objectives heat exchangers fluid mechanics (ducts and pitot tubes) introduction to psychrometrics

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

,,

,,

,,,,

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