Transcript Human Thermal Comfort Analysis

Assessing Thermal Comfort in Deep Underground Mines
By
Maurice N. Sunkpal
MSc. Candidate (Mining Engineering)
Advisor: Dr. Charles Kocsis
Department of Mining and Metallurgical Engineering
Fall-2015
Prelude
Fundamentally, comfort involves a heat
balance (a thermal equilibrium) … where:
heat in
≈
heat out
where “heat in” is provided by metabolism,
radiation, conduction, convection
where “heat out” is via radiation, conduction,
convection, evaporation
Prelude
Climatic Parameters
Air Temperature
Air Humidity
Radiant
Temperature
Air Velocity
Individual Differences
Physiological
Inputs


Metabolic rate
Clothing
Thermal Comfort ≈ Heat
balance
𝑴=𝑪+𝑹+𝑩+𝑬
Yes
In Thermal
Comfort Zone




No
Out of Thermal
Comfort Zone
Age effects
Nationality
Sex effects
Time-of-day
Non-Significant
Outline
1.
2.
3.
4.
5.
Scope
Objective
Introduction
Assessing thermal climates: Related research
Assessing mine climates
• Required sweat rate
• Required Skin wettedness
• Worker safe exposure time
6. Results
6. Conclusion
7. Acknowledgements
Objectives
• Prediction of thermal stress in conditions likely to lead to
excessive dehydration in the human body
• Prediction of exposure times with which physiological
strain is acceptable
• Investigation in the effects of changing the comfort
parameters on
required sweat rate (SWreq),
skin wettedness (w)
duration limit of exposure (DLE)
• Investigation will be used by a graduate student to assess
and determine a heat stress index that will protect U/G
workers in hot mines
Scope
• An analytical evaluation and interpretation of thermal
stress in a hot environment
• A method to predict required sweat rate that the human
body will respond to in underground working conditions
• The effects of physical and environmental parameters
influence on thermal stress experienced by a mine
worker
Introduction
Thermal Comfort:
• “Thermal comfort” describes a person’s psychological
state of mind and is usually referred to in terms of
whether someone is feeling too hot or too cold
• The best that you can realistically hope to achieve is a
thermal environment that satisfies the majority of people
in the workplace, or put more simply, “reasonable
comfort”.
• ASHRAE considers ~80 % persons satisfied as reasonable
Introduction
•
•
The most commonly used indicator of thermal comfort is air
temperature
not a valid or accurate indicator of thermal comfort or
thermal stress
Thermal comfort should always be considered in relation to
other environmental and personal factors
Environmental factors
Personal factors
Air temperature
Radiant temperature
Air velocity
Humidity
Clothing Insulation
Metabolic heat
Introduction
Why is thermal comfort important?
Assessing Hot Environments
• Thermal comfort is defined as being an opinion (essentially an
individual perception)
• A perception (condition of mind) is best assessed by asking workers
(occupants) how they feel
Subjective Evaluation (Asking)
• The traditional 7-point “status” scale:
cold | cool | slightly cool | neutral | slightly warm | warm | hot
• An alternative “action” scale:
Would you prefer: to be warmer | no change | to be cooler
• A post-occupancy evaluation (POE) tool
• If workers are in the environment being evaluated
• Not directly usable as a design tool
• There is no occupied space during design
Assessing Hot Environments
Human Thermal Balance
The most comprehensive method in evaluating thermal environment is by
analyzing the human heat transfer balance equation:
Comfort = S = 0
S = M − (C + R + B + E + K + W)
S = Heat storage in the body
M = Metabolic rate
C = Convective heat generation
R = Radiation heat generation
B = Respiratory heat generation
E = Evaporation heat generation
K = conduction heat generation
W = Mechanical power
Heat
In
Heat
Out
Assessing Hot Environments
Comfort Indices
• One way to evaluate thermal environment is to use thermal
comfort indices, which combines two or more parameters of
thermal climate into one variable.
Empirical Index
Rational Index
Sophisticated indices, which
integrate environmental and
physiological variables; they are
difficult to calculate and are not
feasible for daily use
Direct Index
Simple indices, which are based on
the measurement of basic
environmental variables
e.g. Discomfort index
DI=0.4Tw+0.4Ta+8.3
Assessing Hot Environments
Fangers’ PMV and PPD Models
• A commonly used method for assessing thermal comfort is the
Fangers’
– Predicted Mean Vote (PMV) and
– Predicted Percentage of Dissatisfied (PPD)
Value
Thermal Scale
+3
+2
+1
0
-1
-2
-3
Hot
Warm
Slightly Warm
Neutral
Slightly Cool
Cool
Cold
(ASHRAE, 2005)
The recommended acceptable PMV range for thermal comfort from between -0.5
and +0.5 for an interior space.
Assessing Hot Environments
Limitations of Fangers’ model
• The PMV equation only applies to humans exposed for a long
period to constant conditions at a constant metabolic rate.
• PMV predicted actual thermal sensation most accurately for
clothing insulation in the range 0.3 to 1.2 clo, for activity levels
below 1.4 met (80 W/m2).
Range of validity for ISO 7933, 2004
Parameter
Relative humidity, RH (%)
Air Temperature (tr=ta), (° C)
Air Velocity , va (m/s)
Clothing Insulation (clo)
Metabolic rate, M (W/m2)
Fangers‘ Model
Min
Max
Min
Max
0
15
0
0.1
100
100
50
3
1
450
0
60
27
0.2
1.2
100
0
0.3
Assessing Hot Environments
Sweat rate, Skin wettedness and Maximum exposure time
• Factors that affect state of thermal stress include:
– Maximum rate of sweat production
– Dehydration level
– Level of skin wittedness
• In hot climates (e.g. Underground mines), the heat balance
equation can be rearrange to provide the required evaporation rate
(Ereq) for heat balance (S=0).
𝐸𝑟𝑒𝑞 = 𝑀 − (𝐶 + 𝑅)
(Waclawik and Branny, 2004)
• In hot climates, sweating and its evaporation is the major mode of
rejecting heat from the body to maintain comfort.
Assessing Comfort Using
Sweat rate, Skin wettedness and Maximum exposure time
• The method assesses comfort based on:
– Required Sweat Rate,
– Required Skin Wettedness, and
– Maximum Worker Exposure Time
• These indices are predicted and compared to the allowable maximum under
the prevailing thermal conditions
• A criteria is used for acclimatized and unacclimatized workers
• If required sweat rate is achieved with acceptable dehydration, no limit is
put on heat exposure for the 8 hr. shift
Assessment Methodology
Climate conditions
(ta ,tr, va, RH)
Physical parameters
(clo, M)
Comfort Engine
Evaluate using the human
balance equation
Predict stress/strain (SW,
TLV, w)
Limit Criteria
• Maximum skin wettedness, wmax
–
–
0.85 for unacclimatized
1 for acclimatized
• Maximum sweat rate, Swmax
– SWmax =2.6 X (M-58)
– SWmax unacclimatized = 650 – 1000
g/h
g/h
• Maximum dehydration, Dmax (which is strain criteria)
– Dmax = lies between 3.5 % and 7 % of and average body mass of 75 kg
Dmax
D max = 3900 g (5.2 %)
– Admissible working time =
SWreq
•
Acclimatized workers sweat more, sweat early , able to endure greater water loss
Data Measurement
Climatic conditions with air temperatures in the range of 10 °C to
50 °C and one other parameter varying as indicated
Parameter
Range
Constant value
Relative Humidity, RH (%)
50-100
50,60,..100
Mean Radiant Temperature (tr=ta), (° C)
10-40
Air Velocity , va (m/s)
0-4
0,1,1.5….4
Clothing Insulation (clo)
0-1
0.093
200-340
200-300
Metabolic rate, M (W/m^2)
Data Measurement
Estimating Personal Factors Affecting Comfort
• Physical
– Clothing (specifically its insulation value in “clo”)
– Activity level (specifically metabolic heat production in “met”)
Metabolic
Rate
Rcl, clo,
(m^2)/W
fcl
hcl=1/R,
W/(m^2K)
Rating
No clothing
0.000
1
∞
0
Resting
M≤117 W
Shorts
0.051
1.05
19.6
1
Low metabolic rate
117 < M ≤ 234 W
Shorts and a thin short sleeved shirt
0.078
1.11
13.2
2
Moderate metabolic rate
234 < M ≤ 360 W
Thin trousers, long-sleeved shirt
0.093
1.18
10.8
Thick trousers, long sleeved shirt
0.116
1.28
8.6
3
High metabolic rate
360 < M ≤ 468 W
Overalls, long shirt
0.155
1.28
6.5
4
Very high metabolic rate
M > 468 W
Type of Clothing
(1 Clo = 0.155 m2/W insulation value).
(Waclawik & Branny, 2004)
Activity
(ISO 7243, 1989)
Analysis of Required Sweat Rate
Varying M, RH
a
b
Analysis of Skin Wettedness
Varying M, RH
a
b
Analysis of worker tolerable time
Varying M, RH
a
b
Analysis of required sweat rate
Varying Va, M
a
b
V=1.5 m/s
V=1.5 m/s
Analysis of required skin wettedness
Varying Va, M
M=320
a
b
Analysis of worker tolerable time
Varying Va, M
a
b
Analysis of required sweat rate
Varying Va, M
a
b
Analysis of required skin wettedness
Varying Va, M
0.85
a
b
Analysis of worker tolerable time
Varying M, RH
M=300 W/m2
39-43 ̊C
M=300 W/m2
30-32.5 ̊C
Sensitivity Analysis on Comfort
Varying M and RH
Metabolic Activity (W/m2)
200
220
240
260
280
300
320
340
Air
Temperature Decreasing Ambient Temperature Requirements at the Face (%)
(°C)
Humidity
(%)
36.50
1.87
3.81
5.75
7.79
9.96
11.90
14.07
50
34.80
1.95
3.96
6.03
8.12
10.26
12.44
14.69
60
33.10
2.03
4.15
6.30
8.48
10.71
12.98
15.33
70
31.50
2.16
4.36
6.61
8.87
11.21
13.59
16.03
80
30.00
2.26
4.56
6.92
9.28
11.75
14.24
16.80
90
28.50
2.44
4.80
7.27
9.75
12.32
14.93
17.61
100
Sensitivity Analysis on Comfort
Varying M and Va
RH = 70 %
Metabolic Rate
W/m2
Air Velocity (Va), m/s
0
0.5
1
1.5
2
2.5
3
Increasing Ambient Temperature Requirements at the Face (%)
200
31.23
4.23
5.60
5.89
5.67
5.09
4.35
220
30.72
3.97
5.37
5.70
5.47
4.92
4.17
240
30.17
3.78
5.17
5.54
5.34
4.71
4.04
260
29.58
3.62
5.04
5.41
5.24
4.70
3.96
280
28.96
3.52
4.94
5.32
5.18
4.63
3.87
300
28.32
3.43
4.84
5.26
5.08
4.59
3.81
320
27.64
3.36
4.81
5.25
5.10
4.59
3.80
340
26.95
3.30
4.75
5.23
5.08
4.56
3.71
Sensitivity Analysis on Comfort
Varying Va and RH
Air Velocity (Va), m/s
M=200 W/m2
0
Relative Humidity
Ta (°C)
(RH), %
0.5
1
1.5
2
2.5
3
3.5
4
Increasing Ambient Temperature Requirements at the Face (%)
50
34.53
4.17
5.53
5.82
5.56
5.01
4.26
3.36
2.37
60
32.82
4.20
5.61
5.88
5.67
5.09
4.36
3.47
2.47
70
31.23
4.23
5.60
5.89
5.67
5.09
4.35
3.43
2.43
80
29.74
4.20
5.58
5.85
5.62
5.01
4.24
3.33
2.29
90
28.31
4.20
5.58
5.83
5.55
4.95
4.13
3.18
2.12
100
26.93
4.20
5.53
5.76
5.46
4.79
3.97
2.97
1.89
Sensitivity Analysis on Comfort
Varying Va and RH
Air Velocity (Va), m/s
M=200 W/m2
0
Relative
Humidity (RH),
%
0.5
1
1.5
2
2.5
3
3.5
4
Decreasing Ambient Temperature (°C) Requirements at the Face (%)
50
34.53
35.97
36.44
36.54
36.45
36.26
36.00
35.69
35.35
60
4.95
4.92
4.88
4.90
4.86
4.88
4.86
4.85
4.87
70
9.56
9.51
9.50
9.50
9.47
9.49
9.47
9.50
9.50
80
13.87
13.84
13.83
13.85
13.83
13.87
13.89
13.90
13.95
90
18.01
17.99
17.97
18.01
18.02
18.06
18.11
18.16
18.22
100
22.01
21.99
22.01
22.06
22.09
22.17
22.22
22.30
22.38
Conclusion
• Maximum thermal satisfaction is attainable with higher air velocities than those that
can be obtained at the lower airflow velocity
• The method developed makes it possible to determine changes in environmental
parameters (e.g. Ta, RH, Va) to obtain desirable maximum exposure times in the
working area
• This study analyses and summarizes the thermal stress evaluation indices of ISO
7933 from simulations performed on the mathematical model
• From the simulated results based on the thermal parameters of the environment,
upper working limits of air temperature, activity, humidity, and air velocity can be
determined and recommended
• The limits can be verified by simulation results of subjective evaluation indicators,
including sweat rate, skin wettedness and the upper limit of working time
Conclusion
• Optimum air temperatures are achieved at air velocities of 1.5 m/s. When the air
motion across the skin increases, thermal comfort will increase and that the
optimum air velocity for comfort is 1.5 m/s
• The analysis also observed that humidity contributes a lot more to deviations from
comfort. It is followed by activity level and then airflow velocity. Note that in this study
values for clothing (clo) are kept constant, and Ta is equated to Tr
• Results form this research work was used by a graduate student to assess and
determine a heat stress index that will protect U/G workers in hot mines
Thank You!
?
ACKNOWLEDGEMENT:
Dr. Robert Watters
Dr. Javad Sattarvand
CDC/NIOSH