Resistance in Thermal
Systems
Overview
Resistance in Thermal Systems
1. What is resistance in the thermal
system?
- opposition to the flow of thermal
energy
2. How does heat flow through an object
with low thermal resistance?
- low thermal resistance means heat
can easily flow – high flow rate
Resistance in Thermal Systems
3. What is the effect of heat flow through
an object with high thermal resistance?
- high thermal resistance means heat
cannot easily flow – low flow rate
4. When would we use a substance with high
thermal resistance?
- when you want to keep something hotter or
cooler than its surroundings
Resistance in Thermal Systems
5. What are some examples of materials
with high thermal resistance?
- refrigerated truck – thick walls to
prevent heat from flowing into truck
- styrofoam cups – keep hot drinks hot,
cold drinks cold
- fire-fighting suit – keeps heat from fire
away from fire-fighters
Resistance in Thermal Systems
6. When would we use a substance with low
thermal resistance?
when we need to heat or cool a substance
rapidly
7. What are some examples of materials with
low thermal resistance?
frying pan – heat flows easily through the
pan into the food
car radiator – low resistance allows the
radiator to cool quickly
Resistance in Thermal Systems
8. How do you define thermal resistance?
RT = ΔT
QH
RT = thermal resistance
ΔT = temperature difference T2-T1
(always positive)
QH = heat flow rate
Resistance in Thermal Systems
9. What are some common units for
thermal resistance?
- like the fluid system, there is no single
unit for thermal resistance
- the unit is a combination of a temp over
a flow rate
- common SI – Co/cal/sec
- common English – Fo/Btu/hr
Resistance in Thermal Systems
10. Example – The temperature inside a
house is 24oC when the temperature
outside is 4oC. A furnace supplies 2625
kilocalories to the house each hour.
What is the thermal resistance (RT)
of the house?
Resistance in Thermal Systems
-
RT = ΔT
QH
RT = 24oC - 4oC
2625 kcal/hr
RT = 20oC
2625 kcal/hr
RT = 0.0076oC/kcal/hr
Resistance in Thermal Systems
11. What three factors determine the
thermal resistance of a material?
- thermal conductivity – how well the
material conducts heat
- area – the greater the area, the lower
the resistance
- thickness – the thicker the material,
the greater the resistance
Resistance in Thermal Systems

Recall the factors that
affect the heat flow rate
(QH) across a material of
thickness (l), temp
difference ( T), area (A)
and thermal conductivity
(k)?
• QH = kA
T/l, where
• QH is measured in units such
•
as Btu/hr, or cal/sec
RT = l/kA
Rate in Thermal Systems
11
Resistance in Thermal Systems
12. What is the R-factor for insulation?
- number assigned to indicate the
insulating value of different materials
- the higher the number, the greater
the resistance
Resistance in Thermal Systems
Example – R-11- this means that if
there is a temperature difference of
11 degrees, then one Btu of heat
passes through one square foot per
hour
- doubling the thickness of the
insulation doubles the thermal
resistance
Resistance in Thermal Systems
13. What are some applications of high
thermal resistance?
- used when you want to keep
something hot or cold
- insulation of water pipes – keep the hot
water hot or the cold water from freezing in
the winter
- firebricks are used around fireplace – can
withstand high temperatures
Resistance in Thermal Systems
14. What are some applications of low
thermal resistance?
- used when you want to transfer
energy quickly
- cooling systems for car engine, hydraulic
equipment
- process of soldering wires, welding, making
glass