An ice-cream freezer has three scraper blades, temperature difference30°C, ice density917 kg/m³,latent heat335 kJ/kg and U=2000 kJ/(m²·hour·°C). Maximum ice thickness before scraping is 10 µm. Find minimum shaft rpm.
Per unit area, the latent energy to form thickness δ is $\rho L\delta=917(335)(10^{-5})=3.07195$ kJ/m². Heat flux is $U\Delta T=60000$ kJ/(m²·hour),so formation time is 3.07195/60000 hour=.184317 s. Three blades make3N passes/min, so time between passes is 60/(3N). Set this equal to .184317 to get108.5087 rpm. Thus **109 rpm, B**.
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