Question

Tomatoes are to be dried from 90% moisture content to 12% moisture content in a tunnel drier The tomatoes are dried at a rate of 150kg/hr. The drying air is first heuted from 20°C and 30% relative humidity to 50°C after which it is passed through the drying trays from which it leaves at 90% relative humidity i Sketch the process on a psychrometric chart and outline the air conditions at exit from the drier (3 marks) How much moisture is removed from the tomatoes per hour (3 marks) Determine the required air flow rate (m/min) for the drying operation (4 marks) iv How much energy is required to heat up the air? (2 marks)

Answer 1

Answer and Explanation:

To sketch the process on a psychrometric chart, we need to consider the initial and final air conditions.

1. Initial air conditions:

- Temperature: 20°C

- Relative humidity: 30%

2. Air conditions after heating:

- Temperature: 50°C

- Relative humidity: Unknown (to be determined)

3. Air conditions at the exit from the dryer:

- Temperature: 50°C (same as after heating)

- Relative humidity: 90%

To determine the air conditions after heating, we can use the psychrometric chart. From the initial conditions (20°C, 30% relative humidity), we move along the constant enthalpy line until we reach the temperature of 50°C. At this point, we can read the new relative humidity value on the chart.

To determine the amount of moisture removed from the tomatoes per hour, we need to calculate the difference in moisture content between the initial and final conditions. Given that the tomatoes are dried from 90% moisture content to 12% moisture content, the moisture removed can be calculated as:

Moisture removed = Initial moisture content - Final moisture content

Moisture removed = 90% - 12%

Moisture removed = 78%

Since the tomatoes are dried at a rate of 150 kg/hr, the moisture removed from the tomatoes per hour can be calculated as:

Moisture removed per hour = Moisture removed × Drying rate

Moisture removed per hour = 78% × 150 kg/hr

To determine the required air flow rate for the drying operation, we need to consider the moisture content of the air and the amount of moisture removed from the tomatoes per hour. The required air flow rate can be calculated using the equation:

Required air flow rate = Moisture removed per hour / (Initial moisture content - Final moisture content)

Finally, to determine the energy required to heat up the air, we need to calculate the heat energy using the equation:

Energy = Mass flow rate × Specific heat capacity × Temperature difference

The mass flow rate can be calculated using the drying rate and the moisture content of the tomatoes. The specific heat capacity of air is known. The temperature difference is the difference between the initial and final temperatures.

By following these steps, you will be able to determine the air conditions at the exit from the dryer, the amount of moisture removed from the tomatoes per hour, the required air flow rate, and the energy required to heat up the air.