Thermal Comfort

What do we mean by thermal comfort? –Are you feeling uncomfortable with the temperature in the workplace? –The term “thermal comfort” describes a person’s state of mind in terms of whether they feel too hot or too cold –There’s more to it than just a room temperature

Condition Mean air temperature Mean radiant temperature Mean relative humidity Mean air velocity Cool18.3 (0.5)18.9 (0.6)67.1 (3.7)0.3 (0.1) Neutral24.6 (0.4)25.0 (0.5)47.7 (2.2)0.2 (0.1) Warm31.3 (0.4)31.1 (0.5)31.6 (3.3)0.2 (0.1) OUTSIDE18.0 (1.5)18.9 (1.7)44.9 (6.6)0.1 (0.0)

Definition of thermal comfort ISO 773 &ASGRAE HB: –The condition of mind which expresses satisfaction with the thermal environment” ASHHRAE ” –Thermal environmental conditions for human occupancy: Thermal environmental conditions for human occupancy specifies conditions in which 80% or more of the occupants will find the environment thermally acceptable

Environmental factors ( such as humidity and sources of heat in the workplace) combine with personal factors ( such as the clothing you are wearing and how physically demanding your work is) to influence what is called your “thermal comfort”

What is thermal comfort??? Definition –Thermal comfort is defined in British Standard BS ISO 7730 as: That condition of mind which expresses satisfaction with thermal environment

So the term “thermal comfort” describes a person’s psychological state of mind and is usually referred to in terms of whether someone is feeling too hot or too cold thermal comfort is very difficult to define because you need to take into account a range of environmental and personal factors when deciding what will make people feel comfortable

These factors make up what is known as the “human thermal environment” The best that you can realistically hope to achieve is a thermal environment that satisfies the majority of people in the workplace or put more simply – “reasonable comfort”

HSE considers 80% of occupants as a reasonable limit for the minimum number of people who should be thermally comfortable in an environment

Heat flow Convection Conduction Radiation Evaporation

Under the normally comfortable temperature 18 o C (70 o F), the proportions of body heat loss are as follows: Radiation, conduction and convection 72% Evaporation from skin surface 15% Evaporation from lungs (exhaled air) 7% Warming of air inhaled to lungs 3% Heat expelled in feces and urine 3%

Clothes The insulating value of clothing is measured in CLO units. Table 4.2

Why is thermal comfort important?? Because thermal comfort is psychological, it may affect our overall morale Employee complaints may increase, productivity may fall and in some cases people may refuse to work in a particular environment Some aspects of the thermal environment, such as air temperature, radiant heat, humidity and air movement may also contribute to the symptoms of sick building syndrome

Adapting to thermal environment People employ adaptive strategies to cope with their thermal environment, e.g. donning or moving clothing, unconscious changes in posture, choice of heating, moving to cooler locations away from heat sources, etc.

The problem arise when this choice (to remove jacket, or move away from heat source ) is removed and people are no longer able to adapt In many instances the environment within which people work is a product of the processes of the job they are doing, so they are unable to adapt to their environment

Comfort Standards The ASHRAE comfort zone represents combinations of air temperature and relative humidity that most often produce comfort for a seated adult in shirtsleeves (total 0.6 CLO) in the shade and without noticeable air motion ASHRAE standards

What this is saying is that if you can keep the temperature and humidity levels within the appropriate seasonal ranges, the space is “comfortable.” Actually, within these guidelines, 80% of the occupants are comfortable. Clearly, ranges of as much as 9°F are not equally comfortable given its subjectivity.

There is also the type of space to consider. Optimal comfort for offices, classrooms, laboratories, kitchens, and athletic facilities are different.

ASHRAE STANDARDS

The six basic factors The most commonly used indicator of thermal comfort is air temperature – it is easy to use and most people can relate to it But although it is an important indicator to take into account air temperature alone is neither a valid nor an accurate indicator of thermal comfort or thermal stress Air temperature should always be considered in relation to other environmental and personal factors

The six factors affecting thermal comfort are both environmental and personal these factors may be independent of each other, but together contribute to a worker's thermal comfort

Environmental factors Air temperature Radiant temperature Air velocity Humidity

Personal factors Clothing insulation Metabolic heat

Environmental factors Air temperature –This is the temperature of the air surrounding by the body –It is usually given in degrees Celsius or degree Fahrenheit

Radiant temperature Thermal radiation is the heat that radiates from a warm object Radiant heat may be present if there are heat sources in an environment Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment Our skin absorbs almost as much radiant energy as a matt black object, although this may be reduced by wearing reflective clothing Examples of radiant heat sources include : –The sun –Fire electric heaters –Furnaces –Ovens –Cookers –Dryers –Hot surfaces and machinery –Molten metals, etc

Air velocity This describes the speed of air moving across the worker and may help cool the worker if it is cooler than the environment Air velocity is an important factor in thermal comfort because people are sensitive to it Still or stagnant air in indoor environments that are artificially heated may cause people to feel stuffy It may also lead to a build – up in door Moving air in warm or humid conditions can increase heat losses through convection without any change in air temperature

Small air environment in cool or cooled environments may be perceived as draught If the air temperature is less than the skin temperature it will significantly increase convective heat loss Physical activity also increases air movement, so air velocity may be corrected to account for a person’s level of physical activity

Humidity If water is heated and it evaporates to the surrounding environment, the resulting amount of water in the air will provide humidity Relative humidity is the ratio between the actual amount of water vapor in the air and the maximum amount of water vapor that the air can hold at that air temperature Relative humidity between 40% and 70% does not have a major impact on thermal comfort

In some offices, humidity is usually kept between 40-70% because of computers However, in workplaces which are not air conditioned, or where the climatic conditions outdoors may influence the indoor thermal environment, relative humidity may be higher than 70% on warm or hot humid days

Humidity in indoor environment can vary greatly, and may be dependent on whether there are drying processes (papers mills, laundry etc) where steam is given off High humidity environments have a lot of vapor in the air, which prevents the evaporation of sweat from the skin In hot environment, humidity is important because less sweat evaporates when humidity is high (80%+)evaporates

The evaporation of sweat is the main method of heat loss in humans When vapor – impermeable PPE is worn, the humidity inside the garment increases as the wearer sweats because the sweat cannot evaporates If an employee is wearing this type of PPE ( e.g. asbestos or chemical protection suits) the humidity within the microclimate of the garment may be high

Personal Factors Clothing insulation –Clothing, by its very nature, interferes with our ability to lose heat to the environment –Thermal comfort is very much dependent on the insulating effect of clothing on the wearer –Wearing too much clothing or personal protective equipment (PPE) may be a primary cause of heat stress even if the environment is not considered warm or hot –If clothing does not provide enough insulation, the wearer may be at risk from cold injuries such as frost bite or hypothermia in cold conditions

Clothing is both a potential cause of thermal discomfort as well as a control for it as you feel cold, or remove layers of clothing if you feel warm However, many companies remove this ability for their employees to make reasonable adaptations to their clothing It is important to identify how the clothing may contribute to thermal comfort or discomfort. It may also be necessary to evaluate the level of protection that any PPE is providing Can less or other PPE be used?

Work rate/ metabolic heat The work or metabolic rate, is essential for a thermal risk assessment It describes the heat that we produce inside our bodies as we carry out physical activities The more physical work we do, the more heat we produce The more heat we produce, the more heat needs to be lost so we don’t overheat The impact of metabolic rate on thermal comfort is critical When considering these factors, it is also essential to consider a persons own physical characteristics

a person physical characteristics should always be borne in mind when considering their thermal comfort, a factors such as their size and weight, age, fitness level and sex can all have an impact on how they feel, even if other factors such as air temperature, humidity and air velocity are all constants

Measuring thermal comfort A simple way of estimating the level of thermal comfort in your workplace is to ask the workers or their workplace representatives, such as Unions or employee associations, if the percentage of workers dissatisfied with the thermal environment is above a certain level, you will need to take action

Body energy balance (ASHRAE)

Where M-rate of metabolic heat production, W/m 2 W- rate of mechanical work accomplished, W/m 2 q sk - total rate of heat loss from skin, W/m 2 q res - total rate of heat loss through respiration, W/m 2 C+R-sensible heat loss from skin, W/m 2 E sk -total rate of evaporative heat loss from skin, W/m 2 C res -rate of convective heat loss from respiration, W/m 2 S sk -rate of heat storage in skin compartment, W/m 2 S cr - rate of heat storage in core compartment, W/m 2