It is not uncommon in tropical countries to see local residents set up small-sca

Question

It is not uncommon in tropical countries to see local residents set up small-scale food processing operations to dry fruits or vegetables. In one such facility, air is brought into the building where a dryer for dehydrating cassava is housed. The incoming air has a temperature of 28°C and a relative humidity of 80%. After filtering, the air is heated to 60°C and used to dry the cassava slices in the dryer. While in the dryer, the air loses some of its heat and picks up moisture. The air is exhausted from the dryer with a temperature of 50°C and a relative humidity of 50%.

Questions: (10 marks total) What is the relative humidity of the heated air entering the dryer?(2 marks)

How much moisture was picked up per kilogram of dry air while it was in the dryer? (2 marks)

If the exhaust air from the dryer was cooled, at what temperature would it become saturated? (2 marks)

Being concerned about the environment, the processor feels that she should filter and cool the exhaust air back to its original temperature of 28°C before venting it to the outside atmosphere. If the dryer exhaust air was cooled to 28°C, what would be its relative humidity? To cool the outgoing air, the processor uses an air-to-air countercurrent heat exchanger that takes heat from the outgoing air and uses it to warm the fresh incoming air. In this way, the process becomes more energy efficient. (2 marks)

Why does the processor filter the incoming air that is going to the drying process? (2 marks)

Explanation / Answer

1. THE RELATIVE HUMIDITY OF HEATED AIR ENTERING THE DRYER IS 37.33%.

2 13% moisture was picked by dry air while it was in the dryer?

3. If the exhaust air from the dryer was cooled, at 25°C would it become saturated (100% relative humidity).

4. To cool the outgoing air, the processor uses an air-to-air countercurrent heat exchanger that takes heat from the outgoing air and uses it to warm the fresh incoming air. In this way, the process becomes more energy efficient. hence It is an is an energy recovery ventilation system.

5 The processor filter the incoming air that is going to the drying process so that dirt present in the air can be removed otherwise the will clogging up the system.

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