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Animal Environment & Heat Flow
BSE 2294 Animal Structures and Environments
S. Christian Mariger Ph.D. & Susan W. Gay Ph.D.
Environmental Fundamentals
• Environment is the total of all external conditions that effect the development, response and growth of plants and animals. – Physical factors – Social factors – Thermal factors
• Ventilation is the method of environmental modification for agricultural structures.
Physical Factors
• Space
• Lighting
• Sound
• Gasses
• Equipment
Social Factors
• Number of animals to a pen
• Behavior
Thermal Factors
• Air temperature
• Relative humidity
• Air movement
• Radiation (one type of heat transfer)
Environmental Factors
• Influence:– Animal health– Breeding– Production efficiency – Product quality – Human health – Equipment service life – Building material longevity
Heating and Ventilation Terms
• Heat – the energy transferred from a warmer body to a colder body because of the temperature difference
• Temperature – is a measure of a body’s ability to transfer or receive heat from matter in contact with it.
• Ambient temperature - the temperature of the medium surrounding a body
• British Thermal Unit (Btu) – the quantity of heat required to raise one pound of water one °F
Heating and Ventilation Terms
• Calorie – the quantity of heat required to raise one gram of water one °C
• Specific heat – is the quantity of heat required to raise one pound of material one °F (Units = Btu/lb-°F)
• Sensible heat – is a measure of the energy that accompanies temperature change
• Latent heat – is the heat energy absorbed or released when a material changes phase (ice to water for example)
Sensible and latent heat to change one lb of water from ice to steam
qs = McvΔT
Sensible & latent heat example
• Given a 20 cubic foot water trough that was allowed to freeze to 28° F how many Btu will be required to thaw and warm the water to 40° F.
Sensible & latent heat example
• Find lbs of water.– ρH2O = 62.4 lb/ft3
– 20 ft3 x 62.4 lb/ft3 = 1,248 lbs
Sensible & latent heat example
• Find sensible heat required (Btu) to raise the temp from 28° F to 32° F.– Specific heat of ice = 0.56 Btu/lb - 1° F– 1,248 lbs x 0.56 Btu/lb - 1° F = 699 Btu - 1° F– (32° F – 28° F) x 699 Btu - 1° F = 2,796 Btu
Sensible & latent heat example
• Find the latent heat of fusion for the water.– Latent heat of fusion H2O = 144 Btu/lb
– 1,248 lbs x 144 Btu/lb = 179,712 Btu
Sensible & latent heat example
• Find sensible heat required to raise the temp from 32° F to 40° F.– Specific heat of water = 1.0 Btu/lb - 1° F– 1,248 lbs x 1.0 Btu/lb - 1° F = 1,248 Btu - 1° F– (40° F – 32° F) x 1,248 Btu - 1° F = 9,984 Btu
Sensible & latent heat example
• Sum the Btu’s to find the energy required to raise the temp from 32° F to 40° F.– (32° F – 28° F) = 2,796 Btu– Latent heat of fusion = 179,712 Btu– (40° F – 32° F) = 9,984 Btu– Total = 192,492 Btu
Types of Heat Transfer
Conduction
• Conduction – the exchange of heat between contacting bodies that are at different temperatures or transfer of energy through a material as a result of a temperature gradient.
Conduction is often a heat loss factor as well as a heating factor!
Conduction heat flow
• q = AK (T1 – T2) / L
– A = cross-sectional area of the surface – K = thermal conductivity – L = thickness of the material
– T1 – T2 = ΔT = change in temperature
• q = (A/R) ΔT– R = thermal resistance (L/K)
Conduction example
• Determine the heat transfer through a wall composed of two sheets of ½” plywood (R = 0.62) and 3 ½” of batt insulation (R = 11).
Inside temp = 80° F Outside temp = 20° F
Assume the cross-sectional area “A” is 1ft2
Conduction example
• Find RT for the wall:– Material #1 ½” plywood R = 0.62– Material #2 3 ½” batt insulation R = 11.00 – Material #3 ½” plywood R = 0.62
RT = 12.24
Conduction example
• Find q for the wall:
– q = (A/RT) x (Tinside – Toutside)
– q = (1ft2/12.24) x (80 – 20) = 4.90 Btu/Hour
Heat conduction for a building (qb)
• Calculate the conduction (q) for each building component:
– Ceilings qc - Windows qwi
– Doors qd - Walls qw
– Etc.
• Add all the conductions to find the conduction for the building (qb)
qb = qc + qwi + qd + qw + q...... (in Btu/hr)
Conduction temperature change
• We can also calculate the temperature from one side to the next for each layer in the wall.
• Determine the temperatures at points 2 and 3. – Where T1 – T2 = (q/A) R
T1 = 80° F T4 = 20° FT2 = ? T3 = ?
R2 = 11
R1 = 0.62 R3 = 0.62
Conduction temperature change
• Temp at point 2• T2 = T1 – (q/A) R1
• T2 = 80° F – (4.9/1) x 0.62 = 77° F
• Temp at point 3• T3 = T2 – (q/A) R2 . • T3 = 77° F – (4.9/1) x 11.0 = 23° F
Convection
• Heat transferred to or from a body by mass movement of either a liquid or a gas
Convection
• Convection is often used for interior heating
Radiation
• The exchange of thermal energy between objects by electromagnetic waves.
• Radiant energy is transferred between two bodies in both directions, not just from warmer to cooler.
Radiation
• Here is an example of infra red (IR) radiation being used in an interior heating application
Typical Environmental Effects (dairy cattle example)
Heat stress occurs in animals when their
heat gain is greater than their heat loss.
Body heatMetabolism
Physical activity
Performance
Environment Radiation (sun)
Convection (air)
Conduction (resting surface)
Heat stress has a severe impact on cow performance and health.
IncreasesRespiration rate
Sweating
Water intake
DecreasesDry matter intake
Feed passage rate
Blood flow to internal organs
Milk production
Reproduction performance
Cows are much more comfortable at cooler
temperatures than humans.
Thermal comfort zone
41 – 77 °F
Lower critical temperature
Neonatal calves 55 °F
Mature cows 13 °F
Upper critical temperature
77 – 78 °F
Animals can lose heat by sensible or latent
heat losses.
Sensible heatConduction (direct contact)
Convection (air movement)
Radiation (line of sight)
Latent heatEvaporation (phase change)
As air temperatures increase, animals cannot lose as much sensible heat, so they
pant and sweat (evaporation).
Indirect radiation
Digestive heat
Indirect radiation
Convection
Conduction
Direct radiation
As relative humidity rises, an animal losses
less heat by evaporation.
Evaporation (from skin)
Evaporation (respiratory tract)
NoStress
MildStress
HeatStress
SevereStress
DeadCows
72
80
90
100
110
120
Te
mp
era
ture
(F
)Relative Humidity (%)
0 20 40 60 80 100
How can you tell if a cow is suffering from
heat stress?
Rectal temperaturesAbove 102.5 °F
Respiration rates> 80 breaths per minute
Decreases inDry matter intake
Milk production
How can heat stress be managed?
Shade
Air exchange
Air velocity
Water
Shade lowers the solar heat load from direct
and, sometimes, indirect radiation.
Good air exchange or ventilation of confinement housing is essential to animal comfort.
RemovesHot, moist air
IncreasesConvective heat loss
Recommended1000 cfm per cow
Cows’ cooling ability is improved by increasing the air velocity over the animal’s skin.
RemovesHot, moist air in contact with
the animal
TurbulenceDisrupt the boundary layer
Recommended220 to 440 fpm (2.5 to 5 mph)
Water improves animal cooling through
evaporation.
Watering locationsIncrease in hot weather
Sprinkling systemsWet cow’s hide
Increase direct evaporation
Evaporative cooling padsCools air directly
Cows cooled by convection
Thermal effects on other species
Heat Balance
Heat balance
• To maintain constant room temperature, heat produced by the animals and heaters must equal the heat lost through the building structure and by ventilation.
• Heat gain (Qh) = Heat loss (QT): – Qf + Qs = Qvent + Qb
Heat removed by ventilation (Qvent)
• Ventilation removes heat by replacing warm in side air with cold outside air.
• If humidity is constant we know the specific heat of air.
• If we also know the difference between the outside temp and the inside temp (Δt)
• If we also know how much air is being exchanged in Cubic Feet/Minute (cfm)
• Then we can calculate the heat removed by ventilation.
Heat removed by ventilation (Qvent)
Qvent = (1.1)(Fan rate cfm)(Δt)
Ventilation (Qvent) example
• A building is ventilated at 1,200 cfm. The inside temperature is 65° F and the outside temperature is 15° F. Determine the rate of heat removal.
Ventilation (Qvent) example
• (Qvent) = (1.1) (fan rate) (Ti – To)
• (Qvent) = (1.1) (1,200) (65 – 15)
• (Qvent) = 66,000 Btu/hour
Heat lost through the structure (Qb)
• We have discussed heat lost through structure in terms of thermal resistance (R) and thermal conductivity (K).
• Q = AK (T1 – T2) / L– A = cross-sectional area of the surface – K = thermal conductivity – L = thickness of the material – T1 – T2 = Δt = change in temperature
• Q = (A/R) Δt– R = thermal resistance (L/K)
• Qb = qc + qwi + qd + qw + q......
Heat gain (Qh)
• Heat gain in an animal structure comes from two major sources:– Supplemental heat (Qf) – the heat provided by
various heaters. – Animal sensible heat (Qs) – the heat the
animals give up to the environment. • Conduction • Convection • Radiation • Evaporation (latent heat of vaporization)
Animal sensible heat (Qs)
Assumptions: 1. air velocity 20-
30 fpm
2. humidity 50%
3. surface temp of walls are equal to air temp
Forced ventilation example
• Fifty (50) pigs in the growing stage (100 lbs) are housed at a temperature of 60° F. The cold weather ventilation rate (To = 20° F) is 7 cfm for each animal. The total heat loss for the structure QB = 14,000 Btu/hour and the animal sensible heat Qs = 375 Btu / hour / head. Will supplemental heat (Qf) be required for this structure, if so how much?
Forced ventilation example
• Find the required ventilation:– (# animals) x (cfm/animal) = fan rate – fan rate = (50) x (7 cfm) = 350 cfm
Forced ventilation example
• Find the heat removed by ventilation (Qvent)
– Qvent = (1.1) (fan rate) (Δt)
– Qvent = (1.1) (350) (60 – 20)
– Qvent = 15,400 Btu/hour
Forced ventilation example
• Find the total heat loss (QT)
– (QT) = Qvent + QB
– (QT) = 15,400 Btu/hour + 14,000 Btu/hour
– (QT) = 29,400 Btu/hour
Forced ventilation example
• Find animal sensible heat (Qs)
– (Qs) = (# animals) (Btu/hour – head)
– (Qs) = (50) (375) = 18,750 Btu/hour
Forced ventilation example
• If Qf + Qs = Qvent + QB then:
• Find the supplemental heat (Qf)
– (Qf) = Qvent + QB – Qs
– OR
– (Qf) = QT – Qs
– (Qf) = 29,400 Btu/hour - 18,750 Btu/hour
– (Qf) = 10,650 Btu/hour
Moisture Balance
What about moisture?
• As ventilating air moves through a structure it evaporates moisture from the floor, pits and other wet surfaces.
• As animals breath, moisture is lost from their respiratory system to the air.
• To maintain a desirable temperature, enough moisture must be removed to keep the relative humidity below 70%
Moisture balance
• To maintain a constant rate of moisture:
• Moisture Loss = Moisture production
• The moisture holding capacity of air nearly doubles with each 20° F increase in temperature!
Where does the moisture come from? • Incoming air
• Animal waste
• Animal respiration
• Feed and water
Swine ventilation rates
Air tempering systems
• Tempering warms or cools air before it enters the animal housing portion of a structure.
Air tempering systems
• Tempering systems include: – air make-up systems – air blending systems – heat exchangers – solar collectors – earth tubes – evaporative coolers
Air pre-heating
Air blending
Heat exchangers
Solar collectors
Earth tubes
Evaporative cooling
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