Thermal comfort – definition; main indices used to express thermal comfort and heat stress. General approaches to measurement of temperature, humidity and air velocity

Thermal comfort

Thermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment (ANSI/ASHRAE Standard 55). Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC (heating, ventilation, and air conditioning) design engineers.

Thermal comfort

Thermal comfort is affected by heat conduction, convection, radiation, and evaporative heat loss. Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. It has been long recognised that the sensation of feeling hot or cold is not just dependent on air temperature alone.

Importance of thermal comfort

Thermal discomfort has been known to lead to Sick Building Syndrome symptoms. The combination of high temperature and high relative humidity serves to reduce thermal comfort and indoor air quality. The occurrence of symptoms increased much more with raised indoor temperatures in the winter than in the summer due to the larger difference created between indoor and outdoor temperatures.

Factors determining thermal comfortinclude

Personal factors (health, psychology, sociology & situational factors)

Insulative clothing (Clo Value)

Activity levels (Met Rate)

Factors determining thermal comfortinclude

General FactorsAir temperatureMean radiant temperatureRelative humidity (see also perspiration)

Drifts and ramps in operative temperature

Factors determining thermal comfortinclude

Localized factorsAir movement/velocity (see wind chill factor)

Radiant asymmetryFloor surface temperatures (see underfloor heating)

Air temperature stratification

Metabolism

When measuring metabolic rates, many factors have to be taken into account. Each person has a different metabolic rate, and these rates can fluctuate when a person is performing certain activities, or under certain environmental conditions. Even people who are in the same room can feel significant temperature differences due to their metabolic rates, which makes it very hard to find an optimal temperature for everyone in a given location.

Metabolism

Food and drink habits may have an influence on metabolic rates, which indirectly influences thermal preferences. These effects may change depending on food and drink intake.

Metabolism

Body shape is another factor that affects thermal comfort. Heat dissipation depends on body surface area. A tall and skinny person has a larger surface-to-volume ratio, can dissipate heat more easily, and can tolerate higher temperatures than a more rounded body shape.

Clothing insulation

During cold weather, layers of insulating clothing can help keep a person warm. At the same time, if the person is doing a large amount of physical activity, lots of clothing layers can prevent heat loss and possibly lead to overheating. Generally, the thicker the garment is the greater insulating abilities it has. Depending on the type of material the clothing is made out of, air movement and relative humidity can decrease the insulating ability of the material.

Clothing insulation

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.

Clothing insulation

Clo units can be converted to R-value in SI units (m²·K/W) or RSI) by multiplying clo by 0.155 (1 clo = 0.155 RSI). (In English units 1 clo corresponds to an R-value of 0.88 °F·ft²·h/Btu.) ASHRAE 55-2004 mentioned a Table B1 and Table B2 for more clothing information.

Relative humidity

The human body has sensors that are fairly efficient in sensing heat and cold, but they are not very effective in detecting relative humidity. Relative humidity creates the perception of an extremely dry or extremely damp indoor environment. This can then play a part in the perceived temperature and their thermal comfort. The recommended level of indoor humidity is in the range of 30-60%.

Relative humidity

A way to measure the amount of relative humidity in the air is to use a system of dry-bulb and wet-bulb thermometers. A dry-bulb thermometer measures the temperature not relative to moisture. This is generally the temperature reading that is used in weather reports. In contrast, a wet-bulb thermometer has a small wet cloth wrapped around the bulb at its base, so the reading on that thermometer takes into account water evaporation in the air.

Relative humidity

The wet-bulb reading will thus always be at least slightly lower than the dry bulb reading. The difference between these two temperatures can be used to calculate the relative humidity. The larger the temperature difference between the two thermometers, the lower the level of relative humidity.

Relative humidity

The wettedness of skin in different areas also affects perceived thermal comfort. Humidity can increase wetness on different areas of the body, leading to a perception of discomfort. This is usually localized in different parts of the body and local thermal comfort limits for local skin wettedness differ between different skin locations of the body.

Relative humidity

The extremities are much more sensitive to thermal discomfort from wetness than the trunk of the body. Although local thermal discomfort can be caused from wetness, the thermal comfort of the whole body will not be affected by the wetness of certain parts.

Relative humidity

Recently, the effects of low relative humidity and high air velocity were tested on humans after bathing. Researchers found that low relative humidity engendered thermal discomfort as well as the sensation of dryness and itching. It is recommended to keep relative humidity levels higher in a bathroom than other rooms in the house for optimal conditions.

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. Values are expressed as the Wet Bulb Globe Temperature or Discomfort Index.

Thermal stress

Generally, humans do not perform well under thermal stress. People’s performances under thermal stress is about 11% lower than their performance at normal thermal conditions. Also, human performance in relation to thermal stress varies greatly by the type of task you are completing. Some of the physiological effects of thermal heat stress include increased blood flow to the skin, sweating, and increased ventilation.

Adjustment mechanisms

The body has several thermal adjustment mechanisms to survive in drastic temperature environments. In a cold environment the body utilizes vasoconstriction; which reduces blood flow to the skin, skin temperature and heat dissipation. In a warm environment, vasodilation will increase blood flow to the skin, heat transport, and skin temperature and heat dissipation.

Adjustment mechanisms

If there is an imbalance despite the vasomotor adjustments listed above, in a warm environment sweat production will start and an evaporative cooling mechanism will be provided. If this is insufficient, hyperthermia will set in, body temperature may reach 40∘C and heat stroke may occur. In a cold environment shivering will start, involuntarily forcing the muscles to work and increasing the heat production by up to a factor of 10. If equilibrium is not restored, hypothermia will set in which, can be fatal.

Adjustment mechanisms

Long term adjustments to extreme temperatures of a few days to 6 months may result in cardiovascular and endocrine adjustments. A hot climate may create increased blood volume, improving the effectiveness of vasodilation, enhanced performance of the sweat mechanism, and the readjustment of thermal preferences. In cold or underheated conditions, vasoconstriction can become permanent resulting in decreased blood volume, and increased body metabolic rate.

Effects of natural ventilation of thermal comfort

Many buildings use a HVAC (Heating Ventilation Air Conditioning) unit to control their thermal environment. Recently, with the current energy and financial situation, new methods for indoor temperature control are being used. One of these is natural ventilation.

Effects of natural ventilation of thermal comfort

This process can make the controlled indoor air temperature more susceptible to the outdoor weather, and during the seasonal months the temperatures inside can become too extreme. During the summer months, the temperature inside can rise too high and cause the need for open windows and fans to be used. In contrast, the winter months could call for more insulation and layered clothing to deal with the less than ideal temperatures.

Operative temperature

The ideal standard for thermal comfort can be defined by the operative temperature. This is the average of the air dry-bulb temperature and of the mean radiant temperature at the given place in a room. In addition, there should be low air velocities and no 'drafts,' little variation in the radiant temperatures from different directions in the room, and humidity within a comfortable range.

Operative temperature

The operative temperature intervals varied by the type of indoor location. They also vary by the time of year. ASHRAE has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances.

Operative temperature

For example, a single office in a building has an occupancy ration per square meter of 0.1. In the summer the suggested temperature is between 23.5 (74.3 F) and 25.5 degrees Celsius (77.9 F) , and airflow velocity of 0.18 m/s. In the winter, the recommended temperature is between 21.0 and 23.0 degrees Celsius with an airflow velocity of 0.15 m/s.

Thermal sensitivity of individuals Cold sensitivity

The thermal sensitivity of an individual is quantified by the descriptor FS, which takes on higher values for individuals with lower tolerance to non-ideal thermal conditions. This group includes pregnant women, the disabled, as well as individuals whose age is below 14 or above 60, which is considered the adult range. Existing literature provides consistent evidence that sensitivity to hot and cold surfaces declines with age.

Thermal sensitivity of individuals Cold sensitivity

There is also some evidence of a gradual reduction in the effectiveness of the body in thermoregulation after the age of 60. This is mainly due to a more sluggish response of the counteraction mechanisms in the body that are used to maintain the core temperature of the body at ideal values.

Situational factors include the health, psychological, sociological and vocational activities of the persons.

Gender differences

While thermal comfort preferences between genders seems to be small, there are some differences. Studies have found men report discomfort due to rises in temperature much earlier than women. Men also estimate higher levels of their sensation of discomfort than women

Gender differences

One recent study tested men and women in the same cotton clothing, performing mental jobs while using a dial vote to report their thermal comfort to the changing temperature. Many times, females will prefer higher temperatures. But while females were more sensitive to temperatures, males tend to be more sensitive to relative humidity levels.

Models of thermal comfortWhen discussing thermal comfort, there are two different models that can be used. These are the static model and the adaptive model.

The adaptive model states that there is an optimal temperature for a given indoor environment depending on the outdoor air temperature. It takes into account that humans can adapt and tolerate different temperatures during different times of the year. The optimal temperature for a given time is determined by looking at the mean outdoor temperatures of each month of the year. Also, field studies are performed in these areas to see what the majority of people would prefer as their set-point temperature indoors at different times of the year.

Models of thermal comfort

On the other side, the static model states that the indoor temperature should not change as the seasons do. Rather, there should be one set temperature year-round. This is taking a more passive stand that humans do not have to adapt to different temperatures since it will always be constant.

Models of thermal comfort

More advanced research on thermal comfort considers the heat balance of the human body and calculates sensation and comfort for local body parts.

Thermal comfort in different regions

In different areas of the world, thermal comfort needs may vary based on climate. In China there are hot humid summers and cold winters causing a need for efficient thermal comfort. Energy conservation in relation to thermal comfort has become a large issue in China in the last several decades due to rapid economic and population growth. Researchers are now looking into ways to heat and cool buildings in China for lower costs and also with less harm to the environment.

Thermal comfort in different regions

In tropical areas of Brazil, urbanization is causing a phenomenon called urban heat islands (UHI). These are urban areas, which have risen over the thermal comfort limits due to a large influx of people and only drop within the comfortable range during the rainy season.

Thermal comfort in different regions

Urban Heat Islands can occur over any urban city or built up area with the correct conditions. Urban Heat Islands are caused by urban areas with few trees and vegetation to block solar radiation or carry out evapotranspiration, many structures with a large proportion of roofs and sidewalks with low reflectivity that absorb heat, high amounts of ground-level carbon dioxide pollution that retains heat released by surfaces, great amounts of heat generated by air conditioning systems of densely packed buildings and large amount of automobile traffic generating heat from engines and exhaust.

Thermal comfort in different regions

In the hot humid region of Saudi Arabia, the issue of thermal comfort has been important in mosques where Muslims (followers of Islam, the only religion allowed to operate publicly in Saudi Arabia, according to the Shariah) go to pray. They are very large open buildings which are used only intermittently (very busy for the obligatory noon prayer on Fridays) making it hard to ventilate them properly. The large size requires a large amount of ventilation but this requires a lot of energy since the buildings are used only for short periods of time.

Thermal comfort in different regions

Some mosques have the issue of being too cold from their HVAC systems running for too long and others remain too hot. The stack effect also comes into play due to their large size and creates a large layer of hot air above the people in the mosque. New designs have placed the ventilation systems lower in the buildings to provide more temperature control at ground level. Also new monitoring steps are being taken to improve the efficiency.

Thermal comfort of livestock

Although thermal comfort of humans is the main focus of thermal comfort studies, the needs of livestock must be met as well for better living and production. The Department of Animal Production in Italy produced a study on ewes, which tested rumen function and diet digestibility of ewes chronically exposed to a hot environment. These two bodily functions were reduced by the hot temperatures offering insight that thermal comfort levels are important to livestock productivity.

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. 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 deviated progressively from the ideal, fewer and fewer people were satisfied. This observation could be expressed statistically as the % of individual who expressed satisfaction by comfort conditions and the predicted mean vote (PMV)

Research

This research is applied to create Building Energy Simulation (BES) programs for residential buildings. Residential buildings can vary much more in thermal comfort than public and commercial buildings. This is due to their smaller size, the variations in clothing worn, and different uses of each room. The main rooms of concern are bathrooms and bedrooms. Bathrooms need to be at a temperature comfortable for a human with or without clothing. Bedrooms are of importance because they need to accommodate different levels of clothing and also different metabolic rates of people asleep or awake.

Research

Thermal comfort research in clothing is currently being done by the military. New air-ventilated garments are being researched to improve evaporative cooling in military settings. Some models are being created and tested based on the amount of cooling they provide.

How to Measure Air VelocityThe Fixed Anemometer

Drill a hole in the overhead of the wheelhouse, sufficiently large to pass the cable from the impeller to the wind-display unit. Run the cable from inside the wheelhouse up to the planned location for the impeller. Place the anemometer's impeller as high as possible in your boat. The ideal location will be unobstructed for a full 360 degrees, and above all other instruments or sensors, such as radar or GPS antennae.

Run the cable through the 34mm tubing and connect it to the impeller using the screw connector on the end of the cable. Use the hose clamps to lightly clamp the 34mm-diameter tube to the mast or a rail on the uppermost part of the superstructure of the vessel. Insert the mounting journal of the impeller into the tube and use the screwdriver to tighten the hose clamps to secure the impeller in place.

Use the wire ties to secure the cable to the mast or rail so that the cable follows the shortest route, but is secured against flapping and wind damage.

How to Measure Air VelocityThe Fixed Anemometer

Connect the leads from the wind-display device to the common power bus and common ground. The wind-display unit is a reference device only. It should not obscure equipment that will be used frequently, like radar or the VHF/GMDSS bridge-to-bridge radio.

Connect the cable from the impeller to the wind-display device and, if necessary, secure the cable to available surfaces and appliances with wire ties to prevent tripping hazards. Apply marine silicone caulk around the hole in the wheelhouse overhead on both the inside and outside surfaces.

After the silicone caulk has cured in accordance with the manufacturer's directions, it may be painted.

The wind-display unit will display both wind speed and direction.

How to Measure Air VelocityThe Handheld Anemometer

Find a location aboard the boat that is both high enough to hold the impeller where it will not be obstructed from the apparent direction of the wind, and secure enough to provide good footing.

Hold the impeller into the wind.Read the wind velocity and direction

from the digital display panel on the anemometer.

Humidity

Humidity

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture, called the Absolute humidity.

Humidity

In everyday usage, it commonly refers to relative humidity, expressed as a percent in weather forecasts and on household humidistats; it is so called because it measures the current absolute humidity relative to the maximum.

Humidity

Specific humidity is a ratio of the water vapor content of the mixture to the total air content (on a mass basis). The water vapor content of the mixture can be measured either as mass per volume or as a partial pressure, depending on the usage.

Humidity

In meteorology, humidity indicates the likelihood of precipitation, dew, or fog. High relative humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in a heat index table, used during summer weather.

Absolute humidity

If all the water vapor in one cubic meter of air were condensed into a container, the mass of the water in the container could be measured to determine absolute humidity.

Absolute humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the vapor is saturated at 30 °C.

The absolute humidity changes as air pressure changes.

Relative humidity

Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at those conditions. The relative humidity of air depends not only on temperature but also on pressure of the system of interest.

Specific humidity

Specific humidity is the ratio of water vapor to dry air in a particular mass, and is sometimes referred to as humidity ratio. Specific humidity ratio is expressed as a ratio of grams of water vapor, mv, per kilogram of dry air ma .

EffectsAnimals and plants

Humidity is one of the fundamental abiotic factors that defines any habitat, and is a determinant of which animals and plants can thrive in a given environment.

EffectsAnimals and plants

The human body dissipates heat by a perspiration and evaporation. Heat convection to the surrounding air, and thermal radiation are the primary modes of heat transport from the body. Under conditions of high humidity, the rate of evaporation of sweat from the skin.

EffectsAnimals and plants

Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot dissipate heat by conduction to the air, and a condition called hyperpyrexia results. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs. Physical strength declines, and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected, resulting in heat stroke or hyperthermia.

Human comfort

Humans are sensitive to humid air because the human body uses evaporative cooling as the primary mechanism to regulate temperature. Under humid conditions, the rate at which perspiration evaporates on the skin is lower than it would be under arid conditions. Because humans perceive the rate of heat transfer from the body rather than temperature itself, we feel warmer when the relative humidity is high than when it is low

Human comfort

Some people experience difficulty breathing in high humidity environments. Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety. Sufferers will often hyperventilate in response, causing sensations of numbness, faintness, and loss of concentration, among others.

Human comfort

Air conditioning works by reducing humidity in summer. In winter, heating cold outdoor air can decrease relative humidity levels indoor to below 30%, leading to discomfort such as dry skin and excessive thirst.

Electronics

Many electronic devices have humidity specifications, for example, 5% to 95%. At the top end of the range, moisture may increase the conductivity of permeable insulators leading to malfunction. Too low humidity may make materials brittle. A particular danger to electronic items, regardless of the stated operating humidity range, is condensation

Building construction

Traditional building designs typically had weak insulation, and it allowed air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of condensation forming in and around walls, which encourages the development of mold and mildew.

Measurement

A hygrometer is a device used for measuring the humidity of the air

Measurement

There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or hygrometer. A humidistat is used to regulate the humidity of a building with a dehumidifier. These can be analogous to a thermometer and thermostat for temperature control.

Measurement

Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of future weather forecasts.

Hygrometer

A hygrometer (UK: /haɪˈɡrɒmɪtə/) is an instrument used for measuring the moisture content in the environmental air, or humidity. Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed.

Modern electronic devices use temperature of condensation, or changes in electrical capacitance or resistance to measure humidity changes.

Hygrometer

A dial hygrometer, in this case a hair tension style. Note nonlinear scale.

Types

Metal/pulp coil typeThe familiar metal/paper coil

hygrometer is useful for giving a dial indication of humidity changes, but it appears most often in very inexpensive devices and their accuracy is very limited.

Hair tension hygrometersThese devices use a human or animal

hair under tension

Types

Electronic hygrometers

TypesElectronic hygrometers

℗chilled mirror dewpoint hygrometers

℗capacitive humidity sensors℗resistive humidity sensors℗thermal conductivity humidity sensors

Applications

Besides greenhouses and industrial spaces, hygrometers are also used in some incubators (egg), saunas, humidors and museums. They are also used in the care of wooden musical instruments such as guitars and violins which can be damaged by improper humidity conditions. In residential settings, hygrometers are used to aid humidity control (too low humidity damages human skin and body, while too high humidity favours growth of mildew and dust mite).

Applications

Hygrometers are also used in the coating industry because the application of paint and other coatings may be very sensitive to humidity and dew point. With a growing demand on the amount of measurements taken the psychrometer is now replaced by a dewpoint gauge known as a Dewcheck. These devices make measurements a lot faster but are often not allowed in explosive environments.

Psychrometers

The interior of a Stevenson screen showing a motorized psychrometer

Psychrometers

A psychrometer consists of two thermometers, one which is dry and one which is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer.

Psychrometers

When the air temperature is below freezing, however, the wet-bulb is covered with a thin coating of ice and may be warmer than the dry bulb. Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on a psychrometric chart.

Temperature measurement

A medical/clinical thermometer showing the temperature of 38.7 °C

Temperature measurement

Attempts of standardized temperature measurement have been reported as early as 170 AD by Claudius Galenus. The modern scientific field has its origins in the works by Florentine scientists in the 17th century. Early devices to measure temperature were called thermoscopes.

Temperature measurement

The first sealed thermometer was constructed in 1641 by the Grand Duke of Toscani, Ferdinand II. The development of today's thermometers and temperature scales began in the early 18th century, when Gabriel Fahrenheit adapted a thermometer using mercury and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Technologies

Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid.

Technologies

Temperature increase causes the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated so that one can read the temperature simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint, is the gas thermometer.

Other important devices for measuring temperature include:

ThermistorsResistance Temperature Detecto

r (RTD)

PyrometerLangmuir probes (for electron temperature of a plasma)

InfraredOther thermometers

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