### Heat Losses for a 1.2m L x0.6m W x0.6m T viv

Posted:

**Sun Sep 03, 2017 3:44 am**[TOPIC AUTO-LOCKED DUE TO INACTIVITY]

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/conve ... d_430.html

http://www.physicsclassroom.com/class/t ... t-Transfer

http://www.engineersedge.com/calculator ... _10047.htm

https://www.electronics-cooling.com/200 ... lat-plate/

https://www.physicsforums.com/threads/t ... ow.381511/

http://www.physicsclassroom.com/Class/t ... u18l1e.cfm

http://www.engineeringtoolbox.com/heat- ... d_113.html

k values

http://www.engineeringtoolbox.com/therm ... d_429.html

h value for air

http://www.engineeringtoolbox.com/heat- ... d_113.html

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/conve ... d_430.html

http://www.physicsclassroom.com/class/t ... t-Transfer

http://www.engineersedge.com/calculator ... _10047.htm

https://www.electronics-cooling.com/200 ... lat-plate/

https://www.physicsforums.com/threads/t ... ow.381511/

http://www.physicsclassroom.com/Class/t ... u18l1e.cfm

http://www.engineeringtoolbox.com/heat- ... d_113.html

k values

http://www.engineeringtoolbox.com/therm ... d_429.html

h value for air

http://www.engineeringtoolbox.com/heat- ... d_113.html