# Air Properties and Psychrometrics by liwenting

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```									            Air Properties and
Psychrometrics
MET 150

3/27/2006        Air Properties and Psychrometrics   1
Atmospheric and Gauge Pressure

 Atmospheric pressure
 Measure with a barometer
 Standard pressure (dry air at sea level)
 14.7 psia
 29.92 in. Hg. absolute

 Gauge pressure scale
 Conversion between atmospheric and
gauge pressure

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Pressure Conversions

   What is the absolute pressure if the

   What is the gauge pressure if the

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Pressure in Inches of Water

 Common measure in air distribution
systems
 14.7 psi = 406.9” water = 33.9’ water
 1 psi = 27.68 inches water
 Convert inches water to psi: divide by
27.68
 Convert psi to inches water: multiply by
27.68
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Converting Between Inches Water
and PSI

   What is the pressure in inches water if
the gauge reading is 2.2 psi?

   What is the pressure in psi if the

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Gas Laws

   Dry air is a gas

   Follows specific laws pertaining to
relationships between pressure,
temperature and volume

   Known as the gas laws

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Dalton’s Law

   Total pressure of a mixture of confined
gases is the sum of the pressures each
gas would exert if it alone occupied the
volume of the mixture at the same
temperature.

   Each gas acts independently

   TP = P1 + P2 + … Pn

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Boyle’s Law

   At constant temperature, the pressure on
a given quantity of confined gas varies
inversely with the volume of the gas.

   At constant temperature, the volume of a
given quantity of gas varies inversely
with the applied pressure.

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Boyle’s Law

 Po x Vo = Pn x Vn
 Vn = Vo x Po / Pn
 Pn = Po x Vo / Vn
 Where:
 Po = original absolute pressure (psia)
 Pn = new absolute pressure (psia)

 Vo = original volume (cubic feet)

 Vn = new volume (cubic feet)

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Boyle’s Law Examples:

   What is the new volume of 3 cubic feet of
gas at 25 psig if it is compressed to 40
psig?

   4 cubic feet of gas is expanded from 45
psig to 10 psig. What is the new volume
in cubic feet?

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Charles’ Law

   With a constant pressure, the volume of
a given quantity of confined gas varies
directly with its absolute temperature

   With a constant volume of gas, the
pressure varies directly with its absolute
temperature

   Absolute temperature = oF + 460
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Charles’ Law

   At constant pressure:
 Vo x Tn = Vn x To
 Vn = Vo x Tn / To

 Tn = Vn x To / Vo

 Where:
 To = original absolute temperature
 Tn = new absolute temperature

 Vo = original volume (cubic feet)

 Vn = new volume (cubic feet)

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Charles’ Law

   At constant volume:
 Po x Tn = Pn x To
 Tn = Pn x To / Po

 Pn = Po x Tn / To

 Where:
 To = original absolute temperature
 Tn = new absolute temperature

 Po = original absolute pressure (psia)

 Pn = new absolute pressure (psia)

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Charles’ Law Examples

 What is the new volume of 10 cubic feet
of gas at 50 degrees if the temperature is
raised to 130 degrees at constant
pressure?
 What is the new pressure (in psig) of a
quantity of gas in a cylinder whose
pressure is 30 psig at 40 degrees if the
temperature is raised to 95 degrees?

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Psychrometrics

   Study of dry air and water vapor mixtures

   Condition of the air (temperature and
humidity) affect human comfort

   Dry air a mixture of gases (nitrogen,
oxygen and others)

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Definitions

   Specific volume: how much space 1
pound of dry air occupies.
   At 70oF and sea level: 13.33 ft3/lb
   Specific density: Weight of dry air per 1 ft3
   At 70oF and sea level: 0.075 lb/ft3
   Specific heat: ability to get hot
   At 70oF and sea level: 0.24 Btu/lb/oF

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Sensible Heat

 The amount of heat which, when added
to air, causes a change in temperature
with no change in the amount of
moisture present
 Measured with a thermometer
 Sensible heat = specific heat x specific
density x 60 min/hr x CFM x DT (Btuh)
 Btuh = 0.24 x.075 x 60 x CFM x DT
 Btuh = 1.08 x CFM x DT

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Humidity

 Amount of humidity affects rate at which
perspiration evaporates, making skin
cooler
 Comes from evaporation of earth’s
ocean and other bodies into the
atmosphere
 Inside, comes from cooking, showers,
human respiration and perspiration

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Humidity
   Moisture contained in the air
 Pounds moisture per pound of dry air
 Grains moisture per pound of dry air
 At 70oF and sea level, 7000 grains per pound
of water
 Saturated air: holds all the moisture it can
at that temperature and pressure
 Saturated air at 70oF and sea level
contains 110.5 grains of water or 0.01579
pounds of water per pound of dry air
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Relative Humidity

 Amount of water held by the air as a
percent of what it can hold at that
temperature and pressure
 Expressed as a percentage
 Warmer air will hold more water
 Relative humidity affects comfort
 Winter: 67-76oF, 30% RH
 Summer: 72-81oF, 40% RH

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Comfort Zones
 Dressed in typical
summer or winter
clothing
 Engaged in
sedentary activity
 Air motion in
occupied zone does
not exceed 30 fpm
(winter) or 50 fpm
(summer)
ASHRAE Comfort Chart
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Measuring Humidity

 Sling psychrometers
 Electronic psychrometers
 Hygrometers with thermometers
 Measure both air temperature and
moisture content

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Sling Psychrometer

 Two thermometers – dry bulb and wet
bulb
 Dry bulb measures sensible heat
 Wet bulb thermometer has wick
saturated with distilled water,
evaporation lowers temperature, taking
into account moisture content of air

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Sling Psychrometer

 Insure that wick is wet
 Whirl psychrometer with a steady motion
through surrounding air (30 sec.
minimum)
 Periodically check wet bulb readings
 Accept wet bulb reading when two
 Always read wet bulb before dry bulb
 Do not touch wick when taking reading
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Sling Psychrometer

 Determine approximate relative humidity
from sliding scale
plotting the dry bulb and wet bulb
temperatures on the psychrometric chart
(covered next week)

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Digital Psychrometers and Hygrometers

 Measure dry bulb and wet bulb
temperature directly or measure relative
humidity directly

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Dewpoint

 Temperature at which water vapor in the
air becomes saturated with moisture and
the moisture starts to condense into
water droplets
 If relative humidity is 100%, dewpoint,
wet bulb and dry bulb temperatures are
all the same (no water can be
evaporated from the wet bulb
thermometer)

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Enthalpy

 Total heat content of air and water vapor
mixture
 Measured from pre-determined base
point
 Expressed in Btu/lb
 Find enthalpy of air stream by measuring
dry and wet bulb temperature and
plotting on a psychrometric chart

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Equipment Capacity

 Btu/hr
 Enthalpy difference of air entering and
leaving equipment x specific density of
air x cfm x 60 min/hr
 Btu/hr (Btuh) = 4.5 x cfm x DH

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Example

   Enthalpy of 10,000 cfm of air entering
and leaving a cooling coil are as follows:
 41.5 btu/lb
 22.3 Btu/lb

   What is capacity of unit in Btu/hr? Tons?
 Btu/hr = 4.5 x 10000 x (41.5-22.3) =
864,000 Btu/hr
 864,000 Btu/hr x 1ton/12,000 Btu/hr = 72 T

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Homework/Labs

 Read pages 11.1 – 11.7