CAE 331/513 Building Science Fall 2015

Transcription

1 CAE 331/513 Building Science Fall 2015 Week 5: September 24, 2015 Psychrometrics (equations) Advancing energy, environmental, and sustainability research within the built environment Dr. Brent Stephens, Ph.D. Civil, Architectural and Environmental Engineering Illinois Institute of Technology

2 d SI chart 2

3 d Relative Humidity φ 50% Enthalpy h 44 kj/kg da Dew Point Temp T dew 11.7 C Specific Volume v m 3 /kg da Density ρ 1/v 1.18 kg da /m 3 Wet Bulb Temp T wb 15.5 C Dry Bulb Temp T = 22 C Humidity Ratio W 8.2 g/kg da (i.e., kg/kg) 3

4 IP chart

5 Enthalpy h 30 Btu/lb Specific Volume v ft 3 /lbm Dew Point Temp T dew 40 F Wet Bulb Temp T wb 65 F Humidity Ratio W 5.8 lb/lb da Relative Humidity φ 13% Dry Bulb Temp T 100 F

6 grains/lb: 1 lb = 7000 grains Alternate IP chart (Wang) 6

7 Question: Using these parameters What is the mass of water vapor in this classroom right now? 7

8 d SI chart 8

9 PSYCHROMETRIC EQUATIONS 9

10 Psychrometric equations We can rely on the psychrometric chart for many building science applications Particularly when you don t need precise answers Answers to within a few percent But when we need more precise answers, or when we need to automate engineering calculations We need to rely on the underlying equations that govern moist air properties and processes and make up the psychrometric chart This begins by treating air as an ideal gas 10

11 Treating air as an ideal gas At typical temperatures and pressures within buildings, air and its constituents act approximately as ideal gases Each gas i in the mixture, as well as the entire mixture, will follow the ideal gas law: Ideal Gas Law (Boyle s law + Charles s law) pv = nrt p = pressure (Pa) V = volume (m 3 ) n = number of moles (#) R = gas constant (Pa m 3 /(mol K)) T = absolute temperature (K) 11

12 Air as an ideal gas We can treat air as a composition of ideal gases A bunch of ideal gases acting as an ideal gas For individual gases (e.g., N 2, O 2, Ar, H 2 O, CO 2, pollutant i): P i V = n i RT P i = n i V RT P i = y i P tot P i = partial pressure exerted by gas i n i = # of moles of gas i R, V, T = gas constant, volume, temperature Rearrange so that n i /V is the molar concentration P tot = total pressure of air (atm, Pa, etc.) y i = mole fraction of gas i in air (moles i / moles air) 12

13 Air as a composite mixture Air as an ideal gas P i = y i P tot P tot = P i = PV = nrt n i V RT = RT V n i = RT V n tot 13

14 Calculating the density of air at typical indoor conditions PV = nrt n V = P RT n V = P RT = 1 atm $ atm m 3 & % mol K ' ) 293 K ( 20 C, 68 F n V = 41.6 moles m 3 = moles L ρ air = n V MW air = MW air moles degrees C 14

15 What is the molecular weight (MW) of air? MW air = y i MW i = y N2 MW N2 + y O2 MW O2 + y H2 OMW H2 O +... MW air = 0.781(28 g/mol) (32 g/mol) g/mol ρ air = (29 g mol ) mol L =1.2 g L kg degrees C 3 m Remember this number: density of air is ~1.2 kg/m 3 at 20 C Air density (kg/m 3 ) Density is a function of temperature: ρ air ( T air ) where T air is in degrees C In building applications, where: -15 C < T < 40 C 1.15 kg/m 3 < ρ air < 1.3 kg/m Air temperature ( C) 15

16 Universal gas constant The universal gas constant relates energy and temperature It takes many forms depending on units Universal gas constant PV = nrt Value of R Units (V P T -1 n -1 ) J/(K mol) m 3 Pa/(K mol) L atm/(k mol) m 3 atm/(k mol) ft 3 psi/(r lb-mol) Btu/(lb-mol R) 16

17 Specific gas constants To work with air and water vapor we can also work with specific gas constants (which are functions of molecular weight) Dry air (no water vapor): MW da = g/mol R da = R MW da = J K mol g mol 1000g kg = 287 J kg K R i = R MW i Water vapor alone: MW w = g/mol J R w = R = K mol 1000g MW w g kg = 462 J kg w K mol pv = p ρ = RT i Specific gas constant: 17

18 Air pressure variations The barometric (atmospheric) pressure and temperature of air vary with both altitude and local weather conditions But there are standard values for pressure as a function of altitude that are normally used At sea level, the standard temperature is 15 C and the standard pressure is kpa (1 atm) Temperature is assumed to decrease linearly with altitude Pressure is more complicated p = ( 1 ( )Z) T air = Z pv = p ρ = RT T = temperature ( C) Z = altitude (m) p = barometric pressure (kpa) 18

19 Air pressure variations Chicago, IL Denver, CO Big Sky, MT Breckenridge, CO 19

20 Dalton s law of partial pressures for psychrometrics In an ideal gas, the total pressure can be considered to be the sum of the partial pressures of the constituent gases p = p N2 + p O2 + p H2 O + p CO 2 + p Ar + We can consider moist air as dry air combined with water vapor and break the pressure into only two partial pressures p = p da + p w 20

21 Dalton s law of partial pressures for psychrometrics We can analyze the dry air, the water vapor, and the mixture of each gas using the ideal gas law and assuming they are all at the same temperature p da v da = R da T & p w v w = R w T & pv = RT For each individual gas, a mole fraction (Y i ) can be defined as the ratio of the partial pressure of gas i to the total pressure n i n = p i p = Y i 21

22 Specifying the state of moist air In order to specify the state of moist air, we need total atmospheric pressure, p, the air temperature, T, and at least one other property W, φ, h, p w, or T dew We can use the psychrometric chart We can also use the underlying equations for greater accuracy and automation 22

23 Key concepts: Vapor pressure and Saturation Air can hold moisture (i.e., water vapor) Vapor pressure is a measurement of the amount of water vapor in a volume/parcel of air (ideal gas) p w *Units of pressure, Pa or kpa (partial pressure) The amount of moisture air can hold in vapor form before condensation occurs is dependent on temperature We call the limit saturation p ws *Units of pressure, Pa or kpa 23

24 Relative humidity, φ (RH) The relative humidity ratio, φ, is the mole fraction of water vapor (x w ) relative to the water vapor that would be in the mixture if it were saturated at the given T and P (x ws ) We can also describe RH by partial pressures (ideal gas) Relative humidity is a common measure that relates well to how we perceive moisture in air φ =! # " x w x ws $ & % T,P = p w p ws 24

25 p ws for 0 C< T <200 C (SI units) For p ws, the saturation pressure over liquid water: ln p ws = C 8 T +C 9 +C 10 T +C 11 T 2 +C 12 T 3 +C 13 lnt Note: These constants are only for SI units IP units are different Units: *We will use this equation for most conditions in building science 25

26 p ws for -100 C< T <0 C (SI units) For p ws, the saturation pressure over ice: ln p ws = C 1 T +C 2 +C 3 T +C 4 T 2 +C 5 T 3 +C 6 T 4 +C 7 lnt Note: These constants are only for SI units IP units are different Units: 26

27 Humidity ratio, W (SI units) The humidity ratio, W, is ratio of the mass of water vapor to mass of dry air in a given volume We use W when finding other mixture properties Note 1: W is small (W < 0.03 for most real building conditions) Note 2: W is sometimes expressed in grains/lb where 1 lb = 7000 grains (I don t use this but you will in CAE 464 HVAC Design) UNITS W = m w m da = MW w p w MW da p da = p w p da = p w p p w [ kg w kg da ] x " x #$ = p " p #$ 27

28 Saturation humidity ratio, W s (SI units) At a given temperature T and pressure P there is a maximum W that can be obtained If we try to add any more moisture, it will just condense out It is when the partial pressure of vapor has reached the saturation pressure This maximum humidity ratio is called the saturation humidity ratio, W s From our previous equation we can write: W s = p ws p da = p ws p p ws UNITS [ kg w kg da ] 28

29 Degree of saturation, µ (SI units) The degree of saturation, µ (dimensionless), is the ratio of the humidity ratio W to that of a saturated mixture W s at the same T and P Note that µ and φ are not quite the same Their values are very similar at lower temperatures but may differ a lot at higher temperatures! µ = W $ # & " W s % T,P µ = φ = φ 1+ (1 φ)w s / (0.6295) µ 1 (1 µ) p ws / p 29

30 Specific volume, ν, and density, ρ (SI units) The specific volume of moist air (or the volume per unit mass of air, m 3 /kg) can be expressed as: v = R T da = R T ( W ) da p p w p v (T )( W ) / p If we have ν we can also find moist air density, ρ (kg/m 3 ): ρ = m da + m w V = 1 ( v 1+W ) 30

31 Enthalpy, h (SI units) The enthalpy of a mixture of perfect gases equals the sum of the individual partial enthalpies of the components Therefore, the enthalpy (h) for moist air is: h = h da +Wh g h = enthalpy for moist air [kj/kg] h g = specific enthalpy for saturated water vapor (i.e., h ws ) [kj/kg w ] h da = specific enthalpy for dry air (i.e., h ws ) [kj/kg da ] Some approximations: h da 1.006T h g T h 1.006T +W ( T ) *where T is in C and h is in kj/kg 31

32 Remember: 3 different temperatures T, T dew, and T wb The standard temperature, T, we are all familiar with is called the dry-bulb temperature, or T d It is a measure of internal energy We can also define: Dew-point temperature, T dew Temperature at which water vapor changes into liquid (condensation) Air is maximally saturated with water vapor Wet-bulb temperature, T wb The temperature that a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it Units of Celsius, Fahrenheit, or Kelvin ü The energy needed to evaporate liquid water (heat of vaporization) is taken from the air in the form of sensible heat and converted to latent heat, which lowers the temperature at constant enthalpy 32

33 Dew-point temperature, T dew The dew point temperature, T dew, is the air temperature at which the current humidity ratio W is equal to the saturation humidity ratio W s at the same temperature i.e. W s (p, T dew ) =W When the air temperature is lowered to the dewpoint at constant pressure, the relative humidity rises to 100% and condensation occurs T dew is a direct measure of the humidity ratio W since W = W s at T = T dew 33

34 d Dew Point Temp T dew 11.7 C W = W s at T = T dew 34

35 Dew-point temperature, T dew (SI units) Dew-point temperature, T dew Note: These constants are only for SI units IP units are different 35

36 Wet-bulb temperature, T wb (SI units) Wet-bulb temperature, T wb Requires iterative solving find the T wb that satisfies the following equation (above freezing): W = ( T wb )W s@t wb 1.006(T T wb ) T 4.186T wb = actual W And for T below freezing: W = ( T wb )W s@t wb 1.006(T T wb ) T 2.1T wb = actual W *Where T wb and T are in Kelvin 36

37 Obtaining these data from ASHRAE Tables ASHRAE HoF Ch. 1 (2013) Table 2 gives us W s, v da, v s, h da, and h s directly at different temperatures: 37

38 Obtaining these data from ASHRAE Tables ASHRAE HoF Ch. 1 (2013) Table 3 gives us p ws at different temperatures: 38

39 Revisit example from last class Moist air exists at 22 C dry-bulb temperature with 50% RH at sea level Find the following: (a) the humidity ratio, W (b) dew point temperature, T dew (c) wet-bulb temperature, T wb (d) enthalpy, h (e) specific volume, ν (f) density, ρ Also: (g) degree of saturation, µ 39

40 Psychrometric equations summary φ = p w p ws W = p w p p w µ = W W s ln p ws = C 8 T +C 9 +C 10 T +C 11 T 2 +C 12 T 3 +C 13 lnt Dew point temperature: 40

41 Psychrometric equations summary Wet bulb temperature (iterative solver): W = ( T )W 1.006(T T wb s@t wb wb ) T 4.186T wb = actual W *Where T wb and T are in Kelvin v = R T da = R T ( W ) da p p w p v (T )( W ) / p ρ = m da + m w V = 1 ( v 1+W ) h 1.006T +W ( T ) *where T is in C 41

42 d Relative Humidity φ 50% Enthalpy h 44 kj/kg da Dew Point Temp T dew 11.7 C Specific Volume v m 3 /kg da Density ρ 1/v 1.18 kg da /m 3 Wet Bulb Temp T wb 15.5 C Dry Bulb Temp T = 22 C Humidity Ratio W 8.2 g/kg da (i.e., kg/kg) 42

43 Revisit another example from last class Moist air exists at 30 C dry-bulb temperature with a 15 C dew point temperature Find the following: (a) the humidity ratio, W (b) degree of saturation, µ (c) relative humidity, ϕ (d) enthalpy, h (e) specific volume, ν (f) density, ρ (g) wet bulb temperature, T wb 43

44 Humidity ratio W = p w p p C For a known T dew = 15 C, we know that the actual humidity ratio in the air, W, is by definition the same as the saturation humidity ratio, W s, at an air temperature of 15 C C =W s@t=15 C = p ws@15c = kpa Assume p = kpa (sea level) p ws p p C C =W s@t=15 C = = kg w kg da

45 Degree of saturation Need the saturation humidity T = 30 C: W s@t=30 C = p ws p p C! µ = W $ # & " W s =30 C p ws@15c = kpa W s@t=30 C = = kg w µ = W W s = = 0.39 kg da 45

46 From previous: Relative humidity φ = p w p ws p w@t=30 C = p ws@t=15 C =1.7057kPa p ws@t=30 C = kPa φ = = 0.40 = 40% 46

47 Enthalpy h 1.006T +W ( T ) *where T is in C h 1.006(30) + ( )( (30)) = 57.4 kj kg 47

48 Specific volume and density v (T )( W ) / p v ( )( ( )) / ( ) ρ = 1 ( v 1+W ) = v m3 kg da ( ) =1.157 kg m 3 48

49 Wet-bulb temperature Wet-bulb temperature is the T wb that fits this equation: W = ( T wb )W s@t wb 1.006(T T wb ) T 4.186T wb = where: T = 30 C T wb =? C W s@twb =? = Procedure: Guess T wb, calculate pws for that T, calculate W s for that T Repeat until W calculated based on those values (and original T) in equation above is equal to actual W ( in our case) T wb = 20.1 C p ws p p wb =? 49

50 d Enthalpy h 58 kj/kg da Specific Volume v m 3 /kg da Saturation W W s 0.27 kg w /kg da Relative Humidity φ 40% Dew Point Temp T dew 15 C Dry Bulb Temp T = 30 C Wet Bulb Temp t b 20 C Humidity Ratio W 10.7 g/kg da (i.e., ) 50

51 HW 3 assigned HW 3 assigned on Blackboard today Building an Excel-based psychrometric calculator Due Thursday October 1 51