Title: HVAC Design: Field Trip Wednesday, October 22nd 9 am Location: St. Edward's University
1HVAC Design Field TripWednesday, October 22nd 9
amLocation St. Edward's University
- Hosts
- - Hannah James
- - Wesley Stidham, PE PrincipalShah Smith and
Associates, Inc
2HW3
- Cooling Cycles Problems 1) Book 3.1 (page
69), 2) Book 3.5 ((page 70), 3) Same like 3.5
for R22 with no intercooler4) 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
-
3Objectives
- Learn about heat exchangers (ch.11)
4Heat exchangers
Air-liquid
Tube heat exchanger
Plate heat exchanger
Air-air
5Some 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
6Heat Exchanger Effectivness (e)
Cmcp
Mass flow rate
Specific capacity of fluid
THin
TCout
THout
TCin
Location B
Location A
7Example
What is the saving with the residential heat
recovery system?
Outdoor Air
32ºF
72ºF
72ºF
Combustion products
52ºF
Furnace
Exhaust
Fresh Air
Gas
For e0.5 and if mass flow rate for outdoor and
exhaust air are the same 50 of heating energy
for ventilation is recovered! For e1 ? free
ventilation! (or maybe not)
8Coil Extended Surfaces Compact Heat Exchangers
- Fins added to refrigerant tubes
- Important parameters for heat exchange?
9Overall Heat Transfer
- Q U0A0?tm
- Overall Heat
- Transfer Coefficient
Mean temperature difference
10Heat Exchangers
- Parallel flow
- Counterflow
- Crossflow
Ref Incropera Dewitt (2002)
11Heat Exchanger Analysis - ?tm
12Heat Exchanger Analysis - ?tm
Counterflow
For parallel flow is the same
or
13Counterflow Heat Exchangers
Important parameters
14What about crossflow heat exchangers?
Correction factor
?t for counterflow
Derivation of F is in the book
15- Example
- Calculate ?tm for the residential heat recovery
system if mcp,hot 0.8 mc p,cold - th,i72 ºF, tc,i32 ºF
- For e 0.5 ? th,o52 ºF, th,i48 ºF ? R1.25,
P0.4 ? F0.89 - ?tm,cf(20-16)/ln(20/16)1
7.9 ºF, ?tm17.9 0.8915.9 ºF
16Overall Heat Transfer
Need to find this
17Overall Heat Transfer
Need to find this
18Resistance model
- Q U0A0?tm
- 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
19Example
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 ?tcold mcp,hot ?thot
U0A0?tm From heat exchanger side Q
U0A0?tm ? A0 Q/ U0?tm U0 1/(RInternalRCond
RFinRExternal) (1/100.00201/10) 4.95
Btu/hsfF ?tm 16.5 F From air side Q
mcp,cold ?tcold
200cfm60min/h0.075lb/cf0.24Btu/lbF16 3456
Btu/h Then A0 3456 / (4.9516.5) 42 sf