note on humidity

8/19/2019 Note on Humidity

1/19

HUMIDITY

HUMIDITY refers to the mass (lb. or kg) of water vapour per unit mass of dr air

or moles of water vapour per mole of dr air.

Humidit spe!ifs the amount of water vapor in the air.

Humidit is e"pressed in different was#

1. Absolute Humidity is the mass of water vapor in a given volume of air.

This is e"pressed as grams of water vapor in a !ubi! meter of air. (Is the per!entage

of water vapor a!tuall present in the air).

$"ample# if the water vapor in % !ubi! meter of air weighs &' grams the absolute

humidit of the air is &' grams!ubi! meters (&' gm&).

2. Specific humidity is the mass of water vapor !ompared with the mass of all the

air in the par!el (in!luding vapor).

3. Relative humidity is the absolute humidit divided b the amount of water that

could be present in the air. *elative humidit indi!ates the degree of !omfort or

dis!omfort one feels from the humidit be!ause it indi!ates the amount of

perspiration that !an evaporate from the skin.

4. The mixi! "#tio is the mass of water vapor divided b the mass of dr

air.

+oth spe!ifi! humidit and mi"ing ratio are e"pressed as grams of water

vapor per kilogram of air (gkg).


2/19

De$ %oit&

The dew point is a good indi!ator of the air,s water vapor !ontent. High dew points

indi!ate high water vapor !ontent- !onversel low dew points indi!ate low water

vapor !ontent. ddition of water vapor to the air in!reases the dew point- removing

water vapor lowers it. This is the temperature at whi!h air attains saturation and a

further addition of water vapour whi!h leads to dew formation be!ause of

!ondensation of water vapour. It is rarel used to indi!ate the moisture !ontent of

airatmosphere.

Degree of /aturation#

It is stated as the ration of weight of water vapour in air at a given !onditions to the

weight of the water vappour in air at saturation keeping temperature !onstant. It is

also !alled saturation ratio or per!entage humidit.

Me#su"i! Humidity&

% ps!hrometer is an instrument used to determine the water vapor !ontent

of the air. It !onsists of two thermometers (dr bulb and wet bulb). fter

whirling the instrument the dew point and relative humidit !an be obtained

with the aid of the en!losed table.

0 hgrometer is an instrument designed to measure the air,s water vapor

!ontent. The sensing part of the instrument !an be hair (hair hgrometer) a

plate !oated with !arbon (ele!tri!al hgrometer) or an infrared sensor


3/19

(infrared hgrometer). This instrument !an be used in the field and

greenhouse to measure both temperature and humidit.

%sych"omet"ic 'h#"t

1. It"oductio

The ps!hrometri! !hart shows graphi!all the parameters relating to water

moisture in air. This appli!ation note des!ribes the purpose and use of the

ps!hrometri! !hart

ir !omprises appro"imatel 123 nitrogen 0%3 o"gen and %3 other gases. +ut

air is never dr even in a desert. Two4thirds of the earth5s surfa!e is !overed with

water and this along with other surfa!e water and rain maintain low pressure

water vapor to be suspended in the air making up part of the %3 of other gases.

The ps!hrometri! !hart indi!ates the properties of this water vapor through the

following parameters ea!h of whi!h is e"plained in more detail below#

6 dr bulb temperature

6 wet bulb temperature (also known as saturation temperature)

6 dew point temperature

6 relative humidit

6 moisture !ontent (also known as humidit ratio)

6 enthalp (also known as total heat)


4/19

6 spe!ifi! volume (the inverse of densit)

7or an e"ample on how the ps!hrometri! !hart is used in pra!ti!e please read

8ower 9not5s appli!ation note on measuring enthalp to evaluate the effi!ien! of

an air !onditioning sstem.

2. %"ope"ties o the 'h#"t

2.1 D"y bulb (D)* tempe"#tu"e

:e measure the temperature of the air with a thermometer. Traditional

thermometers have a bulb that !ontains a li;uid that e"pands and a tube indi!ating

the temperature on a s!ale. s the li;uid e"pands it rises up the s!ale.

:hi!hever method is used this measurement is !alled the dr bulb temperature

be!ause the end of the thermometer that is making the measurement has no

moisture on it.

The temperature of the air is measured in


5/19

2.2 +et bulb (+)* tempe"#tu"e

The wet bulb temperature is measured b having the bulb

of the thermometer moist. The moisture evaporates lowering the temperature

re!orded b the thermometer. ?ess moisture in the air will result in a faster rate of

evaporation and therefore a !older reading. In pra!ti!e we !an use an ele!troni!

thermometer and wrap a paper tissue over the thermo!ouple. Make the paper tissue

moist but not too wet that water is dripping from it. Move air over the tissue (or

move the thermo!ouple through the air) so the water evaporates.

:hen the air sample is saturated with water (that is it has %@@3 relative

humidit) no water !an evaporate from the moist tissue so the :+ temperature

will read the same as the D+ temperature. This temperature is therefore also

referred to as the saturation temperature. This temperature is indi!ated b diagonal

lines on the !hart.

2.3 ,el#ti-e humidity (,H*


6/19

This is the ratio of the fra!tion of water vapor in the air to

the fra!tion of saturated moist air at the same temperature and pressure. *H is

dimensionless and is usuall e"pressed as a per!entage. %@@3 *H indi!ates the air

is saturated and !annot hold an more moisture. 8referred values of !omfort for

people are between &'3 and A@3.

?ines of !onstant relative humidit are shown as e"ponential lines on the

ps!hrometri! !hart. The line at %@@3 is referred to as the saturation line.

2.4 De$ poit (D%* tempe"#tu"e

This is the temperature of the air at whi!h a moist air sample rea!hes water vapor

saturation. It is e;uivalent to a wet bulb temperature at %@@3 relative humidit. t

this !ombination of temperature and humidit further removal of heat results in

water vapor !ondensing into li;uid.

pra!ti!al view of the dew point is the temperature

to whi!h air must be !ooled before !ondensation will begin. n e"ample is when

ou take a bottle of beer out of the refrigerator. :ater !ondenses on the outside of


7/19

the bottle onl if the original temperature of the bottle was below the dew point.

The instantaneous temperature of the water is the dew point temperature.

s a sample of air is !ooled its *H !limbs until it rea!hes %@@3 *H (saturated

air). This is the dew point temperature. t saturation the dew point temperature

e;uals the wet bulb temperature whi!h also e;uals the dr bulb temperature and

the *H is %@@3. This temperature is shown as hori>ontal lines on the !hart.

2. Moistu"e cotet

This is also known as the humidit ratio and is usuall designated as :. It is the

proportion of the mass of water vapor per unit mass of dr air. Humidit ratio is

dimensionless but in the U/ it is usuall e"pressed as pounds of moisture per

pound of dr air- elsewhere it ma be e"pressed as grams of water per kilogram of

dr air or as a per!entage.%

The moisture !ontent is the verti!al a"is of the !hart.

2./ 0th#lpy (tot#l he#t*


8/19

$nthalp (usuall designated as h) is the total amount of

heat energ of the moist air and therefore in!ludes the amount of heat of the dr air

and the water vapor in the air. In the appro"imation of ideal gases lines of !onstant

enthalp are parallel to lines of !onstant :+ temperature. Thus the enthalp is

indi!ated b diagonal lines on the !hart.0

In the U/ enthalp is measured in +TU per pound of dr air- elsewhere it is

measured in Boules per kilogram of air.

2. Specific -olume

This is the inverse of densit. /pe!ifi! volume is therefore the volume per unit

mass of the air sample. This is shown as diagonal lines on the !hart.

In the U/ this is measured b !ubi! feet per pound of dr

air- elsewhere it is measured b !ubi! meters per kilogram of dr air.

2. 'omfo"t oes

8eople feel !omfortable within a small range of temperatures and humidities. The

ranges var based on where ou live and on the time of ear. In the northern


9/19

hemisphere people tpi!all wear more !lothes in winter than in summer.

Therefore rooms are maintained at !ooler temperatures in winter than in summer.

8eople are sensitive to humidit be!ause water evaporates from the skin and this

evaporation !ools the bod. t a high humidit water evaporates from the skin

slowl so we feel warmer. t a low humidit water evaporates faster and we feel

!older. The effe!t of per!eived in!rease in temperature with in!rease in humidit is

referred to as the Heat Inde" and is shown graphi!all in the figure.%

This also e"plains wh some people in!rease the

temperature of a room after treating an air !onditioner with the /ntheti!


10/19

*efrigeration =atalst. The treatment with the /ntheti! *efrigeration =atalst

makes the air !older. This !older air !ondenses more water moisture from the air

and lowers the humidit. 8eople !ompensate b raising the temperature and

thereb saving even more mone after treatment with the /ntheti! *efrigeration

=atalst.

The ps!hrometri! !hart shows the two !omfort >ones tpi!all a!!eptable for U/

!iti>ens.

3. 0x#mple uses of the psych"omet"ic ch#"t

4.1 idi! "el#ti-e humidity

Cuestion# iven the ambient temperature is 1@


11/19

pen!il to the right to read the dew point. This is '@


12/19

for developing an understanding of it,s usage.

The basi! information used to !onstru!t the !hart is the water vapor saturation data

(Tsat 8g) whi!h is obtained from steam tables over the range from Tsat J @.@%


13/19

humidit as a parameter to produ!e the various relative humidit !urves (blue

lines) as follows#

where 8 is the standard atmospheri! pressure %@%.&0' Nk8aO.

The saturation !urve (%@@3 relative humidit) also known as the dew point !urve

is drawn as a red line. Poti!e that on the saturation !urve the wet and dr bulb

temperatures have the same values.

The maFor simplifing assumption in the !onstru!tion of the !hart is that the

enthalp of the mi"ture is assumed to be !onstant throughout the adiabati!

saturation pro!ess. This implies that the evaporating li;uid added does not

signifi!antl affe!t the enthalp of the air4vapor mi"ture leading to the !onstant

slope wet bulb temperature enthalp (red) lines defined b#

Pote that on the L J @ a"is (dr air) h J T N


14/19

where the gas !onstant * air J @.021 NkBkg.9O

It is normal pra!ti!e to separate out the overlapping enthalp wet bulb

temperature lines allowing them to be separatel evaluated. Thus we introdu!e an

obli;ue enthalp a"is and enthalp (bla!k) lines as follows#

The four e;uations highlighted above were programmed in MT?+ and used to

plot the simplified ps!hrometri! !harts shown above.


15/19

Sol-ed %"oblem 5 ssume that the outside air temperature is &0


16/19


17/19

Sol-ed %"oblem& Qutside air at &'oneR. Using the 8s!hrometri! =hart neatl plot the re;uired air !onditioning

pro!ess and estimate (a) the amount of moisture removed N%%.'g4H0@kg4dr4air O

(b) the heat removed N(%)4(0) ;!ool J G2kBkg4dr4air O and (!) the amount of heat

added N(0)4(&) ;heat J %@kBkg4dr4air O.


18/19

Sol-ed %"oblem Hot dr air at G@


19/19

This tpe of !ooler is e"tremel popular in hot dr !limates and is popularl

known as a S$#mp 'oole". n interesting appli!ation of using a swamp !ooler to

!ool drinking water in e"tremel hot environments is des!ribed in the posting of

*i!h Qppel in the t :ar blog of the Pew York Times# 6D"i7i! "om Soc7s6.
http://www.consumerenergycenter.org/home/heating_cooling/evaporative.htmlhttp://atwar.blogs.nytimes.com/2009/08/28/drinking-from-socks/http://www.consumerenergycenter.org/home/heating_cooling/evaporative.htmlhttp://atwar.blogs.nytimes.com/2009/08/28/drinking-from-socks/

note on humidity

Documents