You can measure Standard, Actual Dry or Moist Air Specific airflow (depending on the instrument). Some systems have outlets that are identical in size and shape as the outlets at which a (non powered) flow hood was calibrated, and also have such a high total volume that the back pressure caused by the hood is insignificant. You can measure FPM and calculate airflow with the formula FPM x AK factor = CFM. Some systems have inlet (or outlet) grills with a "known" effective area (AK factor) and little to no " system effect" (non-uniform airflow in-to or out-of the grille, register or diffuser), these are easily and accurately measured by a velocity traverse. You can estimate Standard Air Specific airflow. These are easily and accurately measured by pressure drop and the manufactures published data. Some cased/uncased coils are as clean as new with entrances and exits that mimic the conditions at which they were rated, with little to no " system effect" (non-uniform airflow into or out of the coil). These are easily and accurately measured by T.E.S.P. and the manufactures published data. Some furnaces and AHU's have clean as new "insides" with supply and return plenums that mimic the conditions at which the blower data was derived with little to no " system effect" (non-uniform airflow into or out of the fan/coil, or furnace). Some systems have electric heaters and/or a blower motor in the air stream and therefore a known btu/hr output in which to apply the temperature rise formula (be aware and avoid the effect of radiant heat, shield your thermometer or measure "out of the line of site" of the heater). You can calculate Standard Air, or Actual Moist Air Specific airflow with PerFormCalc. In the "real world" there isn't anything much harder to measure than airflow.
No need to change static pressures as Standard Air assumes no change in pressure. "Calculate with Standard Air" should be unchecked.Īn example of a "Standard Air Specific" ton of air conditioning can be calculated with, 80° dry bulb, and 67° wet bulb as the entering air, 60° dry bulb and 86.65% relative humidity as the leaving air, 400 entering or leaving standard CFM for air flow, with the "Calculate with Standard Air" option checked. "Enhancement Factor" and "Compressibility Factor" options should be checked. "Calculate with Standard Air" should be unchecked.Īn example of an actual "Dry Air Specific" ton of air conditioning can be calculated with, 80° dry bulb, and 51.2% relative humidity as the entering air, 61° dry bulb and 82.1% relative humidity as the leaving air, 0 feet for altitude, 400 entering or leaving standard CFM for air flow, -0.25"wc (negative) entering static and 0.11"wc leaving static pressure with the "Moist Air Specific "option unchecked. Conversion factors are listed in ASHRAE RP-1485.įor an example of an actual "Moist Air Specific" ton of air conditioning, input 80° dry bulb, with 51.2% relative humidity as the entering air, 61° dry bulb with 79.7% relative humidity as the leaving air, 0 feet for altitude, 400 Entering actual CFM for air flow, 0.25"wc entering and 0.1"wc leaving static pressure with the "Moist Air Specific", "Enhancement Factor" and "Compressibility Factor" options checked. The results are converted back to IP units for display. Temperature, humidity and pressure inputs in are converted to SI values for calculations of volume, density and enthalpy. Other properties are calculated using equations and Tables from ASHRAE 2009 Fundamentals and 2008 HVAC Systems and Equipment.Įquation of State is (P
The Wet Bulb, Dew Point and Apparatus Dew Point are iterated using a curve fitted Enhancement Factor. The Volume, Density, Enthalpy, Compression Factor and Enhancement Factors.
This Psychrometric Calculator uses formulae from ASHRAE RP-1485 "Thermodynamic Properties of Real Moist Air,ĭry Air, Steam, Water, and Ice" by S.