controlling moisture in commercial buildings · controlling moisture in commercial buildings ......

Controlling Moisture in Commercial

Buildings

Part Load Problems

Raymond E. Patenaude, PE, CIAQP, RPIH

The Holmes Agency, Inc.Consulting and Forensic Engineers

Historical PerspectiveWillis Carrier

“Air conditioning is the control of the humidity of the air by eitherincreasing or decreasing its moisture content. Added to the control ofhumidity is the control of the temperature by either heating or cooling the air, the purification of the air by washing or filtering the air, and the control of air motion and ventilation”

Book cover

Chapter 3 - Managing Ventilation Air

Chapter 5 - Avoiding Bugs, Mold & Rot

Comfort - Keep the dew point under control, and comfort happens at warmer, more energy-efficient temperatures

ASHRAE Std 62.1 - 2004Ventilation for Acceptable

Indoor Air Quality

Ventilation - If it’s not dried… you’re in trouble

Mold - It’s the surface rh that counts.. So keep the dew point down, and things go well

Mold Growth

Enzymes on mold spore combine with surface moisture to dissolve food source… paper, wood, ceiling tileOsmotic pressure causes liquid nutrients to diffuse across spore wall allowing spore to absorb the nutrientsSpore germinates producing filamentous hypheaHyphea grows quickly creating mycelium matMold grows conidia which generates and releases spores into the air

Water ActivityWater activity indicates how much water is biologically available to fungus in its food source.Water activity of 0.8 refers to the amount of water absorbed into a material when the surrounding air is at 80% RHWater activity of a material is very different from the relative humidity of the materialMold growth is a risk when surface relative humidity stays above 85% for extended periods

Moisture Contentand Surface Relative Humidity

Uncontrolled air flow

Buildings which have never reported relative humidity above 65% still may have mold growth

THE MOISTURE PROBLEM

Highest enthalpy occurs at peak dew

point.. NOT peak dry bulb

ASHRAE 2005 Fundamentals listsExtreme DB with MCWB ….example Kansas City, MO (1%) 93db/75wb (107 gr/lb) 68dp

Extreme DP with MCDB (.4%) 75dp/85db (137 gr/lb) (78wb) ….…. …poor

performance at part load!!

THE MOISTURE PROBLEM

ASHRAE 2005 Fundamentals lists extreme DP with MCDB….example Orlando, FL (0.4%) 83db/77dp (142 gr/lb) (79 wb) ….…. …again poor

performance at part load!!

TYPES OF MOISTURE PROBLEMS

Outdoor air85 deg F137 gr/lb72%rh

75 deg F dp Interior Design 75 deg F

Building Envelopewith poor vapor barrier

Dry wall

Vapor transport

Mold growth onbackside of drywall

Ventilation - If it’s not dried… you’re in trouble

ChW EvapCoil

Supply air

VAV Boxwith Heat

Outsideventilation

air

ReturnAir

VAV Supply Fan

VAV AIR CONDITIONINGFull Load

93F db107 gr/lb

44%rh

75F65 gr/lb50%rh

55 F90%rh58 gr/lb

58 F80% rh58 gr/lb

Constant volumeOA Fan

7,710 OA11,940 RA19,650 SA

R R

ASHRAE PSYCHROMETRIC CHART NO.1NORMAL TEMPERATURE

BAROMETRIC PRESSURE: 29.921 INCHES OF MERCURYCopyright 1992

AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR-CONDITIONING ENGINEERS, INC.

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10% RELATIVE HUMIDITY

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SENSIBLE HEAT QsTOTAL HEAT Qt

ENTHALPYHUMIDITY RATIO

∆h∆W

ChW EvapCoil

Supply air

VAV Boxwith Heat

Outsideventilation

air

ReturnAir

VAV Supply Fan

VAV AIR CONDITIONINGPart Load

85F db137 gr/lb

72%rh

75F65 gr/lb50%rh

55 F100%rh65 gr/lb

57 F92% rh65 gr/lb


Resultant Room74F db

61.4% rh

7,710 OA2,440 RA

10,150 SA

R R




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NEW Note # 1 Here...

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∆h∆W

ChW Post Cooling

(sensible)

Supply air

VAV Box(NO Reheat)

ReturnAir

VAV Supply Fan

Outsideventilation

air

OA Pre Treatment with Post CoolingFull Load

93F db107 gr/lb44%rh

75F65 gr/lb50%rh

55 F85%rh55 gr/lb

58 F75% rh55 gr/lb


ChwPre Cool

55F

DXEvap42F

(RemoveMoisture)

DXCond

Reheat75F

DX system

R R




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.002

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.026

.028


20%

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90%

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OA DX pre coolOA reheat

RA eat

MixSA lat post cool Mtr

Room

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-∞∞



∆h∆W

ChW Post Cooling

(sensible)

Supply air

VAV Box(NO Reheat)

Return Air

VAV Supply Fan

Outsideventilation

air

OA Pre Treatment with Post CoolingPart Load


75F65 gr/lb50%rh

55 F71%rh46 gr/lb

57 F65% rh46 gr/lb


ChwPre Cool

55F

DXEvap42F

(RemoveMoisture)

DXCond

Reheat75F

DX system

Room Resultant74F db

46.3% rh

R R




SEA LEVEL

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.002

.004

.006

.008

.010

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.014

.016

.018

.020

.022

.024

.026

.028


20%

30%

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60%

70%

80%

90%

35

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50 55

55 60

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TU

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R P

OU

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DR

Y A

IR

OA eat

OA CW pre cool

OA DX pre cool OA reheat

RA


Room

R R




SEA LEVEL

0

1.0 1.0

∞-

2.04.08.0

-8.0-4.0-2.0-1.0

-0.5-0.4-0.3-0.2-0.1

0.1

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0.3

0.4

0.5

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0.8-2000

-1000

0

500

1000

1500

2000

3000

5000

-∞∞



∆h∆W

ChW Post Cooling

(sensible)

Supply air

VAV Box(NO Reheat)

Return Air

VAV Supply Fan

Outsideventilation

air

OA Pre Treatment with Post CoolingPart Load – reset DX


75F65 gr/lb50%rh

55 F76.5%rh49 gr/lb

57 F70% rh49 gr/lb


ChwPre Cool

55F

DXEvap45F

(RemoveMoisture)

DXCond

Reheat75F

DX system

Room Resultant74F db

49.3% rh

R R




SEA LEVEL

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Y B

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- °

F

.002

.004

.006

.008

.010

.012

.014

.016

.018

.020

.022

.024

.026

.028


20%

30%

40%

50%

60%

70%

80%

90%

35

3540

40 45

45 50

50 55

55 60

6065

65

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75

75

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85

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HU

MID

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RA

TIO

- P

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S M

OIS

TU

RE

PE

R P

OU

ND

DR

Y A

IR

OA eat

OA CW pre cool

OA DX pre cool OA reheat

RA


Room

R R




SEA LEVEL

0

1.0 1.0

∞-

2.04.08.0

-8.0-4.0-2.0-1.0

-0.5-0.4-0.3-0.2-0.1

0.1

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-1000

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-∞∞



∆h∆W

Design Considerations

It’s a system… not just a unitThe habitable space is a plenumUncontrolled air flow – ASHRAE Journal

Don’t operate in a vacuum… it’s a team effortSequence of operation

Control outside air at the sourceControl condensation

ASHRAE Standards and Publications

Available from ASHRAE in Atlanta, Humidity Control Design GuideFor Commercial andInstitutional Buildings


Available from ASHRAE in AtlantaDesign Data for WeatherWeather Data ViewerASHRAE Design Weather Sequence Viewer


Psychrometric Analysis CD

Available at ASHRAE Book Store Long Beach, California

ASHRAE eLearning

On-Demand Interactive Courses in Modules

Online course reference bookOnline self assessmentwww.ashrae.org

controlling moisture in commercial buildings · controlling moisture in commercial buildings ......

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