Conduction - ThermalNet

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CONDUCTION
Heat transport - short distance
1. No mass movement
2. Interaction adjacent molecules
Magnitude - heat transfer
Determined by:
1. thermal conductivity - material
a. Metals high
b. Gases low
2. Contact surface
a. lying down vs. standing up
Resting human or standing animal Heat exchange via conduction - usually small
Often neglected - heat balance studies
Air layer against skin = pathway for conduction
BUT - air has low thermal conductivity
Therefore - small role - total transfer
Heat transfer feet >> ground = small contact area
Hoofed animal - distance between blood vessels &
surface much greater than in feet
Conductive heat transfer increases with animal lying on
cool/wet surfaces
Greater import. under these conditions
1. substrate thermal conductance
2. temperature gradient
3.
area of contact relative to total surface area
Sheep - lying on cold, poorly insulated ground dissipates up to 30% minimum heat production by
conduction.
If temperature of substrate is greater than body temperature
•gradient is reversed - heat inflow via conduction and
increases heat load.
Floor material - important for animals such as pigs
(~20% - pig’s surface - may be in contact with floor)
Thermal capacity - floor material - very important
Density of material x specific heat = thermal capacity
Different materials affect instantaneous fall in temperature
Feeling of warmth - related to instantaneous temperature
drop
Assumed that floor would not feel very cold if:
Instantaneous drop did not exceed 1.6F at Ta = 64.5 F
(wood floor)
40 kg animal - LCT = 11.5 - 13.0 C straw
(thermal insulation + decreases heat flow > floor)
= 14.0 - 15.0 C asphalt
= 19.0 - 20.0 C concrete
Newborn pig at Ta = 10 C - Moving animal from bare
concrete >> straw has same thermal effect on
metabolism as increasing Ta > 18 C.
Heat Transfer - warming food or drink usually insignificant.
If a 20 kg pig drinks 2.5 L/day and water temp. = 20 C
Pig puts 200 kJ into water to raise it to deep body
Temperature
MR = 6700 kJ / day
Therefore - a 3% shift (same found for sheep).
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What is tissue insulation?
Resistance offered to heat flow between heat-producing
tissues and skin surface.
Dependent on:
1.
Thickness - subcutaneous fat
2.
Vasomotor changes - blood flow
Internal thermal conductance = combination - 2 heat
transfer channels
1.
2.
Conduction - heat through muscle and fat layers
Convection - heat transfer via blood
Tissue Insulation (°C x m2 x W-1)
________________________________________________
Calf
0.09
0.04
(vasoconstriction)
(vasodilation)
Adult Steer
0.14
0.04
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•
Cold Ta conductance - not greatly affected by Ta
In theory - fully vasoconstricted at LCT
Minimal peripheral blood flow
Likewise - after vasodilation - little additional increase
in peripheral blood flow
External Insulation
(Animal Coat or Clothing Insuln.) + (Air - Ambient Insuln.)
(Air - Ambient Insuln.) = region between surface of coat
or bare skin and environment
Hair Coat Insulation (°C x m2 x W-1)
________________________________________________
Calf
0.11
0.16
(unerected)
(erected)
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1.
Hair coat insulation increases with coat dept
2.
Air trapped between fibers
3.
Wind velocity decreases amount of trapped air
(mW x m-1 x °C-1)
______________________________________________
Thermal conductivity still air (20°C)
25
Coats - artic mammals
55
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COAT TYPE - IMPORTANT
Decrease in thermal insulation with less still air
Insulation of dead fat - greater than for live fat –
WHY
INSULATION DENSITY
Barnett - used isolated mouse skin
Insulation = 0.069°C/m2/W-1 for a unit hair density
(1 mg/cm-2)
Insulation of mouse coat - using these units = 4 times
that of pig hair of same density and 10 times that of calf
WHY GREATER INSULATION VALUE OF MOUSE FUR?
Possibly - greater # - fine hairs (wt for wt) & more
effective layer - trapped air
Measurement of density as number hairs / unit area may
be more useful comparison - than meas. - density as
weight of hair / unit area
Coat Density - very important
Thick coat - husky dog - so dense - it can sleep
comfortably on snow - with no melting on tips of hairs
• Wind - less effect on dense coats than sparse coats
Dividing line - sparse and dense = 1,000 hairs/cm2
Coat also important - preventing heat gain - hot
environments
External Insulation - Birds
Plumage = effective barrier - heat loss
1.
Down feathers - traps air >> little convection
2.
Contour feathers - distal parts >> windproof cover
•
Chicks without contours - have a problem
Water Problem 1.
thin layer oil - preen gland
2.
+ spaces between finest divisions feather
structure - extremely small
• Both factors >> plumage with high resistance wetting
Thermal insulation - greater in larger birds
1. Able to carry heavier load feathers than small birds
2. Because of radial distribution of feathers - smaller
objects (torso, extremities) have less compact
insulation
Definite benefit during cold exposure
Some birds elevate feathers during expsoure solar radiation
Permits better circulation - air through plumage & incr.
heat loss
Acclimation - warm Ta > decr. plumage - domestic fowl
Some birds - winter plumage may be 30% greater
than summer

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