Avoid thermal 'short cuts' to improve protection efficiency

INTERTEC offers turnkey protection systems, or components for self-build. If you are confguring your own outdoor protection enclosure, and have arrived at this website looking for components such as heaters, then this short article will help you to understand an important challenge in making your design efficient:

Beware of metal enclosures
Specifying an enclosure for field-based instrumentation is not a trivial task. If the enclosure is destined for an extreme environment - such as desert or Arctic regions - starting such a configuration process with one of the commonplace styles of metal enclosures used for electrical panel gear is usually not such a good solution, and can pose problems for the inexperienced.

The vast majority of metal enclosures are used inside buildings and the biggest thermal protection problems that most users face is working out how to dissipate heat to the outside - which itself is usually an environment with a stable temperature such as a factory building. Some of the major enclosure manufacturers offer software to help designers with heat dissipation issues such as these.

Greater effort is required when the environmental conditions are more challenging, involving protection against extremes of cold such as frost and condensation, or temperature regulation.

Adding insulation may not be good enough
Few off-the-shelf enclosures are available with the appropriate degree of insulation to minimize the temperature regulation problem. In any event, just adding insulation is rarely adequate, because of fundamental limitations posed by the basic metal construction. Heat losses are exacerbated by the good heat-conducting properties of a metal enclosure, and often by the typical kind of metal bulkhead fittings used to mount such enclosures as well - which can act as a kind of rudimentary fin.

Holes can cause problems
Moreover, designers almost invariably need to customize the enclosure by cutting access holes - changing the heat loss characteristics substantially. Holes act as thermal short cuts and can account for a large percentage of an enclosure’s heat losses. Another aspect of these access points to the equipment is either an absence of insulation, or ad-hoc insulation arrangements that are often then incorporated in attempts to maintain a level of thermal protection. It's quite common to leave a hole in the same state that it was cut - presumably to have the flexibility to re-install or modify the equipment at some later commissioning or operational stage. If insulation is used, it's often very rudimentary - such as wrapping some mineral wool around the tube or cable. Ideally, purpose-made insulation components are required and these tend to be specialist items that are not readily available.

In these kinds of situation, the combined effect of all the thermal short cuts can account for as much as 80% of heat losses - and have a major impact on the heating system requirements. As extra holes are often added late in the enclosure design phase, to accommodate last-minute improvements to process connections, their effect on overall thermal performance can and frequently does get overlooked. (Equally for many field enclosures, the thermal short cuts have the same negative impact on cooling performance as outside temperature rises significantly - as it would in daytime desert conditions for instance).

Materials matter
Starting a field protection application with the right kind of enclosure materials and construction techniques makes a big difference to the efficiency of protection. As you may already know from looking around this site, INTERTEC uses GRP (glass fibre reinforced polyester) materials as the starting point for enclosures. Enclosure walls are typically made from two sheets of GRP enclosing an inner layer of insulation. For standard protection applications, polyurethane foam is used for the insulation layer, with a choice of thicknesses from 20 to 100 mm depending on the severity of the environment.

Insulation makes a very big difference in temperature performance. Typically, to maintain the same internal temperature, insulated enclosures require just one-sixth of the heating power of uninsulated enclosures. A second, not so obvious point is that conduction heaters are some five times more efficient than convection heaters. So, if an application only requires part of its components to be protected against low temperatures, it will almost certainly prove more cost-effective in the long run to fit a conduction heater.

However, conduction heaters demand good thermal bonding, which means that the item of equipment to be heated must have a flat surface to ensure adequate and efficient heat transfer. In the case of common forms of process instrumentation, such as a pressure transmitter or regulators, there is usually a manifold (which provides the valves to isolate and bleed the process media so that maintenance such as instrument calibration can be carried out) with flat surfaces that are suitable for mounting a block conduction heater. The heater's output is then localized to protecting the instrument (and the flow of the fluid which might be affected by low temperatures). There are purpose-designed metal interfaces for conduction heaters on the market for some popular transmitters (INTERTEC offers one for Emerson's industry-standard Rosemount 3051 for instance).

Position is important
If, as is more likely the case, you need to heat all the equipment inside the enclosure, or are unable to efficiently couple-up a conduction heater, then you will need to employ a convection heater. Hot air rises of course, so the heater should be positioned horizontally below the equipment, near the bottom of the enclosure. But it is important to note that there needs to be sufficient space beneath the heater to permit the free circulation of convection currents, otherwise efficiency can be severely compromised. A good rule of thumb is to leave a minimum of two inches. If the enclosure is large, and the equipment layout permits, then four to eight inches is even better. If it is not possible to position the heater underneath the equipment, there are models designed for vertical mounting next to the equipment – but again, it is essential to pay attention to the 'circle of convection', to ensure that there is an unimpeded flow of air throughout the interior.

Similarly, if cooling components are also required, careful positioning also makes a significant difference to performance.

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GRP enclosures for outdoor protection
GRP cabinets for outdoor protection
GRP shelters for outdoor protection
Heater components
Cooling components
INTERTEC's turnkey design service, SAFE-LINK

Designers almost invariably need to customize field enclosure by making access holes, changing the heat loss characteristics substantially. (INTERTEC's DIABOX shown.)

Specialized components can minimize the effects of thermal short cuts. INTERTEC's ISOPASS foam molding - with nylon fixings - is drilled to accept tube connections.

Convection heating demands care over layout to aid the flow cycle, as in this transmitter enclosure. (INTERTEC's MINIBOX shown.)

An efficient INTERTEC conduction block heater attached to a flat surface on the block-and-bleed manifold in a field transmitter enclosure. (INTERTEC's DIABOX shown.)

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Die Vermeidung von Wärme 'short cuts' vereinfacht das Design.

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