Simple psychrometric calculation
Calculate Btu/hr heat removed from a closed space if 10,000 cfm air enters at
89DBT/73WBT and leaves at 95DBT/78WBT. What is the quantity of moisture / lb air gets
removed? Use psychrometric chart for your calculation.
Air out 95DBT/78WBT
Closed space
Air in 89DBT/73WBT
89DBT/73WBTDBT/
78WBT
How to read psychrometric chart for this post
1. Dry-bulb [DBT] temperature lines: DBT points are given on the x-axis. The vertical lines emerging from
each DBT point on x-axis are DBT lines. It is the ambient temperature of the air.
2. Relative humidity [RH]: Steady RH is represented by the bent lines running from the base left to the
upper right of the graph. RH is the amount of water vapor present in air expressed as a percentage of the
amount needed for saturation at the same temperature.
Psychrometric
chart
3. Wet-bulb temperature [WBT] lines: WBT line emerges from the intersection of DBT and RH and goes
along the diagonal line. The point where this diagonal line intersects the saturation line is WBT. It is an
indicator of moisture air . WBT is the temperature of air if air gives off all its absorbed enthalpy [moisture]
adiabatically and returns to its initial RH.
4. Enthalpy line: This is the extension of the WBT point along the diagonal. The point where extension of
WBT intersects the enthalpy line is the enthalpy in the air for given DBT / RH / WBT. Enthalpy of moist air is
sum of the enthalpy of dry air and enthalpy of water vapor which is latent heat stored in water vapor.
5. Specific volume line: On the psychrometric chart, lines of the constant specific volume are almost vertical
lines with scale values written below the dry-bulb temperature scale and above the upper boundary's
saturation temperature scale. Greater specific volume is an indicator of more moisture in the air. Specific
volume of a substance is the ratio of the substance's volume to its mass. It is the reciprocal of density.
Solution
Heat balance
In air
Air in: 10,000 cfm at 89 DBT/73 WBT
Reference – Red line
Follow the vertical lines emerging from DBT point 89 deg F on x-axis and intersecting digonal WBT line at 73
deg F. The bent line where DBT intersects WBT is RH. Follow DBT-WBT intersection point along the diagonal.
The point where this digonal line intersects the enthalpy line is the enthalpy in the air for given DBT / WBT.
In air enthalpy = 37 Btu/lb [close to]
In air RH = 50% [close to]
Specific volume of in air = 13.85 cu ft / lb [close to] shown within circle below red dot under x-axis.
10,000 cfm in air = 10000/13.85 = 722 lb/min
Heat in = 722*37= 26715 Btu/min
Out air
Air out: 10,000 cfm air at 95DBT/78WBT, this corresponds to about 55RH In the same manner as explained
above, follow blue line and blue dots.
Out air enthalpy = 43 Btu/lb [close to]
Out air RH = 55 [close to]
Specific volume of out air = 14.0 cu ft / lb [close to] shown within circle below blue dot under x-axis.
10,000 cfm out air = 10000/ 14.0 = 714 lb/min
Heat out = 714*43 = 30714 Btu/min
Heat removed / min = [30714-26715] = 3999 Btu/min or = 239940 Btu/hr
Moisture balance
In air moisture = 0.015 lb / lb dry air [ see in humidity ratio axis]
Moisture in= 722*0.015 = 10.83 lb / min
Out air moisture = 0.018 lb / lb dry air [ see in humidity ratio axis]
Moisture out = 714*0.018= 12.85 lb/min
Moisture removed / hr= [12.85-10.83]*60 = 121 lb / hr