Dalton’s Law of Partial Pressures as it Affects Dewpoint Measurements
The Definition of Dalton’s Law
In chemistry and physics, Dalton’s law (also called Dalton’s law of partial pressures) states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
Dalton’s Law of Partial Pressure States:
P(TOTAL) = P1 + P2 + P3+ …… P(N)
WHERE
P(TOTAL) IS THE TOTAL PRESSURE OF THE SYSTEM
P1, P2 , P3 , …… P(N) ARE THE PARTIAL PRESSURES OF THE COMPONENTS
COMPRISING THE GAS STREAM
P(N) = P(TOTAL)Y(N)
WHERE Y(N) IS THE MOLE FRACTION OF THE NTH COMPONENT IN THE GAS STREAM
AS P(TOTAL) CHANGES SO DO THE PARTIAL PRESSURES, I.E. P1 , P2 , P3 , …… P(N)
SINCE DEWPOINT (T) IS ASSOCIATED WITH PARTIAL PRESSURE, AS PARTIAL PRESSURE CHANGES, SO DOES DEWPOINT (T)
HOWEVER, THE NUMBER OF MOLES IN THE GAS REMAINS THE SAME; THEREFORE, IF MOISTURE CONTENT IS MEASURED IN PPMV IT IS NOT AFFECT ED BYPRESSURE
Effects of Pressure on Dewpoint Using Air as an Example:
TOTAL AIR PRESSURE IS THE SUM OF THE PARTIAL PRESSURE OF THE CONSTITUNETS OF AIR:
P(Total) = P(Nitrogen)+ P(Oxygen)+ P(water vapor)+ P(other gasses)
THE WETTER THE AIT THE HIGHER THE PARTIAL WATER VAPOR PRESSURE
IF THE AIR IS COMPRESSED AND THE TOTAL PRESSURE INCREASES, THE PARTIAL WATER VAPOR PRESSURE INCREASES PROPORTIONALLY
THE DEWPOINT IS DEPENDENT ON THE PARTIAL WATER VAPOR PRESSURE THUS IT WILL INCREASE AS WELL