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Theoretical Discussion:
Impacts of Air flow on AC
Performance
ASHRAE Summer Meeting
June 30, 2004
Hugh I Henderson, Jr., P.E.
CDH Energy Corp.
Cazenovia, NY
www.cdhenergy.com
Overview
• Use Manufacturer’s “Catalog” Data to
Show Impact of Airflow on Performance
– efficiency & capacity
• Use Public Domain Steady-State
Refrigeration Model to Evaluate Flow
Impacts
Catalog Data for
3-ton RTU
• Combined “Gross” Coil Performance Data with Fan Curves
– 300 to 500 cfm/ton
• Shows Combined Impact of Flow:
– comp efficiency increases with flow
– fan power increases with flow
9.0
9.2
9.4
9.6
9.8
10.0
10.2
10.4
10.6
10.8
800 900 1000 1100 1200 1300 1400 1500 1600
Airflow (cfm)
Net
EE
R (
Btu
/Wh
) 80/72
80/67
80/62
-
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
800 900 1000 1100 1200 1300 1400 1500 1600
Airflow (cfm)
SH
R (
-)
80/72
80/67
80/62
Fan Performance
Impacts EER
Trends
3-ton, Standard-Fan, Direct Drive
0
0.1
0.2
0.3
0.4
0.5
0.6
800 900 1000 1100 1200 1300 1400 1500 1600
Airflow (cfm)
Un
it S
tati
c P
ressu
re (
in)
Fan
Po
wer
(kW
)
3-ton, Standard-Fan, Direct Drive
0.230
0.235
0.240
0.245
0.250
0.255
0.260
0.265
0.270
0.275
0.280
0.285
800 900 1000 1100 1200 1300 1400 1500 1600
Airflow (cfm)
No
rmali
zed
Fan
Po
wer
(Watt
/cfm
)
• Unit Static Ranges from 0.16 to 0.49 in
• RTU similar to Residential
– direct drive, FC fan
– DX coil & furnace section
• 0.24-0.28 W/cfm
Static
Power
Impact of Flow at
Nominal Conditions 80/67F, 95F
3-ton RTU
97%
98%
99%
100%
101%
102%
800 900 1000 1100 1200 1300 1400 1500 1600
Airflow (cfm)
Rela
tive V
alu
e
GROSS Efficiency
NET Efficiency
3-ton RTU
95%
96%
97%
98%
99%
100%
101%
102%
103%
104%
800 900 1000 1100 1200 1300 1400 1500 1600
Airflow (cfm)
Rela
tive V
alu
e
Power
NET Capacity
• NET Capacity and Power change by ±2-3%
• NET EER changes by less than 1%
Use AC Model to Evaluate Trends
• Public-Domain Refrigeration System Model
– 20+ years of history
– validated by ASHRAE 859-RP, others
– web-interface, parametric run mode
• Default Model Inputs
– 2.5 ton, reciprocating compressor,
– R22, 400 cfm/ton
– supply fan: 0.365 W/cfm (constant value)
Refrigeration
Model
Output
Almost No DB Impact AC Model Performance
55 60 65
Entering WB (F)
8.0
8.5
9.0
9.5
10.0
Ne
t E
ER
(B
tu/W
h)
70DB 72DB
74DB
76DB
78DB
80DB
82DB
EER is a
function of
WB – except
for dry coil
Impact of Entering WB
AC Model Performance
250 300 350 400 450 500 550
Normalized Airflow (cfm/ton)
8.0
8.5
9.0
9.5
10.0
Ne
t E
ER
(B
tu/W
h)
56WB
58WB
60WB
62WB
64WB
66WB
68WB
What Are Typical Entering Conditions?
Nominal: 80F & 51% RH 67F WB
Typical: 75F & 55% RH 64F WB
Mild: 80F & 35% RH 62F WB
…BUT so is…
75F & 48% RH
Dry Climate: 75F & 35% RH 58F WB
Impact of Fan Power (at 64F WB)
AC Model Performance
250 300 350 400 450 500 550
Normalized Airflow (cfm/ton)
8.5
9.0
9.5
10.0
10.5
11.0N
et
EE
R (
Btu
/Wh
) 0.1 W/cfm
0.2 W/cfm
0.3 W/cfm
0.4 W/cfm
0.5 W/cfm
0.365 W/cfm
AC Model Performance
250 300 350 400 450 500 550
Normalized Airflow (cfm/ton)
7
8
9
10
11
Ne
t E
ER
(B
tu/W
h) 0.1 W/cfm
0.2 W/cfm
0.3 W/cfm
0.4 W/cfm
0.5 W/cfm
Impact of Fan Power (at 60F WB)
0.365 W/cfm
AC Model Performance
250 300 350 400 450 500 550
Normalized Airflow (cfm/ton)
7
8
9
10
11
Ne
t E
ER
(B
tu/W
h)
0.1 W/cfm
0.2 W/cfm
0.3 W/cfm
0.4 W/cfm
0.5 W/cfm
Impact of Fan Power (at 58F WB)
0.365 W/cfm
What Fan Power is Typical?
• Same 3-ton RTU offers High Static Option with
Belt Driven Fan
– Fan speed set to provide flow & static
• Fan Power Can Change by Factor of 5 Across the
Range: 0.2 to 1.0 Watt/cfm
cfm/ton 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
300 0.20 0.28 0.38 0.48 0.54 0.63 0.69 0.75 0.80 0.86
333 0.22 0.30 0.38 0.49 0.55 0.63 0.69 0.75 0.80 0.85
367 0.24 0.32 0.36 0.50 0.55 0.61 0.66 0.72 0.77 0.82
400 0.27 0.34 0.41 0.53 0.56 0.61 0.67 0.73 0.78 0.83
433 0.29 0.37 0.46 0.54 0.58 0.64 0.71 0.76 0.82 0.87
467 0.32 0.41 0.48 0.55 0.60 0.66 0.73 0.79 0.84 0.90
500 0.35 0.44 0.51 0.57 0.63 0.70 0.76 0.83 0.89 0.95
Unit Static (in)
Normalized Fan Power (Watts/cfm)
Riding Fan
Curve
What if I Don’t Need the Latent
Capacity in My Dry Climate?
• Don’t Worry Your Not Getting Much…. COIL1_TEST_4B_10B_16B_22B 08/30/02 07:42:04 Cycle #1 (Comp ON time: 45.0 minutes)
0 20 40 60 80 100
time (minutes)
-20
-10
0
10
20
30
Capacity (
MB
tu/h
)
Compressor
Sensible
Latent Addition
Latent Removal
Off-Cycle
Evaporation
Sensible Latent
EER ~
Sensible EER
Summary
• Airflow has Very Little Impact on EER near
Nominal Conditions
– but more impact at dry coil conditions
• The Variation of EER with Airflow
Depends on Fan Power
• Best Airflow Rate Depends on Climate and
Equipment Characteristics
