script thermal comfort 2013

7/28/2019 Script Thermal Comfort 2013

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Laboratory: measurement techniques THERMAL COMFORT

1 A.Thewes/A.Degens

THERMAL COMFORT

Laboratory: measurement techniques


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2 A.Thewes/A.Degens

1. What is thermal comfort?

Although the human can acclimate on different external conditions, there is a range, therange of comfort, within the human feels most comfortable. Thermal comfort is defined

as the feeling, that express satisfaction with the environment.

Figure 1: Thermal comfort in dependent on physiological, intermediate and physical factors

The first requirement on an acceptable thermal indoor climate is that the person is in

thermal equilibrium (That means, the person does not know, if a higher or lower

temperature is more comfortable). The ASHRAE-Standard 55 define the thermal

comfort as: Thermal comfort is that condition of mind, which expresses satisfaction

with the thermal environment..

PRIMARY FACTORS

ADDITIONAL FACTORS

SECONDARY FACTORS

Insulative

clothing

Activity level

Air temperature

Mean radiant

temperature

Relative humidity

Air movement


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3 A.Thewes/A.Degens

The warmth sense will affected by e.g. type of activity, clothing or room temperature.

Generally there are 21 different parameters. All factors can be combined in many

different ways to create a comfortable thermal climate for a person. However there aremany factors which depend heavily on the individual in room, such as age, sex and

constitution (Figure 1). Therefore it is impossible to specify a thermal environment that

will satisfy everybody.

The standard DIN EN ISO 7730 standardizes an analytical method based on the PMV-

PPD index included the six main parameters (activity, clothing, air temperature, radiant

temperature, air velocity and relative humidity), which are summarized to one value

(PMV predicted mean vote). This heat balance model was developed by Fanger in

1972. This allows estimation based on the following Seven-Point-Scale:

+3 (hot), +2 (warm), +1 (slightly warm), 0 (neutral), -1 (slightly cool),-2 (cool),-3 (cold)

The PMV-value is an index, which predicts the average value for the climate

estimation by a large group. A PMV-value of zero means thermal neutrality and

well-being. The thermal balance is reached, when in the body generated heat is equal

to the released heat. The PMV can be used to verify, if the available thermal

environment correspond to the comfort criteria.

According to Fanger, the PMV is calculated with the equations (1) (4):


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4 A.Thewes/A.Degens

Where:

M metabolic rate [W/m2

] tabular value (Figure 26 Annex a)W mechanical performance [W/m2] mostly 0 W/m2

Icl clothing insulation [m2*K/W] tabular value (Figure 27-Annex b)

fcl clothing area factor

ta air temperature [C] measured value

tr mean radiant temperature [C] measured value

var relative air velocity [m/s] measured value

pa partial pressure of water vapor [Pa] measured value as rel. humidity [%]

hc convective heat transfer coefficient [W/(m2*K)]

tcl surface temperature of persons clothing [C]

The equations fcl and tcl could be solved iterative.

Independent of these equations, the PMV-value can be estimated using tables (Annex

c) of standard EN ISO 7730. For this, there are different combinations of activity,

clothing, operative temperature and relative air velocity.

Some estimations deviate around the value of thermal neutrality. Therefore, it is useful

to predict the number of people, who feel the environment as too warm or too cold.

Hence the quality of thermal environment can specify as predicted percentage ofdissatisfied (PPD-Index predicted percentage of dissatisfied). This percentage

depends on the predicted mean vote (PMV).

However, Fanger doesnt ask the people for their satisfaction with the thermal

environment, but he decided that persons, who indicated a PMV-value of -3, -2, +2 or

+3, are dissatisfied. The PPD-index of Fanger is a quantitative prediction of the number

of dissatisfied people in regard to the thermal environment and is only based on

assumptions. Furthermore, he defined, if the human is in thermal balance (PMV = 0),

5% of all occupants can always be expected to be dissatisfied (Figure 2). Therefore it isimpossible to specify a thermal environment that will satisfy everybody.


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The PPD-index can be calculated using following equation. It is necessary to know the

PMV-value:

PPD = 100 95 * exp(-0,03353 * PMV4 0,2179*PMV2)

Figure 2: Correlation between PMV and PPD according to Fanger

The PMV model of Fanger was expanded by studies of Mayer. Mayer asked the

persons not only about the thermal sensation, he asked about the thermal satisfaction

(thermal comfort) too. In the process he noted, that a vote of -1 (slightly cool) on the

PMV scale classified already discomfort. Then he modified the correlation between

PMV and PPD so that there is a better accord with his measurements. So the optimum

is at a PMV value of +0,4 and a minimal percentage of dissatisfied of 16%. (Figure 3)


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6 A.Thewes/A.Degens

Figure 3: Correlation between PMV and PPD according to Mayer

Thermal dissatisfaction could have other reasons too. For example unwanted cooling or

heating of one particular part of the body. Therefore such a discomfort is known as

local discomfort.

One possible cause is a local convective cooling of the body (draught).If the room air

temperature is higher than the temperature of the moved air, which flow only from one

side against the human body, there are a great percentage of dissatisfied persons. Thispercentage of dissatisfied persons (DR = draught rating) can be calculated using

following equation:

Where:

DR = percentage dissatisfied persons by draught [%]

ta,I = local air temperature [C]

va,l = local average air velocity [m/s], < 0,5 m/s

Tu = local turbulence intensity [%], 10% - 60%, if the exact value is not available,

generally can be calculated with 40%.

if va,l > 0,05 m/s then va,l = 0,05 m/s

if DR > 100% then DR = 100%


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The turbulence intensity is a value of the uniformity or non-uniformity of air-flow. To

calculate the turbulence intensity Tu is it necessary to have information about the

standard deviation sv of the air velocity.

Where:

sv = standard deviation of instantaneous value of the air velocity

= mean air velocity

A second cause of local discomfort is vertical temperature differences between thehead and ankle level. The following graph shows the percentage of dissatisfied persons

(PD = percentage of dissatisfied) as a result of vertical temperature differences.

The graph (Figure 4) shows, that a uniform temperature with a maximum difference of 1

degree Celsius, between head and ankle level, is most comfortable.

Figure 4: Local discomfort as a result of vertical temperature differences

Alternative the percentage of dissatisfied can be calculated:

Where: PD = percentage of dissatisfied [%]

= air temperature differences between head and ankle level [C]


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8 A.Thewes/A.Degens

Also local discomfort and a percentage of dissatisfied (PD = percentage of

dissatisfied) can be estimated as a result of an asymmetric radiation environment

or rathertoo high or too low floor temperatures.

Figure 5: Local thermal discomfort as a result of a warm or cold floor

Figure 5 demonstrates that the human feels most comfortable at a floor temperature of

25C. The diagram is based on studies with persons who working in standing and

sitting positions.

The already mentioned discomfort as a result of asymmetric radiant temperature is

mainly caused by warm ceilings and cold floors. In Figure 6 is shown the percentage of

dissatisfied persons due to asymmetric radiation.


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Figure 6: Local thermal discomfort as a result of asymmetric radiant temperature

Where:

PD = percentage of dissatisfied [%]

= asymmetric radiant temperature [C]1 = warm ceilings

2 = cool walls

3 = cool ceilings

4 = warm walls

For old listed buildings the phenomena of local discomfort is very important. For

example, they can often not be sufficiently thermally insulated although the rooms are

used as offices or residences.

In a long-term evaluation can be differentiated between three categories of general

comfort. The favored conditions of a room can be chosen from the following table on

the basis of the categories A, B and C. Category A comes up to high expectations and

on the other hand category C moderate expectations. Each category allows a maximum

percentage of dissatisfied persons (Figure 7). As well the maximum percentages of

dissatisfied due to local discomfort are defined. It should be noted, that all criteria of a

category should be met.


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Figure 7: Three categories of ambient conditions

In every room there is an optimal operative temperature corresponding to PMV = 0. The

operative temperature is a uniform temperature of a hypothetical black room in which

the human body would lose the same amount of heat by radiation and convection as it

would in some actual environment. It results from the mean air temperature and

radiation temperature and depends on the clothing and the activity of persons in the

room.

In the following figure is only shown the graph of category A. The operative

temperatures are the same for all categories, only the acceptable range varies.

Figure 8: The optimal operative temperature as a function of activity and clothing. The shaded areas

inform about the comfort range around the optimal inside temperature

X = clothing insulation [clo]

Y = metabolic rate [met]

Y = activity in W/m


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The diagram is valid for a relative humidity of 50%. As well an air velocity lower than 0,1

m/s in the room is assumed.

One met is a metabolic unit and describes the energy consumption of people. So, 1

met is equal to 58 W/mskin surface, and the average skin surface of adult human is 1,7m

(Annex a Figure 26).

Example:

The starting point for the estimation of the optimal operative temperature of a room is to

know which expectations regarding thermal comfort there are. If we consider there are

high expectations, category A values applies (Figure 8). If the persons work in sitting

positions (office work = 1,2met) and wear mean clothes (1 clo), the result is an optimal

operative temperature of 21C 1C.

For different room types, there are defined thermal criteria. The estimation of optimaloperative temperature is dependent on standard activity levels and clothing insulation.

During the summer the closing insulation is 0,5 clo and 1,0 clo (Figure 27-30, Annex b)

during the winter. Therefore in the following table, there are differences between the

criteria of summer and winter.


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Figure 9: Thermal criteria for different room in different building types

All the above models are based on rooms with air conditioning plants. In a study of the

ASHRAE society, several office buildings with natural window ventilation or air

conditioning were analyzed. In the buildings with air conditioning there was a good

correlation between the rating by the occupants and the PMV-model, whereas in

buildings with natural ventilation were more deviations between ratings and model.

Brager und de Dear found that the occupants of natural ventilated buildings are more

tolerant regarding to the room temperature. They found a 80% acceptability within a 4K

temperature spread for mechanical ventilated buildings and within 7K for natural

ventilated.

While the standard DIN EN ISO 7730 evaluates the range of comfortable temperature

dependent on ambient conditions, the ASHRAE standard define a correlation between

operative temperature and monthly mean ambient temperature (Figure 10).

This approach of ASHRAE standard is only to apply by ambient temperatures between

10C and 33,5C.

A second requirement is a light sitting activity (1 to 1,3met).


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Figure 10: comfortable operative temperature of natural ventilated rooms ASHRAE Standard 55

2. Installing the measurement techniques and analysis

The measuring system includes a globe-thermometer, a humidity-/temperature sensor

and a thermo-anemometer. The height of the sensors should be approximate the same

height than the head of a sitting person (approx. 1,10m).

Figure 11: Measurement techniques

It is possible with this measuring setup to measure all the physical parameters needed

for assessing and evaluating thermal comfort. The data acquired from the series

measuring the radiant temperature (globe-thermometer), room temperature, room air


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flow and humidity, and the necessary input parameters (e.g. clothing insulation,

activity level and mechanical power) are used together to calculate the PMV (predicted

mean vote) and the PPD (predicted percentage of dissatisfied) values as per EN ISO7730 using the AMR WinControl software.

The operative temperature is the sensed room Temperature

and can be calculate as the arithmetic mean of air- and radiant

temperature. To determine the radiant temperature it is

necessary to measure the globe temperature. To measure the

globe temperature is used a matt black copper bulb with a

standard diameter of approximate 15cm to absorb the radiant

heat of surrounding objects. Inside is a Pt100-sensor (Resistor

based sensor).

The air space inside of the black bulb makes sure that the

absorbed heat influences are mixed inside the bulb and so

create a uniform temperature which is measured by the precise

sensor.

Figure 12: Globe-Thermometer

The operative temperature is to = a*ta + (1-a)*tr

Where ta = air temperaturetr = radiant temperature

a = 0,5 if air velocity v < 0,2 m/s

a = 0,6 ifv = 0,2 0,6 m/s

a = 0,7 ifv = 0,6 1,0 m/s

If the globe-temperature tg, the air temperature ta and the air velocity va are measured, it

is possible to calculate approximately the radiant temperature using following equation:

[( )

( )]

The air velocity can be measured using a mechanical anemometer (e.g. rotating vanes)

or using a thermo-anemometer (e.g. hot wire anemometer). For this measurement

system we use a thermo-anemometer (Figure 13)


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Figure 13: thermo-anemometer

The anemometer consists of a probe tube with heated miniature thermistor, cooled

down by the air flow. The change in resistance is a measure for the air velocity. As this

measurement strongly depends on the ambient temperature a further precision NTC

resistance is used to measure and automatically compensate the ambient temperature.

The measuring range of air velocity is 0,01 to 1 m/s.

For an interpretation as per standard EN ISO 7730, must be assigned all measured

values of room temperature, radiant temperature, humidity and air velocity by selection

of measuring channels (Figure 14) in the Software AMR Win Control.


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Figure 14: PMV assistant of Software WinControl

During the measurement of air velocity it is important to make sure, that not the current

value but the average value of each cycle will be saved. As per DIN 1946 Part 2 is

requested, that the duration of each cycle is minimum 100s.

For the mean radiant temperature there are three different possibilities, to measure,to estimate, or to link. The two last ones allow a simplified calculation, if for example

the ambient influences are negligible or assumed as constant. For an exact calculation

the method to measure is to choose. This method allows the measurement of the

radiant temperature using appropriate equipment or to create complex analytic

relationships with user defined equations in an output channel. This could be for

example the above mentioned equation to calculate the radiant temperature with the

measured values of air temperature, globe temperature and air velocity as necessary

inputs.

Finally according to ISO 7730 information about metabolic rate, mechanical

performance and clothing insulation values are necessary:


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Possible outputs are the PMV index or the PPD value:


The results can be displayed as table or diagram (Figure 17 and 18):


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Figure 18: Output of PMV and PPD using Software WinControl as line diagram


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The necessary fresh air demand, to comply the CO2-limits, depends on quality of fresh

air and type of activity.

Figure 21: supply air flow depending on activity


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Max von Pettenkofer (1858) defined that the human is the main source of air pollution.

Therefore the necessary air exchange rate is based on the number of persons in theroom.

New researches have shown that there are many other sources of pollution in a

building.

For the assessment of indoor air quality, it would be desirable to measure all pollutants

and odours. Because this is very complex, Fanger (1990) introduced two new units:

1 Olf

1 Dezipol = 1 Olf / (10l*s)

10 Dezipol = 1 Pol

Figure 22: Definition of olf and dezipol.

One Olf is the sensory pollution strength from a standard person, defined as anaverage adult with a hygienic standard equivalent of 0,7 baths per day and whose skin

has a total area of 1,7 sqm. Each other source of pollution in a room (furniture, carpet,

building materials,) is to convert in a sensory pollution strength from a number of

standard persons (Figure 23).

The average strength of a pollution source in buildings is:

0,2 olf/m2 in buildings with low pollution

0,6 olf/m

2

if there are approx. 40% smokers in the building


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Figure 23: Sources of pollution illustrated in olf

Because there arent applicable measuring instruments, the odour strength is evaluated

by trained test persons.

One Dezipol is defined as the unit for perceived air quality caused by a standard

person (1 Olf) in a room, which is ventilated by 10l/s pure air. OnePol defined a room,

which is ventilated by only 1l/s pure air. That means very bad air quality.

Example:

A room is ventilated by an air conditioning plant which supplies air flow rate of 10 l/s.The ambient air is polluted by 1 dezipol. In addition, 1 olf by the ventilation system is to

note. Therefore the supply air is polluted by 2 dezipol. The occupants and the room

produce 1 olf each. Finally, the perceived indoor air quality is 4 dezipol.

Figure 24: Perceived air quality


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The following two graphs show the dependence on the percentage of dissatisfied as a

result of the air quality. Where the left graph shows the dependence on unit pol,

illustrate the right graph the dependence on air exchange rate. Both graphs describethe reciprocal value of the other.

It is evident, that the percentage of dissatisfied decrease with increasing air exchange

rate and constant odour strength. The nonlinear behavior is explainable by the number

of grumbler, which are always dissatisfied even if the best air quality is in the rooms.

Figure 25: percentage of dissatisfied in dependence on the air quality or the air exchange rate.

The left graph represents the solutions of the following equation:

Where:

PD = percentage of dissatisfied [%]

C = air quality [dezipol]


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The required supply air flow rate to evaluate a defined air quality in dezipol can be

calculated using following equation:

Where:

V = required supply air flow rate [l/s]

Ci = requested air quality [dezipol]

CZU = quality of supply air [dezipol]

V = effectiveness of ventilation

G = total strength of pollution [olf]

Exercise:

A seminar room with a surface of 100 m2 shall be ventilated, that a perceived room air

quality of 1,4 dezipol is existing (=20% dissatisfied). The room is occupied by 50

persons. The strength of pollution per person is 1 olf, because they are working in a

sitting position. The strength of pollution due to materials in room is 0,3 olf/m2.

The quality of supply air is assumed to 0,2 dezipol. The effectiveness of ventilationshould be V = 1.

Calculate the required supply air flow rate!


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5. Summary

Finally, an overview shows the most important values of thermal comfort, inclusivesome limit and reference values:

PMV (= predicted mean vote) = 0 means thermal neutrality

PPD (predicted percentage of dissatisfied) = 5% if thermal neutrality

Local thermal discomfort:

o Due to drafts < 0,2 m/s

o Due to vertical temperature differences = T < 2C

o Warm/cold floors = optimal floor temperature 25C

o Asymmetric radiation temperature = warm ceilings and cold walls

should be avoided!

Clothing insulation value - office work = 1,0 clo = 0,155 mK/W (winter)

= 0,5 clo = 0,08 mK/W (summer)

Metabolic rate sedentary work = 1,2 met = 70 W/m

(reference to 1,7m skin surface)

Relative humidity = 45% is ideal value

30 70 % comfortable

according to DIN EN 7730

Operative temperature, DIN EN 7730 = during winter 20C 24C

= during summer 23C 26C

Adm. Air velocity according to DIN 1946 = 20 22C 0,13 m/s

Strength of pollution source = 1 olf (standard person)

= 6 olf (smoker)

optimal CO2-concentration in rooms = 800 1200 ppm


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6. Standards

DIN EN ISO 7730; 2005-11: Ergonomics of the thermal environment analyticaldetermination and interpretation of thermal comfort using calculation of the PMV and PPD

indices and local thermal comfort criteria

ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy

DIN EN ISO 9920; 2005-02: Ergonomics of thermal environment Estimation of thermal

insulation and water vapor resistance of a clothing ensemble

DIN ISO EN 8996: 2005-01: Ergonomics of thermal environment Determination of

metabolic rate

DIN EN 15251: 2007-08: Indoor environmental input parameter for design and assessment

of energy performance of buildings addressing indoor air quality; thermal environment;

lighting and acoustics

DIN EN ISO 10551: 2002-01: Ergonomics of the thermal environment Assessment of

the influence of the thermal environment using subjective judgment scales

DIN 1946-2: Ventilation and air conditioning in workrooms and public areas (since May

2005 replaced by DIN EN 13779: 2005 with reference to DIN EN ISO 7730)


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7. Exercise: Questionnaire

a.) Determine the thermal insulation of your clothes! (Annexe b)

b.) How would you rate the indoor climate of the seminar room?

very neithernor very

1.) warm cold

2.) dry air humid air

3.) fresh air exhausted air

4.) uncomfortable comfortable

temperature temperature

5.) bright dark

6.) air draft no air movement

7.) total indoor climate comfortable

uncomfortable


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b. Table for estimation of clothing insulation values

Figure 27: Thermal insulation values of typical clothing combinations DIN EN ISO 9920; 2005-02


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Figure 28: Thermal insulation factors of typical clothing and the necessary changing of the

optimum operative temperature DIN EN ISO 9920; 2005-02


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Figure 29: Thermal insulation factors of typical clothing and the necessary changing of the

optimum operative temperature DIN EN ISO 9920; 2005-02

Figure 30: Thermal insulation of chairs DIN EN ISO 9920; 2005-02


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c. Table to calculate the PMV-Index

The PMV-values of the following table are valid for a relative humidity of 50%.Further you may only work with this table, if the difference between the air

temperature and the operative temperature is lower than 5K. In this case, the

uncertainty of the tabular values is better than 0,1 PMV.


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d. HX diagram (Mollier)

Figure 32: hx Diagram

script thermal comfort 2013

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