COMFORTWhat is meant by thermal comfort? To have "thermal comfort" means that a person wearing a normal amount of clothing feels neither too cold nor too warm. Such comfort is important both for one's well-being and for productivity in office work, and can be achieved only when the air temperature, humidity and air movement are within the specified range often referred to as the "comfort zone".Where air movement is virtually absent and when relative humidity can be kept at about 50%, the ambient temperature becomes the most critical and debated factor for maintaining thermal comfort. Unfortunately, however, temperature preferences vary greatly among individuals and there is no one temperature that can satisfy everyone. Nevertheless, it is fair to say that an office which is too warm makes its occupants feel tired; on the other hand, one that is too cold causes the occupants' attention to drift, making them restless and easily distracted. Maintaining constant thermal conditions in a building is important. Even minor deviation from comfort may lead to impaired performance and safety. Conversely, workers already under stress are less tolerant of uncomfortable conditions

What temperature should an office be? A general recommendation is that the temperature be held constant in the range of 21C - 23C. In summertime when outdoor temperatures are higher it is advisable to keep air-conditioned offices slightly warmer to minimize the temperature discrepancy between indoors and outdoors.What humidity level and air velocity should an office be? When relative humidity is kept at about 50%, office workers have fewer respiratory problems (specifically in the winter) and generally feel better. Higher humidity makes the office feels "stuffy". More important, it can contribute to the development of bacterial and fungal growth (especially in sealed buildings).Humidity lower then 50% causes discomfort by drying out the mucous membranes, contributing to skin rashes, and causing some electrostatic disturbances to both office equipment and their users.Air velocities below 0.25 metres/second do not create any significant distraction even in tasks requiring sustained attention.In general, what temperature is 'right' for various activities? Table 1 summarizes some typical responses to various temperatures.Table 1

TemperatureCResponses

25Optimal for bathing, showering. Sleep is disturbed

24People feel warm, lethargic and sleepy. Optimal for unclothed people.

22Most comfortable year-round indoor temperature for sedentary people.

21Optimum for performance of mental work.

18Physically inactive people begin to shiver. Active people are comfortable.

Thermal ComfortThe main influences, which affect human comfort, are:1.Temperature2.Air Movement3.Radiation4.Humidity

1. Temperature

Attempts have been made in the past to assess human comfort by considering temperatures defined in certain ways.Suitable temperatures are between 21oC and 23oC.In warmer countries room temperatures up to 25oC can be acceptable.The air temperature can be measured with a mercury-in-glass thermometer.The average room temperature should be taken near the middle of the room, well above floor level but not above head height.The Mean Radiant Temperature is the mean temperature of a room due to surfaces radiating heat into the room as if all the air is exhausted.The Environmental Temperatures are the assumed temperatures inside (tei) and assumed temperatures outside (teo) a building. The inside environmental temperature (tei) is a combination of radiant and air temperatures i.e.

tei = 2/3 tr + 1/3 tawhere:tr = mean radiant temperatureta = air temperature (dry bulb)EXAMPLE 1If tr = 23oC and ta = 20oCThen: tei = 2/3 x 23 + 1/3 x 20

tei = 15 1/3 + 6 2/3

tei = 22oC

The inside environmental temperature (tei) is a fairly good guide to comfort if there are no unusual draughts or humidity conditions in a room.The outside environmental temperature (teo) is a complex combination of outside air temperature and the readiness of a surface to receive radiant heat, from solar radiation or to radiate heat outwards.

Comfortable Temperature

What temperature is comfortable or optimum?About 20oC to 22oC is OK for some sedentary situations, but if activity is taking place then a lower temperature is necessary, for example, in a factory the air temperature of 16 oC to 17 oC may be satisfactory.It is difficult to generalize since some factories such as high-tech operations may require a higher room temperature.Generally speaking if the occupants of a room are sedentary or mostly seated then they will not generate as much body heat those who are working very strenuously. Therefore the design room temperature varies with activity.Also clothing has to be considered. In winter time room occupants may wear heavier clothes than in the summer and some accommodation of the seasons may be necessary in deciding room conditions.Another factor that should be considered is the age of the room occupants. Small babies and elderly people require a higher room temperature, up to 24oC, to feel comfortable.Room design temperature should be carefully considered and the CIBSE guide has an appropriate table to guide the engineer.

2. Air Movement

Large air movements in rooms can cause discomfort especially if the air is cold in winter time. Cooler air tends to travel at floor level and can cause discomfort at the ankles.Very low levels of air movement can also cause a feeling of discomfort and stuffiness in a room especially if the ceiling height is low and the dry bulb temperature is too high in summer.Air velocities between 0.10 m/s and 0.45 m/s are generally acceptable, but this depends on conditions such as dry bulb temperature, humidity and clothing.To allow for dry bulb temperature the graph below gives acceptable values for comfort.

For general use in buildings where the air temperature is suitable a figure of 0.15 m/s can be used for acceptable air velocity.Air velocities less than 0.10 m/s can cause a feeling of discomfort as can higher values over 0.45 m/s at which draughts can result.Less than 0.01 m/s results in stagnant conditions.As a general rule higher values of air movement are more acceptable in summer than in winter.Older buildings tend to have areas such as cracks around doors and windows where air can be admitted to the inside thus causing increases in overall air movement in rooms. Modern buildings, on the other hand, can be so well sealed that little air movement is the result. Badly designed ventilation systems can also be the cause of high air velocities in rooms and care should be taken when designing air diffuser systems with exit velocity and throw values.Air movement can be measured with a hot wire anemometer to ascertain if comfort is compromised.

3. Radiation

Radiation is completely independent of any intermediate medium and will occur just as readily across a vacuum as across an air space.The intensity of radiation varies with the square of the distance between the point of origin and the receiving surface. In a room with four walls, a floor and a ceiling there will always be an exchange of radiant heat energy if all the surfaces are at different temperatures and different textures.If radiant heating is used in a room then there will be an exchange of radiant energy from the heater to the room surfaces and occupants. It is possible to feel uncomfortable in a room with radiant heating, particularly if overhead heating panels emit radiant heat downwards onto the head. Similarly it is possible to feel uncomfortable if a room surface is cold and the body radiates heat to that surface. This can happen when people occupy an unheated building and the walls and other surfaces are cold. Even when the central heating system has been on for a while the air temperature may be satisfactory but the surfaces are still at a temperature much less than the air temperature, thus causing an excessive radiant heat exchange from body to surfaces.

In general the dry bulb temperature should not exceed the mean radiant temperature of the surroundings in summer. In winter the dry bulb temperature should be less than the mean radiant temperature. This means that in winter the mean radiant temperature should be higher than the dry-bulb. In practice this is difficult to achieve since external walls and windows are at a lower temperature than the air inside a room unless radiant panels are attached to walls.4. Humidity

Humidity is the amount of moisture in air.This moisture is also known as water vapour.Also the moisture in air can be regarded as low pressure steam.Unlike the other measures of moisture, relative humidity the most familiar term is not an absolute measure of moisture content. Rather, as its name suggest, it is a measure only of the relative amount of moisture contained by air. More specifically, it is the moisture content of the air relative to the maximum amount of moisture which air at a given dry bulb temperature can hold when saturated. If the weatherman says the relative humidity is 70 percent and the temperature is 23oC it means that the air contains 70 percent of the moisture it could possibly hold at 23oC. Relative humidity is not really considered to be of vital importance in human comfort since body tolerances are quite wide. We cab tolerate a low humidity of about 40%, but with lower values complaints are made of dry skin and dryness of the eyes.From 40% to 60% is usually regarded as comfortable. At above 80% it may become uncomfortable especially if activity increases and perspiration becomes less easy.

One effect of a considerable rise in humidity is that occupants feel a few degrees warmerthan they really are if the air temperature is already quite high.The amount of moisture or water vapour or steam in air is very small.At 0oC 1 kg of air can contain 3.7 grams of moisture.At 20oC the amount of moisture increases to 14.4 grams, therefore warmer air can hold more moisture.The relative humidity is a ratio of vapour pressures.It is the amount of moisture contained in air expressed as a percentage of the maximum which could be contained in air at a given temperature.

RH %

= x 100See Calculation of Air Properties section in the Properties of Air part of the Science notes in this site.Comfort RecommendationsThe CIBSE guide A Section 1.3 gives details of recommendations on suitable winter and summer temperature ranges, outdoor air supply rates, filtration grades, maintained illuminances and noise ratings for a range of room and building types.

The Table below is an example of these recommendations.

Building /RoomWinter Dry Resultant Temperature range for activity and clothing levelsSummer Dry Resultant Temperature range for activity and clothing levelsAir Supply per person (l/s/person)Filtration GradeIlluminance(lux)Noise Rating (NR)

Temperature (oC)ActivityClothingTemperature (oC)ActivityClothing

Conference / Boardrooms22-231.11.023-251.10.658F6-F7300/50025-30

Dwellings - Bathrooms26-271.20.2526-271.20.2515G2-G4 (Extract)100-

Dwellings - Bedrooms17-190.92.523-250.91.20.4 to 1.0 AC/h to control moistureG2-G4

10025

Dwellings - hall/stairs/landings19-241.80.7521-251.80.65--100-

Dwellings - kitchen17-191.61.021-231.60.6560 l/sG2-G4 (Extract)30040-45

Dwellings - living rooms22-231.11.023-250.91.20.4 to 1.0 AC/h to control moistureG2-G4

50-20030

Dwellings - toilets19-211.41.021-231.40.65More than 5 Ac/hG2-G4

100-

Offices - executive21-231.20.8522-241.20.78F750030

Offices - general21-231.20.8522-241.20.78F6-F750035

Offices open plan21-231.20.8522-241.20.78F6-F750035

Restaurants / Dining rooms22-241.10.924-251.10.658F5-F750-20035-40

Notes on Table

The summer comfort temperatures given apply to air conditioned buildings. Reference should be made to the table of recommended illuminances in the Code for Interior Lightingand CIBSE Lighting Guides for design guidance on specific applications.

Thermal IndicesSince there a quite a few comfort indicators, some more important than others, many attempts have been made to devise indices which combine some or all of these variables into one value which can be used to evaluate how comfortable people feel.The index that has been adopted by CIBSE is the Dry resultant Temperature.Dry Resultant TemperatureDry resultant temperature combines air and mean radiant temperatures into a single index temperature, as follows:

Where;t c =dry resultant temperature (C), t ai=inside air temperature (C)t r =mean radiant temperature (C) v =air speed (m/s).

The above equation is simplified at indoor air speeds below 0.1 m/s, as shown below.

t c= 0.5 t ai + 0.5 t r

Fanger's ResearchFanger researched comfort criteria in Denmark and produced a series of comfort charts and tables based on subjective tests which considered the metabolic rate for various activities and the clothing worn.The volume of data provided enables comfort predictions to be made with a high level of accuracy.Predicted Mean Vote (PMV) and Predicted Percentage Dissatisfied (PPD)

These indices are more complicated than the Dry Resultant Temperature.They combine the influence of air temperature, mean radiant temperature, air movement and humidity with that of clothing and activity level into one value.The PMV index is a method of predicting if occupants will feel comfortable in a room.Research was carried out in various locations to find out if people felt comfortable and this is used to determine levels of comfort.The PMV index may be defined as the mean value of the votes of a large group of persons, exposed to the same environment with identical clothing and activity.The predicted percentage dissatisfied (PPD) as a function of predicted mean vote (PMV).The predicted percentage dissatisfied (PPD) can be obtained from the PMV using the following equation.PPD = 100 95 exp [(0.03353 PMV 4 + 0.2179 PMV2)]The graph below shows typical comfort zones for winter and summer.

SIX FACTORS FOR COMFORT OUTDOORS

There are six factors that influence how a person will feel when going outside. They are sunlight, wind, evaporational cooling, temperature, humidity and clothing. The combination of these six factors determines whether a person feels cold, warm, comfortable or uncomfortable. Let's take a look at each of the six factors.

Direct sunlight makes a person feel warmer because electromagnetic radiation is being embedded directly into the skin. If the temperature feels uncomfortably cool in the shade, standing in direct sunlight will make one feel warmer.

Wind makes a person feel cooler especially when the wind is blowing over moistened skin. This effect is very apparent if you have gotten out of a swimming pool on a windy and dry day. The wind evaporates moisture from the body. Since evaporation is a cooling process and absorbs latent heat away from the body, the person feels colder. Skin always has moisture on it. Just like a tree transpires, the human body is constantly having water evaporated from it. Wind intensifies this process. A hot day with a breeze will feel more comfortable than a hot day with calm wind. Wind and evaporational cooling are closely linked. The higher the wind, the greater the amount of evaporational cooling, especially if air is dry.

Perhaps the most important factor in determining comfort is temperature. If the temperatures are cold, the human body conducts energy to the surrounding air and gradually loses heat (you shiver and feel cold!). If temperatures are too warm, excess heat builds in the body and the body has trouble releasing that heat to the surrounding air (water loss rate from your skin increases from sweat and you feel hot!).

The humidity is important because it determines the overall loss of water from your body. If the air is dry, the effect of evaporational cooling on the body is maximized. Evaporational cooling and a wind breeze can partially or completely offset temperatures that would normally be considered uncomfortably hot. When the humidity is high, the effect of evaporational cooling is reduced. Because of this, heat builds in the body. At the same temperature, a humid day will feel more hot and uncomfortable than a dry day.

The last factor is clothing. Clothing can obviously make you feel comfortable on a day that is considered warm or cold. Clothes are added to counter the chill in the air. The wind chill value is only relevant to exposed skin. There are variables the wind chill index does not consider including direct sunlight and some assumptions in the wind chill equation do not mirror reality perfectly. On a cold day it is best to dress in layers. The goal is to maximize the heat between the skin and the clothes on a cold day. On a hot day, white clothes and loose fitting clothes are the best. Everyone has a slightly different temperature they consider being the "comfortable temperature". This range for any one person tends to be from 68 to 78 F.

All the six factors mentioned go into determining how a person will feel. The combination of all these factors is so complex that no formula using all these factors has been developed. The two that are commonly used today by weathermen are the wind chill and heat index. Wind chill considers wind and temperature while the heat index considers heat and humidity. These two indices do not take into account several other factors that determine how one will feel. The heat index does not consider wind and direct sunlight.

In summary, when a person is outside, if you feel cold you can step into the sunlight, reduce the wind, increase the temperature, increase the humidity, and increase clothing. If you feel hot, you can step out of direct sunlight, increase the wind, decrease the temperature, decrease the humidity and take off clothes. Off course, we can not control all these variables that occur in the atmosphere except for three ways: (1) wear proper clothing (2) go inside to a comfortable building (3) evaporational cooling through adding water to the skin surface on a hot day.MODES OF HEAT TRANSFERHeat Transfer is the transfer of energy from one body to another due to a temperature difference between the bodies. The bodies may be solids or flowing fluids as in a heat exchanger.There are three fundamental methods of heat transfer:Conduction, Convection and Radiation.Heat is transferred by conduction within a body or substance by direct molecular communication. It is characterised by a continuously decreasing temperature in the direction of heat flow and by the absence of motion within the substance. It is the only mechanism of heat transfer within solids. When a steel plate is heated on one side, the other side becomes warm by conduction.Moving fluids can transfer heat from one body or region to another by convection. When the fluid motion is due to a density difference arising from a temperature difference, the heat transfer is termed natural convection.When a container of water is heated on a stove, the hot low-density water near the bottom rises and transfers heat to the upper regions of the container by natural convection.Where the fluid flow is produced by a fan, pump, or any mechanism other than temperature differences within the heat transfer device, the process is termed forced convection.In a car radiator heat is transferred from the water (circulated by the pump) to the radiator surface by forced convection. Forced convection also accounts for the transfer of heat from the radiator surfaces to the air (flow due to fan and/or vehicle movement).Heat transfer by radiation involves a wave action similar to light transmission. A hot body can raise the temperature of the medium separating the two bodies. Heat can be transferred by radiation through a vacuum, most gases and some liquids. The sun transmits solar energy to the earth by radiation; a hot stove heats surrounding objects primarily by radiation.ConductionThe Fourier equation may be used to assess the amount of heat transfer by conduction.The Fourier equation in this form is used for non-composite structures i.e. one layer of thickness.Q=( k / l ) x A x TWhere;Q=Heat transfer by conduction (Watts)k=Thermal conductivity of material (W/m deg.C)A=Area of material (m2)T=Temperature difference across the material (deg.C)l=Thickness of material (m)Since k / l = 1 / R, where R is resistance to heat flow (m2deg.C/W), the above equation may also be written;Q=( 1 / R ) x A x TWhere;Q=Heat transfer by conduction (Watts)R=Resistance to heat flow (m2deg.C/W)A=Area of material (m2)T=Temperature difference across the material (deg.C)Also this can be written;

Q=A x T / RIn building heat losses: 1/ R = U value (W/m2degC) or Thermal transmittance value.Therefore:

Q= U . A . TWhere a wall is composed of several different materials an overall resistance and overall U value of the composite structure is required. This will include the internal and external surface resistances giving the expression;

Q= Uoverall . A . TFor more details of heat losses see Thermal Transmission section in Heating notes.Example 1Calculate, using the Fourier equation, the heat transferred by conduction through a steel boiler wall if the boiler surface area is 0.6m2.Compare this with stainless steel and copper alloy.Use the thermal conductivity values for; Steel (1% carbon), Stainless steel 316 and Copper alloy 11000 in the Table of Solid Material Properties in this section.DATAAverage gas side hot surface temperature = 85oCAverage water side temperature = 80oCThermal conductivity Steel (1% carbon) = 43 W/m KThermal conductivity Stainless steel 316 = 17 W/m KThermal conductivity Copper alloy 11000 = 388 W/m K.Boiler wall thickness = 6mmQ=(k / l ) x A x TWhere;Q=Heat transfer by conduction (Watts)k=Thermal conductivity of material (W/m deg.C)A=Area of material (m2)T=Temperature difference across the material (deg.C)l=Thickness of material (m)Therefore for Steel (1% carbon) ;Q=( 43 / 0.006 ) x 0.6 x ( 85 80 )

Q=( 7166.66 ) x 0.6 x ( 5 )

Q=21,500 Watts = 21.5 kW.Therefore for Stainless steel 316;Q=( 17 / 0.006 ) x 0.6 x ( 85 80 )

Q=( 2833.33 ) x 0.6 x ( 5 )

Q=8,500 Watts = 8.5 kW.Therefore for Copper alloy 11000;Q=( 388 / 0.006 ) x 0.6 x ( 85 80 )

Q=( 64,666.66 ) x 0.6 x ( 5 )

Q=194,000 Watts = 194 kW.Therefore we can conclude that Stainless steel does not transfer as much heat by conduction in the boiler as Steel (1% carbon) and Copper alloy transfers a lot more heat than the other two metals.