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Psychrometric Chart: Moist Air Properties in HVAC Processes

Explore the use of psychrometric charts to determine psychrometric properties of moist air in HVAC processes, including heating, cooling, and humidification. Learn how to calculate relative humidity, humidity ratio, specific enthalpy, specific volume, wet bulb temperature, and dew point temperature.

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Psychrometric Chart: Moist Air Properties in HVAC Processes

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  1. Lecture 34 The Psychrometric Chart Psychrometric Properties in EES Air Conditioning Processes

  2. Moist Air Properties Using a combination of the ideal gas and real-fluid models along with ideal mixing we have discovered, Note: The textbook puts a ‘#’ sign on the enthalpy to remind us that its mass basis is per lbm of dry air in the mixture. Applying this notation to the heating problem from Lecture 33,

  3. Moist Air Properties Question: Is there an easier way to determine h# ? Answer: YES!  As with pure fluids, a real-fluid mixture formulation has been developed for the dry air-water vapor mixture. The formulation uses the real-fluid behavior of the components along with real mixing. As such, the resulting formulation is complex. Fortunately, it can be displayed graphically in a series of charts known as the psychrometric charts. The psychrometric charts are published by ASHRAE. They were drawn at the Center for Applied Thermodynamic Studies (CATS) at the University of Idaho. There are 5 IP (English unit) charts, and 7 SI charts.

  4. Psychrometric Properties The Psychrometric Chart

  5. Chart No. 1 is available on the BbLearn website!

  6. Wet Bulb and Dry Bulb Temperatures Wet bulb and dry bulb temperatures are thermodynamic properties of the moist air mixture. They are measured with a device known as a psychrometer.

  7. Psychrometric Properties using the Chart Given: Moist air mixture in our classroom; T, Twb measured with a sling psychrometer. Find: The following psychrometric properties of the mixture using the Psychrometric Chart relative humidity humidity ratio specific enthalpy specific volume dew point temperature

  8. Psychrometric Properties in EES Given: Moist air at 1 atm, 85°F, 40% relative humidity Find: The following psychrometric properties of the mixture using EES humidity ratio specific enthalpy specific volume wet bulb temperature dew point temperature

  9. Air Conditioning Processes

  10. The Five HVAC Processes • Heating • With and without humidification • Adiabatic humidification • Cooling • With and without dehumidification • Adiabatic mixing of moist air streams • Evaporative cooling

  11. Heating Without Humidification • Constant humidity ratio • Relative humidity decreases • Outlet relative humidity may be too dry to be comfortable

  12. Heating with Humidification

  13. Adiabatic Humidification The location of state 2 (2a, 2b, or 2c) depends on the state of the water being injected for humidification

  14. Cooling Without Dehumidification This process is not very common in HVAC systems. Often, the surface of the heat exchanger is below the dew point which causes water to condense

  15. Cooling With Dehumidification If Twater is not given, it is common to assume that it is equal to T2

  16. Adiabatic Mixing 1-2-3 are on a straight line!

  17. Evaporative Cooling This is the adiabatic humidification process when the water used for humidification is colder than T1 This system works very well in hot, dry climates. Notice that there can be a significant increase in the humidity levels (both relative humidity and humidity ratio).

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