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ASHRAE Region V CRC August 12, 2005 Hugh I. Henderson, Jr., P.E. CDH Energy Corp. Cazenovia, NY Understanding Dehumidification Performance at Part Load in Commercial Applications DOE/NETL Project #DE-FC26-01NT41253
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Page 1: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

ASHRAE Region V CRC August 12, 2005

Hugh I. Henderson, Jr., P.E.CDH Energy Corp.Cazenovia, NY

Understanding Dehumidification Performance at Part Load in

Commercial Applications

DOE/NETL Project #DE-FC26-01NT41253

Page 2: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Overview

o DOE Research Project Results Ø field tested various commercial and residential

cooling systemsØ detailed laboratory tests of DX and CHW coilsØ Findings: latent capacity degrades at part load

conditions

o Implications for equipment manufacturers & design engineers?Ø Calcs at sensible or latent design conditions are

not enoughØ Staging is key

Page 3: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Steady-State Cooling Coil Performance

o Cooling coil performance is well understood at steady-state conditions Ø Coils provide both sensible cooling and

moisture removal (latent cooling) Ø SHR: sensible heat ratio…..fraction of total

cooling that is sensible Ø Colder coil surfaces remove more moistureØ Reduced air flow provides more moisture

removal (for DX coils)Ø Higher condenser/outdoor temperatures

increase SHR

Page 4: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

40 45 50 55 60 65 70 75 80 85Dry Bulb Temperature (F)

0.000

0.005

0.010

0.015

0.020

Abs

olut

e H

umid

ity R

atio

(lb

/lb)

SHR: 0.80

Sat

. Tem

p =

42.

4Total: 2.0 tons (coil)Power: 2.5 kW EER: 9.6 Btu/Wh

Typical AC Coil Performance

Page 5: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

-

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 (

-)

80/7280/6780/62

Catalog Data for 3-ton RTU

o SHR is decreases with… Ø Lower air flow cfm/tonØ Higher WB (actually RH)

Nominal Rating Point: SHR ~ 0.71

Page 6: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

DX Part Load Performance

o AC cycles compressor ON and OFF based on a space thermostat

o The portion of time the coil operates (i.e., the runtime fraction) is longer when cooling loads are greater

RTF = ON(ON + OFF)

o How do sensible and latent capacity vary under cyclic conditions?

Page 7: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Sensible and Latent Capacity With Continuous Supply Air Fan Operation

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 100time (minutes)

-20

-10

0

10

20

30

Cap

acity

(M

Btu

/h)

Compressor

Sensible

Latent Addition

Latent Removal

Off-Cycle Evaporation is Adiabatic Process:

Sensible ≈ Latent Transition pt

Page 8: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Sensible Heat Ratio (SHR) vs Efficiency Debate

q Some in the industry assert that high efficiency AC have lowered SHRsØ ARI studies of industry test data have shown steady-state SHRs

remain near 0.75 …. unchanged from the past

q Part-load latent degradation may explain some of this disconnectØ newer equipment may use larger coils and lower suction

temperatures to achieve the same SHRØ steady-state performance is unchanged….but part-load

characteristics may be affected

Page 9: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Lab & Field Test Results

&

Eng. Model Development

Page 10: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Latent Degradation Concepts

0 20 40 60TIME (minutes)

La

tent

Cap

acity

Delay Time (to)time for first condensate to fall from pan

QL

Qe

Useful Moisture RemovalMo

Evaporation

twet = Mo/QL

γ = Qe/QL

Mo ≈ Evaporation

Page 11: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

LHR Degradation Model

o Borrowed approaches used to develop part-load efficiency degradation function for SEER test procedure

o Used same part-load assumptions (Cd):Ø AC at startup described by a time constantØ Cycling rate driven by thermostat curve

o Additional latent assumptions:Ø Coil surfaces hold a fixed amount of moisture (Mo)Ø Off-cycle coil is like an evaporative cooler

Page 12: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction (-)

0.70

0.80

0.90

1.00

1.10

SH

R

Latent Degradation Model with:

twet = 720 secγ = 1.07Nmax = 3.6 cyc/hτ = 75 seclinear decay

Measured Data:

• averaged into 4 hour intervals

• entering RH: 57% to 63%

• entering air temp: 68F to 72F

• loop temps: 65F to 90FSteady State SHR = 0.76

Continuous supply air fan operation

Comparing LHR Model to Field Tests

o Henderson (1998) compared the model results to field data collected on a 3-ton residential geothermal heat pump

Page 13: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

0

20

40

60

80

100

120

140

160

180

2 3 4

Number of Rows

Occ

uren

ces

Packaged

Air Handlers

Evap Coils

Equipment Survey

o Reviewed specifications for 500 commercial and residential AC units

o Goal was to determine Ø range of common coil

geometriesØ Variation by equipment type

o Typical DX AC coil is 3-rows, 15 fpi

0

20

40

60

80

100

120

140

160

180

200

10 11 12 13 13.5 14 14.5 15 16 17 18

Fin Spacing (fpi)

Occ

uren

ces

PackagedAir Handlers

Evap Coils

Page 14: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Field Testing

q Six field test sites were recruited:

Residential (5 units at 4 sites)

Ø 5-ton single-speed residential DX system in Connecticut

Ø Two single-speed DX units (2.5-ton and 3-ton) at a Virginia residence

Ø 3.5-ton residential DX system in Florida (single-speed condensing unit with variable speed air handler)

Ø 3-ton residential DX system in Florida (two-speed condensing unit with variable speed air handler)

Commercial (3 units at 2 sites)

Ø 10-ton commercial rooftop DX unit in Boston (2 stage)

Ø Commercial constant-air-volume chilled water coil in Florida (3 ton)

Ø Commercial variable-air-volume chilled water coil in Florida (7 ton)

Page 15: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

COIL2_TEST_4B_10B_16B_22B_25B 11/14/02 10:07:57 Cycle #2 (Comp ON time: 45.0 minutes)

0 20 40 60 80 100

time (minutes)

0

5

10

15

20

25

Cap

acity

(MB

tu/h

)

16.3 min

QC= 6.7

QS= 21.8

QL= 7.4

Integration Pt

11.9 (gamma = 1.60)

Integrated Moist (delay of 1.0 min)

Mass twet

Sens (lb & min): 1.98 17.0

Lat (lb & min): 1.97 16.9

79.9 F, 60.4 F dp, 51.5 % Run 4 1.5 hz, 76.6 psi, 967 cfm, 30.04 in Hg

Actual Laboratory Results

Condensate

Sensible

(+) Latent (psych)

(-) Latent (psych)

Page 16: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Constant Fan Cyclic Tests for COIL2 (80db, 60dp)

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction (-)

0.8

1.0

Sen

sibl

e H

eat R

atio

(-)

Steady State SHR = 0.755 (based on condensate)

twet= 17.3, gamma= 1.50, exp

1st Cycle2nd Cycle3rd Cycle

Laboratory Testing – SHR Degradation

Constant Fan Mode

LHR Model matches measured data at nominal entering conditions

(80F db, 60.4F dp)

Page 17: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Constant Fan Cyclic Tests for COIL2 (75db, 64dp)

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction (-)

0.50

0.60

0.70

0.80

0.90

1.00

Sen

sibl

e H

eat R

atio

(-)

Steady State SHR = 0.533 (based on condensate)

twet= 11.2, gamma= 0.49, exp

1st Cycle2nd Cycle3rd Cycle

Laboratory Testing – SHR Degradation (cont.)

Constant Fan Mode

Model also matched measured data at other entering conditions!

(75F db, 64F dp)

Page 18: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Auto Fan Cyclic Tests for COIL5

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction (-)

0.6

0.8

1.0

Sen

sibl

e H

eat R

atio

(-)

Steady State SHR = 0.680 (based on condensate)

1st Cycle2nd Cycle3rd Cycle4th Cycle5th Cycle

Laboratory Testing – SHR Degradation (cont.)

AUTO Fan Mode

Coil 5 showed impact in AUTO Fan Mode!

Page 19: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Field Testing – SHR Degradation

Significant AUTO fan degradation!

Sweetser - Downstairs AC Unit

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction

0.8

1.0S

ensi

ble

Hea

t Rat

io (-

)

Steady State SHR = 0.855

Auto Fan ModeConstant Fan Mode (08/18/02 to 09/04/02)

(Model: twet=15 min, gamma=2.2, Nmax=3)

Page 20: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Measured Performance Parameters

Size

Fin Surface

Area

Retained Moisture Mass

Cond Delay Time

twet

(tons) (ft2) (lb) (lb/kft2) (min) (min) Coil 1 (Slanted slab, 3 row, 13 fpi, plain fins, orifice)

2.9 243.8 2.1 8.6 13.5 16.5

Coil 2 – Normal Flow (A-coil, 3 rows, 15.5 fpi, lanced sine-wave fins, TXV)

2.4 237.8 2.0 8.4 16.3 17.0

Coil 2 – Low Flow (A-coil, 3 rows, 15.5 fpi, lanced sine-wave fins, TXV)

1.4 237.8 2.0 8.4 32.5 29.0

Coil 4 (vert. slab, 2 rows, 14 fpi, wavy fins, orifice)

1.8 137.4 1.9 13.8 23.5 18.5

Coil 5 (slanted slab, 4 rows, 12 fpi, wavy fins, orifice)

2.3 162.7 1.4 8.6 11.5 9.5

Coil 6 (A-coil, 3 rows, 13 fpi, wavy fins,TXV)

1.7 231.1 2.7 11.7 34.0 33.

Coil 6 – High Flow (A-coil, 3 rows, 13 fpi, wavy fins,TXV)

2.0 231.1 2.7 11.7 27.0 27.

Coil 8 – Chilled Water (vert. slab, 4 rows, 10 fpi, wavy fins,46°F CHW)

1.5 135.0 1.4 10.4 26.5 26.5

Notes: 1-

2- 3-

Cooling capacity includes sensible and latent cooling at nominal conditions. Nominal conditions correspond to ASHRAE Test A test point. Surface area is gross fin area (coil face area x coil depth x fin spacing x 2). Condensate delay time and twet are at nominal conditions.

Page 21: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

- 2 4 6 8 10 12 14 16 18 20

Korte & Jacobi (1997)

Lab - Coil 1

Lab - Coil 2

Lab - Coil 3

Lab - Coil 4

Lab - Coil 5

Lab - Coil 6

Lab - Coil 7

Lab - Coil 8

Residential 3.5 ton

Rooftop 10 tons

CHW Coil 2.9 ton

Henderson (1998)

Coil Moisture Retention (lb per 1000 ft2)

Lab + Field: Retained Moisture

Estimated in Field

Lab Measurements

Moisture per total gross fin area (gross area = face area x depth x fin spacing x 2)

Page 22: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Heink - Humidity Inside vs. Outside

0 50 100 150

Outdoor Humidity (gr/lbm)

0

20

40

60

80

100

Indo

or H

umid

ity (

gr/lb

m)

Auto Fan ModeConstant Fan Mode

Space Humidity – Impact of Constant Fan

o Large penalty for constant fan operation

o Even for a dual-capacity AC unit

Page 23: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Impact on Space Humidity Levels

Room Humidity - July 13

50

55

60

65

70

75

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24Hour of Day

Spa

ce H

umid

ty (%

RH

)

TRNSYS Model of Hotel Room in Hawaii

Constant Fan

AUTO Fan

Page 24: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Chilled Water Coils

Page 25: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Chilled Water Coil Performance

CHW Performance at 400 cfm/ton

0 2 4 6 8 10Chilled Water Flow (gpm)

0.50

0.60

0.70

0.80

0.90

1.00

Sen

sibl

e H

eat R

atio

(-) 80F / 60Fdp

80F / 68Fdp

75F / 56Fdp

75F / 64Fdp

Nominal Operating Point

Chilled water coils behave similar to direct expansion (DX) coils …..even though they are controlled differently

• gpm varies to provide capacity

Page 26: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Latent Degradation - CW CoilsConditions: IN= 75F, 0.010lb/lb, TCH=44F, fpi= 8, rows= 4

0.2 0.4 0.6 0.8 1.0Total Capacity (tons)

0.6

0.8

1.0

Sen

sibl

e H

eat R

atio

(-)

LHR curve for DX EquipmentAssuming: 960s, 1.5, 45s, 3 cyc/h, linear, SHR_steady-state=0.75 (at 400 cfm/t)

CW Coil (ASHRAE CCDET Model)

Page 27: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

What Can Equipment Manufacturers

& Design Engineers Do?

Page 28: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

What Control Strategies Help?

Do fan delay strategies help?q No…not until about a 10 minute delay

Two-speed or staged capacity systems?q Yes…increases runtime fraction (depends on coil split &

fan control)

Reduced airflow during off cycle?q Yes…would minimize off cycle evaporation

Cycle the fan with the compressor?q Yes…whenever possible

Page 29: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Impact of Fan Delay

•Fan was turned off for 2-, 5-, and 10-minutes after the cooling cycling to “allow moisture to drain from the coil”.

• No significant “drainage” occurs

Impact of Fan Delay

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction (-)

0.80

0.85

0.90

0.95

1.00

Sen

sibl

e H

eat R

atio

(-)

Steady State SHR = 0.823 (based on condensate)

10-min delay

5-min delay

2-min delay

No fan delay

Coil on/off times (min): 7/17.5 10/10 20/6.7 30/6

Page 30: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Impact of Proper AC SizingConstant Fan Cyclic Tests for COIL2 (80db, 60dp)

0.0 0.2 0.4 0.6 0.8 1.0Runtime Fraction (-)

0.8

1.0

Sen

sibl

e H

eat R

atio

(-)

Steady State SHR = 0.755 (based on condensate)

twet= 17.3, gamma= 1.50, exp

1st Cycle2nd Cycle3rd Cycle

“Normal” Sizing

50% Oversized

Normally Sized

Oversized

Page 31: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Impact of Equipment Staging

q Specify 2-Stage RTUsq Most RTUs spend majority of operating time at less

than 50% runtime…with no latent capacityq Continuous operation in first stage provides more

latent removal at part load..assuming face-spilt coils

q Operate in continuous fan only when requiredq Consider cycling fan with compressor when ventilation

is not requiredq Consider using lower air flow rate when cooling is off

Page 32: Understanding Dehumidification Performance at Part Load in ...cloud.cdhenergy.com/presentations/lhr_humidity_aug05.pdfSensible Heat Ratio (SHR) vs Efficiency Debate qSome in the industry

Summary

q It takes 10 to 30 minutes after startup for moisture removal to begin

q A typical coil holds ½ to 1 lb per tonq With constant fan, latent removal disappears

below half loadq Chilled water coils have similar part load

performanceq To maintain latent capacity

q Cycle fan with the compressor (when possible)q Slow the fan down during off cycleq Add more capacity stages


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