Summary 7S4X0 Building physics and building services engineering
Heat
Chapter 5
Heat is internal energy, which is stored in all matter and can be defined as the sum of the kinetic
(molecules vibrating) and potential energy(body expands) of the molecules
Heat transport is triggered by temperature differences, heat transfers from the place with a high
temperature to a place with a low temperature.
Ways of heat transport:
- Convection: the passing of heat through liquids or gasses (through air). Free
convectionmovement by air temp. differences
Heat flow rate/heat flux by convection h=heat transfer coefficient
External convective heat transfer coefficient= 20 W/m2K
Convective heat transfer coefficient for vertical surfaces= 2.5 W/m2K
Convective heat transfer coefficient for horizontal surfaces= 5 W/m2K
Convective heat transfer coefficient for downward directed heat flow= 0 W/m2K
- Ventilation: heat flow from the interior to the exterior
Heat flow by ventilation from the inside to the outside
V=volume flow rate/ventilation rate in m3/s and p*c=1200 J/m3K
- Radiation: straling, at the surface of each object K>0 internal energy is converted into
electromagnetic radiation, so the temp of the surface becomes lower. But if there is no temp
difference in the room the surface will not become colder, because it also absorbs radiation.
Heat flux by radiation and is the weighted average radiation temp.
Hr means heat transfer coefficient for radiation, h2=5 W/m2K
o Emittance (uitzending) of a surface
e=emission factor, constant for each material Q=Stefan bolzmann 5.67*10-8
There are two types of radiation:
1. Long wave radiation (thermal radiation), emitted by surfaces like floors walls ceilings. All
objects have the same absorption with long wave radiation
2. Short wave radiation from the sun (solar radiation), the absorption of objects is
depending on its color
There are three options with thermal radiation on a transparent surface(glass): absorption
reflection and transmission
Translucent/non-transparent materials like concrete only has absorption and reflection
Long wave radiation only reflects and absorbs, heat stays in the material
- Conduction: transfer of heat by microscopic particles and movement of electronics within a
body (through floor or walls)
, Heat flux by conduction and is thermal conductivity, porous materials
absorb moisture which has a higher so the heat conduction increases
Chapter 6
The mean temperature of our body’s core is 27 C. To warm up the body there is going on a process
called metabolism (unit met) in which chemical energy is converted into heat. The metabolism is
higher when you are active.
Our body has a heat balance: over long periods of time the amount of heat generated is equal to the
amount of heat delivered
Heat balance : the heat flux produced is the sum of emitted + accumulated(ophopen),
in buildings under stationary conditions we set accumulated flux=0. It can also be written as
in which is heat flux by evaporation(of sweat)
Heat flux by convection is convection via skin and convection via
respiration(breathing). Skin convection is , respiration convection is
Heat flux by radiation is
The thermal resistance of clothing which plays a role in heat transfer by radiation and convection is
Rclo and its unit is clo 1 clo=0.155 m2K/W.
Heat flux by evaporation is L= vaporization heat water=2500kJ/kg G=moisture flow
Heat flux by evaporation is evaporation from diffusion + breathing + sweating
We can neglect the heat flux by conduction in the heat balance because are small quantities because
it only happens at the contact surface(feed and fingertips)
Thermoregulation: the body’s system to regulate heat and to be able to keep the core temp at 37C.
When the heat emission is too high the effects are vasoconstriction(reduction of blood flow), goose
bumps, chattering teeth(use of muscles). When the emission is too low the effects are
vasodilatation(temperature rise so more convection and radiation), sweat secretion
Thermal comfort: state of mind which expresses satisfaction with the thermal environment. Personal
factors that influence it are metabolism and clothing. Environmental factors the influence it are air
temperature, weighted radiant temperature, air speed.
There can be differences in the temp at the height of the head and the height of the feed. Reasons
are hot air goes up and cold air goes down. Heat flow by radiation to or from our body doesn’t take
into account asymmetry. Parts that are not covered with clothing are sensitive to draught(tocht), a
local excessive heat loss by convention. Draught is a combination of air velocity and air temperature.
Heat transfer by conduction may have an influence on the thermal comfort. Sitting on a cold
wooden bench is more comfortable than on an equally cold concrete bench, since a wooden bench
adopts faster to the body temperature than a concrete bench. It depends on each material heat
, penetration(doordringe) coefficient c=specific heat. Now you can calculate the contact
temperature
Heat emission by radiation is a function of the weighted mean radiant temperature(temp of walls)
and the heat transfer by convection is a function of the air temperature and the relative air speed.
The operative temperature is a measurement of the combined effect of the air and the weighted
mean radiant temperature on the thermal sensation: b is depending on de va
relative air velocity(table). Combination of a low radiant temperature and a high air temperature or
a high radiant temperature and a low air temperature gives a desired operative temperature
Chapter 7
Heat concept: which role we want heat to play in our design
Northern orientated spaces are cool, there is a lack of direct sunlight. Southern and east and
western orientated spaces become warm through solar radiation. East-orientated space in the
morning, a west-orientated space in the afternoon
Conservatories (serres) are unheated spaces made of glass, the indoor temperature depends on the
weather
Solar protection is also part of the heat concept, we can use different types of sun shading. And
cooling is also a part of the heat concept, opening windows to increase air velocity or mechanical
cooling. Heating systems are also part of the heat concept, wood stove for example.
Thermal condition and thermal comfort of a space can be influence with architectural measures like
thermal insulation, orientation of window openings, barriers, glass surface area, ventilation.
The better the thermal insulation of a partition (tussenschotten), the more their surface on the
interior side of a room will approach the indoor temperature. Insulating partitions will make it easier
to obtain the desired thermal conditions, even under extremely varying outdoor climate conditions
In the winter the solar radiation on the southern façade is higher than on the eastern and western
facades. In the summer the radiation on the western and eastern façade is higher than on the south
because of the higher elevation angle of the sun.
Barriers, like other buildings and vegetation can influence the heating of a surface by the sun.
Barriers that are designed to block the sun (overhangs) are called sun blinds.
G-value is the solar heat gain value, this indicates the part of the incident solar energy falling on a
window/construction(including shading) that enters the compartment. Always between 0 and 1. A
type of solid sun shading is tinted glazing. A decrease in G-values often means a lower LT-value, so a
smaller amount of light enters the room and there is a less attractive outside view. Fixed sun shading
is often used, disadvantage is that the sun’s angle changes and rooms are experiences as dark,
adjustable sun shading is a good alternative. Internal shading is not effective in preventing the
heating of a room, external sun shading is.
Night ventilation is sometimes used in office buildings that are not equipped with mechanical
cooling systems, the air during night is colder.
