389H_NO_5_psychrometrics.ppt

7/30/2019 389H_NO_5_psychrometrics.ppt

1/31

Forced Convection

External turbulent flow over a flat plate Nu = h m L/k = 0.036 (Pr ) 0.43 (Re L0.8 9200 ) ( /w )0.25

External turbulent flow (40 < Re D


2/31

Natural Convection

Common regime when buoyancy is dominant Dimensionless parameter Rayleigh number

Ratio of diffusive to advective time scales

Book has empirical relations for Vertical flat plates (eqns. 2.55, 2.56)

Horizontal cylinder (eqns. 2.57, 2.58) Spheres (eqns. 2.59) Cavities (eqns. 2.60)

Pr

T gH T H g Ra

/T2

33

H = plate height (m, ft)

T = temperature (C, F)

Q = heat transfer rate (W, Btu/hr)

g = acceleration due to gravity (m/s 2, ft/min 2)

T = absolute temperature (K, R)

Pr = Prandtl number


3/31

Phase Change Boiling

What temperature does water boil under idealconditions?


4/31

Radiation

Transfer of energy by electromagneticradiation Does not require matter (only requires that the

bodies can see each other) 100 10,000 nm (mostly IR)


5/31

Surface Radiation Issues

1) Surface properties are spectral, f( )

Usually: assume integrated properties for two beams:

Short-wave and Long-wave radiation

2) Surface properties are directional, f( )

Usually assume diffuse


6/31

Radiation emissionThe total energy emitted by a body,

regardless of the wavelengths, is given by:

Temperature always in K ! - absolute temperatures

emissivity of surface = 1 for blackbody

Stefan-Boltzmann constant

-

4

AT Qemited


7/31

Short-wave & long-wave radiation

Short-wave solar radiation 3mm Glass is not transparent Depends on surface temperature


8/31

Radiation Equations

2

2

2

1

211

1

42

411

21111

)(

A A

F

T T AQ

22

21

2111

3

22

21

2111

21

42

41

111

4

111

)()(

A A

F

T

A A

F

T T T T

h avg r

t AhQ r rad

Q1-2 = Q rad = heat transferred by radiation (W, BTU/hr) F 1-2 = shape factor

hr = radiation heat transfer coefficient (W/m 2/K, Btu/hr/ft 2/F) A = area (ft 2, m 2)

T,t = absolute temperature (R , K) , temperature (F, C)=


9/31

Combining Convection and Radiation

Both happensimultaneously on asurface Slightly different

temperatures

Often can use h = h c + h r


10/31

Humidity Ratio, W

W = m w /ma Degree of saturation, = W/W s

Humidity ratio is hard to measure, but veryuseful in calculations What are units?

Is W a function of temperature? What aboutW s?

W s = humidity ratio at saturationma = mass of dry air mw = mass of water vapor


11/31

Relative Humidity

= x w /xw,s = P w /P ws

Function of T

Easy to measure and useful in some contexts, butoften need to know temperature as well

W W s

622.0622.0

m

x = mole fraction P = pressure

= degree of saturationW = humidity ratio


12/31

Dew-point temperature, t d

Temperature at which condensation will form Under appropriate surface conditions

Vapor is saturated = ? W s( P , t d ) = W


13/31

Wet-bulb temperature, VBT ( t*)

Temperature of wet surface or

Temperature at which water, by evaporating into the

air, will bring air to saturation adiabatically

* superscript is designation that variable is evaluatedat the wet-bulb temperature

Note, distinct from that measured by a sling psychrometer

Section 9.5


14/31

Tables for Moist Air (P = 1 atm)

Tables A.4 in your text Ability to get W s for calculations

Subscripts: a = dry air, s = saturated air

v = v a+v as

h = h a+h as

s = s a+s as


15/31


16/31

Psychrometric Chart

Need two quantities for a state point Can get all other quantities from a state point

Can do all calculations without a chart Often require iteration Many digital psychrometric charts available

Can make your own

Best source is ASHRAE fundamentals (Chapter 6) Also in your text (back cover fold-out)


17/31Ref: Tao and Janis (2001)


21/31

Examples

What is enthalpy of air in the classroom rightnow?

Condensation on windows when taking ashower

How cold does it have to be outside for condensation toform on windows?

Assumption is that windows are the same temperature asoutside air 80 F, RH = 80%


22/31


23/31

Alternate calculation for W

PV = mRT (IGL)

What do we know about R ratio?

P = P w + P a

w

a

a

w

T R

V P

T R

V P

a

w

R

R

P

P

m

mW

a

a

w

w

w

w P P

P W 622.0

R = gas constant P = pressureV = volumeT = absolute temperatureW = humidity ratio

Subscripts: w is water vapor, a is dry air


24/31

Calculation of psychometric quantities

For an ideal gas, hda = c pa dT, h w = c pwdT

So, hda = c p,da t which assumes a reference state of 0F or 0 C Tables A4 Note different reference

hw = c pwt + h g0 h = c p,da t + W(c pwt + h g0 )Or you can use: h = c pt + Wh g0 , c p = c p,da + Wc pw

c p = specific heath = enthalpy

T = absolute temperaturet = temperatureW = humidity ratio

Subscripts: w is water vapor, a is dry air, g is

saturated water vapor


25/31

Adiabatic mixing

Governing equation hmQhmout in

External heat


26/31

Sensible heating

t cmQ p


27/31

Dehumidification by Cooling


28/31

Real Dehumidification Process


29/31


30/31


31/31

Summary

Describe psychrometric quantities Given any two psychrometric quantities,

calculate any other quantity Use Tables A4 or psychrometric charts to look

up psychrometric quantities Calculate psychrometric quantities at non-

standard conditions

Documents

389H_NO_5_psychrometrics.ppt