1. HVAC Design: Field Trip Wednesday, October 22nd 9 am Location: St
HVAC Design: Field Trip Wednesday, October 22nd 9 am Location: St. Edward's University
Hosts:
- Hannah James
- Wesley Stidham, PE Principal Shah Smith and Associates, Inc

2. HW3
Cooling Cycles Problems: 1) Book: 3.1 (page 69), 2) Book: 3.5 ((page 70), 3) Same like 3.5 for R22 with no intercooler 4) Book 3.9 (pages 70-71) 5) Book 4.3 (page 110) Heat exchanger problems: 6) Book Problem 11.1 (page 326), 7) Problem 11.7 (page 327) Due March 23th

3. Objectives
Learn about heat exchangers (ch.11)

4. Heat exchangers Air-liquid Tube heat exchanger Air-air
Plate heat exchanger

5. Some Heat Exchanger Facts
All of the energy that leaves the hot fluid enters the cold fluid
If a heat exchanger surface is not below the dew point of the air, you will not get any dehumidification
Water takes time to drain off of the coil
Heat exchanger effectivness varies greatly

6. Heat Exchanger Effectivness (ε)
C=mcp
Mass flow rate
Specific capacity of fluid
THin
TCout
THout
TCin
Location B
Location A

7. Example:
What is the saving with the residential heat recovery system?
Outdoor Air
32ºF
72ºF
72ºF
Combustion
products
52ºF
Exhaust
Furnace
Fresh Air
Gas
For ε=0.5 and if mass flow rate for outdoor and exhaust air are the same
50% of heating energy for ventilation is recovered!
For ε=1 → free ventilation! (or maybe not)

8. Coil Extended Surfaces Compact Heat Exchangers
Fins added to refrigerant tubes
Important parameters for heat exchange?

9. Overall Heat Transfer Q = U0A0Δtm Mean temperature difference
Transfer Coefficient
Mean temperature
difference

10. Heat Exchangers Parallel flow Counterflow Crossflow
Ref: Incropera & Dewitt (2002)

11. Heat Exchanger Analysis - Δtm

12. Heat Exchanger Analysis - Δtm
Counterflow
For parallel flow is the same or

13. Counterflow Heat Exchangers
Important parameters:

14. What about crossflow heat exchangers?
Δtm= F·Δtm,cf
Correction
factor
Δt for
counterflow
Derivation of F is in the book:
………

15. Example:
Calculate Δtm for the residential heat recovery system if : mcp,hot= 0.8· mc p,cold
th,i=72 ºF, tc,i=32 ºF
For ε = 0.5 → th,o=52 ºF, th,i=48 ºF → R=1.25, P=0.4 → F=0.89
Δtm,cf=(20-16)/ln(20/16)=17.9 ºF, Δtm=17.9 ·0.89=15.9 ºF

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

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

18. 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

19. 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