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

PostPosted: Sun Sep 03, 2017 3:44 am
by kingofnobbys
[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

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

PostPosted: Sun Sep 03, 2017 5:07 pm
by CooperDragon
I bought mine because it was large, required no construction, was relatively affordable, and had front opening doors. At the time it made sense to get but I was a noob reptile keeper... I'd probably attempt to build my own if I had another shot at it but I'm pretty awful at that sort of thing. I leave the top off completely. It certainly doesn't hold in much heat but I don't expect it to. I just let the overhead lighting keep things in appropriate ranges. The power of my lighting is probably much more than would be needed for an enclosed wooden viv. I do like having the top open so I can raise the lighting as high as I want. That allows me to change the size of the basking area and keeps the lights well out of the way of climbing areas.

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

PostPosted: Tue Sep 05, 2017 9:01 am
by charmander16
Wood is clearly better for larger vivs, but for smaller starter vivs glass makes sense. I think 40 gallon and less vivs could easily overheat with the typical bulbs used.

The larger the enclosure the more better insulation makes sense. I think 40 gallons is about as large as you would want an all-glass enclosure. You can of course make it work with larger all-glass enclosure's is just requires more heat.

But that's why most people do use "wood" (or wood like) material for larger vivs.

The real issue is not so much material, but rather starting people off on smaller vivs to begin with.

Ideally, people would start with a minimum of 8 square foot enclosures, because that's saves money in the long-run, but that's a more expensive way to start and it's easier for sellers to get people into the hobby at the lower starting cost and with a smaller, less imposing, viv.

And you see that here on the boards and it happened to me too. I think many fewer people would buy a bearded dragon if it was clear that the needed to start off with a minimum 4 foot long, $400-$500 (new including lights) setup.

Those starter setups cost about $150 to $200, which gets more people in the door, but really its $150 of wasted money because virtually everything needs to be replaced within 6 months.