# What is humidity

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```					What is humidity?
What is humidity?

Humidity – Water Vapor within a given space.

Moisture – 1. Water Vapor in the air 2. Water
in a medium such as soil or insulation; but
not bulk water or flowing water.
What is humidity?
Relative Humidity – 1. The ratio of the partial
pressure or density of the water vapor in air to
the saturation pressure, respectively, at the
same dry bulb temperature and barometric
pressure. 2. The ratio of the mole fraction of
water vapor to mole fraction of water vapor
saturated at the same temperature and
barometric pressure.

Simplified – How full (of moisture) is the air.
-Based on temperature
?Relative? Humidity

100% R.H.           25% R.H.

35° F               72° F
?Relative? Humidity

100% R.H.

35° F          72° F               120° F
What is humidity?
Humidity Ratio (mixing ratio) – The ratio of the
mass of the water vapor to the mass of dry air.

Examples:
•Grains of moisture per pound of dry air (gr/lb)
•Pounds of moisture per pound of dry air (lb/lb)
•Europe uses grams of moisture per kilogram of
dry air (g/kg)

7000 grains = pound
___1000 grams = 1 kilogram
Relative Humidity             0.06
(420)
Conversion

100% R.H.       0.04
(280)

50% R.H.        0.02
(140)

35° F        72° F                120° F
What is humidity?
Dry-bulb temperature – The temperature of air
indicated by an ordinary thermometer.

Wet-bulb temperature – 1. The temperature of air
as indicated by a thermometer. 2. In a psychrometer,
that indicated when the the bulb is covered with a
water-saturated wick over which air is caused to flow at
approximately 900 ft/min (4.5 m/s) to reach equilibrium
temperature of water evaporating into air when the heat
of vaporization is supplied to the latent heat of air.
Steam
Nozzle

Location: New York City
Mixed Air:        20,000 cfm
Outside Air:      4,000 cfm (20%)
Space Design:     72º F @ 35% R.H.

Using Nortec Engineering Manual 03-124B. gr/ft³ method
Outdoor Design (Table 4): 0º F @ 61% R.H.
Saturated 0ºF (Table 3):  0.48 gr/ft³
Saturated 72ºF (Table 3): 8.59 gr/ft³

= OA cfm * (indoor design grains – outdoor design grains) * conversion factor(s)
= 4000 ft³ /min * (8.59 gr/ft³ x 35% - 0.48 gr/ft³ x 61%) x 60 min/hr x 1 lb/7000 gr
= 4000 ft³ /min x 2.7137 gr/ft³ x 1 lb/7000 gr x 60 min/hr
= 93 lb/hr

Simplification
Outside air cfm x Delta grains x 60 min  7000 gr
Using Nortec Engineering Manual 03-124B. gr/ft³ method

Bowling Alley in Albuquerque New Mexico
10,000 cfm with 20% O.A.

What is Outside Air (O.A.) Volume        2000
_____________ cfm
_____F ______          4.44/5.20
What is the Indoor Design (Table 7) 73-75 @ 50-55 % R.H. =_________ gr/ft³
0
What is the Outdoor Design (Table 4) _____F @ 46-68 % R.H. =_________ gr/ft³
______        0.22/0.33
How much moisture are we adding (Calculate)             Δ = _________ gr/ft³
4.22/4.98

_______ cfm (O.A.) x _________ grains (Δ/ft³ ) x 60  7000 = ________ lb/hr
2000                4.22/4.98                                72 - 85

What do you use if there is a range for Indoor design in Table 7?
What do you use if there are multiple R.H. values in Table 4?
Cont..

Bowling Alley in Albuquerque New Mexico

Bonus Question:

Why is humidification important in this example?
______________________________________________________________
Prevent Cracking/Splitting wood on lanes.
______________________________________________________________
Protect Computers from Static
______________________________________________________________
Improved human health and comfort (customers and staff)
______________________________________________________________
Why humidify?

For People!
Why Humidify?

Healthier Environment

Increased Comfort

Reduced Absenteeism

\$ Higher Productivity \$
Health and Comfort
Optimum
Zone
Bacteria

Viruses

Fungi

Mites

Allergic rhinitis
and asthma

Respiratory
infections 1

Chemical
interactions

Ozone
production
10         20        30        40         50        60       70        80   90
1. Insufficient data above
Per cent relative humidity
50 per cent R.H.

E.M. Sterling, Criteria for Human Exposure to Humidity in Occupied Buildings, 1985 ASHRAE
HOW WARM DO YOU FEEL
HEAT INDEX TEMPERATURE
ºF
T    125        123   141
E    120        116   130   148
M    115        111   120   135   151
P    110        105   112   123   137 150
E    100        105   113   123   135 149
R     95         99   104   110   110 120 132 144
A     90         85    87    90    93  96 100 106           113   122
T     85         80    82    84    86  88   90   93          97   102   108
U     80         75    77    78    79  81   82   85          86    88    91
R     75         70    72    73    74  75   76   77          78    79    80
E     70         65    66    67    68  69   70   70          71    71    72
% RH
10    20    30    40  50   60   70          80    90   100
RELATIVE HUMIDITY
Source: National Oceanic & Atmospheric Administration
Indoor Air Quality
“Research conducted in
twelve public schools showed a
20% reduction in absenteeism
in all schools when relative
humidity was raised from
20% to 35%.”
Where to Humidify
General office areas
Nursing homes
Schools
Day Care centers
Lawyers offices
Communication centers
Medical clinics
Real estate offices
Cooperate office centers

Wherever large numbers of people gather.

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