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# Objectives - PowerPoint PPT Presentation

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

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Presentation Transcript

• Finish heat exchangers

• Air Distribution Systems

• Diffuser selection

• Duct design

• fluid dynamics review

• 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

• NTU – absolute sizing (# of transfer units)

• ε – relative sizing (effectiveness)

AHU

M

For the problem 9 HW assignment # 2

(process in AHU) calculate:

a) Effectiveness of the cooling coil

b) UoAo value for the CC

Inlet water temperature into CC is coil is 45ºF

OA

CC

CC

(mcp)w

steam

RA

tc,in=45ºF

Qcc=195600Btu/h

tM=81ºF

tCC=55ºF

• 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

• Chapter 11

- From 11.1-11.7

• Redo fin theory

• Energy balance on fin surface, water film, air

Introduce Lewis Number

• Digression – approximate enthalpy

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

• Very parallel procedure to dry coil problem

• U-values now influenced by condensation

• See Example 11.6 for details

• Describe room distribution basics

• Select diffusers

• Supply and return duct sizing

Forced driven air flowDiffusers

Grill (side wall)

diffusers

Linear diffusers

Vertical

Horizontal one side

Valve diffuser

swirl diffusers

ceiling diffuser

wall or ceiling

floor

Perforated ceiling diffuser

Jet nozzle diffuser

Round conical ceiling diffuser

Square conical ceiling diffuser

Wall diffuser unit

Swirl diffuser

Floor diffuser

Auditorium diffuser

Linear slot diffuser

DV diffuser

External louvre

Smoke damper

http://www.titus-hvac.com/techzone/

http://www.halton.com/halton/cms.nsf/www/diffusers

Low mixing Diffusers Displacement ventilation

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)

Diffuser Selection Procedure

• Select and locate diffusers, divide airflow amongst diffusers

• 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)

• Pick throw, volumetric flow from register catalog

• Check noise, pressure drop

• Find Q sensible total for the space

• Select type and number of diffusers

• Find V for each diffuser

• Find characteristic length

• Select the diffuser from the manufacturer data

• Qtot = 38.4 kBTU/hr

• Δh = 9.5 BTU/lbma

omission in text

• Static pressure

• Velocity pressure

• Total pressure – sum of the two above

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

Duct Design

Frictional Losses of velocity

Non-circular Ducts of velocity

• Parallel concept to wetted perimeter

Dynamic losses of velocity

• Losses associated with

• Changes in velocity

• Obstructions

• Bends

• Fittings and transitions

• Two methods

• Equivalent length and loss coefficients

Loss Coefficients of velocity

ΔPt = CoPv,0

Example 18.7 of velocity

• Determine total pressure drop from 0 to 4