Mech/Elec ALS Lesson One - Basic Thermal Processes: Summary

Summary

Different modes of heat transfer through the building skin.

Heating load calc:

Qtot = Qc + Qs + Qi

Cooling load calc:

Qtot = Qp + Qm + Qi + Qcltd + Qr

temperature gradient inside walls: calc. using comparative resistances and overall temp. drop

Highlights:

Temperature = a Measure of stored heat energy, temperature is never transferred, only heat is

Sensible heat = Transferred heat that causes a change in temperature

latent heat = heat that causes a change of state as in melting or freezing water

Specific heat = the amount of heat a material can store compared to the storage capacity of water

(BTU)British Thermal Unit = Amount of heat energy required to raise one pound of water by one degree Fahrenheit.

Radiation = process of heat transferring between two objects NOT in contact and NOT shielded from each other

Emissivity = ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to radiate absorbed energy. In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02

Transmissivity = measure of how easily a material allows radiant energy to pass through it.

(MRT) mean radiant temperature = average radiant temperature of your surroundings, independent of air temp.

Convection = Convection in the most general terms refers to the movement of molecules within fluids (i.e. liquids and gases). Hot air rising is an example. Directional heat transfer.

Conduction = heat transfer process which occurs when objects are in direct contact. Not directional.

Conductivity = Each material has a specific level conductivity (K) of heat.

Resistivity = Inverse of conductivity, each material has a resistance (R) value, the rating used to indicate the amount of resistance to heat transfer a given material has.

U Value = the calculated "conductance" of a wall assembly which represents all the interactions of the internal materials including some radiation and convection. The reciprocal of R values, the lower the U value the better.

phase change = The process by which a substance transforms from solid to liquid or liquid to gas, as well as transformations in the reverse direction. (caused by heat transfer)