Heat Transfer Flashcards
What is thermal comfort?
- Thermal comfort is the least effort required to maintain the human body’s thermal balance.
- However, maximum comfort in a building cannot be achieved as it is subjective to each user.
- Therefore, the designer must provide the best indoor climate possible within the range of thermal comfort.
- To maintain a constant temperature, heat generated by the body must be transferred away at the same rate that is generated.
Which factors influence thermal comfort?
Factors involved with buildings include:
- Radiation from surfaces
- Humidity
- Air temperature
- Level of clothing
- Activity
Personal and psychological factors include:
- Age
- Gender
- Health Conditions
- Acclimatisation
- Expectations
What is operative temperature?
Operative temperature is the average of the air temperature and the mean radiant temperature.
Which is the main factor which determines the metabolic heat generation?
The level of activity has the greatest effect on metabolic heat generation.
What is the average heat emission of an adult?
58W/m2 which is known as 1 Met.
Which factors affect metabolic heat generation?
- Activity
- Gender
- Age
- Weight
- Fitness
What is the unit for the insulation of clothing?
0.155m2K/W is the equivalent to the thermal resistance of a winter business suit, which is known as 1 Clo.
How can we increase thermal comfort?
If too cold:
- Exercise to generate metabolic heat.
- Alter our clothing.
- Block drafts.
If too hot:
- Open a window.
- Lower the blinds.
- Turn on a fan.
What is acclimatisation?
- Over time we acclimatise to the environmental conditions around us.
- This is shown by how when the mean outdoor temperature is low, the comfortable indoor temperature doesn’t have to be as high.
What is the law of conservation of energy?
- “Energy is neither created nor destroyed, but can be converted from one form to another”
- For energy to be balanced, the gains must equal the losses, or else temperature will tend to infinity or absolute zero.
What is most of the high-grade (concentrated) energy converted into?
Low-grade heat energy through the waste product of different appliances.
What is energy flow?
- Energy can flow from a place which is hotter, to another cooler place when they are put in thermal contact.
- This causes ordered states (where heat is organised into distinct hot and cold areas) to become more disordered (where heat is evenly distributed).
- This results in thermal equilibrium and at this point, heat flow stops.
What are the different mechanisms of heat transfer?
Humans and buildings:
- Conduction
- Convection
- Radiation
Humans only:
- Evaporation (through respiration and perspiration)
What is conduction?
- Heat is transferred through direct contact between molecules which collide.
- When these collisions occur, the hotter molecules with more energy transfer some of their energy to the cooler molecules.
- There is no change in basic position of molecules.
What is convection?
- Heat is transported because when a fluid is heated, it expands and becomes less dense and therefore lighter.
- This fluid then rises above the heavier, cooler fluid taking heat away in the process.
- There is a change in basic position of molecules.
What is radiation?
- Heat is transferred at long wavelengths known as infra-red.
- The higher the absolute temperature, the more heat/energy a body radiates.
- Transmission doesn’t depend on molecules - instead wavelength of radiation.
What is evaporation?
- Heat is transported as air which takes moisture from damp surfaces.
- To do this, the bonds holding together the water must be broken which requires energy, allowing the water to evaporate and cook the surface.
- There is a change in basic position of molecules.
What are bioclimatic charts?
- Bioclimatic charts are used to show a central range of comfort conditions.
- It also highlights problems with conditions outside of this range to help you to incorporate different design features into the building.
What are the different types of heat loss?
Fabric heat loss (conductive):
- Walls
- Windows
- Roofs
- Floors
Air heat loss (convective):
- Controlled ventilation
- Uncontrolled infiltration (drafts/gaps)
How can we limit heat loss?
We can limit heat loss by specifying limits for:
- The maximum thermal transmittance of constructional elements.
- The permeability of the building envelope to limit heat loss through infiltration.
- The percentage of glazing in walls in order to limit summer overheating.
What is the equation for total heat loss?
What is the equation for fabric heat loss?
What is the equation for heat loss of an element?
What is the equation for thermal resistance of an element?
What is the equation for total thermal resistance?
What is the equation for U value?
What are the different types of resistance?
Internal surface resistance (Rsi):
- Deals with climate, wind and rain.
External surface resistance (Rso):
- Deals with convection heat flow.
Airspace resistance (Rc):
- Controls the flow of air through the building element.
What is conductivity?
- A measure of the rate at which heat is conducted through a material of given thickness.
- Symbol = λ
- Unit = W m-1 K-1
What is resistivity?
- A measure of the resistance to heat flow of a material for a given thickness.
- Symbol = r
- Unit = m K W-1
- r = 1 / λ
What is conductance?
- The conductivity of a material divided by its thickness.
- Symbol = C
- Unit = W m-2 K-1
What is resistance?
- A measure of resistance to heat flow for a given thickness.
- Symbol = R
- Unit = m2 K W-1
- R = 1 / C
What is transmittance (U value)?
- Similar to conduction but considers surface and air space resistances.
- Symbol = U
- Unit = W m-2 K-1
What is the equation for total air heat loss?
What is the derivation for the equation of total air heat loss?
What is the equation for the energy requirements per degree of heating?
How do you convert the energy requirements per degree of heating (WK-1) into kWh/day?
This is done for building energy cost calculations.
What influence does building form have on energy consumption?
- Heat is lost through the surface area of buildings (m2) via conduction and infiltration.
- Thermal energy is stored in the volume of the building (m3).
Therefore, the surface area to volume ratio is an indication of how much heat will be lost by a building:
- A large SA:Vol will have a large amount of heat loss.
- A small SA:Vol will have a small amount of heat loss.
What are heating degree days?
- If a building needs an average of 1 degree of heating over 1 day, then this is the equivalent of 1-degree day.
- A cooling degree day is when the building needs to be cooled by 1 degree over 1 day.
- Heating degree days are calculated from the daily difference between mean temperature and the defined base temperature (which is 15.5oc in the UK).
- Degree day = time period of heating x average magnitude of heating.
