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Purpose of Thermal Design? Why do we undertake thermal design?
Write your own reasons We’ll discuss them.
Thermal DesignObjective: Control heat flow to:2. Design Proper HVAC systems
1. Maintain comfortable indoor conditions2. Reduce heating/cooling loads, which reduces
operating costs3. Understand Building Envelope Loads
1. Control vapor movement/condensation2. Design to accommodate contraction/expansion of
building materials and sealant joints
How we Control Heat Transfer Roof – Insulate & Reflect Walls – Insulate, Reflective, seal gaps,
control vapor movement Windows – Insulate, Emissivity, seal
gaps, control vapor Doors – Insulate, Seal gaps Foundations – Insulate, avoid water
The Mechanisms of Heat Transfer Now we’ll look at how heat actually
moves. This is almost all in Beall – it should be
a review.
How Heat Moves
Cooler areaHeat flow
Temperature
High Pressure Low Pressure
Warmer area
Mass flowPressure
Conduction Conduction : direct
transfer by contact of solid, liquid or gas
Q=A*Delta-T / R Analog to Electrical
Resistance Current = Voltage/R Circuits follow directly
Linear if constant and in plane. Very complex if have
time variance or complex shapes
Convection Convection: transfer
of heat by the movement of air or water Heat moves with the
mass of fluid. Warmer or colder replaces original.
Complex - Simplify
RadiationNon-Linear
•Highly Complex
•We must simplify
•Do so with “equivalent temperatures” – empirically derived
4TQ
Radiation BehaviorAbsorptance0.25 ---> 0.95Reflectance0.1 ---> 0.95Emittance 0.08 ---> 0.95Transmittance (calculated)Note that Reflection +
emittance + transmittance is equal to 100%
Evaporation Phase change from solid or fluid to gas
Takes energy to do so, thus cools the materials it’s on
Complex calculation, dependent on temperature, Rh, material properties, air flow etc.
Usually simplify – look at long term.
Simplifications of Complexity Simplify to linear behavior Consider one-dimensional situation Ignore time variation Use “effective” properties
Emperically derived simplifications Convective behavior converted to R Surface Temperature Air gap behavior
More Complex 3-D complexity – usually try to ignore it
Beall does deal with it slightly with ties analysis Time effects
Daily Sun motion
Yearly Sun Path variation Shading variation from vegetation
Material Properties Thermal Mass Time variation – degradation Temperature & Humidity variation
Calculations Wall Gradient Calculator Spreadsheet
Thermal gradient across a wall Saturation vapor pressure across a wall Actual vapor pressure across a wall Joint width necessary to address component
movements and construction tolerances. Thermal Bridging Wind pressure on a wall in both PSF and inches of
water
Factors affecting thermal performance - continued
Mass Heat migrates through solid materials from the hot
side to the cooler side. The time of delay involving absorption of the heat is called thermal log.
The amount of energy necessary to raise material temp is proportional to the wt of the material.
Heavy materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration. This characteristic is called thermal storage capacity. It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies.
Major Insulation Types1. Loose
(fibers,chips) - fill insulation (poured, blown)
2. Flexible and semi-rigid (batt, blanket)
3. Rigid (wood, fiberglass board)
4. Formed-in-place (urethane foam)
Loss of Thermal ResistanceIt is recommended to have TRR be greater than
80%. Less than 80% insulation is considered wet. See table 3.13 in text.
Thermal Resistance Ratio:TRR = wet thermal resistivity
dry thermal resistivity
Effects of InsulationImprove the thermal performances of building walls and
roofs by reducing both conductive heat flow through the section and corrective heat flow in air spaces:
2. Results in more comfortable indoor air temp and less fluctuation
3. Reduces cooling/heating loads
Thermal efficiency of insulation depends on:6. Thermal resistance R7. Stability over time (R value dimensional stability)8. Resistance to deterioration9. Securing attachments
Effects of Thermal BridgingA thermal bridge occurs when a subject of high
thermal conductivity penetrates a material of low thermal conductivity (insulation) increasing the rate of heat flow at the penetration.
To account for thermal bridging correction factors (<1.0) should be used. Example 1 - Use table 3.9 in text for correction of R value (0.5 ---> 0.38).
Better yet, calculate it.