section 2 of 6 - carl munters · munters technical training - october 2016 assessing mold risk -...

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Section 2 of 6

Applied Psychrometrics

• Psychrometric chart - your energy map

• Key psychrometric variables for humidity

• Predicting building condensation

• Hot weather

• Cold weather

• Predicting moisture sorption and mold growth

Munters Technical Training - October 2016

Maps help us understand where we are and how far we have to go to our destination

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Long distance... lot of effort

Distance quantifiedby the map’s scale


Psychrometric chart helps us understand the current energy state of air, and how much work we’ll have to do to change it

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Long distance... lot of effort

Required work is quantifiedby the chart’s (many) scales


Each axis quantifies a different property of the air

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Dry bulb temperature

Humidity ratio

Relative humidity

Dew point temperature

Wet bulb temperature


Consider typical indoor conditions

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75°F50% Relative Humidity


75°F Dry bulb temperature

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The temperature of the air as measured by a thermometer that has a dry sensing bulb


50% Relative humidity

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The amount of water vapor in the air... compared to its maximum capacityat that temperature.

A relative (not absolute) measurement of humidity


Grab your chart and plot the point - 75°F, 50% RH

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Humidity ratio = 0.0092 lb/lb = 65 gr/lb

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The weight of water vapor in the air... compared to the weight of dry air alone(lbw ÷ lba)

An “absolute”measurement of humidity

(0.0092 lbw ÷ 1 lba) x 7000 gr/lb = 65


Dew point temperature = 55°F

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

The dew point is another “absolute” measurement of humidity


Predicting indoor condensation (hot weather)

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In other words...

What is the dew point of air at 83°F, 151 gr/lb = ___?__)

At design conditions in Hampton, VA, any outdoor air sucked into the cavities above ceilings is 83°F and 151 gr/lb.

How cold does the ceiling surface have to be to produce condensation?


Assessing mold risk - School summer vacation

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In Hampton, VA, outdoor air infiltrating into an empty school during the summer time has a dew point of 79°F.

Thermostat is reset to save energy. 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 when the dew point is 79°F and the dry bulb temperature is 85°F ?

RH = __?__


Mold risk on building surfaces - Hotel room

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Using the chart to find RH at the SURFACE

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Dew Point =19.6°CSurface = 20.7°C


Helps explain common mold growth in hotels(and underlines the importance of air-tight duct connections)

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Predicting cold weather condensation in walls

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In Calgary, Alberta, small amounts of air are leaking outward from the swimming pool enclosure @ 85°F, 60% RH.

That air will condense moisture when it contacts a surface with a temperature of __?__.

In other words, what is the dew point of air at 85°F, 60% RH ?

Dew point = __?__


Wet bulb temperature = 62.5°F

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The temperature of the air as measured by a thermometer with a wet sensing bulb.

The temperature at which water will begin to freely evaporate into the air.


Finding humidity from DB + WB bulb temperature

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During hours when the outdoor temperature is that hot in Phoenix, the average wet bulb temperature is 70°F

How much humidity is in that outdoor air at peak dry bulb conditions?

In other words, what is the dew point, humidity ratio and relative humidity of air at 110°F, 70°F WB ?

Dew point = __?__ Humidity ratio = __?__ RH = __?__

In Phoenix, AZ, the design dry bulb temperature is 110°F.


Psychrometric summary for air at 75°F, 50% RH

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Dry bulb: 75°

RH: 5

0%

Humidity ratio: 65 gr/lbDew point: 55°F

Wet bulb: 62.5°F

section 2 of 6 - carl munters · munters technical training - october 2016 assessing mold risk -...

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