1.08 is a constant for density at sea level. cfm is the volume of conditioned air D t is the temperature difference between the supply air and the room control temperature Equation 6: Equation for latent cooling load: Q = .68 * cfm * GR Where: Q = Load in Btu/H .68 is the latent load constant GR = difference between absolute humidity between indoor humidity/area and outdoor humidity/area Room Load Calculations
Engineering Guidelines - Fan Coils B | B10 FAN COILS One of the four major elements of thermal energy and comfort is humidity. Psychrometrics uses thermodynamics properties to analyze conditions and processes involving moist air. A detailed study of psychrometrics can be found in Chapter 1 of ASHRAE 2009 Fundamentals Handbook. This section is a summary of how knowledge of psychrometrics can be used to maximize space comfort and system performance. Atmospheric Air (the air that you breathe), contains many gaseous components including water vapor and containments. Dry Air is atmospheric air with all moisture removed and is used only as a point of reference. Moist Air is a combination of dry air and water vapor and can be considered equal to atmospheric air for this discussion. A psychrometric chart ( Figure 115 ) is a graphical representation of the thermodynamic properties of moist air. There are several charts available to cover all common conditions. The one shown here is taken from ASHRAE 2009 Fundamentals Handbook, Chapter 1 and illustrates conditions of 32 to 100 B F at sea level. The Dry-bulb Temperature (DBT), is the temperature measured using a standard thermometer. Dry-bulb is also known as the sensible temperature. The Wet-bulb Temperature (WBT), is the temperature measured using a ‘wetted’ thermometer. Wet-bulb is used to determine the moisture content of air. The Absolute Humidity (AH), is the vapor content of air. It is described in terms of moisture per lb of dry air or grains of moisture per lb of dry air. AH is also referred to as ‘moisture content’ or ‘humidity ratio.’ There are 7000 grains in a lb. of water. The Relative Humidity (RH), is the vapor content of air. It is described as the percentage of saturation humidity at the same temperature (%). The goal for optimum space comfort is 30-35% for heating conditions, and 45-60% for cooling conditions. Saturation humidity is the maximum vapor content (lb/lb) per lb dry air that air can hold at a fixed temperature. The Dew Point Temperature (DPT), is the temperature at which vapor begins to fall out of air to form condensation. DPT is the temperature at which a state of saturation humidity occurs, or 100% RH. It is also known as the saturation temperature. Figure 115. Psychrometric Chart Psychrometrics 10 15 20 30 35 40 45 50 55 55 60 60 Enthalpy - Btu per Pound of Dry Air 15 20 25 30 35 40 45 50 Enthalpy - Btu per Pound of Dry Air Saturation Temperature, °F 35 40 45 50 55 60 65 70 75 80 85 90 95 100 115 Dry Bulb Temperature, °F .002 .004 .006 .008 .010 .012 10% Relative Humidity 20% 30% 40% 50% 60% 70% 80% 90% 35 35 40 40 45 45 50 50 55 55 60 60 65 65 70 70 75 75 80 80 85 Wet Bulb Temperature, °F 85 90 12.5 13.0 13.5 14.0 Volume - cu. ft. per lb. Dry Air 14.5 15.0
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