SYSTEMS - MECHANICAL Flashcards
3 WAYS to transfer heat
Convection - transfer of heat through movement of gas or liquid. Only when air temperature surrounding person is less than the body skin temperature, around 85°F. body heat air, hot air rises, replaced by cooler air
Radiation - heat transfer through electromagnetic waves from one surface to colder surface.
Conduction - transfer of heat through direct contact between two objects of different temperatures
General comfortable temperature
between 69 and 80°F, tolerable from 60 to 85°F depending on the relative humidity
Emissivity
Emissivity - of an object is a measure of its ability to absorb and then radiate heat. Shiny objects have low emissivity
Emittance
Emittance - of an object is ratio of the radiation emitted by object or material to that emitted by a black body at the same temperature. Shiny objects have low emissivity
Mean Radiation temperature MRT
weighted average of the various surface temperatures in a room and the angle of exposure of occupant to the surfaces, as well as any sunlight present. all room surfaces and the temperatures and positions must be taken into account.
Thermal conductivity K
Thermal conductivity K
Rate At Which Heat Passes Through 1 SF Of 1 Inch Thickness Of Material When Temperature Differential Is 1°F
Conductance C
Rate At Which Heat Passes Through 1 sf Of Thickness Of Material Other Than One Inch When Temperature Differential Is 1°F
R-value Formula
Measure of resistance to heat flow through a given thickness of material. So the higher the R-value, the more thermal resistance the material has and therefore the better its insulating properties
R = 1 / C
Overall coefficient of heat transmission U Formula
U = 1 / sum R
CONDUCTANCE OF THE WHOLE ASSEMBLY
ΔT
- determined by subtracting outdoor design temperature from desired indoor temperature, usually 70°F
Heat Infiltration Formula
transfer of air into building through open doors, cracks, around windows, flues, vents etc
Air Infiltration Heat Loss = Room Volume * ΔT * Air Changes / hr * 0.O18
ΔT=Design Temperature Difference
Design cooling load factor DCLF - formula
Area Of Glazing * DCLF = Heat Gain through Glazing
DCLF - Design cooling load factor
Heat gain through lighting
1 W equals 3.41 Btu/ hr
(for fluorescent lighting W of ballast must be included)
BTU
heat required to raise the temperature of 1 LBM water by 1°F
Enthalpy
total heat of substance, including latent heat and sensible heat
Specific heat
Number of BTUs required to raise the temperature of specific material by 1°F
Minimize First Cost:
Single Duct Constant Air Volume (CAV) or through wall packaged terminal units
Minimize Operating Costs:
VAV, Single Duct CAV, Hydronic Convectors, Closed Loop heat pump
Control Air Quality/Velocity
VAV (all types), Single Duct Constant Air Volume (CAV), Multizone
Individual Control:
VAV, Constant Air Volume (CAV) Reheat, Multizone, Air-water induction, fan-coil terminals, through wall packaged terminal units
Minimize System Noise:
All Air Systems (except induction) and Hydronic Convectors
Minimize Visual Impact:
Any all-air system
Min floor space or floor height:
Through wall packaged terminal units, induction systems, and hydronic convectors
Minimize maintenance:
single duct Constant Air Volume (CAV), and hydronic convectors
Avoid Chimney
Electric boilers, through wall packaged terminal units, closed-loop heat pumps
Min Construction:
through wall packaged terminal units
Balance point temperature:
temp at which the building does not require mechanical heating or cooling
sensible heat
the temperature of the air
latent heat
the moisture content of the air
Temperature at a point in a wall - Formula
Toutside + [(Rvalue outside of point in wall/Rvalue total) x ΔT]
The average R-value of sloped insulation
The average R-value of sloped insulation is the average of the thickest and thinnest values.