MAHENDRA INSTITUTE OF TECHNOLOGY Mahendhirapuri
of saturation v) dew point temperature and vi) enthalpy of the mixture.
Department Of Mechanical Engineering
UNIT-V UNIT -V (PSYCHROMET PSYCH ROMETRY) RY) 1. Air at 16˚C and 25% relative humidity passes through a heater and then through a humidifier to reach final dry bulb temperature of 30˚C and 50% relative humidity. Calculate the heat and moisture added to the air. What is the sensible heat factor? 2. In an adiabatic mixing of two streams, derive the relationship among the ratio of mass of streams, ratio of enthalpy change and ratio of specific humidity change. 3.Saturated air at 20˚C at a rate of 1.167m 3/s is mixed adiabatically with the outside air at 35˚C and 50% relative humidity at a rate of 0.5m 3/s. Assuming adiabatic mixing condition at 1 atm, determine specific humidity, relative humidity, dry bulb temperature and volume flow rate of the mixture. 4. A room 7m x 4m x 4m is occupied by an air water vapour mixture at 38˚C. The atmospheric pressure is 1bar and the relative humidity is 70%. Determine humidity ratio, DPT, mass of dry air and mass of water vapour. If the mixture of air-water vapour is further cooled at constant pressure until the temperature is 10˚C. Find the amount of water vapour condensed. 5. Air at 20˚C, 40% RH is mixed adiabatically with air at 40˚C, 40%RH in the ratio of 1kg of the former with 2kg of later. Find the final condition (humidity & enthalpy) of air. Draw the process in chart also as diagram. 6. In a laboratory test, a sling psychrometer recorded dry bulb and wet bulb temperatures as 303K and 298K respectively. Calculate i) vapour pressure ii) relative humidity iii) specific humidity iv) degree
7. 1 kg of air at 313K dry bulb temperature and 50% relative humidity is mixed with 2kg air at 293K dry bulb temperature and 203K dew point temperature. Calculate the temperature and specific humidity of the mixture. 8. Describe the adiabatic cooling process and deduce the expression for its enthalpy. enthalpy. 9. A steam of air at 101.32KPa, 18˚C, and a relative humidity of 30% is flowing at the rate of 14.15m 3/min. A second stream at 101.32KPa, 38˚C and a relative humidity of 50% is flowing at the rate of 8.5m3/min. The two streams are mixed adiabatically to form a third stream at 101.31KPa. Determine the specific humidity, relative humidity and the dry bulb temperature of the third stream. 10. Atmospheric air at 1.0132bar has DBT of 32˚C and a WBT of 26˚C. Compute i) partial pressure of water vapour ii) specific humidity iii) dew point temperature iv) relative humidity v) degree of saturation vi) density of air in the mixture vii) density of vapour in the mixture and viii) enthalpy of the mixture. Use thermodynamic table only. 11. Explain the process of cooling dehumidification of air. 12. 30m3/min of moist air at 15˚C DBT and 13˚C WBT are mixed with 12m3/min of moist air at 25˚C DBT and 18˚C WBT. Determine DBT and WBT of the mixture assuming the barometric pressure is one atmosphere. 13. What is the lowest temperature that air can attain in an evaporative cooler, if it enters at 1 atm, 302K and 40 percent relative humidity?
14. Consider a room that contains air at 1 atm, 308K and 40 percent relative humidity. Using the Psychrometric chart, determine: the specific humidity, the enthalpy, the wet-bulb temperature, the dew point temperature and the specific volume of the air. 15. An air-conditioning system is to make in outdoor air at 283K and 30percent relative humidity at a steady rate of 45m 3/min and to condition it to 298K and 60percent relative humidity. The outdoor air is first heated to 295K in the heating section and then humidified by the injection of hot steam in the humidifying section. Assuming the entire process takes place at a pressure of 100KPa, determine i) the rate of heat supply in the heating section and ii) the mass flow rate of the steam required in the humidifying section. 16. A 5m x 5m x 3m room contains air at 25˚C and 100KPa at a relative humidity of 75 percent. Determine i) The partial pressure of dry air ii) The specific humidity iii) The enthalpy per unit mass of the dry air, and iv) The masses of the dry air and water vapour in the room. 17. The dry and the wet-bulb temperatures of atmospheric air at 1 atm (101.325KPa) pressure are measured with a sling psychrometer and determined to be 25˚C and 15˚C respectively. Determine i) The specific humidity ii) The relative humidity, and iii) The enthalpy of the air using thermodynamic relations. 18. What is sensible heat? How is the sensible heat loss from a human body affected by the i) skin temperature ii) environment temperature and iii) air motion. 19. Saturated air leaving the cooling section of an air-conditioning system at 14˚C at a rate of 50m 3/min is mixed adiabatically with the outside air at 32˚C and 60 percent relative humidity at rate of 20m3/min. Assuming that the mixing process occurs at a pressure of 1 atm, determine the specific humidity, the relative humidity, the dry bulb temperature, and the volume flow rate of mixture.
20. With the aid of suitable diagram discuss the sensible heating processes. 21. Consider a room that contains air at 1 atm, 35˚C and 40 percent relative humidity. Using psychometric chart, determine i) The specific humidity, ii) The enthalpy iii) The wet-bulb temperature iv) The dew-point temperature and v) Specific volume of air. 22. Why does the enthalpy of an air-vapour mixture remain constant during an adiabatic saturation process? 23. Explain the process of heating with humidification of air, with the help of sketches of schematic of air duct with the elements involved as well as the representation on Psychometric chart. 24. 1kg of air at 40˚C DBT and 50% RH is mixed with 2kg of air at 20˚C DBT and 20˚C dew point temperature. Calculate the specific humidity, enthalpy and the dry bulb temperature of the mixture. 25. A Sling Psychrometer reads 40˚C DBT and 36˚CWBT. Find the humidity ratio, relative humidity, dew point temperature, specific volume and enthalpy of air. 26. Explain the process of cooling dehumidification of air.
