Unit 1: C1 Heat Flashcards
what is Air Temperature
A measure of how hot or cold the air is
How is temperature expressed
In Degrees Celsius
What is typical room temperature
20-25 Degrees
What is the scientific way to describe temperature
- Degrees Kelvin
2. 0 kelvin is equal to 273 degrees Celsius
what instrument is used to measure air temperature
thermometer
What is thermal radiation
Heat that radiates from a warm object
What are examples of radiant heat sources ?
- The sun
- Fire
- ovens
- kiln walls
5, dryers
6, hot surfaces nad machinery
What is mean radiant temperature
Average surface temperature of surfaces enclosing a space
How can radiant heat be assessed
Using a globe thermometer and the reading is known as a globe temperature
what is humidity
the amount of water vapour present in the air
Why is humidity in our homes bad?
- Because it causes mould, moisture and creates condensation on our windows
- High humidity influences homes and health
- low humidity helps spread the rate of infection
Humidity chart
>70% poor high humidity levels >60% and <70% fair >30% and <60% maintain your healthy levels >25% and <30% fair <25% poor low humidity levels
Higher temperature leads to high humidity
When does humidity change
- Atmosphere gains or losses water vapor
- Evaporation
- Temperature changes
- Lower temperature - relative humidity changes
- Raise temperature - relative humidity decreases
what does a whirling hygrometer measure
- Humidity
2. heat
What is air velocity / movement
The speed of air moving
What are the effects of still/stagnant air
If artificially heated may cause people to feel stuffy
What are the effects of air movements in cool or cold environments
Draught
What instrument measures air movement
Anemometer
or
kata thermometer (measure time it takes for air too cool)
Explain one way a thermostat controls temperature
- The thermostat measures the temperature of the air (1) therefore interprets the information to turn on/off heating system (1)
- Thermostats can contain a heat activated switch (1) which opens and closes a circuit to control heat
What are factors that can affect human comfort
- Metabolism
- Clothing insulation
- Gender
What are the three mechanisms of heat transfer
- Conduction
- Convection
- Radiation
What is conduction
the process by which heat is directly transmitted through a material of a substance when there is a difference of temperature
Give a material with a high kt Value (Thermal conductivity)
- Copper
- Diamond
- Silver
4 Gold
5 Aluminium
Give a material with a low kt value (Thermal conductivity)
- Wood
2. Rubber
What is the equation for heat loss
Heat Loss - Thermal conductivity x area x change in temperature / thickness
What is convection
Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion
What is forced convection
The fluid is forced to flow in a tube or over a surface by external means such as a fan
What is natural convection
if the motion is caused by buoyancy forces induced by density differences due to the variation of temporaries in the fluid
What is radiation
The emission/transmission of energy in the form of waves or particles through space or material medium
What is fabric heat loss and what is the building fabric
- Transfer of heat through the external shell of the building
- Foundations, walls/ceilings, roofs doors, windows, insulation
What is ventilation heat loss
Uncontrolled flow of air through penetrations in the building fabric caused by pressure difference’s generated across these openings by the action of wind and temperature
Sate factors of heat gain in a building
- Heating systems
- The sun
- Heat produced by people
- Cooking
- Use of hot water
What is archived by ventilations
- Ventilation allows the moist air produced y the occupants by cooking, bathing, showering and domestic washing to escape before it causes harm to the building fabric
- Ventilation rides buildings of indoor air pollution
What is the formula to calculate fabric heat loss
Qf (rate of fabric heat loss) = U (U-value) x Area x Change in temperature
What is the equation for ventilation heat loss
Qv ( Rate of ventilation loss (w)) = Cp (Volumetric specific heat capacity of air (J/m³K)) x N (hrs) (Air infiltration from the room) x V Volume of the room (m³) x change in temperature (C) / 3600
Qv= Cp x N x V x change in temp / 3600
What re factors that contribute to heat gains
Colour if external surface (1) produces heat gains caused by radiant heat as the sun warms the surface of the building (1)
Occupants (1) as the internal heat gain will be influenced by the activities (1)
What are casual heat gains
Heat gains from occupants and equipment in the building
What are the factors that contribute to heat gains and losses
- Insulation of building
- Surface area of external shell
- Exposure and impact of local climatic conditions on a building
4 .Temperature difference between inside and outside - Air change rate
- Building use
How is heat loss reduced through windows
By double glazed windows. double glazed widows add and extra pane of glass. the air trapped between the two panes acts as an insulator and reduces heat loss by conduction
Give a drawback of double glazed windows
- Expensive
2. Difficult to break in emergencies
How can heat loss through walls be reduced?
Through the cavity in the wall.
- The air trapped in the cavity acts as an insulator and reduces any heat loss due to conduction through the bricks.
- Heat loss can also occur due to convection within the cavity. Plastic foam insulation can be pumped into the cavity to prevent this.
How can heat loss from radiators be reduced ?
Some of this heat energy is absorbed by the wall that the radiator is attached to, and so the wall heats up.
Heat loss from a radiator can be reduced by placing shiny foil between the wall and the radiator. The foil prevents heat radiation from reaching the wall by reflecting it back into the room
This method of insulation is very cheap to install.
How can heat loss through doors be reduced
A draught is the movement of air due to a convection curre
A lot of heat energy can be lost from a house due to draughts escaping through gaps under doors and around windows.
Draught excluders are hairy or spongy strips that can be used to close the gaps around doors and windows.
What is a thermal bridge
A thermal bridge are areas of the building fabric which has a higher thermal transmission than other parts of the fabric, resulting in a reduction in the overall thermal insulation of the structure.
What occurs as a result of thermal bridging
- An increase in solar heat gains during the summer
- Reduction in internal surface temperatures
- Cold spots occurring within the building
- An increased risk of both surface and interstitial condensation, which may result in mould growth and pattern staining
- Reduction in indoor air quality (due to condensation and mould growth)5.
- Damage to building components
What three ways can heat be transferred ?
- Conduction
- Convection
- Radiation
What is the formula for heat flow
Q = (lambda)/d . A . (t1- t2) (W)
Lambda (W/mK) - lambda - is the thermal conductivity of the material
d (metres) is the thickness of the material
A (m2) is the area of the material, measured perpendicular to the flow of heat
t1 and t2
- increase the area of through which heat flows, more will flow
- increase the thickness of material through which heat flows, less will flow
- change the material through which heat flows, you change the rate of flow
What is the formula for resistance
Resistance = Thickness (M) / Lambda (Thermal Conductivity)
What is formula for total resistance
Rtotal (m².k/m) = Rsi + R1 + R2+ R3 .. Rso
What is the formula to calculate u-values
1/R
