Psychrometrics

Abstract

Psychrometrics is concerned with the measurement of moisture in gases, particularly in air. Psychrometrics can be used to answer such questions as:

Problems

• 13.1 Ambient air at 20 °C and 80% relative humidity was heated to 90 °C. What is the specific volume, humidity ratio, specific enthalpy, and wet bulb temperature of the heated air?

• 13.2 A process requires air at 30 °C at a relative humidity of 40%. The available air is at 35 °C with a relative humidity of 60%. To make the available air suitable for the process, it is passed through a unit that dehumidifies by cooling to the dew point, followed by a reheating unit. The flow rate of air entering the device is 1 m³/s at 1 atm.

• a. What is the mass flow rate of the air?

• b. What is the rate of heat removal in the dehumidifying unit?

• c. What is the rate of moisture removal in the dehumidifying unit?

• d. What is the rate of heat addition in the heating section?

• e. What is the volumetric flow rate of the finished air?

• 13.3 Air is available at 10 °C and 80% relative humidity. To condition the air to 40 °C and 50% relative humidity, it is first passed at a flow rate of 1 m³/s through a heater and then a humidifier where steam at atmospheric pressure is injected.

• a. At what mass flow rate must steam be injected?

• b. At what temperature must the air leave the heating section?

• c. At what rate must heat energy be added in the heating section?

• d. What is the volumetric flow rate of the air leaving the humidifier?

• 13.4 Air at 50 °C and 10% RH was used for drying tortilla chips in a dryer. At the dryer exit, the air is found to be totally saturated. Compute the amount of water picked up by the air.

• 13.5 If air at 50 °C and 20% RH passes through high-moisture food at a flow rate of 10 m³/s, determine the maximum rate of moisture removal that can be achieved.

• 13.6 Air at 20 °C and 30% RH is heated to 60 °C before being used in a food dryer. During drying of a food material, 32 kg of water is removed every hour and the air leaves the dryer at 45 °C and 70% RH. What is the required flow rate of air?

• 13.7 An air stream having a volumetric flow rate of 2.5 m³/s, a relative humidity of 60%, and a dew point of 1 °C is electrically heated to 55 °C and used for drying 250 kg of cut fruits with an initial moisture content of 80% on wet basis. If the dryer is operated for one hour and the air exists the dryer saturated, compute the following:

• a. The amount of water removed and the final moisture content of the cut fruits on wet and dry basis.

• b. The electrical cost of heating, assuming electrical energy costs $ 0.08/kW-h.

• 13.8 On two separate nights, the sky is so clear that maximum radiational cooling can take place. Below are the sundown temperatures and relative humidity for each day. If the dew point is reached above the freezing temperature, dew will form. If the temperature falls below freezing before the dew point is reached, frost will form. Which day is more likely to have frost? At what temperature will the dew or frost start to form in each case?

Day	Temp. (°C)	RH
1	45	50%.
2	40	90%

• 13.9 To conserve energy, a 10 m³/s flow rate stream of exhaust air at 55 °C and 30% RH is mixed with 20 m³/s of ambient air at 30 °C and 60% RH. Determine the relative humidity, dry bulb temperature, moisture content, and dew point of the air mixture.

• 13.10 Air stream A flows at 0.65 m³/s at a temperature of 10 °C and 25% RH. Air stream B flows at 1.1 m³/s at a temperature of 20 °C and 50% RH. Both are at 1 atm pressure. The two streams are mixed adiabatically.

• c. What is the volumetric flow rate of the mixture?

• d. What is the temperature of the mixture?

• e. What is the moisture content of the mixture?

• f. What is the RH of the mixture?