Announcements

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Announcements

• Tonight ASHRAE Student Chapter Meeting

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Objectives

• Finish with review thermodynamics
• Learn about Psychrometrics

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p-h diagram
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Ideal gas law

• Pv RT or PV nRT
• R is a constant for a given fluid
• For perfect gasses
• ?u cv?t
• ?h cp?t
• cp - cv R

M molecular weight (g/mol, lbm/mol) P
pressure (Pa, psi) V volume (m3, ft3) v
specific volume (m3/kg, ft3/lbm) T absolute
temperature (K, R) t temperature (C, F) u
internal energy (J/kg, Btu, lbm) h enthalpy
(J/kg, Btu/lbm) n number of moles (mol)
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Mixtures of Perfect Gasses

• m mx my
• V Vx Vy
• T Tx Ty
• P Px Py
• Assume air is an ideal gas
• -70 C to 80 C (-100 F to 180 F)

Px V mx RxT Py V my RyT
What is ideal gas law for mixture?
m mass (g, lbm) P pressure (Pa, psi) V
volume (m3, ft3) R material specific gas
constant T absolute temperature (K, R)
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Mass-Weighted Averages

• Quality, x, is mg/(mf mg)
• Vapor mass fraction
• f v or h or s in expressions below
• f ff x ffg
• f (1- x) ff x fg

s entropy (J/K/kg, BTU/R/lbm) m mass (g,
lbm) h enthalpy (J/kg, Btu/lbm) v specific
volume (m3/kg) Subscripts f and g refer to
saturated liquid and vapor states and fg is the
difference between the two
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Properties of water

• Water, water vapor (steam), ice
• Properties of water and steam (pg 675 685)
• Alternative - ASHRAE Fundamentals ch. 6

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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 saturation ma mass of
dry air mw mass of water vapor
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Relative Humidity

• F xw/xw,s Pw/Pws
• Function of T
• Easy to measure and useful in some contexts, but
  often need to know temperature as well

x mole fraction P pressure µ degree of
saturation W humidity ratio
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Dew-point temperature, td

• Temperature at which condensation will form
• Under appropriate surface conditions
• Vapor is saturated
• F ?
• Ws(P, td) W

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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
  evaluated at the wet-bulb temperature
• Note, distinct from that measured by a sling
  psychrometer
• Section 9.5

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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 vaµvas h haµhas s saµsas
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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)

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Alternate calculation for W

• PV mRT (IGL)
• What do we know about R ratio?
• P Pw Pa

R gas constant P pressure V volume T
absolute temperature W humidity
ratio Subscripts w is water vapor, a is dry air
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Calculation of psychometric quantities

• For an ideal gas,
• hda ?cpadT, hw ?cpwdT
• So, hda cp,dat which assumes a reference state
  of 0
• F or 0 C Tables A4
• Note different reference
• hw cpwt hg0
• h cp,dat W(cpwt hg0)
• Or you can use
• h cpt Whg0, cp cp,da Wcpw

cp specific heat h enthalpy T absolute
temperature t temperature W humidity
ratio Subscripts w is water vapor, a is dry
air, g is saturated water vapor
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Adiabatic mixing

• Governing equation

External heat
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Sensible heating
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Dehumidification by Cooling
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Real Dehumidification Process