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