MICRO-AIR Artic Air 24 Guia de Instalação descarregar pdf (Página 61)

A plot of the energy use with increasing insulation thickness curve, would show that the

amount of energy saved as insulation thickness increases also tails off past a certain point.

More insulation is good, but a point of diminishing returns is reached as more insulation is added.

The heat load is not evenly distributed in the box. The temperature differential is greatest at

the bottom of the box and for this reason the bottom generally carries the greatest heat load. It

makes sense to install an extra inch or two of insulation at the bottom of the box, and if

necessary reclaim the extra space by using less at the top.

Notice that a freezer of a given size will have a lot more heat to remove than a refrigerator.

Keep in mind that it also takes relatively more power to remove each BTU of heat at the colder

temperature.

Super Insulation

Many boats simply do not have room to apply the insulation thickness guidelines above and

still have room for a box of the desired volume. So called Super Insulation, or vacuum insulation

panels (VIPs) can provide the answer. There are two basic types of VIP on the market at

present. The first type of panel uses a special one inch thick foam core from Dow Chemical,

called Instill™.

Dow has recently discontinued the manufacture of these foam cores and when the existing

inventory is gone, these panels will no longer be available.

The foam cores for this type of panel are wrapped in a vacuum barrier membrane material.

When the panels are pulled down in a vacuum and sealed, they achieve an insulating value

close to R 30. This product was originally developed for shipping applications and was

reasonable in cost.

On the down side, the vacuum membrane around foam core panels punctures easily. Even if

the membrane can be maintained intact, the internal pressure rises over time. The pressure rise

occurs because of the slow diffusion of air through the membrane and possibly some outgassing

of the foam material comprising the core. The insulation value of these panels goes down to

about R 3 per inch when they are no longer holding a vacuum. They still may be a good option if

only short term performance life is required or provision can be made in the box design to easily

replace the panels. There are some technical options to increase the service life of these panels

such as adding packets of a chemical called a getter when the panel is manufactured. Even so,

there were no manufacturers of this type of panel that offered a performance life warranty.

A second VIP technology is available that uses aerogel as a core material. Aerogel is a

powdery silica based material which has an R value of around 9 per inch at atmospheric

pressure. Glacier Bay, Inc., uses this core material in BARRIER ULTRA-R super insulation

panels. BARRIER ULTRA-R panels have an R value of 50 per inch. They come with a 25 year

performance warranty against loss in R value. This is possible because the aerogel core

chemically adsorbs gas molecules that pass through the vacuum barrier membrane. This getter

activity allows the panel to maintain its high vacuum level and R value over an extended time.

Boxes built with BARRIER ULTRA-R have a total wall thickness of about two and one quarter

inches. Although the initial cost of this material is higher than other insulation options, it is often

chosen for new boat construction and box retrofit projects because usable space can potentially

be doubled or more for a given external volume. When coupled with the long performance life

and superior energy performance, it may offer the most value.

Both types of VIP panels have to be custom built to fit the application. If they were to be

cut or punctured after manufacture, they would lose a high percentage of their performance. Use

of these high tech materials is not an either/or option. VIPs can be used in hatches or high heat

load areas while the rest of the box is constructed of conventional insulation.

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