objectives heat exchangers fluid mechanics (ducts and pitot tubes) introduction to psychrometrics
TRANSCRIPT
Objectives
• Heat exchangers
• Fluid mechanics (ducts and pitot tubes)
• Introduction to psychrometrics
Any Questions/Administrative Issues?
• Syllabus
• Website
• Homework
• Lecture notes/videos• Feedback?• Is no PDF okay?
Indoor Air 2005
• Very successful and engaging conference
• Excellent talks and discussion
• Trends for the future• Indoor secondary pollutants• PCO (mixed feelings)• Whole-building approaches/solutions• Focus on lesser-developed nations
Photo Album
Heat Exchangers
• Counterflow
• Crossflow
• Cocurrent flow
Ref: Incropera & Dewitt (2002)
Shell-and-Tube Heat Exchanger
Ref: Incropera & Dewitt (2002)
Heat Exchanger Analysis
Heat Exchanger Analysis
• We want to know the energy exchange between two fluids• But, the temperature difference is not constant
• Q = UAΔtm
• The trick is coming up with appropriate expression for Δ tm
Q= heat transferred (W, BTU/hr) UA = area × U-value (W/K, Btu/hr/F)Δtm = mean temperature difference (°F, °C)
Heat Exchanger Analysis
ocih
icoh
ocihicohm
tt
tt
ttttt
,,
,,
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ln
Counterflow
b
a
bam
t
t
ttt
ln
Parallel
Fluid Flow in Pipes
• Analogy to steady-flow energy equation• Consider incompressible, isothermal flow• What is friction loss?
gD
LVf
2
2
Pitot Tubes
Summary
• Use relationships in text to solve conduction, convection, radiation, phase change, and mixed-mode heat transfer problems
• Calculate components of pressure for flow in pipes and ducts
Any questions about review material?
• Where are we going?• Psychrometrics
• Psychrometric terms
• Using tables for moist air
• Application of IGL
• Using psychrometric charts
• 7.1 – 7.5, 7.7
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
Summary
• Review material in chapter 2
• Describe psychrometric quantities