Objectives

- Finish heat exchangers

- Air Distribution Systems - Diffuser selection- Duct design

- fluid dynamics review

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

,

,

Example 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

steamRA

OA

Qcc=195600Btu/h

tM=81ºF

tCC=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

Reading Assignment

• Chapter 11- From 11.1-11.7

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)

Overall Heat Transfer Coefficients

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

Air Distribution System Design

• Describe room distribution basics

• Select diffusers• Supply and return

duct sizing

Forced driven air flowDiffusers

Linear diffusersGrill (side wall)

diffusers

Horizontal one side

Vertical

Diffusers types

swirl diffusers

wall or ceiling

floor

Valve diffuser

ceiling diffuser

Diffusers

http://www.titus-hvac.com/techzone/http://www.halton.com/halton/cms.nsf/www/diffusers

Perforated ceiling diffuser Jet nozzle diffuser Square conical ceiling diffuser Round conical ceiling diffuser

Wall diffuser unit Swirl diffuser Floor diffuser Auditorium diffuser

DV diffuser External louvre Smoke damper Linear slot diffuser

Low mixing Diffusers Displacement ventilation

18.7

Diffuser Selection Procedure

• Select and locate diffusers, divide airflow amongst diffusers

V = maximum volumetric flow rate (m3/s, ft3/min)Qtot = total design load (W, BTU/hr)Qsen = sensible design load (W, BTU/hr)ρ = air density (kg/m3, lbm/ft3)Δt = temperature difference between supply and return air (°C, °F)Δh = enthalpy difference between supply and return air (J/kg, BTU/lbm)

ΔtQ

ΔhQV sentot

Find Characteristic Length (L)

Indicator of Air DistributionQuality

• ADPI = air distribution performance index• Fraction of locations that meet criteria:

• -3 °F < EDT < 2 °F or -1.5 °C < EDT < 1 °C• Where, EDT = effective draft temperature

• Function of V and Δt (Eqn 18.1)

• EDT=(tlocal-taverage)-M(Vlocal-Vaverage) , M=7 °C/(m/s)

ADPI considers ONLY thermal comfort (not IAQ)

Ideal and Reasonable Throws

Select Register

• Pick throw, volumetric flow from register catalog• Check noise, pressure drop

Summary of Diffuser Design Procedure

1) Find Q sensible total for the space

2) Select type and number of diffusers 3) Find V for each diffuser 4) Find characteristic length5) Select the diffuser from the manufacturer data

Example 18.3

• Qtot = 38.4 kBTU/hr

• Δh = 9.5 BTU/lbma

ΔtCQ

ΔhQV

p

sentot

omission in text

Pressures

• Static pressure

• Velocity pressure

• Total pressure – sum of the two above

Relationship Between Static and Total Pressure

2

22

21 VVPP st

• Total and static pressure drops are proportional to square of velocity

• Plot of pressure drop vs. volumetric flow rate (or velocity) is called system characteristic

Duct Design

gDLVf

2

2 sPD

LVfg 2

2

System Characteristic

Electrical Resistance Analogy

Frictional Losses

Non-circular Ducts

• Parallel concept to wetted perimeter

Dynamic losses

• Losses associated with• Changes in velocity• Obstructions• Bends• Fittings and transitions

• Two methods• Equivalent length and loss coefficients

Loss Coefficients 20

1

0

1

VV

CCΔPt = CoPv,0

Example 18.7

• Determine total pressure drop from 0 to 4

Conversion Between Methods

fDCL

gVC

gDVL

f

eq

eq

0

20

2

22

Reading asignement

• Chapter 18• 18.1-18.4 (including 18.4)