dataPraha={{10,(1/35)},{50,(1/10)}};
dataMB={{20,(1/72)},{75,(1/30)}};

mm=0.001;
cm=10mm;
mSTeckou=0.5;
hustota=1000;
VSTeckou=(mSTeckou/hustota);
d=4mm;
a=50cm;
Print["v= ",v=(VSTeckou/(d*a))," m.(s^(-1))"];
cp=4186;
U=230;

fitMB=Fit[dataMB,{1,T[x]},T[x]];
rce=hustota*cp*v*T'[x]==fitMB*((((U/d)))^2);
vstupniTeplota=10;
resMB=DSolve[{rce,T[0]==vstupniTeplota},T[x],x][[1]];
Tzadana=70;
Print["vykon= ",mSTeckou*cp*(Tzadana-vstupniTeplota)/1000," kW"]
teplotaMB=T[x]/.resMB;
delkaMB=x/.FindRoot[teplotaMB==Tzadana,{x,0.1}];
Print["delka ohrivace= ",delkaMB," m"]

fitPha=Fit[dataPraha,{1,T[x]},T[x]];
rcePha=hustota*cp*v*T'[x]==fitPha*((((U/d)))^2);
vstupniTeplota=10;
resPraha=DSolve[{rcePha,T[0]==vstupniTeplota},T[x],x][[1]];
Plot[T[x]/.resPraha,{x,0,0.3delkaMB},AxesLabel->{"x [m]","T [°C]"},PlotStyle->{Red,Thick},GridLines->Automatic,PlotRange->All]