In drying casein, the dryer is found to consume 4 m 3 /h of natural gas with a calorific value of 800 kJ/mole. If the throughput of the dryer is 100 kg of wet casein per hour, drying it from 55% moisture to 10% moisture, estimate the overall thermal efficiency (percent or ratio of heat needed to heat used) of the dryer taking into account the latent heat of evaporation only. Assume that the latent heat of evaporation is 2257 kJ kg-1 (at 100 °C), and the natural gas to be at standard temperature and pressure at which 1 mole occupies 22.4 liters.