12. Psychrometrics
Transcript of 12. Psychrometrics
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Air is a mixture of components in a gaseous
state, typically comprised of approximately:
Properties & components of air
It is within this 1%ofother gasses where
water vapour can reside.
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Moisture in air
The capacity of an air mixture to holdwater vapour is limited.
The amount of water vapour the
air mixture will holdis primarilydependent on its temperature the higher the temperature, the
more water vapour it can hold.
If water is permitted to evaporate into a closed container of air, anequilibrium is reached when no further net evaporation will occur,
and the air mixture is said to be
fully saturated.
Vapour pressure
The total pressure of the air mixture is comprised of the
sum of the partial pressures of each of its components.
Ptotal
= Poxygen
+ Pnitrogen
+ Pwater vapour
+ . . .
Thepartial pressure of the water vapourcomponent is knownas the vapour pressure. The higher the value of the vapourpressure, the greater the actual amount of water the air
contains (measured in grams per cubic metre of air).
When air is fully saturated, it is said to have
reached its saturation vapour pressure.
These saturated values have been determined,
and can be plotted as a function of temperature.
Saturated vapour pressure vs. temperature Saturated vapour pressure vs. temperature
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Relative humidity
Relative humidityis another means of quantifying the
amount of moisture in the air.
It is the ratio of the actual amountof moisture in the air
compared to the maximum amountit could holdunder
the same conditions, usually represented as a percentage.
For example, a relative humidity value of 60% indicatesthat the air mixture contains 60% of the maximummoisture it is able to contain at that temperature.
Relative humidity
Since the amount of moisture air is capable of absorbing is a function of
its temperature, a value of relative humidity is only meaningful if thetemperature of the air is also known hence the term relative humidity.
A reading of 100% relative humidity
indicates the air is fully saturated.
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An instrument comprised of
two thermometers, one whosebulb remains dryand the otherwhose bulb is maintained wet
using a dampened cloth.
This is known as the wet bulb temperature.
As the instrument is swung inthe air, the temperature of the
wet bulb thermometer graduallydrops due to evaporation, untilan equilibrium temperature is
eventually reached.
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If during the daytime at 25C the Relative Humidity (RH) is 50%, what
would be the anticipated RH at night if the temperature falls to 0C,assuming no change in moisture content of the air?
Example: Exterior vapour pressures
Saturation vapour pressure for 25C is 3,167 Pa (from table), therefore
a 50% RH indicates a vapour pressure of:
pv = 0.50 x 3,167 Pa = 1,583 Pa
The saturated vapour pressure at night for 0C is 611 Pa, therefore:
RH at night = 1,583 Pa / 611 Pa = 2.59 = 259% > 100% RH
Therefore condensation (dew or frost) will occur until RH is broughtdown to 100%.
a) using Saturated Vapour Table:
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If during the daytime at 25C the Relative Humidity (RH) is 50%, what
would be the anticipated RH at night if the temperature falls to 0C,assuming no change in moisture content of the air?
Example: Exterior vapour pressures
b) using Psychrometric Chart:
Find 25C dry bulb temperature on the chart, move up to 50% RH.
Move horizontally left until saturation line is reached.
Follow saturation line down to a dry bulb temperature of 10C. duringwhich time condensation will occur to remove moisture from the air
while the RH remains at 100%.
Example: Water vapour in air
a) Find the surface temperature at which condensation will begin when
the indoor condition is 23C with 60% RH.
1) Using Saturated Vapour Table, saturation pressure at 23C is 2,809 Pa.
Partial pressure pw = 0.60 x 2,809 Pa = 1,685 Pa
Temperature having saturation pressure of 1,685 Pa is 14.8C.
2) Using Psychrometric Chart, at a temperature of 23C, move verticallyup the chart to a RH of 60%.
Move horizontally left until saturation temperature is reached.
Move down to read dew point temperature of15C.
Example: Water vapour in air
b) For the same air temperature (23C), determine the surface
temperature at which condensation will begin when the RH is 80%.
1) Using Saturated Vapour Table, saturation pressure at 23C is 2,809 Pa.
Partial pressure pw
= 0.80 x 2,809 Pa = 2,247 Pa
Temperature having saturation pressure of 2,247 Pa is 19.5C.
2) Using Psychrometric Chart, at a temperature of 23C, move verticallyup the chart to a RH of 80%.
Move horizontally left until saturation temperature is reached.
Move down to read dew point temperature of19.5C.
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A) Winter in Yellowknife:
With exterior conditions of 28.5C and 90% RH, and interiorconditions of 20C and 40% RH, what are the interiorand exteriorvapour pressures, and in which direction will diffusion occur?
Example: Vapour pressures
Exterior:From Saturated Vapour Table, saturation vapour pressure at 28.5Cis 0.044 kPa, therefore:
Vapour pressure with 90% RH = 0.90 x 0.044 kPa = 0.040 kPa
Interior:
Similarly, saturation vapour pressure at 20C is 2.337 kPa, therefore:
Vapour pressure with 40% RH = 0.40 x 2.337 kPa = 0.935 kPa
Vapour diffusion will occur from interior ---> exterior
Example: Vapour pressures
B) Summer in Yellowknife:
With exterior conditions of 16.5C and 60% RH, and interiorconditions of 25C and 40% RH, what are the interiorand exteriorvapour pressures, and in which direction will diffusion occur?
Exterior:
From Saturated Vapour Table, saturation vapour pressure at 16.5Cis 1.877 kPa, therefore:
Vapour pressure with 60% RH = 0.60 x 1.877 kPa = 1.126 kPa
Interior:
Similarly, saturation vapour pressure at 25C is 3.167 kPa, therefore:
Vapour pressure with 40% RH = 0.40 x 3.167 kPa = 1.27 kPa
Vapour diffusion will occur from interior ---> exterior
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Example: Vapour pressures
A) Winter in Montreal: With exterior conditions of 15C and 80% RH,
and interior conditions of 20C and 40% RH, what are the interiorand exterior vapour pressures of water, and which direction willdiffusion occur?
Exterior:
From table, saturation vapour pressure at 15C is 165.2 Pa,therefore vapour pressure with 80% RH = 0.80 x 165.2 Pa = 132 Pa
Interior:
Saturation vapour pressure at 20C is 2,337 Pa, therefore vapour
pressure with 40% RH = 0.40 x 2,337 Pa = 935 Pa
Vapour diffusion will occur from interior ---> exterior
B) Summer in Montreal: With exterior conditions of 30C and 80% RH,
and interior conditions of 25C and 50% RH, what are the interiorand exterior vapour pressures of water, and which direction willdiffusion occur?
Example: Vapour pressures
Exterior:
From table, saturation vapour pressure at 30C is 4,243 Pa, therefore
vapour pressure with 80% RH = 0.80 x 4,243 Pa = 3,394 Pa
Interior:
Saturation vapour pressure at 25C is 3,167 Pa, therefore vapour
pressure with 50% RH = 0.50 x 3,167 Pa = 1,584 Pa
Vapour diffusion will occur from exterior ---> interior
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Monthl Temperature & Vapour Pressure An lysis V ncouver
A B C D E F G
month Tdry bulb Tdewpoint MC VPsat VPactual RH
C C g/kgdry air Pa Pa %
Jan 2.5 0.0 3.8 726 624 86
Feb 4.5 2.0 4.5 836 703 84
Mar 5.5 2.5 4.7 897 727 81
Apr 8.5 4.5 5.2 1103 861 78
May 12.0 7.5 6.5 1395 1074 77
Jun 15.0 10.5 8.0 1698 1290 76
Jul 17.5 12.5 9.1 1995 1494 75
Aug 17.0 13.0 9.4 1930 1505 78
Sep 14.0 11.0 8.3 1591 1321 83
Oct 10.0 7.5 6.5 1221 1026 84
Nov 6.0 3.5 4.7 929 799 86
Dec 4.0 2.0 4.3 807 711 88
from table from table
horizontal onpsychrometric
chart fromT
DPto
right axis
from VPsat
table
using TDB
l from VPsat
table
using TDP
col F / col E
Monthl Temperature & Vapour Pressure An lysis Montreal
A B C D E F G
month Tdry bulb Tdewpoint MC VPsat VPactual RH
C C g/kgdry air Pa Pa %
Jan -10 -13.5 1.1 260 190 73
Feb -9.0 -13.0 1.8 284 199 70
Mar -2.5 -7.0 3.1 497 343 69
Apr 5.5 -0.5 5.6 903 587 65
May 13.0 5.5 9.4 1497 913 61
Jun 18.5 12.0 13.5 2130 1405 66
Jul 21.0 14.5 15.8 2486 1666 67
Aug 19.5 14.0 14.4 2267 1609 71
Sep 14.5 10.0 10.4 1651 1238 75
Oct 8.5 4.0 7.0 1110 821 74
Nov 2.0 -1.5 4.4 706 550 78
Dec -7.0 -10.0 2.1 338 264 78
from table from table
horizontal onpsychrometric
chart fromT
DPto
right axis
from VPsat
table
using TDB
l from VPsat
table
using TDP
col F / col E
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