Objectives

• Finish with Heat exchangers

• Learn about Specifics of VAV and DOAS systems

Heat exchanger performance (Book Section 11.3)

• NTU – absolute sizing (# of transfer units)

• ε – relative sizing (effectiveness)

Criteria

NTU

ε P RP

cr

hphcpc cmcm ,, hphcpc cmcm ,,

cpc

oo

cm

AU

,

hph

cpc

cm

cm

,

,

hph

oo

cm

AU

,

cpc

hph

cm

cm

,

,

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, air

Introduce Lewis Number

3. Digression – approximate enthalpy

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

Energy and mass balances

• Steady-state energy equation on air

• Energy balance on water

• Mass balance on water

4. Fin analysis for wet fins

Heat conduction only occurs in y-direction through water film

Overall Heat Transfer Coefficients

• Very parallel procedure to dry coil problem

• U-values now influenced by condensation

• See Example 11.6 for details

Wet Surface Heat Transfer

• If you know dry surface heat transfer• Reynolds number changes – empirical

relationships

• Approximate wet-surface

• Does a wet or a dry coil have higher or lower heat exchange?

• Does a wet or a dry coil have higher or lower pressure drop?

HW4(posted on the website)Homework Assignment # 4 Pat A

DUE: April 9, 2013

 

•Problem 11.1 (page 326)

•Problem 11.7 (page 327)

• Designs Problem

For the problem #9 from the HW assignment 2 (design process in AHU) calculate the needed a) Effectiveness of the cooling coil b) UoAo value for the CC Assume Inlet water temperature into CC is coil is 45ºF

Homework Assignment # 4 Pat B

DUE: April 16, 2013

 

•Problem 18.13

•Problem 18.16

•Problem 18.19

•Problem 18.20

•Problem 18.27

Sensible and Enthalpy wheels

VAV Example

»http://www.trane.com/commercial/library/vol242/v24b6.asp

-One system

-Multiple Spaces/Zones

»- VAV box for each space/zone

VAV System Terms & Equations(you can find them in ASHRAE Standard 62)

• Fraction of minimum outdoor air in space/zone supply airXi = VOAi / VSAi

• Critical space

Space with maximum Xi , Z=max(Xi)

• Fraction of outdoor air in system supply air

X = sum(VOA ) / sum(VSA)

• Required Fraction of outdoor air:

Y=X/(1+X-Z)

Other Issues Related to VAV

• Reheaters (sizing and control)

• Fan control

• Minimum supply air

• Occupancy sensors

DOAS System

• www.doas.psu.edu

DOAS with multi-split systems

»Fresh air?

DOAS fresh air configurations

DOAS fresh air configurations

Issues Related to DOAS

• Split of sensible and latent load

• Selection of hydronic system

• Winter vs. summer operation • Winter operation with DX systems (heat pump)