Comfort
Content
Moist Air
Properties of Moist Air
If air and water vapour are present together in a confined space, a
balanced condition will be reached where the air has become saturated
with water vapour. If the temperature of the mixture is known, then
the pressure of saturated vapour will be the pressure of steam at this
temperature.
mv
P = Pa + Pv
:
ω=
T (o C ) Psat (kPa)
ma
0
0.610
Moist Air & Thermal Comfort
Dr. M. Zahurul Haq
Professor
Department of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)
Dhaka-1000, Bangladesh
10
15
20
25
30
35
40
45
50
[email protected]
http://teacher.buet.ac.bd/zahurul/
ME 415: Refrigeration & Building Mechanical Systems
1.224
1.704
2.337
3.166
4.246
5.628
7.384
9.593
12.35
ω=
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
1 / 19
mv
ma
P≡
Pa ≡
Pv ≡
ω≡
ma ≡
mv ≡
=
c Dr. M. Zahurul Haq (BUET)
Moist Air
Pv V /Rv T
Pa V /Ra T
Total (standard) pressure
Partial pressure of dry air
Partial pressure of water vapour
Humidity ratio
Mass of dry air
Mass of water vapour
= 0.622
Pv
Pa
= 0.622
Moist Air & Thermal Comfort
pv
P −Pv
ME 415 (2011)
2 / 19
ME 415 (2011)
4 / 19
Moist Air
25o C:
For air at
Pv = 3.166 kPa −→ Pa = 97.159 kPa: saturated air.
3.166
⇒ ωs = 0.622 97.159
= 0.02026 kg water/kg dry air.
If mv = 0.01013 kg water/kg dry air −→ µ = 0.5.
ω
saturation or degree of saturation, µ ≡ ω
s
The proportions of moisture can also be expressed as the ratio of
the vapour pressures, and is then termed relative humidity:
relative humidity, φ ≡
Pv
Pv ,sat
⇒ ω = 0.01013 −→ Pv = 1.65 kPa ⇒ φ = 1.65/3.166 = 52
Dew-point temperature, Tdp is
defined as the temperature at which
condensation begins when the air is
cooled at constant pressure.
e957
e956
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
3 / 19
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
Moist Air
Moist Air
Wet Bulb/Dry Bulb Temperatures
ASHRAE-1989 Correlations
φ=
Psat (Twb ) − Pm
Psat (Tdb )
Psat (Tdb ) ≡ saturated pressure corresponding to Tdb
Psat (Twb ) ≡ saturated pressure corresponding to Twb
Pm ≡ partial pressure of vapour due to depression of Twb below
Tdb .
Twb − 273.2
Tdb − Twb
1+
Pm = P
1514
873
e971
e970
If unsaturated air comes in contact with water at the same temperature, there will
be change in vapour pressures. As a result, some of the water will evaporate. The
resultant water temperature drop will provide a temperature difference, and a
thermal balance will be reached where the flow of sensible heat from the air to the
water provides the latent heat to evaporate a part of it. The equilibrium
temperature is Wet-bulb Temperature.
Moist Air & Thermal Comfort
c Dr. M. Zahurul Haq (BUET)
ME 415 (2011)
5 / 19
. Estimate relative humidity of air if Tdb = 30o C & Twb = 25o C.
. Atmospheric air at 30o C and 50% RH is compressed isothermally to
1.0 MPa. Estimate the psychrometric conditions of the compressed air.
c Dr. M. Zahurul Haq (BUET)
Thermal Comfort
Moist Air & Thermal Comfort
ME 415 (2011)
6 / 19
ME 415 (2011)
8 / 19
Thermal Comfort
Thermal Comfort
Typical Climate Conditions
Comfort zone according to ISO 7730
Comfort zone according to ASHRAE 55-1992
Thermal comfort is defined as that condition of mind which expresses
satisfaction with the thermal environment. Factors affecting thermal
comfort are: temperature, thermal radiation, humidity, air speed,
personal activity and clothing.
e959
c Dr. M. Zahurul Haq (BUET)
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
7 / 19
Moist Air & Thermal Comfort
Thermal Comfort
Thermal Comfort
Bangladesh Outdoor Design Condition
Ventilation: Outdoor Air Requirements
To meet metabolic requirements of occupants.
e993
To dilute the indoor air contaminants, odours and pollutants to
maintain an acceptable air quality.
Summer design is based on 4 summer month from June to
September (Total = (30+ 31+ 31+ 30)* 24 = 2928 hrs.)
2.5% means that temperature exceeds the value (33o C) only 2.5%
of all the hrs in 4 months = 73.2 hr.
Winter: December to January (Total 2160 hrs).
To support any combustion process or replace the amount of
exhaust air required in laboratories, manufacturing processes or
rest rooms.
To provide make-up of amount of ex-filtrated air required when
positive pressure is to be maintained at the conditioned space.
97.5% means temperature falls below the value for 54 hrs.
For ordinary buildings, it is customary to design based on level of
2.5% in summer and 97.5% in winter.
To add to or remove heat/moisture from the space.
⇒ To,max = 33o C, DR = 11o C =⇒ To,av = To,max − DR/2 = 27.5o C.
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
9 / 19
c Dr. M. Zahurul Haq (BUET)
Thermal Comfort
Energy release per unit area of skin surface as a result of oxidative
process in the living cell is the metabolic rate. It depends on the
intensity of physical activities.
Metabolic rate (W/m2 )
Mechanical work performed
Convective + radiative, or sensible heat loss from skin
Evaporative heat loss from skin surface
Evaporative heat loss from respiration
1 met = 58.24 W/m2
Mechanical work, W = µM , µ = 0.2 to 0.24
Activity
Sleeping
Office work, reading or writing or seated
Domestic work, cooking
Walking, 2 mph
Walking, 4 mph
Heavy machine work
In an air conditioned space, a steady-state thermal equilibrium is
usually maintained between the human body and the indoor
environment.
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
10 / 19
Metabolic Rate, M
M = W + C + R + Esk + Eres
=
=
=
=
=
ME 415 (2011)
Thermal Comfort
Steady State Thermal Equilibrium
M
W
C +R
Esk
Eres
Moist Air & Thermal Comfort
ME 415 (2011)
11 / 19
c Dr. M. Zahurul Haq (BUET)
met
0.7
1.0
1.6 - 2.0
2.0
3.7
4.0
Moist Air & Thermal Comfort
ME 415 (2011)
12 / 19
Thermal Comfort
Thermal Comfort
Seven Factors Influencing Thermal Comfort
1
Activity level
2
Clothing
3
Expectation
4
Air temperature
5
Radiant temperature
6
Humidity
7
Air speed
ASHRAE Comfort Zone
e974
c Dr. M. Zahurul Haq (BUET)
e264
Moist Air & Thermal Comfort
ME 415 (2011)
13 / 19
c Dr. M. Zahurul Haq (BUET)
Thermal Comfort
14 / 19
Outdoor Air Requirements
to meet metabolic requirements of occupants
As per ASHRAE 55-1992 & 90.1-1999
(o C)
c Dr. M. Zahurul Haq (BUET)
ME 415 (2011)
Thermal Comfort
Indoor Design Conditions
Summer
Winter
1. Commercial & public bld.
2. Health care bld.
Moist Air & Thermal Comfort
T
23.5 to 26.0
φ (%)
30 to 65
20.5 to 23.5
20.5 to 23.5
20 to 65
30 to 65
Moist Air & Thermal Comfort
ME 415 (2011)
to dilute the indoor air contraminants, odors and pollutants to
maintain an acceptable air quality.
to support any combustion process or replace the amount of
exhaust air required in laboratories, manufacturing processes or
rest rooms.
to provide makeup of amount of exfiltrated air required when
positive pressure is to be maintained at the conditioned space.
15 / 19
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
16 / 19
Thermal Comfort
Thermal Comfort
Outdoor Air as per ASHRAE 62-1992
Application
Dining/conference room, office spaces, lounges
Retail stores, transport waiting rooms, class rooms
Hospitial patient rooms, residences
Smoking lounges
Common Air Contaminants
L/s per person
10
7
15
30
e265
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
17 / 19
ME 415 (2011)
19 / 19
Thermal Comfort
Filters for Air Cleaning Devices
1
Viscous impingment type
2
Dry screen type
3
Electrostatic precipitators
4
Air washers
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
18 / 19
Properties of Moist Air
If air and water vapour are present together in a confined space, a
balanced condition will be reached where the air has become saturated
with water vapour. If the temperature of the mixture is known, then
the pressure of saturated vapour will be the pressure of steam at this
temperature.
mv
P = Pa + Pv
:
ω=
T (o C ) Psat (kPa)
ma
0
0.610
Moist Air & Thermal Comfort
Dr. M. Zahurul Haq
Professor
Department of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)
Dhaka-1000, Bangladesh
10
15
20
25
30
35
40
45
50
[email protected]
http://teacher.buet.ac.bd/zahurul/
ME 415: Refrigeration & Building Mechanical Systems
1.224
1.704
2.337
3.166
4.246
5.628
7.384
9.593
12.35
ω=
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
1 / 19
mv
ma
P≡
Pa ≡
Pv ≡
ω≡
ma ≡
mv ≡
=
c Dr. M. Zahurul Haq (BUET)
Moist Air
Pv V /Rv T
Pa V /Ra T
Total (standard) pressure
Partial pressure of dry air
Partial pressure of water vapour
Humidity ratio
Mass of dry air
Mass of water vapour
= 0.622
Pv
Pa
= 0.622
Moist Air & Thermal Comfort
pv
P −Pv
ME 415 (2011)
2 / 19
ME 415 (2011)
4 / 19
Moist Air
25o C:
For air at
Pv = 3.166 kPa −→ Pa = 97.159 kPa: saturated air.
3.166
⇒ ωs = 0.622 97.159
= 0.02026 kg water/kg dry air.
If mv = 0.01013 kg water/kg dry air −→ µ = 0.5.
ω
saturation or degree of saturation, µ ≡ ω
s
The proportions of moisture can also be expressed as the ratio of
the vapour pressures, and is then termed relative humidity:
relative humidity, φ ≡
Pv
Pv ,sat
⇒ ω = 0.01013 −→ Pv = 1.65 kPa ⇒ φ = 1.65/3.166 = 52
Dew-point temperature, Tdp is
defined as the temperature at which
condensation begins when the air is
cooled at constant pressure.
e957
e956
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
3 / 19
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
Moist Air
Moist Air
Wet Bulb/Dry Bulb Temperatures
ASHRAE-1989 Correlations
φ=
Psat (Twb ) − Pm
Psat (Tdb )
Psat (Tdb ) ≡ saturated pressure corresponding to Tdb
Psat (Twb ) ≡ saturated pressure corresponding to Twb
Pm ≡ partial pressure of vapour due to depression of Twb below
Tdb .
Twb − 273.2
Tdb − Twb
1+
Pm = P
1514
873
e971
e970
If unsaturated air comes in contact with water at the same temperature, there will
be change in vapour pressures. As a result, some of the water will evaporate. The
resultant water temperature drop will provide a temperature difference, and a
thermal balance will be reached where the flow of sensible heat from the air to the
water provides the latent heat to evaporate a part of it. The equilibrium
temperature is Wet-bulb Temperature.
Moist Air & Thermal Comfort
c Dr. M. Zahurul Haq (BUET)
ME 415 (2011)
5 / 19
. Estimate relative humidity of air if Tdb = 30o C & Twb = 25o C.
. Atmospheric air at 30o C and 50% RH is compressed isothermally to
1.0 MPa. Estimate the psychrometric conditions of the compressed air.
c Dr. M. Zahurul Haq (BUET)
Thermal Comfort
Moist Air & Thermal Comfort
ME 415 (2011)
6 / 19
ME 415 (2011)
8 / 19
Thermal Comfort
Thermal Comfort
Typical Climate Conditions
Comfort zone according to ISO 7730
Comfort zone according to ASHRAE 55-1992
Thermal comfort is defined as that condition of mind which expresses
satisfaction with the thermal environment. Factors affecting thermal
comfort are: temperature, thermal radiation, humidity, air speed,
personal activity and clothing.
e959
c Dr. M. Zahurul Haq (BUET)
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
7 / 19
Moist Air & Thermal Comfort
Thermal Comfort
Thermal Comfort
Bangladesh Outdoor Design Condition
Ventilation: Outdoor Air Requirements
To meet metabolic requirements of occupants.
e993
To dilute the indoor air contaminants, odours and pollutants to
maintain an acceptable air quality.
Summer design is based on 4 summer month from June to
September (Total = (30+ 31+ 31+ 30)* 24 = 2928 hrs.)
2.5% means that temperature exceeds the value (33o C) only 2.5%
of all the hrs in 4 months = 73.2 hr.
Winter: December to January (Total 2160 hrs).
To support any combustion process or replace the amount of
exhaust air required in laboratories, manufacturing processes or
rest rooms.
To provide make-up of amount of ex-filtrated air required when
positive pressure is to be maintained at the conditioned space.
97.5% means temperature falls below the value for 54 hrs.
For ordinary buildings, it is customary to design based on level of
2.5% in summer and 97.5% in winter.
To add to or remove heat/moisture from the space.
⇒ To,max = 33o C, DR = 11o C =⇒ To,av = To,max − DR/2 = 27.5o C.
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
9 / 19
c Dr. M. Zahurul Haq (BUET)
Thermal Comfort
Energy release per unit area of skin surface as a result of oxidative
process in the living cell is the metabolic rate. It depends on the
intensity of physical activities.
Metabolic rate (W/m2 )
Mechanical work performed
Convective + radiative, or sensible heat loss from skin
Evaporative heat loss from skin surface
Evaporative heat loss from respiration
1 met = 58.24 W/m2
Mechanical work, W = µM , µ = 0.2 to 0.24
Activity
Sleeping
Office work, reading or writing or seated
Domestic work, cooking
Walking, 2 mph
Walking, 4 mph
Heavy machine work
In an air conditioned space, a steady-state thermal equilibrium is
usually maintained between the human body and the indoor
environment.
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
10 / 19
Metabolic Rate, M
M = W + C + R + Esk + Eres
=
=
=
=
=
ME 415 (2011)
Thermal Comfort
Steady State Thermal Equilibrium
M
W
C +R
Esk
Eres
Moist Air & Thermal Comfort
ME 415 (2011)
11 / 19
c Dr. M. Zahurul Haq (BUET)
met
0.7
1.0
1.6 - 2.0
2.0
3.7
4.0
Moist Air & Thermal Comfort
ME 415 (2011)
12 / 19
Thermal Comfort
Thermal Comfort
Seven Factors Influencing Thermal Comfort
1
Activity level
2
Clothing
3
Expectation
4
Air temperature
5
Radiant temperature
6
Humidity
7
Air speed
ASHRAE Comfort Zone
e974
c Dr. M. Zahurul Haq (BUET)
e264
Moist Air & Thermal Comfort
ME 415 (2011)
13 / 19
c Dr. M. Zahurul Haq (BUET)
Thermal Comfort
14 / 19
Outdoor Air Requirements
to meet metabolic requirements of occupants
As per ASHRAE 55-1992 & 90.1-1999
(o C)
c Dr. M. Zahurul Haq (BUET)
ME 415 (2011)
Thermal Comfort
Indoor Design Conditions
Summer
Winter
1. Commercial & public bld.
2. Health care bld.
Moist Air & Thermal Comfort
T
23.5 to 26.0
φ (%)
30 to 65
20.5 to 23.5
20.5 to 23.5
20 to 65
30 to 65
Moist Air & Thermal Comfort
ME 415 (2011)
to dilute the indoor air contraminants, odors and pollutants to
maintain an acceptable air quality.
to support any combustion process or replace the amount of
exhaust air required in laboratories, manufacturing processes or
rest rooms.
to provide makeup of amount of exfiltrated air required when
positive pressure is to be maintained at the conditioned space.
15 / 19
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
16 / 19
Thermal Comfort
Thermal Comfort
Outdoor Air as per ASHRAE 62-1992
Application
Dining/conference room, office spaces, lounges
Retail stores, transport waiting rooms, class rooms
Hospitial patient rooms, residences
Smoking lounges
Common Air Contaminants
L/s per person
10
7
15
30
e265
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
17 / 19
ME 415 (2011)
19 / 19
Thermal Comfort
Filters for Air Cleaning Devices
1
Viscous impingment type
2
Dry screen type
3
Electrostatic precipitators
4
Air washers
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
c Dr. M. Zahurul Haq (BUET)
Moist Air & Thermal Comfort
ME 415 (2011)
18 / 19
