Chapter-2 Understanding Psychrometrics - Air Conditioning and

57 Pages·2013·3.46 MB·English

႔ Chapter-2 Understanding Psychrometrics
Chapter-2 Understanding Psychrometrics

-
( )
( ) Dry Bulb ၊ Wet Bulb ၊ Relative Humidity(RH)၊ p f u ၊ Dew ၊
Enthalpy S a a Psychrometrics (physical meaning)

(ဂ) a ဂ air property)

(ဃ) P a ဂ air property)
ဂ (properties f a )
( ) Sensible heat ၊ a a a a ႔ p )

( ) mixed air) ဂ (properties)

( ) Psychrometric equation) ၊ a tota a a
႔ ႔
၂.၁ Psychrometric
American Society of Heating, Refrigerating, and Air conditioning Engineers (ASHRAE)
႔ C (temperature)၊
(humidity)၊ ႔ (cleanliness) ႔ (distribution) ႔
(treat) air conditioning p )
p f a a a u a u temperature,
u , a a u u f pa

air con ႔ ႔
Comfort a Industrial air conditioin
Comfort Air Conditioning
(confortable) ႔ Industrial Air
Conditioin ၊ ၊ p
႔ (air treatment) ႔
႔ ဂ properties of air)

Psychro ဂ
a p ) a u u ap instrument)
႔ Psychrometrics
a - a ap ႔ a
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Air Conditioning and Mechanical Ventilation (Vol. 1) ႔

၂-၁ ႔ (dry air) ႔ a ap ) a
႔ moist air
႔ (dry air) ႔( water vapor) ႔ mixture M
႔ M ႔ (dry air)
႔
႔ ႔
(dry air) ႔

၂-၂ ( )
ဂ (properties of air) ႔
air conditioning system ႔ (treat)

ဂ (properties of air) (table) ႔ ဂ (chart)
ဂ ( chart) ဂ (properties of air)
Psychrometric Chart C a line) ႔

Air conditioning process

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႔ Chapter-2 Understanding Psychrometrics
SI ဂ (chart) IP ဂ (chart)
chart

Psychrometric c a a
u ) cooling process heating process
Psychrometric c a
Air con ဂ Psychrometrics
႔ air conditioning system
(trouble shooting) ႔
႔ c C a (line)
(physical meaning)

၂-၃ Psychrometric c a
Constant Enthalpy ႔ constant Wet Bulb ႔ (parallel)
ဂ a )
p u )
( a )
ဂ ႔
႔ C a

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Air Conditioning and Mechanical Ventilation (Vol. 1) ႔
- u p
p
p a a u p
႔ a - ႔
line)
straight line) process curve)
p process)
႔
႔

၂-

(air) (water)
(water) ၊ (snow)၊ hail ႔ (solid form)
႔ (rain drops)၊ (mist) ႔ ( liquid form)
(indoor) ၌ ႔ ( vapor form) ႔
႔ Sup a L -Pressure Steam
႔

႔ ၊

ဂ
(properties of air) ဂ properties of air)
P a
႔ ႔ ႔ ႔
(evaporation) ႔ ႔ (condensation)

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႔ Chapter-2 Understanding Psychrometrics
၂ ၁ ၁ ႔ (Evaporation)
(hot air) ႔
႔ ႔ ႔ (evaporation)

႔ evaporation

႔ ႔ (evaporation) ႔
႔ (evaporation) a )
Vaporization ႔ ႔
႔ vaporization) a a )

၂.၁.၂ ႔ ႔ (Condensation)
႔
႔ ႔ ၌ ႔ ႔
c a (e ap a ) ႔
႔ condensation) a a )

႔ (water vapor content) Dry Bulb
႔ (water
vapor content) Dry Bulb Dry Bulb
႔ Wet Bulb
႔ (water vapor content)

႔( water vapor) Dry Bulb
႔( water vapor) Wet
Bulb

Saturated Air and Unsaturated Air
၌ ႔ (water vapor
content) Saturated Air
႔( water vapor) Unsaturated Air
႔( water vapor) ( ) unsaturated air

၂.၂ (Standard Air)
ဂ ႔ ၊
ဂ ႔ ၂၁ ၊ ႔ ႔ ၁ ႔ a )
၊ ၊ ၊
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Air Conditioning and Mechanical Ventilation (Vol. 1) ႔
(standard)

(sea l ) 29.921 in.Hg (atmospheric
p u ) ၀) ဂ (70°F) S a a a S a
L )

Standard atmospheric pressure 1013.25 mbar (101 325 Pa)
mass ဂ (specific properties) ႔
(unit w f a )

၂-၅ ႔ ၂-၆
၂.၃ (Specific Heat of Air)
heat)
h a ) p f a
Specific heat a u f a )

mass) 1°F ႔ C
Specific Heat
ဂ k ) 1°C (energy)
kilocalorie) (1 lb) 1°
energy) BTU (British Thermal Unit) Spec f a
a ႔ sp f a

႔ a ) p f a p a u )
႔ p f a
Cp =1.006 kJ/kg K
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႔ Chapter-2 Understanding Psychrometrics
Cp =1.006 kJ/kg K ႔ ဂ ( dry air ) ဂ
celsius (1°C) heat add)
68°F (20°C) (air) (water) ႔ (heat energy)

ဂ celsius heat
remove)

၂- 1 BTU 1 kcal ၂- 1 kcal
Air Water
Density at 68°F (lb/ ft3) 0.075 62.4
(air)၊ (water) (ice) ႔ specific IP SI ႔
Specific heat
Substance Conventional metric and Yard-pound system S.l metric system
(Btu/lb °F) (kJ/kg K)
air 0.243 1.005
Water 1.0 4.187
Ice 0.504 2.110
Wood 0.327 1.369
Iron 0.129 0.540
Mercury 0.0333 0.139
Alcohol 0.615 2.575
Copper 0.095 0.398
(1 lb) 1° )
႔ (1 lb of dry air) 1°
)
႔ ႔ specific heat

႔ ႔ )

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Air Conditioning and Mechanical Ventilation (Vol. 1) ႔
Total heat of moist air
Sensible heat + Latent heat = Total heat
႔ (dry air) + ႔ (water vapor) =
(water vapor) ႔ p a

Latent Heat ႔ (water vapor)
[Latent heat is the energy stored in water vapor.]
Sensible heat ႔ (dry air)
[Sensible Heat is the energy stored in dry air.]

၂. Da ’ La
(mixture) (total pressure) mixture
(substance) (partial pressure)
Da ’ La a p u ua u f pa a p u

P = Total pressure
Pa = Partial pressure of dry air
Pw = Partial pressure of water vapor
25°C (moist air) (total pressure) a
Saturated vapor partial pressure a ႔ (dry air) partial
pressure a
Partial pressure of dry air (Pa) = 31.66 mbar
Partial pressure of saturated vapor (Pw) = 971.59 mbar
Total(standard) pressure (P) = 1013.25 mbar
Humidity ratio ႔ (partial pressure of water vapor) ႔
(partial pressure of dry air) ႔

- ၌ ႔ (partial pressure of water vapor) p a
barometric pressure(atmospheric pressure) p humidity
ratio
Da ’ La
Pa = P – Pw
Pa = 14.6-0.2 = 14.4 Psia
Humidity ratio

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႔ Chapter-2 Understanding Psychrometrics
႔ ႔ (water vapor) partial pressure Humidity Ratio

Humidity ratio 0.0086 ႔ ႔ (dry air)
(1lb) ႔ (water vapor content)

႔ (dry air) ႔(water vapor) 60.2 grain

Grains Psychrometrics 7000
grains 1 lb SI ႔ (dry air) ဂ
(kg) IP ႔ (dry air) (lb)

( )

RH = Relative Humidity %
Pw = Partial pressure of water vapor at Dry Bulb temperature
Pws = Satuation pressure of water vapor at Dry Bulb temperature
၂.၅ Psychrometric C a

၂-၉ Psychrometric C a ၂-၁၀ Psychrometric C a
၂.၅.၁ Dry Bulb (Temperature)

Dry Bulb
temperature) ၊
႔၌ Dry
Bulb (temperature) Dry Bulb
(temperature) DB

၂-၁၁ Dry Bulb
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Air Conditioning and Mechanical Ventilation (Vol. 1) ႔
Constant Dry Bulb p a u C a
(vertical l ) (vertical line) ၌ Dry Bulb
(temperature)

၂-၁၂ Dry Bulb ၊ u Dew ၂-၁၃ S Dry Bulb Wet Bulb
Point ႔ ႔

Psychrometric c a
(vertical line) Dry u (temperature line)

၂-၁ Dry u (temperature line) ၂-၁၅ Dry u
၂.၅.၂ Wet Bulb (Temperature)
Wet Bulb ဂ ႔
Wet Bulb ၊
( a ) Wet Bulb
cooling effect) Wet Bulb Dry u
Wet Bulb WB
Wet Bulb ႔ (evaporation)
Wet Bulb Dry Bulb Wet Bulb
႔ (amount of moisture)
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