Entering Air

Dry Bulb

Wet Bulb Dew Point Rel. Humidity

°F

Leaving Air

Dry Bulb

Wet Bulb Dew Point Rel. Humidity

°F

Omit this input to have the leaving air calculated at the same moisture content as the entering air. Defaults to saturated if equal moisture is not possible.

Airflow

The Barometric Pressure reported by weather stations is adjusted to sea level and therefor not the actual pressure at higher elevations. Obtain actual pressure or use Altitude. Altitude Barometer

sCFM is Standard CFM, density = 0.075 lbda/ft³. Measured specifically in dry air units of mass (pounds). Use this when the entered airflow is obtained from charts, tables or equations representing or derived from "Standard Air" conditions (typically 70°F dry air at sea level). ᴀCFM is Actual CFM, the density varies with pressure, temperature and moisture content. Use this when the entered airflow is measured at its actual density or directly from the actual velocity × area. The entered CFM will be converted (as selected below) to "Actual Moist Air" or "Standard Air" specific units of mass for the calculated outputs. Actual Dry Air Specific units are used as default. If Airflow input is blank, this option specifies the CFM/Ton and CFM per kBtu/h calculations. Entering sCFM₁ Entering ᴀCFM₁ Leaving sCFM₂ Leaving ᴀCFM₂

ft³/min

Entering₁

Leaving₂

lb/ft3 Sp. Density ρ

Static Pressure

ft3/lb Sp. Volume v

btu/lb Sp. Enthalpy h

psia Total Pressure P

TESP: Total External Static Pressure. Measured entering and leaving the furnace or fan/coil (air-handling) unit. Entering is negative and Leaving is positive. PD: Pressure Drop across coil. Measured entering and leaving the coil. Both Entering and Leaving are positive. Select areas of non-turbulent flow for measurements.

Enhancement Factor f

Compressibility Factor Z

ft3/mol Molar Volume V

Applies the Ferrel Psychrometric Equation ps`- pw =A∙p∙(t-t`)∙(1.00115∙t`) to aspirated/sling or non aspirated psychrometers when "Wet Bulb" temperature is used as a humidity input. A = 0.000666 when wet-bulb is aspirated and not frozen A = 0.000594 when wet-bulb is aspirated and frozen A = 0.000799 when wet-bulb is non aspirated and not frozen A = 0.00072 when wet-bulb is non aspirated and frozen When "Electronic" is selected, Wet bulb = the Thermodynamic Wet Bulb Temperature (like a typical chart). Aspirated/Sling Non aspirated Electronic

lb/mol Total Air Mass M

lb/mol Dry Air Mass Mda

If checked, results are calculated in units of Moist Air mass & volume. This is used for the highest accuracy, it is necessary for high temperatures and humidity. If not checked, results are calculated in units of Dry Air and the moisture it could theoretically carry. This is commonly used for system design, and is what typical psychrometric charts and applications use. Accuracy of Dry Air Specific units is adequate for the temperature & humidity ranges of HVAC&R. Also, it is used to select whether the measured ᴀCFM airflow input is "Moist Air Specific" or "Dry Air Specific".

psiaW Water Vapor pressure pW

Describes the enhancement of the saturation pressure of real moist air compared to the saturation pressure of pure water.

ps = f ∙ pw,s psiaS Saturated Vapor pressure pS

The deviation from ideal-gas behavior to real-gas behavior can be accounted for by using the compressibility factor Z, If Z equals 1 it indicates that the real-gas is behaving ideally, and the further from unity Z becomes indicates the more deviation in real-gas behavior from ideal-gas behavior. Z = (P·V) / (R·T) = 1 + Bm / V + Cm / V ²

Absolute Humidity as a Dry Air Specific Ratio: Pounds (mass) of water vapor per Pounds (mass) of Dry Air. Multiply by 1,000,000 for ppmW lbw/lbda Water Vapor ∕ Dry Air Mass W

If checked, calculations will be made with Standard Air = 0.075lbda/ft³ for both the entering & leaving conditions. All units of air mass (/lb) will default to Dry Air Specific.

Absolute Humidity as a Moist Air Specific Ratio: Pound (mass) of water vapor per Pound (mass) of Moist Air. Symbol ξw in ASHRAE RP-1485 A.K.A. Mass Fraction lbw/lba Water Vapor ∕ Moist Air Mass γ

If not checked, absolute humidity is displayed as percentage of moisture by volume. Overridden if Moisture in Dry Air Specific Volume is greater than 100%.

Absolute Humidity as Parts water vapor Per Million parts Dry Air (or of the Total Mixture) by volume. ppmV  Moisture

Absolute Humidity as Grains of water per volume of air. gr/ft3 Grains of Moisture by Volume

For Air as an ideal-gas, the equation of state is P·V = n·R·T Considered accurate enough for HVAC calculations at typical conditions. In 2010, ASHRAE revised it's psychrometric model of moist air as a “Real Gas" mixture by using the Virial Equation of State of (P·V) / (R·T) = 1 + Bm / V + Cm / V²

ft3/mol 2nd virial mixing coefficient Bm

R = Gas Constant (a.k.a. the molar, universal, or ideal gas constant) Imperial Units from ASHRAE RP-1485

ft6/mol2 3rd virial mixing coefficient Cm

This Psychrometric Calculator uses formulae from ASHRAE RP-1485 "Thermodynamic Properties of Real Moist Air, Dry Air, Steam, Water, and Ice" by S. Herrmann, H.-J. Kretzschmar, and D.P. Gatley to calculate the Volume, Density, Enthalpy, Compression Factor and Enhancement Factors. The Wet Bulb, Dew Point and Apparatus Dew Point are iterated using a curve fitted Enhancement Factor. Other properties are calculated using equations and Tables from ASHRAE 2009 Fundamentals and 2008 HVAC Systems and Equipment.

Equation of State is (P·V) / (R·T) = 1 + Bm / V + Cm / V ². Temperature, humidity and pressure inputs in are converted to SI values for calculations of volume, density and enthalpy. The results are converted back to IP units for display. Conversion factors are listed in ASHRAE RP-1485.

For 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. "Calculate with Standard Air" should be unchecked.

An 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. "Enhancement Factor" and "Compressibility Factor" options should be checked. "Calculate with Standard Air" should be unchecked.

An 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. No need to change static pressures as Standard Air assumes no change in pressure.

AIRFLOW

In the "real world" there isn't anything much harder to measure than airflow. But sometimes it can be easy...

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.

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). These are easily and accurately measured by T.E.S.P. and the manufactures published data. You can estimate Standard Air Specific airflow.

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 pressure drop and the manufactures published data. You can estimate Standard Air Specific airflow.

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 measure FPM and calculate airflow with the formula FPM x AK factor = CFM.

Some systems have a straight length of duct at least 7.5 duct diameters downstream and at least 2.5 duct diameters upstream from the blower and any fittings, turns or other flow obstructions where the flow is uniform, these are easily and accurately measured by a velocity traverse. You can measure FPM and calculate airflow with the formula FPM x Area = CFM.

Some systems have a long enough length of straight duct that the static pressure drop can be easily and accurately measured by a high precision manometer. You can calculate airflow from the pressure drop, length, duct type (and compression for flex duct) using the AllDuct Calculator.

Some fossil fuel furnaces will have a combustion analysis performed to determine the "actual" steady state efficiency, and the "actual" btu content from the fuel supplier, in order to determine the "actual" btu/hr input by either "clocking the meter" or calculating fuel delivery rate by measuring pump pressure and nozzle size and then calculate the "actual" steady state output....(whew!) All of this is required in order to accurately apply the temperature rise formula to a fossil fuel system. This is rarely worth the effort.

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