PSYCHROMETRICS. A bit of new; a bit of review. Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 1. What is Psychrometrics?

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1 PSYCHROMETRICS A bit of new; a bit of review Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 1 What is Psychrometrics? A word manufactured from the Greek psychros (cold) and Middle English meter (a measurer) Describes the study of moist air properties These properties can be addressed via equations (a serious hassle), tables (a moderate hassle), or via the psychrometric [or psych] chart (relatively easy) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 2 1

2 Why Psychrometrics? Building occupants are immersed in moist air Doing something to moist air is the basis of all climate control systems, including: Passive climate control (architectural) Active climate control (mechanical) Psychrometrics can be used to define conditions (state points) Psychrometrics can be used to analyze processes (changes in conditions) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 3 Moist Air Properties fundamental, independent, define psych chart axes combined, co-dependent, reside in interior of psych chart derived Dry bulb temperature (deg F) Specific humidity (lbs / lb) Relative humidity (%) Wet bulb temperature (deg F) Enthalpy (Btu / lb) Specific volume (cu ft / lb) Dew point temperature (deg F) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 4 2

3 The Psychrometric Chart arbitrary limit absolute limit congestion zone state point SOURCE: ASHRAE Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 5 Dry Bulb Temperature Defines the horizontal axis of the psych chart Vertical lines on chart are lines of equal temperature Measured with a standard thermometer or thermocouple deg F A key factor for comfort A key (often the only) measure for air-conditioning control The sensible heat axis Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 6 3

4 Specific Humidity Also called (aka) Absolute Humidity Humidity Ratio Defines the vertical axis Horizontal lines are lines of equal absolute moisture content Not measurable in the field Pounds of moisture per pound of dry air (lb/lb) The latent heat axis Not used directly as an air-conditioning control measure Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 7 Wet Bulb Temperature A combined sensible and latent property Conveys information about the heat and moisture content of the air Measured with a sling psychrometer or a digital device deg F An index for some HVAC decisions (such as cooling tower design) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 8 4

5 wet-bulb temperature Sling Psychrometer dry-bulb temperature The wet bulb thermometer reading is affected by evaporation Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 9 Relative Humidity A combined sensible and latent property Ratio of the amount of moisture in the air to the maximum amount that could be held at the given temperature Measured with a hygrometer Expressed in % Key element of comfort & indoor air quality An index for thermal comfort and mold growth potential Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 10 5

6 Enthalpy A (relative) measure of the total (sensible + latent) energy content of air at a given condition A derived property Btu per pound of air Differences in enthalpy quantify the energy cost of changing the condition of air (of undertaking a psychrometric process such as heating or dehumidifying) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 11 Enthalpy versus Entropy Enthalpy: an expression of the total heat content of moist air increasing enthalpy requires the addition of energy (heat) Entropy: a measure of the unavailability (degradation) of heat energy entropy is ever-increasing (more disorder means more entropy) unless a system intervenes to increase order The second law of thermodynamics is an expression of the universal law of increasing entropy. In simple terms, it is an expression of the fact that over time, differences in temperature, pressure, and density tend to even out in a physical system Wikipedia [no perpetual motion; no free lunch] Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 12 6

7 Specific Volume The inverse of density Cubic feet per pound of air Not field measurable Rarely used for thermal comfort based applications Note similarity to the slope of wet bulb lines Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 13 Dew Point Temperature The temperature of a surface or material that will cause condensation upon contact with air at some defined condition A derived property (not measurable) deg F Critical to architectural design of non-condensing surfaces and assemblies (and to dehumidification of air in an HVAC system) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 14 7

8 Some Effects of Condensation Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 15 Psychrometric Processes What Can Be Done To Air Sensible processes (these change temperature) Latent processes (these change moisture content) Primary processes (are horizontal or vertical) Secondary processes (are diagonal) Passive processes (i.e., evaporative cooling) Active processes (i.e., sensible & latent cooling) Single-equipment processes (i.e., sensible heating) Multiple-equipment processes (i.e., sensible heating & humidification) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 16 8

9 Psychrometric Processes Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 17 Psychrometric Chart Usage read enthalpy read wet bulb temperature read relative humidity read humidity ratio find dew point temperature read dry-bulb temperature read specific volume Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 18 9