Humidity Control - ASHRAEof 40 Humidity Control: Principles and Practice Illinois ASHRAE Chapter -...

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Humidity Control Principles and Practice

Including Advice From A Famous Bank Robber

Distinguished Lecture

Lew HarrimanMason-Grant ConsultingPortsmouth, NHMasonGrant.com

Thursday, May 16, 13

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Humidity Control: Principles and PracticeIllinois ASHRAE Chapter - May 2013

Outline

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Outline

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1. Examples of common humidity control problems


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Outline

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2. Principles of humidity control


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Outline

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3. 5-Step design process for humidity control


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Outline

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3. 5-Step design process for humidity control

4. Summary


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•Who do you work for?

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• HVAC Design consulting firm

• Architectural design firm

• Design-build or Energy Service company

• Contracting or HVAC service company

• Facility manager/building operator

• Utility: Power or gas company


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Great Clinic building, great HVAC system... So why did it grow mold 3 times in five years?

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Indoor dew point = 12.0°C (54°F) - Excellent!


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School - Plenty of system capacity!... Why mold?

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New training building and new HVAC equipment... Why mold and equipment failures?

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Fundamental Principles of Humidity Control

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• Humidity control costs money. Define the PURPOSE for humidity control before you begin any project.

• Dehumidification performance of equipment must equal or exceed the building’s humidity loads. Therefore...

• Quantify all humidity loads and...• Quantify dehumidification performance


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Quick refresher...Psychrometrics for humidity control

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75°F50% RH


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50% RH - Half of the maximum amount at 75°FA relative measurement—it depends on temperature

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75°F50% RH


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Humidity ratio - “absolute” humidity

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75°F50% RH65 gr/lb


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Dew point: The temperature at which moisture begins to condense out of the air (also “absolute”)

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75°F50% RH65 gr/lb55°F


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Example: Predicting condensation risk on cold supply air diffusers

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supply air diffusers

78°F65% RH


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78°F65% RH

78°F


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78°F65% RH

78°F

65% RH


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78°F65% RH

78°F

65% RH65°DewPoint


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Exercise: Predicting condensation from humid air infiltration into air conditioned buildings

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In Chicago, summertime outdoor air being sucked into the building cavities is 84°F, 134 gr/lb.


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How cold does an indoor wall surface have to be to produce condensation?


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How cold does an indoor wall surface have to be to produce condensation?

In other words - What is the dew point of air at 84°F, 134 gr/lb = ___?__)


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Predicting condensation risk inside interior walls and above ceilings

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and above ceilings

84°F134 gr/lb


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and above ceilings

84°F134 gr/lb

78°F


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and above ceilings

84°F134 gr/lb

78°F

134 gr/lb


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and above ceilings

84°F134 gr/lb

78°F

134 gr/lb75°DewPoint


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Designing for humidity control - 5-part sequence

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Defining the purpose of the project

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Why insist the client define the purpose?• If client has no problem... no reason to spend

money on humidity control.• If consequences are not defined… impossible to

say what that budget must be… and impossible to make rational decisions on the owner’s behalf.

A good project purpose definition includes what...• Results are expected• Happens when the results are NOT achieved• Shortcoming is acceptable


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Poor project definition

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“Control the humidity within the usual limits for a hotel”

Does not define what the system must accomplish...• Human comfort?• Control everywhere?… or just in the pool area?• Both high and low limits?• Always? Usually? Only when rooms are

occupied?


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Good definition of project purpose

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“Control the humidity in the hotel so that...”

• Neither condensation nor high rh can occur in building cavities that would allow mold growth or other moisture damage to the building or its furnishings.

• Guest rooms, meeting rooms, dining rooms and offices are held within the ASHRAE comfort zones during both summer and winter seasons

• The system achieves these goals during all but 2% of the hours in a typical year.


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Step 2 - Establish control level that achieves purpose for humidity control

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55°F Dew Point


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Step 3 - Calculate the humidity loads

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Largest loads?Ventilation/makeup air and infiltration

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Retail building in Chicago - Humidity load estimate

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Ventilation to dilute contaminants from people


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Ventilation to dilute contaminants from products and furnishings


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Ventilation to dilute contaminants from products and furnishings

Air leakage into a “tight” building


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Peak humidity loads - ASHRAE Research 1997Correcting 100 years of error - Now we know better

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How to find the peak design dew point?ASHRAE Fundamentals - 1997 or later

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...for Cooling load calculations


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...for Cooling load calculations

...for dehumidification load calculations, and DOAS system design


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Chicago - ASHRAE peak humidity design conditions

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Chicago - ASHRAE peak humidity design conditions

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0.4% Dry Bulb 0.4% Dew Point Differenceat peak dew point

92° DB 74.6° MCWB

84°DB77.7°MCWB

-8°F DB

101 gr/lb 134 gr/lb +33 gr/lb


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Chicago load at peak dry bulb vs. peak dew point

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65 lb/h at peak Dry Bulb


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65 lb/h at peak Dry Bulb

110 lb/h at peak Dew Point


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Step 4 - Size and select the dehumidifier

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Cool-Reheat Dehumidifier


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Desiccant Dehumidifier



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Desiccant Dehumidifier

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Measuring Dehumidification

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How to measure?

Humidity ratios(gr/lb or g/kg)

before and after the dehumidifier


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Finding the humidity ratio from temperature and RH

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75°F50% RH


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75°F50% RH

75°F


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75°F50% RH

75°F

50% RH


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75°F50% RH

75°F

50% RH65°gr/lb


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Dehumidification equation

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Where:!–  lbs/h = Pounds of water vapor per hour!–  CFM = Standard cubic ft/min!–  gr/lb1

= Higher humidity ratio!–  gr/lb2 = Lower humidity ratio!–  7000 = grains of water vapor in a pound!

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Where to locate the DH in the system?

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Need expert advice?... consult an expert!

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Need expert advice?... consult an expert!

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Reporter:

“Willie, it’s a Federal Crime! why rob BANKS?”


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Willie might ask you... where’s the humidity load?

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Dry where the load is - DOAS controls humidity

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Typical layout - Big box retail

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Add return air connection for humidity control during unoccupied hours and vacations

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Summary - Humidity Control Principles and Practice

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• First and most important step: Define the purpose for humidity control


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• To control humidity:


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• Quantify humidity loads and


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• Quantify DH equipment performance


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• Biggest humidity load?... Ventilation and makeup air, BY FAR the largest loads


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• Biggest humidity load?... Ventilation and makeup air, BY FAR the largest loads

• For easy humidity control - Dry the incoming air in a dedicated outdoor air system (DOAS)


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Further ASHRAE Resources

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Lew Harriman2011-2013 Chair, ASHRAE TC 1.12

(603) 431-06352011-2013 Chair, ASHRAE TC 1.12

Humidity ControlDesign Guide

Guide for Buildings inHot and Humid Climates

ASHRAE Handbook(Applications)


Humidity Control - ASHRAEof 40 Humidity Control: Principles and Practice Illinois ASHRAE Chapter -...

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