kingofnobbys
BD.org Sicko
Why do so many people insist in using glass vivs with effectively open tops ?
It's not because they are better except for the petshops who flog them to every noob reptile keeper who walks in the door.
Lets do some very basic heat transfer calculations ( conduction and natural convection , will ignore radiant heat and forced convection for simplicity ).
CONSIDER A GLASS VIV w/ MESH LID
sides = 0.6m x 0.6m = 0.36m^2 ( 2 of these )
glass surfaces = 1.2m x 0.6m = 0.72m^2 ( 3 of these )
total glass surface area = 4x 0.72 = 2.88 m^2 = A
typical glass thickness = 6.5mm = 0.0065m = t
k value for ( soda ) glass = 0.96 W/m/K approx. @ 30 degC
Assume heat difference warm face to cold face = 35 degC to 10 deg C (very chilly)
==> DT = 25 deg C
Apply Fourier's Law of Heat Conduction loss , Q = kA(DT)/t = 0.96x2.88x25/0.0065 = 10633W
loss by free conductive through "open top" = hADT
h for air = 10 W/m^2K
A = 0.6x1.2= 0.72
so approx. heat loss by warm air escaping through lid = 10x 0.72x 25 = 180W
TOTAL FOR GLASS VIV WITH OPEN TOP = 10813W
CONSIDER A TIMBER VIV
TIMBER ( 3/4 in Plywood , front glass door )
k plywood = 0.14 W/m/K
Area of timber
BTM + BACK + TOP all 0.6x1.2 = 3x0.72 m^2
Ends again 0.6x0.6 = 0.36m^2 each
total timber area = 4x0.72m^2
thickness = 0.75x0.025 =0.019m
Assume 35 degC inside , 10 degC outside , DT = 25 degC (Again)
Fouriers Law again : heat loss through timber surfaces = 0.14x(4x0.72)x25/0.019 = 537.6W
Heat loss through front doors ( simply assume equiv 1 sheet of glass 1.2x0.6 )
Fouries Law again : heat loss through front glass doors = 0.96x0.72x25/0.0065 = 2658W
Total for this viv = 3196W
clearly much more heat must be provided via a basking globe , heatpad , CHE or combination of these to maintain the temperature in the glass viv (which has an essentially open top) cf a timber viv with glass front doors.
The difference in heating required = 10813 - 3196 = 7617 W
Please feel free to comment and let me know if I've made any errors (I've been retired for nearly 20 years and am a wee bit rusty on my heat transfer).
Simple improvement for those stuck with a glass viv with a mesh lid , replace the mesh lid with a plywood lid .
The effect of a 3/4 in plywood lid :
Heat loss through timber lid (same dt, same thickness as above)
= 0.14x 0.72x 25/0.0188 = 134W
cf loss by natural convection through mesh lid = heat loss by warm air escaping through lid = 10x 0.72x 25 = 180W
==> 180-135 - 45W less heat lost simply by covering the top of the glass viv with a 3/4 in thick plywood lid.
Apply a buggerfactor (engineering jargon) to compensate for additional heat loss via radiant heat losses through the opening at the top of approx 1.5 for radiant loss through the opening = (1.5x180) - 135 = 135W !!
This is worth chasing especially in winter.
SOME SOURCES
http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html
http://www.physicsclassroom.com/class/thermalP/Lesson-1/Rates-of-Heat-Transfer
http://www.engineersedge.com/calculators/convection_of_known_surface_area_calculator_10047.htm
https://www.electronics-cooling.com/2001/08/simplified-formula-for-estimating-natural-convection-heat-transfer-coefficient-on-a-flat-plate/
https://www.physicsforums.com/threads/thermal-values-for-a-double-pane-window.381511/
http://www.physicsclassroom.com/Class/thermalP/u18l1e.cfm
http://www.engineeringtoolbox.com/heat-loss-buildings-d_113.html
k values
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
h value for air
http://www.engineeringtoolbox.com/heat-loss-buildings-d_113.html
It's not because they are better except for the petshops who flog them to every noob reptile keeper who walks in the door.
Lets do some very basic heat transfer calculations ( conduction and natural convection , will ignore radiant heat and forced convection for simplicity ).
CONSIDER A GLASS VIV w/ MESH LID
sides = 0.6m x 0.6m = 0.36m^2 ( 2 of these )
glass surfaces = 1.2m x 0.6m = 0.72m^2 ( 3 of these )
total glass surface area = 4x 0.72 = 2.88 m^2 = A
typical glass thickness = 6.5mm = 0.0065m = t
k value for ( soda ) glass = 0.96 W/m/K approx. @ 30 degC
Assume heat difference warm face to cold face = 35 degC to 10 deg C (very chilly)
==> DT = 25 deg C
Apply Fourier's Law of Heat Conduction loss , Q = kA(DT)/t = 0.96x2.88x25/0.0065 = 10633W
loss by free conductive through "open top" = hADT
h for air = 10 W/m^2K
A = 0.6x1.2= 0.72
so approx. heat loss by warm air escaping through lid = 10x 0.72x 25 = 180W
TOTAL FOR GLASS VIV WITH OPEN TOP = 10813W
CONSIDER A TIMBER VIV
TIMBER ( 3/4 in Plywood , front glass door )
k plywood = 0.14 W/m/K
Area of timber
BTM + BACK + TOP all 0.6x1.2 = 3x0.72 m^2
Ends again 0.6x0.6 = 0.36m^2 each
total timber area = 4x0.72m^2
thickness = 0.75x0.025 =0.019m
Assume 35 degC inside , 10 degC outside , DT = 25 degC (Again)
Fouriers Law again : heat loss through timber surfaces = 0.14x(4x0.72)x25/0.019 = 537.6W
Heat loss through front doors ( simply assume equiv 1 sheet of glass 1.2x0.6 )
Fouries Law again : heat loss through front glass doors = 0.96x0.72x25/0.0065 = 2658W
Total for this viv = 3196W
clearly much more heat must be provided via a basking globe , heatpad , CHE or combination of these to maintain the temperature in the glass viv (which has an essentially open top) cf a timber viv with glass front doors.
The difference in heating required = 10813 - 3196 = 7617 W
Please feel free to comment and let me know if I've made any errors (I've been retired for nearly 20 years and am a wee bit rusty on my heat transfer).
Simple improvement for those stuck with a glass viv with a mesh lid , replace the mesh lid with a plywood lid .
The effect of a 3/4 in plywood lid :
Heat loss through timber lid (same dt, same thickness as above)
= 0.14x 0.72x 25/0.0188 = 134W
cf loss by natural convection through mesh lid = heat loss by warm air escaping through lid = 10x 0.72x 25 = 180W
==> 180-135 - 45W less heat lost simply by covering the top of the glass viv with a 3/4 in thick plywood lid.
Apply a buggerfactor (engineering jargon) to compensate for additional heat loss via radiant heat losses through the opening at the top of approx 1.5 for radiant loss through the opening = (1.5x180) - 135 = 135W !!
This is worth chasing especially in winter.
SOME SOURCES
http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html
http://www.physicsclassroom.com/class/thermalP/Lesson-1/Rates-of-Heat-Transfer
http://www.engineersedge.com/calculators/convection_of_known_surface_area_calculator_10047.htm
https://www.electronics-cooling.com/2001/08/simplified-formula-for-estimating-natural-convection-heat-transfer-coefficient-on-a-flat-plate/
https://www.physicsforums.com/threads/thermal-values-for-a-double-pane-window.381511/
http://www.physicsclassroom.com/Class/thermalP/u18l1e.cfm
http://www.engineeringtoolbox.com/heat-loss-buildings-d_113.html
k values
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
h value for air
http://www.engineeringtoolbox.com/heat-loss-buildings-d_113.html