Department of Mechanical Engineering ME 322 Mechanical Engineering Thermodynamics

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1 Department of Mechanical Engineering ME 3 Mechanical Engineering Thermodynamics 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, H, h ha T hw T Pw ma 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 37, P 4 psia T 65F 0.50 V 300 cfm Q P T 4 psia 0F Q m h h h h a a w a w a Q m h h

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. 3

4 Department of Mechanical Engineering ME 3 Mechanical Engineering Thermodynamics Psychrometric Properties The Psychrometric Chart

5 5 Chart No. 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. 6

7 Psychrometric Properties using the Chart Given: Moist air mixture in our classroom; T, T wb measured with a sling psychrometer. Find: The following psychrometric properties of the mixture using the Psychrometric Chart (a) relative humidity (b) humidity ratio (c) specific enthalpy (d) specific volume (e) dew point temperature T dp h T wb v T dp T 7

8 Psychrometric Properties in EES Given: Moist air at atm, 85 F, 40% relative humidity Find: The following psychrometric properties of the mixture using EES (a) humidity ratio (b) specific enthalpy (c) specific volume (d) wet bulb temperature (e) dew point temperature 8

9 Department of Mechanical Engineering ME 3 Mechanical Engineering Thermodynamics 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 0

11 Heating Without Humidification Q H Q m h h H a h h Constant humidity ratio Relative humidity decreases Outlet relative humidity may be too dry to be comfortable T T

12 Heating with Humidification Q H ' h water Q m h m h m h H a water water a m m m w water w mwater mw mw m m m a a a h h ' Q m H a h h h water ' ' T T' T

13 Adiabatic Humidification water m h m h m h a water water a m m m w water w mwater mw mw m m m a a a a b c h h h water 0 The location of state (a, b, or c) depends on the state of the water being injected for humidification T 3

14 Cooling Without Dehumidification Q C Q m h h C a h h 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 T T ' T 4

15 Cooling With Dehumidification Q C water h Q m h m h m h c a water water a m m m w water w mwater mw mw m m m a a a h Q m C a h h h water If T water is not given, it is common to assume that it is equal to T T T dp T 5

16 Adiabatic Mixing (cold) m h m h m h a a a3 3 m m m a a 3 a3 m m m a a a3 (hot) h h 3 h 3 3 (warm) 3 m h m h m m h a a a a 3 ma h h3 m h h a 3 ma ma ma ma 3 ma 3 m a 3 h h h 3 3 h are on a straight line! 6

17 Evaporative Cooling This is the adiabatic humidification process when the water used for humidification is colder than T water m h m h m h a water water a m m m w water w mwater mw mw m m m a a a h h h water 0 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). T T 7

For an ideal gas mixture, Dalton s law states that the sum of the partial pressures of the individual components is equal to the total pressure.

For an ideal gas mixture, Dalton s law states that the sum of the partial pressures of the individual components is equal to the total pressure. 1 PSYCHROMETICS Psychrometry is the study of the characteristics of moist air. We will see soon that evaporation of moisture from the skin can have a significant impact on thermal comfort. The rate of