APPENDIX 2
Cold Box Design and Insulation
The box insulation you choose or inherit will affect several aspects of your box performance.
These are the usable volume you will end up with, the heat load and corresponding energy use,
how long you can expect the insulation to perform, and the cost.
It is easy to multiply the surface area of your box interior by the expected heat gain per square
foot, making assumptions regarding the quality of the insulation and the expected temperature
difference, but such an exercise will leave you under predicting real world heat loads by several
times. There are several reasons for this.
For one, it is interesting to note that not all of the heat that has to be removed from the box
comes through the insulation. A significant portion is caused by the oxidation of the food,
otherwise known as spoilage. Therefore, a box full of food will have a higher heat load than an
empty one. The heat load from this source is less at freezer temperatures than at refrigerator
temperatures.
Other factors will include edge effect, which refers to heat that flows around the edges of
insulation panels, and air leakage through loose fitting hatches and drains, the heat added
through the addition of warm food items and the frequency and duration of opening the box.
Movement of Heat
Some discussion of insulation function and the movement of heat is useful to lay a foundation
for evaluating the many choices available.
The function of insulation is to slow down the movement of heat from the outside to the
interior of the box. Heat energy moves by three mechanisms. These are radiation, convection
and conduction.
Radiation is the electromagnetic transfer of heat energy. At refrigerator temperatures it is the
least important source of heat gain, probably less than three percent of the total heat load on the
box.
Convection is heat transfer through the movement of gas molecules. Good convection flow in
a box will result in a more even box temperature.
Conduction is the transfer of heat through the motion of the molecules in a solid. Most of the
heat flow into a box is through a combination of conduction and convection.
The resistance of an insulation material to heat flow is commonly measured in units called R
value. R value is the reciprocal of the K number. K is defined as the number of BTUs which will
move through one one square foot of one inch thick insulation material with one degree F
temperature difference between the two sides. The larger the K number, the higher the thermal
conductivity of the material. The higher the R value, the better insulator it is.
The R value for a given material is not constant, but varies depending on the temperature the
material is tested at. Doubling the thickness of a material doubles the conductive heat resistance
of the material, but does not double the radiant heat resistance. For practical purposes at
refrigeration temperatures, this difference can be ignored and the R values of like thicknesses of
insulation can be added together.
The measurement of R value can be done in several ways. The most appropriate for our
purposes is ASTM 518 standard for “Steady-State Heat Flux Measurements and Thermal
Page 58
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