Environmental ControlsEnvironmental ControlsLecture 6

Thermal Comfort

TerminologyTerminologyAbbrev. UnitsPsychrometrics --- ---Relative Humidity RH (%RH)Dry Bulb Temperature DBT (ºF)Wet Bulb Temperature WBT (ºF)Dew Point Temperature DPT (ºF)Standard Comfort Zone --- ---

PsychrometricsPsychrometricsMeasurement of moisture content and air temperature.

Relative Humidity

Relative HumidityRelative HumidityQuantity of moisture held in the air relative to the maximum amount that can be held (%). Warm air holds

more moisture Moisture

condenses from the air as it cools

Relative HumidityRelative HumidityWhat is 100% RH?

Fog

Dry Bulb TemperatureDry Bulb TemperatureAir temperature as read from a standard thermometer

Dry Bulb Temperature ºF

Wet Bulb TemperatureWet Bulb TemperatureAir temperature as read from a moistened thermometer

Wet Bulb Temperature

Dew Point Temperature ºF

Dew Point TemperatureDew Point TemperatureTemperature at which RH is 100%

Dew Point TemperatureDew Point TemperatureFor air at 75ºF DBT/65 ºF WBT,what is the dewpoint temperature?

Dew Point Temperature ºF

60ºF

Standard Comfort ZoneStandard Comfort ZoneConditions where the typical occupant is comfortable for the given intent of a space.

[L: p. 65, fig. 4.8a]

Influenced by Metabolism Migration Heat flow Clothing

Thermal Comfort FactorsThermal Comfort Factors

MetabolismMetabolismMetabolism affects perception of thermal comfort

Muscular activity Digestive process Daily cycle Health Age Gender

MigrationMigrationBody attempts to maintain core temperature (98.6ºF) by shifting blood flow

Vasodilation Vasoconstriction

Heat FlowHeat FlowMultiple mechanisms Convection Conduction Radiation Evaporation

Heat Flow – Convection Heat Flow – Convection Air movement caused by pressure and density changes Wind is formed by air

moving from high to low pressure areas

Warming air rises Cooling air falls

Heat Flow – Conduction Heat Flow – Conduction Caused by direct contact between two objects of different temperature Heat always flows from

warm to cool

Heat Flow – Radiation Heat Flow – Radiation Radiant heat is exchanged by two objects of different temperatures Heat always flows

toward cooler object Surface temperature

controls interaction Seasonally variable

Radiant Heat FlowRadiant Heat FlowSeason variations can change direction of radiant heat flow

Mean Radiant Temperature Mean Radiant Temperature (MRT)(MRT)

MRT approach is used in Europe as a means to lower operating temperatures

Heat Flow – Evaporation Heat Flow – Evaporation Water changes from liquid to gas and this phase change extracts heat Respiration Perspiration

Compounds and/or mitigates the effects of other mechanisms

ClothingClothing