1. Objectives Finish with heat exchangers (ch.11)
Start with Air-distribution systems

2. Overall Heat Transfer
Q = U0A0Δtm
Need to find this

3. Resistance model Q = U0A0Δtm From eq. 1, 2, and 3:
We can often neglect conduction through pipe walls
Sometime more important to add fouling coefficients
R Internal
R cond-Pipe
R External

4. Example
The air to air heat exchanger in the heat recovery system from previous example has flow rate of fresh air of 200 cfm.
With given: Calculate the needed area of heat exchanger A0=?
Solution: Q = mcp,cold Δtcold = mcp,hot Δthot = U0A0Δtm
From heat exchanger side: Q = U0A0Δtm → A0 = Q/ U0Δtm
U0 = 1/(RInternal+RCond+RFin+RExternal) = (1/ /10) = 4.95 Btu/hsfF
Δtm = 16.5 F
From air side: Q = mcp,cold Δtcold =
= 200cfm·60min/h·0.075lb/cf·0.24Btu/lbF·16 = 3456 Btu/h
Then: A0 = 3456 / (4.95·16.5) = 42 sf

5. For Air-Liquid Heat Exchanger we need 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
tF,m

6. Fin Theory pL=L(hc,o /ky)0.5 k – conductivity of material
hc,o – convection
coefficient
pL=L(hc,o /ky)0.5

7. 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

10. Heat exchanger performance (11.3)
NTU – absolute sizing (# of transfer units)
ε – relative sizing (effectiveness)
Criteria
NTU ε P RP cr

12. Air Distribution System Design
Describe room distribution basics
Select diffusers
Supply and return duct sizing

13. Forced driven air flow Diffusers
Grill (side wall)
diffusers
Linear diffusers
Vertical
Horizontal one side

14. Diffusers types Valve diffuser swirl diffusers ceiling diffuser
wall or ceiling
floor

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

16. Low mixing Diffusers Displacement ventilation

19. 18.7

20. 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)