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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

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objectives
Objectives
  • Finish heat exchangers
  • Air Distribution Systems
    • Diffuser selection
    • Duct design
      • fluid dynamics review
fin efficiency
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
Heat exchanger performance (11.3)
  • NTU – absolute sizing (# of transfer units)
  • ε – relative sizing (effectiveness)
example problem
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
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
Reading Assignment
  • Chapter 11

- From 11.1-11.7

analysis of moist coils
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
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
Air Distribution System Design
  • Describe room distribution basics
  • Select diffusers
  • Supply and return duct sizing
forced driven air flow diffusers
Forced driven air flowDiffusers

Grill (side wall)

diffusers

Linear diffusers

Vertical

Horizontal one side

diffusers types
Diffusers types

Valve diffuser

swirl diffusers

ceiling diffuser

wall or ceiling

floor

diffusers
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

diffuser selection procedure
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 distribution quality
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
Select Register
  • Pick throw, volumetric flow from register catalog
  • Check noise, pressure drop
summary of diffuser design procedure
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
example 18 3
Example 18.3
  • Qtot = 38.4 kBTU/hr
  • Δh = 9.5 BTU/lbma

omission in text

pressures
Pressures
  • Static pressure
  • Velocity pressure
  • Total pressure – sum of the two above
duct design
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
non circular ducts
Non-circular Ducts
  • Parallel concept to wetted perimeter
dynamic losses
Dynamic losses
  • Losses associated with
    • Changes in velocity
    • Obstructions
    • Bends
    • Fittings and transitions
  • Two methods
  • Equivalent length and loss coefficients
loss coefficients
Loss Coefficients

ΔPt = CoPv,0

example 18 7
Example 18.7
  • Determine total pressure drop from 0 to 4
reading asignement
Reading asignement
  • Chapter 18
    • 18.1-18.4 (including 18.4)
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