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UVM Extension AgEngineering Blog

Psychrocalc

Posted: August 16th, 2015 by Chris Callahan

Psychrocalc is a free Excel(TM) add-in that makes psychrometric calculations easier. Currently the following calculations are supported with Excel(TM) friendly functions:

  • RH_TDB_TWB (TDB, TWB, P) – Calculates relative humidity (RH, %) for a known dry-bulb (TDB, F) and wet-bulb (TWB, F) temperature at a given pressure (P, PSIA)
  • RH_TDB_W (TDB, W, P) – Calculates relative humidity (RH, %) for a known drybulb (TDB, F) and humidity ratio (W, lbw/lbda) at a given pressure (P, PSIA).
  • TDB_RH_W (RH, W, P) – Calculates drybulb (TDB, F) for a known relative humidity (RH, %) and humidity ratio (W, lbw/lbda) at a given pressure (P, PSIA).
  • TWB_TDB_RH (TDB, RH, P) – Calculates wet-bulb temperature (TWB, F) for a known dry-bulb (TDB, F) and relative humidity (RH, %) at a given pressure (P, psia).
  • TDP_TDB_RH (TDB, RH, P) – Calculates dew-point temperature (TDP, F) for a known dry-bulb temperature (TDB, F) and relative humidity (RH, %) at a given pressure (psia)
  • W_TDB_TWB (TDB, TWB, P) – Calculates humidity ratio (w, lbw/lbda) for known dry-bulb temperature (TDB, F) and wet-bulb (TWB, F) at a given pressure (P, psia)
  • T_Wsat (Wsat) – Calculates the temperature (Tsat, F) corresponding to a given saturation humidity ratio (w, lbw/lbda)
  • Wsat_T (T) – Calculates the saturation humidity ratio (w, lbw/lbda) for a given temperature (Tsat, F)
  • Tsat_P (P) – Calculates the saturation temperature (Tsat, F) of water vapor for a given pressure (Psat, PSIA)
  • Psat_T (T) – Calculates the saturation pressure (Psat, psia) of water vapor for a given temperature (Tsat, F)
  • Pvap_TDB_RH (TDB, RH, P) – Calculates the vapor pressure of water (Pvap, psia) in air for a given dry bulb temperature (TDB, degF), RH (%) and total pressure (P, psia) as inputs.
  • hfg_T (T) – Calculates the specific enthalpy of evaporation at a given temperature (T, degF).
  • Interpolate (x, x1, x2, y1, y2) – Calculates y (unknown) given x and four known table lookup values (x1, x2, y1, y2) using a linear interpolation.
  • hda_T (T) – Calculates specific enthalpy of dry air (BTU/lb) at a specified temperature (T, degF).
  • hsa_T (T) – Calculates specific enthalpy of saturated air (BTU/lb) at a specified temperature (T, degF).
  • ha_T_RH (T,RH) – Calculates specific enthalpy of a air water mixture (BTU/lb) at a specified temperature (T, degF) and relative humidity (RH).
  • Tda (h) – Calculates a specific temperature for dry air of a specific enthalpy.
  • vda_T (T) – Calculates specific volume of dry air (ft3/lb) at a specified temperature (T, degF).
  • vsa_T (T) – Calculates specific volume of saturated air (ft3/lb) at a specified temperature (T, degF).
  • va_T_RH (T,RH) – Calculates specific volume of a air water mixture (ft3/lb) at a specified temperature (T, degF) and relative humidity (RH).

The calculations follow ASHRAE Handbook – Fundamentals. 2005. Chapter 6.

Download Excel(TM) Add-in. Follow this guide to activate the add-in in Excel(TM) after download. Please note: Install the Add-in before trying to open the sample spreadsheet below.  When using the functions above, you should not have to include a full path to the add-in.  Mac users: An error may occur with spaces being inserted into the path.  To resolve this, delete the path and use only the function name.

Download Example Excel(TM) Worksheet. This is an Excel(TM) workbook that includes an example worksheet with the Psychrocalc functions demonstrated.  It also includes data from Table 2 and Table 3 of the ASHRAE reference.

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