Moisture transfer (moist air) Csaba Szikra senior research fellow TUB Faculty of Architecture Department of Building Energetics and Services

Transcription

1 Moisture transfer (oist air) Csaba Szikra senior research fellow TUB Faculty of Architecture Deartent of Building Energetics and Services 2010

2 Psychroetry Moist air Psychroetry involves the study of the roerties of oist atosheric air, the basic ediu in building hysics and air-handling ractise a binary ixture of dry air and low ressure water vaour. Atosheric air is a ixture of dry air and any aount of water vaor. Coosition of dry air are Nitrogen, Oxygen, Argon, Carbon dioxide and others. The aount of water vaor in oist air is variable, ranging fro nearly zero to axiu 30 g water vaor er kg dry air deending on teerature and ressure at saturation. Moist air is treated as a ixture of indeendent erfect gases, each obeying the erfect gas equation of state.

3 Psychroetry Dalton s Law The air in the atoshere and within buildings ay be considered as a two-coonent gas ixture of dry air and water vaour. Take the box heretically divided by a artition. One art contains air gas olecules, the other art oisture ones. The ressure on the box wall is roortional to the nuber of olecules. Let us reove the heretic artition. Now, both gases strive to coletely fill the whole box caacity, as if the other gas were non existent. The box wall is of course acted uon by ressure fro both gases: the ressures of the oisture and air gases a and are roortional to the nuber of oisture and air olecules, resectively. Dalton's law (also called Dalton's law of artial ressures) states that the total ressure exerted by a gaseous ixture is equal to the su of the artial ressures of each individual coonent in a gas ixture. This eirical law was observed by John Dalton in 1801 and is related to the ideal gas laws. In building engineering only an atosheric air considered as gas ixture of dry air and oisture, thus: 0 = a + 0 is an overall ixture ressure of dry air and oisture P a are the ressure of the coonents or artial ressure of the coonents

4 Psychroetry Moisture content (g) g = a = R R a g = a a = Moist air is a binary (two-coonent) ixture of dry air and water vaour. Moisture content of a oist air is defined as the ratio of the ass of water vaour to the ass of dry air. Moisture content is exressed as the nuber of kilogras of water vaour er kilogra of a dry air (g/kg, kg/kg). The aount of water vaour varies fro zero (dry air) to a axiu that deends on teerature and ressure. Substituting ideal gas equation to the definition of oisture content, exressing a by 0 - and substituting secific gas constant of air and oisture, oisture content can be exressed by artial ressure of the oist. Moisture content is also referred to as huidity ratio, ixing ratio, or secific huidity, concentration of water vaour in air.

5 Psychroetry Saturation vaour ressure ( s ) s t The axiu vaour ressure which can exists in any given teerature is the saturation vaour ressure. Under this condition the water vaour in the ixture can coexist in neutral equilibriu with surface of water at the sae teerature and the oist air said to be saturated with vaour. Saturation vaour ressure is a function only of teerature and is indeendent of the dry air which siultaneously occuies the sae sace. The value of saturation ressure is given in s -t diagra or stea tables. in s -t diagra is often used for describing oisture behaviour of coosite building aterials. Saturation ressure can exressed (based on the reviously develoed equation) by oisture content: x = s x

6 Psychroetry Relative huidity P [Pa] s s t[ C] Relative huidity is defined as the ratio of the artial ressure of water vaor in a gaseous ixture of air and water vaor to the saturated vaor ressure of water at a sae teerature. That is, a ratio of how uch energy has been used to free water fro liquid to vaor for to how uch energy is left. Relative huidity is exressed as a ercentage and is calculated in the following anner: RH = s ( t) ( t)

7 Psychroetrics dew oint P [Pa] s t[ C] Associated with relative huidity is dew oint (if the dew oint is below freezing, it is referred to as the frost oint). Dew oint is the teerature at which water vaor saturates fro an air ass into liquid or solid usually foring rain, snow, frost, or dew. Dew oint norally occurs when a ass of air has a relative huidity of 100%. This haens in the atoshere as a result of cooling through a nuber of different rocesses. Vízellátás - Csatornázás

8 Psychroetrics dry and wet bulb teeratures The dry-bulb teerature is the teerature of air easured by a theroeter freely exosed to the air but shielded fro radiation and oisture. In construction, it is an iortant consideration when designing a building for a certain cliate. It one of "the ost iortant cliate variables for huan cofort and building energy efficiency". Wet-bulb teerature the teerature read fro wet bulb theroeter; Wet-bulb teerature is easured using a theroeter that has its bulb wraed in cloth called a sock that is ket wet with water via wicking action. Such an instruent is called, a wet-bulb theroeter. At relative huidities below 100%, water evaorates fro the bulb which cools the bulb below abient teerature. To deterine relative huidity, abient teerature is easured using an ordinary theroeter, better known in this context as a dry-bulb theroeter. At any given abient teerature, less relative huidity results in a greater difference between the dry-bulb and wet-bulb teeratures; the wet bulb is colder. The recise relative huidity is deterined by finding one's wet-bulb and dry-bulb teeratures on a sychroetric chart Assann s sychroeter Vízellátás - Csatornázás

9 Psychroetrics Secific Enthaly (h) heat content er unit ass, is the su of the internal (heat) energy of the oist air in question, including the heat of the air and water vaor within h = c t + x ( c t + r0) 1+ g a c a Secific heat of dry air at constant ressure [kj/kgk] = 1 c Secific heat of overheated stea at constant ressure [kj/kgk] = 1.86 r 0 secific enthaly of water[kj/kg] = 2500

10 Psychroetrics Vaour diffusion Let us take a box divided by a artition. Let its resective divisions contain different gas ixtures of air gas and oisture olecules. The overall ressures on the artition are equal, but on one side, artial ressure of the air gas, on the other side, that of the oisture gas is the higher. If the artition is not coletely tight but gas olecules can ass through, then the difference between artial ressures of the air gas gas akes air gas gas to flow to the oosite division, and so does oisture gas, in site of the overall ressures of the ixtures being equal. This is the background (the driving echanis) of the vaour diffusion through building eleents. Vízellátás - Csatornázás

11 i Calculation of oisture transfer Exale 1 ti=20 C Rhi=65% Psi=2338Pa i=0,65*si=1520pa te=-2 C Rhe=90% Pse=516a e=0,65*si=464pa internal convection b h W/K k W/ 2 K R 2 K/W T C d g/s,ma Rd s,2,ma/g - 8 0,125 1,8 t iw=t i- t i= 18, ineral 1 0,05-0,057 0,877 12,3 t 12=t iw- t 1= 5,9 0,08 0, wool 2 brick 0,30-0,577 0,520 7,3 t 23=t 12- t 2= -1,4 0,05 6, e external convection ,042 0,6 t e=t ew- t e -2, U=1/SuR q=(ti-te)/sur Teerature distribution SuR: 1,564 22,00 SuRd 6,625 0,639 W/ 2 K 14,1 W/ 2 =(i-e)/surd: 159,40 P Pa Distr. Part. Press Exale1: Calculate the artial ressure dsitribution of a two layer wall! Heat calculation: R i =1/h i ; R 1 =b 1 /k 1 ; R 2 =b 2 /k 2 ; R e =1/h e R= R i + R 1 + R 2 + R e q=(t i -t e )/ R t I =q R I ; t 1 =q R 1 ; t 2 =q R 2 ; t e =q R e Checking: t=t i -t e Partial ressure calculation: R d1 =b 1 /d 1 ; R d2 =b 2 /d 2 R d = R 1 + R 2 i = si RH i; ; e = se RH e =( i - e )/ R d I =0; 1 = R d1 ; 2 = R d2 ; e =0; t iw =t i -dt i ; iw = i t 1,2 =t iw -dt 1 ; 1,2 = iw -d 1 t ew =t 1,2 -dt 2 =t i +dt i ; ew = 1,2 -d 2 = e Vízellátás - Csatornázás

12 Calculation of oisture transfer Exale 2 Exale2: Draw the odified artial ressure distirbution curve

13 Psychroetrics chart 1 A sychroetric chart is a grah of the hysical roerties of oist air at a constant ressure. The chart grahically exresses how various roerties relate to each other, and is thus a grahical 'equation of state'. The therohysical roerties found on ost sychroetric charts are: Dry-bulb teerature is that of an air sale, as deterined by an ordinary theroeter, the theroeter's bulb being dry. Wet-bulb teerature is that of an air sale after it has assed through a constant-ressure, ideal adiabatic saturation rocess, that is, after the air has assed over a large surface of liquid water in an insulated channel. In ractice, this is the reading of a theroeter whose sensing bulb is covered with a wet sock evaorating into a raid strea of the sale air. Dew oint teerature is that at which a oist air sale at the sae ressure would reach water vaor saturation. At this saturation oint, water vaor would begin to condense into liquid water fog or (if below freezing) solid hoarfrost, as heat is reoved. The dewoint teerature is easured easily and rovides useful inforation, but is norally not considered an indeendent roerty. It dulicates inforation available via other huidity roerties and the saturation curve. Relative Huidity is the ratio of the ole fraction of water vaor to the ole fraction of saturated oist air at the sae teerature and ressure. The notion that air "holds" oisture, or that oisture dissolves in dry air and saturates the solution at soe roortion, is an erroneous (although widesread) concet. Huidity Ratio is the roortion of ass of oisture resent in a unit ass of air at the given conditions (DBT, WBT, DPT, RH, etc.). Secific Enthaly sybolized by h, also called heat content er unit ass, is the su of the internal energy of a therodynaic syste. These values corresond to the saturated state and are to be read arallel to WBT values. Secific Volue, also called Inverse Density. Volue er unit ass. The versatility of the sychroetric chart lies in the fact that by knowing two indeendent roerties of soe oist air, one of which is the chart's ressure, the other roerties can be deterined. Changes in state, such as when two air streas ix, can be odeled easily and soewhat grahically using the correct sychroetric chart for the location's air ressure or elevation relative to sea level. For locations at or below 2000 ft (600 ), a coon assution is to use the sea level sychroetric chart. Vízellátás - Csatornázás

14 Psychroetrics chart 2 Vízellátás - Csatornázás