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Comfort > Thermal Comfort > Flashcards

Thermal Comfort Flashcards

1
Q

Why thermal comfort?

A

“Energy efficient building is defined as one which provides the specified indoor environment for the least use of energy.”

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2
Q

What is thermal comfort?

A

Definition:

That condition of mind which expressed satisfaction with the thermal environment

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3
Q

What are the 3 perceptions of human comfort?

A

Physical
Physiological
Psychological

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4
Q

Physical perception of human comfort?

A

Body as a heat producing machine cannot deal with the senses toon of comfort or discomfort and the body’s own response

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5
Q

Physiological perception of human comfort?

A

Mechanisms by which the body try’s to maintain temperature,

I.e. Sweating, constriction, dilation of blood vessels

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6
Q

Psychological perception of human comfort ?

A

Indirect measurement of the sensation of comfort.

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7
Q

What does thermal comfort depend on?

A

Air temperature
Air velocity
Surface temperature
Humidity

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8
Q

Body’s thermal balance values?

A

Radiation 45%
Convection 30%
Evaporation 25%

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9
Q

The body stays at 37 degrees by changing the blood flow from the centre to the skin where most heat loss occurs. What does this occur through?

A 
Vc = Vasoconstriction 
Vd = vasodilation
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10
Q

For body to be state of thermal equilibrium with surroundings, the rate of heat loss must equal that of heat generation.
This is mathematically expressed as:

A

M- W = E+R+C+S

M= metabolic rate
W= rate of performance of external work
E= total rate of heat loss by evaporation
R= total rate of radiant interchange
C= total convective heat loss rate
S= body heat storage
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11
Q

Heat loss by convection depends on?

A
  • the average surface temperature of the body
  • the area of the body exposed to air movement
  • the temperature of the air
  • the rate of air movement
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12
Q

Radiant Heat transfer depends on?

A
  • the body surface temperature
  • the surface temperature of the surroundings
  • effective radiating area of the body
  • emissivity of the body and surrounding surfaces
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13
Q

Heat loss by Evaporation depends on?

A
  • the direction and speed of air movement
  • the difference in vapour pressure between sweat and the surrounding air
  • the wetted area of the body
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14
Q

What two conditions must be fulfilled to maintain thermal comfort?

A
  • Heat produced must equal heat lost.

* Signals from Heat and Cold sensors must neutralise each other.

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15
Q

To determine heat loss by the three methods FOUR measurable factors can be considered to affect heat exchange between a body and its surrounding, what are they?

A

Air Temperature
Mean radiant Temperature
Humidity
Relative air speed

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16
Q

Comfort Equation Equation 1 =
M - W = E ± R ± C ± S

M - W = H + Ec + Cres + Eres

A

where
• H is the dry heat loss from the body via convection, conduction and radiation,
• EC is the evaporative heat exchange at the skin,
• Cres is the respiratory convective heat exchange (dry respiration) and
• Eres is the evaporative heat exchange (latent respiration).
• M – metabolic rate, W/m2
• W – effective mechanical power, W/m2

17
Q

Heat loss by Convection depends on:-

A
  • the average surface temperature of the body
  • the area of the body exposed to air movement
  • the temperature of the air
  • the rate of air movement
18
Q

Radiant heat transfer depends upon:-

A
  • the body surface temperature
  • the surface temperature of the surroundings
  • effective radiating area of the body
  • emissivity of the body and surrounding surfaces
19
Q

Heat loss by Evaporation depends on:-

A
  • the direction and speed of air movement
  • the difference in vapour pressure between sweat and the surrounding air
  • the wetted area of the body
20
Q

Conditions for Thermal Comfort Two conditions must be fulfilled to maintain Thermal Comfort:

A
  • Heat produced must equal heat lost.

* Signals from Heat and Cold sensors must neutralise each other.

21
Q

Measurable Factors To determine heat loss by the three methods FOUR measurable factors can be considered to affect heat exchange between a body and its surroundings

A
  • Air temperature
  • Mean radiant temperature
  • Relative air speed
  • Humidity
22
Q

M - W = E ± R ± C ± S

M - W = H + Ec + Cres + Eres

A

where
• H is the dry heat loss from the body via convection, conduction and radiation,
• EC is the evaporative heat exchange at the skin,
• Cres is the respiratory convective heat exchange (dry respiration) and
• Eres is the evaporative heat exchange (latent respiration).
• M – metabolic rate, W/m2
• W – effective mechanical power, W/m2

23
Q

H is either measured directly, or can be calculated using:

A

H= Tsk - Tcl / Icl

  • tsk – mean skin temp
  • tcl – clothing surface temp
  • Icl – clothing insulation
24
Q

Air and surface temperatures

A

A heating system does not supply heat to a person. Because the surface temperature of the body is higher than comfortable ambient temperatures, the heating system simply reduces the heat loss from the body to an acceptable amount.
The amount of heat lost by convection depends upon solely on the air temperature. The amount of heat lost by radiation depends solely on the surface temperature of the surroundings. For best comfort, air temperatures should be lower than the mean radiant temperature.

25
Q

Draught is the most common complaint indoors.

A
  • What is felt is Heat Loss
  • Heat Loss is dependent on the average Air Velocity, Temperature and Turbulence
  • High Turbulence is more uncomfortable, even with the same Heat Loss.

The sensation of Draught depends on the air temperature. At lower air temperatures a higher number will be dissatisfied..

26
Q

What is vertical air temperature difference?

A

Vertical Air Temperature Difference is the difference between Air Temperature at ankle and neck level.

27
Q

Floor Temperature

A

Acceptable floor temperatures ranging from 19 to 29 ˚C. The graph is made on the assumption that people wear “normal indoor footwear”.

28
Q

Radiation Asymmetry

A
  • Radiant Temperature Asymmetry is perceived uncomfortable

* Warm ceilings and cold walls cause greater discomfort

29
Q

Relative Humidity

A

Generally, we are not sensitive to small variations in HUMIDITY. We only begin to suffer adverse effects when the air is either dry or humid. In normal circumstances, humidity in the range 40 - 70%RH is acceptable.

30
Q

TWO personal factors also affect the interchange of heat between a body and its surroundings

A
  • Activity Level - metabolic rate

* Clothing - insulation of clothes

31
Q

Metabolic Rate

A
  • Measurement of the conversion of food to heat and work.
  • Heat output results from the oxidation of food
  • Heat output / O2 (litre) varies slightly with food consumption
  • More exact if take CO2 and urinary N2.
  • Normal diet – 1litre O2 = 20.6kJ heat + Work.
32
Q

Clothing Calculation of Insulation in Clothing:

A

1 Clo = Insulation value of 0.155 m2 oC/W Results achieved and documented in the US, See Fanger (1982) Thermal comfort. Examples of clo values for individual clothing items can be found in Appendix B.

33
Q

Calculation of Clo Value (Clo)

A

Clo values are added together, so for example, a short sleave shirt of clo=0.19, plus pants (clo=0.04), shorts (clo=0.11), socks (clo=0.02) and shoes (clo=0.02) equates to a total clo value for the person of 0.38.

34
Q

Predicting Thermal Comfort

A

A number of different methods have been used to determine thermal comfort based upon physical, physiological and psychological measurements. The most common and the one used in the EN standard is PMV – predicted mean vote. It is based upon the sensation of comfort and ranges from -3 (Cold) to +3 (Hot) with zero representing neutral.

35
Q

PMV And PPD PMV-index

A

(Predicted Mean Vote) predicts the subjective ratings of the environment in a group of people, but it is difficult to determine what the magnitude implies. From experiments with samples of people and dissatisfaction was measured when people voted +3, +2, -2, -3. The figure below shows how the predicted percentage dissatisfied (PPD) changes with PMV. Note that the PPD increases more rapidly as the mean vote deviates from zero..

Comfort (1 decks)

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