btu/lb. F)
+//From figure 21.3:
+T1 = 300                        //Temperature of hot fluid leaving exchanger ( F)
+T2 = 540                        //Temperature of hot fluid entering exchanger ( F)
+T3 = 60                         //Temperature of cold fluid leaving exchanger ( F)
+T4 = 300                        //Temperature of cold fluid entering exchanger ( F)
+
+//Calculation:
+DSh = m*cP*log((T1+460)/(T2+460))      //Entropy for hot fluid (Btu/ F)
+DSc = m*cP*log((T4+460)/(T3+460))      //Entropy for cold fluid (Btu/ F)
+DSa = DSh+DSc                   //Entropy for one exchanger (Btu/ F)
+DSt = DSa*2                     //Total entropy change (Btu/ F)
+
+//Result:
+printf("The entropy chage is : %.4f Btu/ F .",DSt)
+if (DSt>0) then
+    printf("There is a positive entropy change.")
+else 
+    printf("There is a negative entropy change.")
+end