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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)

Example problem

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

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

- 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)

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

Grill (side wall)

diffusers

Linear diffusers

Vertical

Horizontal one side

Diffusers

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

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)

Select Register

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

Summary of Diffuser Design Procedure

- 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

Pressures

- Static pressure
- Velocity pressure
- Total pressure – sum of the two above

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 DesignSystem Characteristic of velocity

Electrical Resistance Analogy of velocity

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

Conversion Between Methods of velocity

Reading asignement of velocity

- Chapter 18
- 18.1-18.4 (including 18.4)

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