PS1Climate Psychrom Comf PDF

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SOLA3010: Low Energy Buildings and Photovoltaics

Problem Set 1 Psychrometry & Human Comfort 1.

Summarise the potentially uncomfortable climate conditions that a building designer needs to address in the following Australian climate types as defined by the Bureau of Meteorology? a. Hot Humid b. Hot Dry c. Temperate

2.

What is the difference between dry air and air that we find in the atmosphere?

3.

What is the difference between specific humidity and relative humidity?

4.

A tank contains 21kg of dry air and 0.3 kg of water vapour at 30 °C and 100 kPa total pressure. Given that the saturation pressure of water at 30°C is Pg = Psat @ 30°C = 4.2469 kPa, and the Gas constant for dry air Ra is 0.287 kJ/kg, Determine a) the specific humidity, b) the relative humidity, and c) the volume of the tank.

5.

In summer, water condenses on the outer surface of a glass filled with water. How can you explain this phenomenon?

6.

On the psychrometric chart, when are the dry-bulb, wet-bulb and dew-point temperatures identical?

7.

A sling hygrometer gives a dry-bulb temperature of 25 ˚C and a wet-bulb temperature of 18 ˚C. Use a psychrometric chart to determine the following properties of the moist air: a. The relative humidity b. The moisture content (specific humidity) c. The enthalpy of the dry air & water vapour mixture d. The dew-point temperature

8.

Plot the following two cases on a psychrometric chart: a. Air at 35 ˚C DBT and 30 % relative humidity is passed through an evaporative cooler and leaves at 100% humidity. b. Air at 35 ˚C DBT and 60 % relative humidity is passed through an evaporative cooler and leaves at 100% humidity. What can you conclude about the effectiveness of evaporative cooling in humid climates?

9.

How much water vapour is added to 20 kg of atmospheric air (including water vapour) going from 10 ˚C, 50 % saturation to 25 ˚C, 60 % relative humidity?

10. The air in a building has a dry-bulb temperature of 27 ˚C and is at 70 % relative humidity. How warm do the walls have to be to prevent condensation? 11. The air inside a room during winter is at 20°C DBT and 40% relative humidity. If the temperature of the inside surface of a window is 9°C, will condensation form on the window glass? 12. Atmospheric air contains 2.0 kg of dry air with a specific enthalpy of 20 kJ/kg and 0.01 kg of water vapour with a specific enthalpy of 2 700 kJ/kg. What is the specific enthalpy of the atmospheric air? 13. Air enters an evaporative cooler at 1 atm, 35°C and 20% relative humidity at a rate of 4m3/min. It leaves with a relative humidity of 90%. Determine a) the exit temperature of the air, and b) the required rate of water supply to the evaporative cooler. 14. 75 m3/min of chilled air at 14 ˚C DBT and 13 ˚C WBT is mixed with 25 m3/min of outside air at 35 ˚C DBT and 25 ˚C WBT. Use a psychrometric chart to find Find a) the specific humidity (moisture content) and specific enthalpy of the mixture by calculation, and b) the DBT and WBT of the mixture by using a psychrometric chart. 15. Moist air at 25 °C dry-bulb and 50 % saturation, undergoes a process so that its condition is changed to 40 °C dry-bulb and 30 % saturation. Use a Psychrometric Chart to determine the change in specific enthalpy for the process. Page 1 of 1...