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Human thermal comfort is defined by ASHRAE as the state of mind that expresses satisfaction with the surrounding environment (ASHRAE Standard 55). The American Society of Heating Refrigerating and Air Conditioning Engineers ( ASHRAE; pronounced 'ash'-'ray' is an international technical society for all individuals Maintaining thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers. HVAC (pronounced either "H-V-A-C" or occasionally " H-vak " is an Initialism or Acronym that stands for " Heating

Thermal comfort is affected by heat conduction, convection, radiation and evaporative heat loss. Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter from a region of higher Temperature to a region of lower Convection in the most general terms refers to the movement of molecules within Fluids (i Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate thus maintaining thermal equilibrium with the surroundings. Any heat gain or loss beyond this generates a sensation of discomfort. [1] It has been long recognised that the sensation of feeling hot or cold is not just dependent on air temperature alone.

Contents

Factors determining thermal comfort

Factors determining thermal comfort include:

Thermal stress

The concept of thermal comfort is closely related to thermal stress. This attempts to predict the impact of solar radiation, air movement and humidity for military personnel undergoing training exercises or athletes during competitive events. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. Values are expressed as the Wet Bulb Globe Temperature or Discomfort Index. [2][3]

Operative temperature

The ideal standard for thermal comfort can be defined by the operative temperature. In the study of human thermal comfort, the operative temperature is one of several parameters devised to measure the air's cooling effect upon a Human body. This is the average of the air dry-bulb temperature and of the mean radiant temperature at the given place in a room. Mean Radiant Temperature ( MRT) is the uniform surface temperature of a black enclosure with which an individual exchanges the same heat by radiation as the actual environment In addition, there should be low air velocities and no 'drafts', little variation in the radiant temperatures from different directions in the room, the humidity has to be in a comfortable range, and the air temperatures in a height of 0. 1 m above the floor should not be more than 2 °C lower than than the temperature at the place of the occupant's head. Also the temperatures should not change too rapidly - neither across the space nor with time.

In addition to environmental conditions, thermal comfort depends on the clothing and activity level of a person. The amount of clothing is measured against a standard amount that is roughly equivalent to a typical business suit, shirt, and undergarments. Activity level is compared to being seated quietly, such as in a classroom.

This standard amount of insulation required to keep a resting person warm in a windless room at 70 °F (21. 1 °C) is equal to one Clo. Clo units can be converted to R-value in SI units (K/(W/m²) or RSI) by multiplying Clo by 0. The R value or R-value is a measure of Thermal resistance (K·m²/W used in the building and Construction industry 155 (1 Clo = 0. 155 RSI). (In imperial units 1 Clo corresponds to an R-value of 0. 88 °F ft²hr/Btu. )

Research

These factors were explored experimentally in the 1970s. Many of these studies, led to the development and refinement of ASHRAE Standard 55, and were performed at Kansas State University by Ole Fanger and others. Kansas State University, commonly shortened to K-State, is an institution of higher learning located in Manhattan, Kansas, in the United States Povl Ole Fanger ( July 16, 1934 – September 20, 2006) was an expert in the field of the health effects of indoor environments Perceived comfort was found to be a complex interaction of these variables. It was found that the majority of individuals would be satisfied by an ideal set of values. As the range of values continued to deviate from ideal, increasingly fewer people were satfisfied. This observation could be expressed statistically as the % of individual who expressed satisfaction by comfort conditions and the predicted mean vote (PMV)

See also

References

  1. ^ Welcome To Home Energy Magazine Online
  2. ^ http://www.jstage.jst.go.jp/article/indhealth/44/3/388/_pdf
  3. ^ About the WBGT and Apparent Temperature Indices

External links

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