Objectives Describe psychrometric quantities Given any two psychrometric quantities, calculate any other quantity Use Tables A4 or psychrometric charts.

Objectives

• 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

• Evaluate psychrometric processes (8.1-8.4)

Humidity Ratio, W

• W = mw/ma

• Degree of saturation, µ = W/Ws

• Humidity ratio is hard to measure, but very useful in calculations

• What are units?

• Is W a function of temperature? What about Ws?

Ws = humidity ratio at saturationma = mass of dry airmw = mass of water vapor

Relative Humidity

• Φ = xw/xw,s = Pw/Pws

• Function of T

• Easy to measure and useful in some contexts, but often need to know temperature as well

W

Ws

622.0

622.0

x = mole fractionP = pressureμ = degree of saturationW = humidity ratio

Dew-point temperature, td

• Temperature at which condensation will form• Under appropriate surface conditions

• Vapor is saturated

• Φ = ?

• Ws(P, td) = W

Wet-bulb temperature, t*

• Temperature at which water, by evaporating into the air, will bring air to saturation adiabatically

• * superscript is designation that variable is evaluated at the wet-bulb temperature

• Note, distinct from that measured by a sling psychrometer• Section 9.5

Tables for Moist Air (P = 1 atm)

• Tables A.4 in your text

• Ability to get Ws for calculations

• Subscripts:• a = dry air, s = saturated air

v = va+µvas

h = ha+µhas

s = sa+µsas

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)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Examples

• What is enthalpy of air in the classroom right now?

• Condensation on windows when taking a shower

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

– Assumption is that windows are the same temperature as outside air

– 80 °F, RH = 80%

Alternate calculation for W

• PV = mRT (IGL)

• What do we know about R ratio?

• P = Pw + Pa

w

a

a

w

TRVP

TRVP

a

w

R

R

P

P

m

mW

a

a

w

w

w

w

PP

PW

622.0

R = gas constantP = pressureV = volumeT = absolute temperatureW = humidity ratio

Subscripts: w is water vapor, a is dry air

A diversion…

• For an ideal gas,• ha = ∫cpadT, hw = ∫cpwdT

• So, ha = cpat which assumes a reference state of 0 °F or 0 °C – Tables A4• Note different reference

• hw = cpwt + hg0

• h = cpat + W(cpwt + hg0)

• h = cpt + W∙hg0, cp = cpa + Wcpw

cp = specific heath = enthalpyT = absolute temperaturet = temperature W = humidity ratio

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

Approximating Psychrometric Processes

• Δhs = cpΔt, ΔhL = hwΔW=(cpwt + hg0) ΔW

• Where do these come from?

• Δh = h2 – h1, Δt = t2 – t1, ΔW = W2 – W1,

• t = (t1 + t2)/2, W = (W1 + W2)/2

• Δh = Δhs + ΔhL

• Major result• Δhs = cpΔt , ΔhL = hwΔW

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

Psychrometric Processes

• Adiabatic mixing of two streams

• Sensible heating and cooling

• Dehumidification by cooling

• Humidification

• Evaporative cooling

Adiabatic mixing

• Governing equation hmQhmoutin

Sensible heating

tcmQ p

Dehumidification by Cooling