Objectives

• Finish heat exchanger • Heat exchanger performance• Dry HX vs. Vet heat exchanger • Example

Air-liquid Tube heat exchanger

Plate heat exchanger

Heat exchangers

Air-air

Fin Efficiency

• Assume entire fin is at fin base temperature• Maximum possible heat transfer• Perfect fin

• Efficiency is ratio of actual heat transfer to perfect case

• Non-dimensional parameter

Heat exchanger performance (11.3)

• NTU – absolute sizing (# of transfer units)• ε – relative sizing (effectiveness)

Criteria

NTU

ε P RP

cr

hphcpc cmcm ,, hphcpc cmcm ,,

cpc

oocmAU

,

hph

cpc

cmcm

,

,

hph

oocmAU

,

cpc

hph

cmcm

,

,

ExampleFollowing HW2 problem

For the problem 9 HW assignment # 2 (process in AHU) calculate:a) Effectiveness of the cooling coilb) UoAo value for the CCInlet water temperature into CC is coil is 45ºF

AHU

M CC

RA

OA

Qcc=…..Btu/htM=81ºFtCC=55ºF

CC

(mcp)w

tc,in=45ºF

Summary

• Calculate efficiency of extended surface• Add thermal resistances in series• If you know temperatures

• Calculate R and P to get F, ε, NTU• Might be iterative

• If you know ε, NTU• Calculate R,P and get F, temps

Analysis of Moist Coils

1. Redo fin theory 2. Energy balance on fin

surface, water film, airIntroduce Lewis Number

3. Digression – approximate enthalpy

4. Redo fin analysis for cooling/ dehumidification (t → h)

1. Redo Fin Theory

• Same result pLpLtanh

2. Energy and mass balances

• Steady-state energy equation on air• Energy balance on water• Mass balance on water• Rewrite energy balance on water surface

• Reintroduce hg0 (enthalpy of sat. water vapor at 0 °C or °F)

3. Define heat-transfer coefficient for wet surface

4. Fin analysis for wet finsHeat conduction only occurs in y-direction through water film

w

w

ocw

pa

ky

hbcwoh

,

1,

Overview of Procedure

1. Same approach as for dry fin with addition of conduction through water film

2. Define “fictitous moist air enthalpy” define at water surface temperature

3. Define heat-transfer coefficient

4. Develop new governing equation

Lx

xΔhΔh

xΔhΔh

ykh

xΔh

BFFFF

F

woF

@0

0@

dd

dd

,2

2,

Overall Heat Transfer Coefficients

• Very parallel procedure to dry coil problem• U-values now influenced by condensation• See Example 11.6 for details

Approximate Expression for Mean Enthalpy Difference

1,,2

2,,1

1,,22,,1

lnrs

rs

rsrsm

hhhh

hhhhh

h1 enthalpy of entering air stream

h2 enthalpy of leaving air stream

hs,R,1 fictitious enthalpy of saturated air at entering refrigerant temp.

hs,R,2 fictitious enthalpy of saturated air at leaving refrigerant temp.

Wet Surface Heat Transfer

• Approximate dry and wet wet-surface area and apply relevant equations

Reynolds number changes – empirical relationships

Questions:• Does a wet or a dry coil have higher or lower heat

exchange?• Does a wet or a dry coil have higher or lower pressure

drop?