humidity control i: design tips and traps library/professional...2020/05/11 · lead author of...

Humidity Control I:

Design Tips and Traps

Learning Institute

© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

Copyright Materials

Copyright © 2017, 2020 by ASHRAE. All rights reserved. No part of this presentation may be reproduced without written permission from ASHRAE, nor may any part of this presentation be reproduced, stored in a retrieval system or transmitted in any form or by any means (electronic, photocopying, recording, or other) without written permission from ASHRAE.

ASHRAE has compiled this presentation with care, but ASHRAE has not investigated and ASHRAE expressly disclaims any duty to investigate any product, service, process, procedure, design or the like, that may be described herein. The appearance of any technical data or editorial material in this presentation does not constitute endorsement, warranty or guaranty by ASHRAE of any product, service, process, procedure, design or the like. ASHRAE does not warrant that the information in this publication is free of errors. The user assumes the entire risk of the use of the use of any information in this presentation.


AIA/CES Registered Provider

ASHRAE is a Registered Provider with The American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.

This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.


Understanding the InstructorMark Nunnelly, PE, CxA, LEED APNunnelly and Associates, Inc., Birmingham, AL

•38 years of experience: HVAC design, application engineering, commissioning, retro-commissioning, troubleshooting, building diagnostics, energy management, and utility energy program management

•Certified commissioning authority(CxA)•ASHRAE Distinguished Lecturer: Europe, South Asia,

North + South America•ASHRAE course developer and instructor beginning

in 2005: Humidity control and dehumidification—mechanical DH, solid desiccant and liquid desiccant


Understanding the Course Developer

Lew Harriman: Mason-Grant Consulting ASHRAE Fellow Past Chair of ASHRAE Technical Committee 1.12

(Moisture Management in Buildings) Chair of 2013 and 2016 ASHRAE Mold Position

Document Committee Lead author of ASHRAE Humidity Control Design

Guide and the ASHRAE Guide for Buildings in Hot & Humid Climates

40 years in humidity and moisture control in buildings—10 with manufacturer, 30 as consultant


Learning Objectives

Understand why and how humidity creates problems such as mold and bacterial growth in buildings.

Understand the five-step process that helps make humidity control design more simple and certain.

Understand and calculate regional variation of the dehumidification loads that govern the design of successful humidity control systems.

Understand basic equipment alternatives for achieving humidity control.


Understanding This Audience

Which category best describes your current everyday work? HVAC designer Architectural designer Building energy management Building operations or management HVAC equipment or controls service Other?


Audience Experience and Needs

What is your personal experience? Never or hardly ever dealt with humidity control

design

Sometimes involved with humidity control issues Frequently involved with humidity control

Examples?

What specific questions would you like to have answered during this course?


Course Outline

1. Humidity Control For Design: The Simplest Possible Overview

2. Mold Risk Reduction: Why It Grows and How to Prevent It

3. HVAC Design for Humidity Control

4. Estimating Dehumidification Loads

5. Equipment and Controls


SECTION 1Humidity Control for Design:

The Simplest Possible Overview

��


How to achieve humidity control...

��


Dehumidification must be greater than the humidity load

��

��


Example 1—Clinic MoldHumidity load was greater than

dehumidification


Example 2—School moldHumidity load was greater than

dehumidification


Example 3—Training buildingHumidity load was greater than

dehumidification


Section 1 SummaryHow Designers Achieve Humidity Control

��

��


SECTION 2Mold Risk Reduction:

Why It Grows and How to Prevent It

Mold growth depends on moisture in materials, not RH of the air

Make HVAC designs effective by eliminating misunderstandings about mold


Common mold growth misunderstandings:

Mold will grow when humidity exceeds 70% rh

Mold will not grow until moisture condenses on the surface

To avoid mold growth, increase air circulation

Mold grows in dark spaces; it can’t grow in bright light


Mold Needs Moisture

The mildew square reminds us thatall fungi need moisture in their food sources


Mold Growth Cycle (1 of 4)

Mold growth starts on a damp food source.

When a spore lands on a damp food source, the enzymes on the spore’s surface react with the damp food, breaking it down into a kind of “soup” of nutrients.

Slowly, that nutrient soup is pulled into the spore, sucked in by the dry material inside the spore.

Source: ASHRAE Humidity Control Design Guide.



Given nutrients, the spore germinates.

After the nutrient soup is pulled into the spore, the fungus has the resources it needs to germinate and grow. Unlike plants, fungus does not need sunlight—it just needs damp food.

The filaments that grow out of the spore also have enzymes on their surfaces. So they can help dissolve more of the damp food and absorb its nutrients, accelerating the mold growth.




Growth speeds up as available water increases.

The filaments grow thicker, extending the reach of the fungus over any food source damp enough to dissolve.

All that fungal chow time generates additional water—just like human metabolism generates water in the form of urine. The metabolic water helps the fungus dissolve even more of the food source—so the fungus grows faster.




Growth stops when food dries out.

Growth slows or stops entirely when the food source runs out or when it dries up enough to keep the fungus from dissolving it.

At that point, the fungus often puts its efforts into reproduction, generating billions more spores, which drift off into the air. These land on other damp food sources, where the fungus can grow again.



Better Understanding of Mold Growth

Mold grows when humidity is over 70% rh:

• True: Only if the water activity of the nutrient surface is over 0.7

• False: If RH is measured only in the air

Mold will not grow until moisture condenses on the surface:

• Partly True: More moisture = faster mold growth (and more of it)

• False: Long‐term high moisture content of a material supports lots of mold growth

To avoid mold growth, increase air circulation:

• True: Only if the circulating air is dry

• False: If the dew point of the circulating air is nearly the same as the surface temperature

Mold grows in dark spaces, not in bright light

• True: Only if the dark surfaces are damp

• False: Unless the light dries the surfaces


Section 3HVAC Design for Humidity Control

Psychrometrics of humidity control

Five-step design process


Psychrometrics of Humidity Control

Dry-bulb temperature

Humidity ratio

Relative humidity

Dew point


Dry-Bulb Temperature

The temperature of air as measured by a thermometer with a dry sensing bulb


Relative Humidity

The amount of water vapor in the air, compared to its maximum capacity at that dry-bulb temperature

Relative measurement, not absolute


Humidity Ratio

The weight of water vapor divided by the weight of the dry air (lbw/lba – an absolute measurement)

lbw/lba × 7000 = grains of water per lb of air


Dew Point

The temperature at which the moisture contained in the air will begin to condense

Another absolute measurement of moisture


Applied Psychrometrics #1Summertime Condensation

In Cincinnati, Ohio on a design day, outdoor air being sucked into the building cavities is 83°F, 135 gr/lb. How cold does an inside wall surface have to be to produce condensation?

In other words, what is the dew point of air at 82°F, 135 gr/lb? __________75°F (24°C)


Applied Psychrometrics #2Humidity in Vacant Buildings

In Cincinnati, OH, outdoor air infiltrating into an empty school during the summer has a dew point of 75°F. If the AC system keeps the building at 85°F, what is the probable relative humidity indoors?

In other words, what is the rh of air at 85°F db with a 75°F dpt? ______73%


Five-Step Design Procedure


Step 1: Purpose of the Project

Why insist the client define the purpose? 1. If client has no problem, no reason to spend money

on humidity control.

2. If consequences are not defined, it is 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


Poor Definition...

“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?


Good Definition:

“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.


Elements of a Clear Project Purpose

What results are expected? What benefits occur and what problems are

avoided when humidity is under control?

What costs are incurred when results are not achieved?

How many hours of increased risk are acceptable per year?


Step 2: Set Control Levelsand Design Conditions

What humidity level will achieve the project purposes? RH range or dew-point range? High limit only? Low limit only? Both high and low limit? When does it matter? All the time? Just when occupied?

Just when unoccupied?

How many hours each year can be above design loads? (hours of increased risk) 2% of 8760 = 175 hr 1% of 8760 = 88 hr 0.4% of 8760 = 35 hr


Helping the Client Figure Out the Appropriate Control Level

1. Work from the client’s definition of the project purpose! Discuss and understand client’s past experiences Use any guidelines from client’s own industry

2. Consult references on the effects of humidity ASHRAE Humidity Control Design Guide ASHRAE Handbook series ASTM Manual 40, Moisture Analysis and Condensation

Control in Building Envelopes


Helping the Client Decide on Outdoor Design Conditions

Work from the client’s definition of the project purpose. How often do problems occur without control? What are the economic consequences, if any?

(hundreds of dollars or millions?)

Ask the client to decide how many hours of increased risk are acceptable. First, obtain costs and performance. It’s the client’s decision and important that they realize

that fact!


Step 3: Estimate Humidity Loads

Identify all load sources

Focus on the big ones (and don’t get wound up in the small ones)

Recognize that there are few absolutes;“engineering judgment” must be frequently applied


Typical Humidity Load Sources

Number of people in the space Ventilation per occupant Infiltration through the exterior wall Door activity Humid material brought into the space Wet surfaces Vapor permeation


Step 4: Size and Select Equipment

Make sure the equipment will remove the peak load.

More important: make sure equipment will modulate to remove the load for the other 99.6% of its operating hours! i.e., Avoid buying a semi tractor with only a key switch

instead of an accelerator pedal. It may be powerful, but it’s not likely to provide a smooth ride.


Step 5: Select and Locate Sensors

Dew-point-based control is usually best No significant temperature dependence Dew point stays more constant than rh

RH sensors need very careful placement Temperature dependence makes readings swing widely Best to co-locate db and rh sensors, convert to dew point, and

control on that value

Fact to keep in mind: all sensors are incorrect allof the time Reduce the error with calibration after installation and

annually thereafter Know what the error and response lag are and adapt system

operation accordingly


Use the Five-Step Procedure(For More Certain Results)


Dehumidification Must Be Greater than the Humidity Load

��

��


Section 4Estimating Dehumidification Loads

Outdoor design conditions Sensible cooling design conditions Dew point DH design conditions

8 DH load sources and their nature

The estimating spreadsheet

Exercise—retail clothing store


People

Respiration

Perspiration

Wet clothing


Ventilation

Reduces occupant exposure to Human-generated contaminants Product-generated contaminants


Ventilation DH Load Depends onOutdoor Design Values

Principal load factor: the difference between humidity ratio indoors versus outdoors

Until 1997, ASHRAE did not have correct values for peak outdoor humidity ratio—only peak outdoor dry bulb and mean coincident wet-bulb temperature (MCWB)

Peak DH load occurs at peak outdoor humidity ratio (peak dew point, not peak dry bulb)


Psych Chart & Design Values

Peak Dew Point85° 146 gr/lb

Peak Dry Bulb92°, 116 gr/lb

Indoors78°, 65 gr/lb


HVAC interactions with the building enclosureBEC-Charleston - May 2010

ASHRAE Dehumidification Design Values

...for cooling load calculations

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


What’s the Difference in Cincinnati?

25 lb/h

25 lb/h


54

2019 News—Major Change to ASHRAE Humidity Guidance for Design

ASHRAE Standard 62.1Ventilation for Acceptable Indoor Air

Quality

The ONLY ASHRAE standardthat establishes any upper limit for

indoor humidity

OLD Limit—65% RHWhat dry-bulb temperature? (Not Defined)


55

ASHRAE Std 62.1-2019 – MAX 60F DPTDuring both Occupied AND UNOCCUPIED Operation


56

Humidity Control Design Requirement - Paragraph 5.10 of Std 62.1-2019

• For SYSTEM DESIGN… no higher than a 60F(15C) dew point• 60F dew point design limit applies for BOTH occupied and unoccupied system operation• In effect… designs need to include equipment and controls that will provide DH

independently from cooling.

• Exception: Overnight periods of less than 12 hours need not be limited by 60F dpt…provided that RH does not exceed 65% during those hours.

• However: This requirement applies to designs, so that owners have systems that CAN control humidity…. Whether or not he/she choses to DO SO.. is up to the owner.


Infiltration

Driven by air pressure differences Between 0.1 and 0.6 cfm/ft2 of wall surface,

and leaky ducts (#3) are a major cause


Door Openings

Door loads are intermittent

Driven by occupancy

Vestibules greatly reduce load


Humid Products

Moisture content of materials changes with relative humidity

Moisture adsorbed from high humidity outdoors is released into indoor air held at lower humidity


Wet Surfaces

Floor cleaning (infrequent)

Rain/snow on shoes (infrequent)

Decorative fountains and ornamental pools (continuous)

Swimming pool (continuous, high load but activity dependent)


Vapor Permeation

Driven by vapor pressure difference

Continuous, but very, very small

Don’t obsess about vapor permeation except in pools, hospitals, and museums


Retail BuildingTypical Peak Load Profile


Looking at Load Elements

Note how the ventilation air dominates to total load



… but the infiltration load is also large.



The load from people is small,



...and the load from door activity depends on the number of customers



The load from humid product is small


Looking At Load Elements

The load from wet carpet is small


Looking At Load Elements

The load from permeation is small


Bottom Line On Loads

Quite a lot of water vapor needs to be removedfrom the air, every hour!


Let’s Look At the Spreadsheet

The “control panel”is on the summary sheet


The Workbook Contains Four Sheets

1. Summary2. Weather City3. Walls & Floor4. Calculations


Weather City Sheet

Data used in the calculations goes here

Values for a few other citiescan be copied from these columnsand pasted into the calculation cells


Walls & Floor Sheet

A very simple,square building!

For permeation(just exterior walls)

For infiltration(50% of the total wall surface)

Used to calculate retail ventilation


Calculation Sheet

These are the basisof the summary graph

The summary values “roll up”from these calculations

White cells indicateuser inputs

Comments explain each line


Now Let’s Try It Out:Peak Sensible Versus

Peak Dew-Point Design Values (Cincinnati)

______ lb/h at peak sensible cooling design

______ lb/h at peak dew-point DH design

Observations or conclusions?

67 lb/h

116 lb/h


What Is The Effect OfChanging The Location?

_______ lb/h in Cincinnati

______ lb/h in Seattle, WA

______ lb/h in Phoenix, AZ


116 lb/h

34 lb/h

90 lb/h


Permeance

______ lb/h for painted block (1.6 perm)

______ lb/h for unpainted block (2.4 perm)

______ lb/h if no exterior wall other than unpainted gypsum board (50 perm)


116 lb/h

117 lb/h

141 lb/h


Ventilation Rate

______ lb/h for Std 62.1 retail ventilation and 75 occupants

______ lb/h for Std 62.1 retail ventilation and zero occupants (stop all ventilation after hours)


116 lb/h

74 lb/h


Humidity Control Level

______ lb/h @ 75F 60%

______ lb/h @ 78F, 45%


116 lb/h

116 lb/h

65 gr. lb

65 gr. lb


Humidity Loads—Summary

Ventilation is the big load!

The only reason for ventilation is for people.

To reduce operating cost, eliminateventilation when the building is notoccupied!


SECTION 5Equipment and Controls

Dehumidifiers

Humidity sensors


Typical System ConceptDedicated Outdoor Air System (DOAS)

Dry the ventilation air deeply enough to supply air at “neutral” or lower moisture

Responds to a humidity controller, not a thermostat


Typical System Hardware


Dehumidifiers


Mechanical Dehumidifiers

Advantages Higher energy efficiency (Btu/lb

removed) than most desiccants in many applications

Better understood by most service mechanics than desiccants

Equipment costs less than desiccants in small sizes and in warm/humid applications (e.g., pools)

Limitations Poor cool weather performance

versus desiccants Custom package for optimal DH and

energy performance (higher costs, longer delivery)


Desiccant Dehumidifiers Advantages

Better performance. Dries deeply in all weather conditions. Superb capacity and faster response than mechanical DH

Can use low-cost or waste heat for reactivation (e.g., summer boiler capacity)

Condenser heat configurations usually less costly and more energy efficient than mechanical DH (e.g., ventilation air/DOAS)

Limitations Needs lots of reactivation heat! Noncondenser heat units less energy

efficient than mechanical DH for most applications

Converts moisture to sensible heat, so supplemental cooling is usually required


General Suggestionsfor Applying Dehumidifiers

Don’t size them based on supply airflow; size them for moisture removal. Dehumidifiers dry air very deeply. Dry part of the air,

then blend the dry air back into the supply. This results in a smaller, less-costly unit.

Measure and control the airflow through the unit. Otherwise it will not perform.

Clean all filters at least every month. Dehumidifiers are very costly when used as air filters Low air flow = less DH capacity than intended


Suggestions forMechanical Dehumidifiers

Condensate drain and trap are critical Large enough diameter to avoid dirt clogs (at least 1.5 in.) Stem height equal to fan pressure plus 1.0 in. for negative

traps For positive pressure traps, bowl height = fan pressure

plus 0.5 in.

Needs smooth airflow at inlet When ducted, be sure to have long straight section or flow

straightener at inlet to avoid performance problems.

Don’t run the unit during construction Wallboard dust turns to concrete inside the deep, wet

dehumidification coil


Suggestions forDesiccant Dehumidifiers

Airflow rate is critical! Measure and control it on both process and reactivation

sides

Consider heat exchanger for post-cooling Reduces equipment and operating costs

Waste heat for preheating reactivation Two-stage heating for reactivation saves operating cash


Humidity Sensors

Response time

Range versus accuracy

Calibration

Location

Constant RH or constant dew point?


Response Time

When humidity changes, sensors require time to respond

Response times vary Expensive sensors respond in

less than a minute; lower cost sensors need many minutes

Wet air to dry air takes longer After condensation, response

time can lengthen to hours, depending on temperature and moisture in the new airstream


Range Versus Accuracy

Below 20% rh, the usual commercial-grade sensors are very inaccurate

For better accuracy in low humidity, use industrial-grade units


Calibration

All sensors are inaccurate all the time. To improve accuracy, calibrate in place!

Worst case: Outdoor air Highly variable Frequent condensation Heavy particulate load

When to calibrate? During commissioning, at least Once a year, before humidity matters Outdoor air, more frequently


Sensor Location

Locate the sensor where humidity matters the most: Museum—near the artwork Gymnasium—near the floor

Avoid locations where relative humidity swings widely: Near the door Near an outside window In the supply air Near heating or cooling coils Near heat/moisture sources (coffee pot)


Constant RH…or Constant Dew Point?

RH may actually vary less when controlling on dew point Dew point stays more constant than rh Fewer calls for changes, conditions stay more constant

Easy to convert RH signal to dew point using building automation software

Most humidity control applications are dew-point dependent, not rh dependent


Dew-Point-Dependent Applications Are Most Common

All human comfort applications are driven by vapor pressure difference between skin and air Offices, schools, hospitals, restaurants, hotels, etc.

Ice rinks, supermarkets, mold prevention in hotels, pool DH, all driven by condensation on cold surfaces


High dew point is a betterindicator of mold risk than the rh

54% rh... should be no problem!54% rh... should be no problem!

...but look at all the mold...but look at all the mold

Problem: the dew point is close to...Problem: the dew point is close to...

...the surface temperature......the surface temperature...

...so rh at the surface is over 90%...so rh at the surface is over 90%


However, Storage Corrosion Protection Applications Are

RH-Dependent

Dry storage in unheated warehouses

Layup of industrial or military equipment or spare parts


Summary of Key Points

Clearly defining the purpose of the humidity control design improves results, saves time, and avoids problems.

To control humidity, control the moisture content of the ventilation air.

To reduce mold risk, keep the indoor dew point low (below 60°F [15°C]).


ASHRAE Resources Humidity Control Design Guide

512 pages, in color, with hard cover ISBN 1-883413-98-2 ashrae.org (404) 636-8400

Guide for Buildings in Hot & Humid Climates (2nd Edition) 316 pages, in color, with hard cover ISBN 978-1-933742-43-4 ashrae.org (404) 636-8400


Questions?

Mark Nunnelly, Nunnelly & Associates, Inc.(205) 425-0060 (office)(205) 516-4064 (cell)

[email protected] or ----

Lew Harriman, Mason-Grant Consulting,(603) 431-0635

[email protected]


Evaluation and Certificate

ASHRAE values your comments about this course. You will receive your Certificate of Attendance when you finish the online course evaluation form using the link below. Please use the linkto also access supplemental files: https://www.ashrae.org/2020may21

Be sure to add your appropriate license numbers. If you have any questions about ASHRAE Certificates, please

contact Kelly Arnold, Coordinator Professional Development at [email protected].

If you have any questions about ASHRAE courses, please contact Tiffany Cox, Professional Development Course Administrator, at [email protected].


Explore ASHRAE Learning InstituteCourses

https://www.ashrae.org/instructor-led-courses

Find a Topic that Fits You: Commissioning Energy Efficiency Environmental Quality HVAC&R Applications Standards and Guidelines

See all the ways to learn and grow with ASHRAE at https://www.ashrae.org/professional-development/learning-portal


ASHRAE Certification

More than 3,000 certifications earned to-date Elevate your reputation among peers, in the workplace and among clients NEW! Digital Badging:

Embedded metadata uniquely linked to you Shareable in electronic media, including LinkedIn and email Instant recognition, with real-time, third-part verification

Visit www.ashrae.org/certification to learn more


humidity control i: design tips and traps library/professional...2020/05/11 · lead author of...

Documents