PsychroCoilCalc™ - Psychrometric and HVAC&R Process Calculator Updated 4/13/17 Provide inputs in the blue boxes, other inputs optional.
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Entering Air
Dry Bulb
°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.
Leaving Air
Dry Bulb
°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.
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.
ft³/min
Entering Leaving
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/(lb·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, Wetbulb = the Thermodynamic Wet Bulb
Temperature (like a typical chart)
lb/(lb·mol) Total Air Mass M
lb/(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 Theroretically carry. This is commonly used for
system design, and is what typical psychrometric charts and
applications use.
It's accuracy is adequate in the temp. & hum. 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
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
lbw/lba Water Vapor ∕ Moist Air Mass γ
Absolute Humidity as a Dry
Air Specific Ratio: Pounds
(mass) of water vapor per
Pounds (mass) of Dry Air.
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.
ps = f ∙ pws
psias Saturated Water Vapor pressure ps
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%.
ft³/(lb·mol) 2nd virial mixing coefficient Bm
ft⁶/(lb·mol)² 3rd virial mixing coefficient Cm
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.
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.
In 2010, ASHRAE revised it's psychrometric model of moist air
as a “Real Gas" mixture by using the Virial Equation of State
to determine compressibility factor as
Z = (P·V) / (R·T) = 1 + Bm / V + Cm / V²

Absolute Humidity as Parts water
vapor Per Million parts Dry Air (or
of the Total Mixture) by volume.
ppmv     Moisture
R = Gas Constant (a.k.a. the molar, universal, or ideal gas constant)
Imperial Units from ASHRAE RP-1485
Absolute Humidity as Grains
of water per volume of air.
Same for Dry or Moist Air
Specific
gr/ft³        Grains

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 Volume, Density, Enthalpy, Compression Factor and Enhancement Factor. Dew Point is calculated using equations from the Buck Research Manual (1996). 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 leaving 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, 60.5° dry bulb and 84.32% 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" should be 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 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 Standard, Actual Dry or Moist Air Specific airflow (depending on the instrument).

Some systems have a straight length of duct at least 7.5 duct diameters downstream and at least 3 duct diameters upstream from 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 be 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.

If you're not dealing with any of the above described systems, then a calibrated blower (Duct Blaster), powered flow hood (Flow Blaster), or calibrated flow resistance (True flow) may be viable options. You can measure Standard, Actual Dry or Moist Air Specific airflow (depending on the instrument).
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