Introduction to Energy Management

Week/Lesson 5 Psychrometrics: The Properties of Air

After completing this chapter, you will be able to:  Define psychrometry, the psychrometric chart and psychrometrics  Describe the composition of air  Explain Boyle’s, Charles’ and Dalton’s laws  Read a psychrometric chart  Use the psychrometric chart to determine the effects of latent and sensible heat on air Psychrometrics: The Properties of Air

Psychrometry — the study of air, temperature and water vapor relationships Psychrometric chart — graphical representation of these relationships Psychrometrics — combination of the concepts of psychrometry and the psychrometric chart Psychrometrics: The Properties of Air

The Composition of Air  Dry air — 77% nitrogen, 22% oxygen, 0.04% carbon dioxide, 0.96% other gases  Water vapor — evaporation and transpiration  Airborne impurities Smoke, dust, pollen, bacteria, noxious gases Particulate matter or gases Psychrometrics: The Properties of Air

Predicting Air’s Behavior  Boyle’s Law — Gas shrinks in volume when pressurized at constant temperature  Charles’ Law — Gas expands when heated at constant pressure  Dalton’s Law of Partial Pressures — Total pressure of a gaseous mixture is the sum of the pressures of the individual gases Psychrometrics: The Properties of Air

Water Vapor in Air — Latent Heat of Vaporization  Equal to 970 Btus/lb of water vapor  Heat added to water causes evaporation  Heat removed from water vapor causes condensation Psychrometrics: The Properties of Air

Defining Humidity  Humidity — airborne moisture  Specific humidity — weight of water vapor in dry air  Relative humidity — percentage of moisture saturation in air  Dew point — point at which air has a relative humidity of 100% Psychrometrics: The Properties of Air

Measuring Humidity  Wet bulb temperatures  Dry bulb temperatures  Sling psychrometer  Psychrometric table  Psychrometric chart Psychrometrics: The Properties of Air

Example 5-1  Dry bulb temperature = 92ºF  Wet bulb temperature = 89ºF  Use Percent Relative Humidity Table  Difference between temperatures is 3ºF  The point of intersection is 89% RH Psychrometrics: The Properties of Air

Other Psychrometric Properties of Air  Specific Volume of air – cu ft/lb dry air  Enthalpy – Btu/lb dry air Introduction to the Psychrometric Chart  Plotted graph of tables of moist air properties  Standardized at sea level pressure Psychrometrics: The Properties of Air

Constructing the Psychrometric Chart  Dry bulb temperature scale along the bottom Constant dry bulb represented by vertical lines  Moisture measurements along the right side Constant moisture represented by horizontal lines  Dew point line Upward curved line at left of chart Also known as the 100% humidity line Psychrometrics: The Properties of Air

Examining the Psychrometric Chart  When two properties are known, the others can be read from the chart Dry and wet bulb temperatures Relative humidity and dew point Enthalpy Specific volume Psychrometrics: The Properties of Air

Example 5-3  Wet bulb temperature of 61 degrees  Dry bulb temperature of 75 degrees  Intersection of these two lines 42% relative humidity Psychrometrics: The Properties of Air

Dew Point The temperature which moisture starts to condense out of the air. Dew point is also known as saturation temperature. Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.

Air temperatureRelative humidityAir movement Surface temperatures Air quality (IAQ) Lighting quality quantity Acoustics Security Building Envelop -The Third Skin The 3 rd. skin moderates the differences between the natural environment and the built environment 75 degrees 65 degrees

MOISTURE CONTENT (BTU/LB AIR ) dew point Saturation Line 30° 40° 50° 60° 70° 80° Temperatures are read at the Saturation line from a set point to the left along humidity ratio lines.

The combined affects of environmental conditions on comfort Air temperature Relative humidity Air movement Surface radiation

Dry Bulb Temperature Reading Change in quantity of water in the air The psychrometric chart Charts the combined affects of air temperature and moisture content

Breakdown of the Lines Dry Blub Relative Humidity Dew Point 100% Saturation Enthalpy BTU / lb Wet Blub Grains of Moisture Specific Volume

50% RH Psychrometric Chart Definition of Relative Humidity The amount of moisture in the air expressed as a percentage of the maximum amount of moisture the air can hold at the given temperature. Example: at dry bulb temp. of 77 and wet bulb temp. of 63, the air is holding 50% of it’s maximum capacity at 77 degrees. Enter dry bulb reading Enter wet bulb reading

warming cooling drying Adding moisture

warming cooling drying Adding moisture Warm & Dry Cool & Humid Winter Summer Energy efficient zones Winter: higher humidity off-sets lower furnace thermostat setting Summer:lower humidity off-sets higher AC thermostat setting

50% RH Air is 100% saturated with moisture and condensation begins The saturation curve

50% RH Air is 100% saturated with moisture and continues to cool Moisture removed by condensation What if cooling continues ?

50% RH Air is 100% saturated with moisture and continues to cool Moisture removed by condensation Same air is reheated to 70 degrees and no moisture added – RH of the room goes down (dryer)

Dry Bulb Temperature Reading Change in quantity of water in the air Chart Exercises What is the RH in the classroom? Dry bulb ________ Wet bulb _________ RH = _________

Dry Bulb Temperature Reading Change in quantity of water in the air Chart Exercises Based on the current room RH, how cool would the walls need to get to start noticing condensation ? Dry bulb ________ Wet bulb _________ RH = _________

Chart Exercises Assume the room is at pt. “A” when class begin and we are comfortable, which direction would room condition tend to move toward as a response to our presents ? Mechanically: (Summer) we need to (add or remove heat) we need to (add or remove moisture) A - Room starts out here B C D E