12. psychrometrics

7/29/2019 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 diffusion will occur from interior ---> exterior


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:


Interior:

Similarly, saturation vapour pressure at 25C is 3.167 kPa, therefore:




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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


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?


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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12. psychrometrics

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