When designing or configuring enclosure protection for sensitive field-based equipment, it pays to consider the entire system—including the enclosure material, its insulation and heat transfer characteristics—rather than focusing simply on specifying appropriate heating (and/or cooling).
Treating the design problem as an inter-related system yields significant benefits for outdoor enclosure applications, especially in harsh environments. For example, a holistic approach has particular benefits in avoiding the 'cold-spots' that can quickly lead to problems from condensation. Plus, it delivers much more stable and controllable operating environments for instrumentation, which can be important in many process control applications, such as process analyzers.
Insulation is key
Specifying an enclosure for sensitive field-based control and instrumentation equipment is not a trivial task. Good insulation is critical to almost all outdoor equipment protection applications. If the enclosure is destined for an environment with extreme conditions, such as a desert or an Arctic region, starting the configuration process with one of the common styles of metal enclosures used for electrical panel gear is usually not such a good solution, and can pose problems for the inexperienced.
The 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 exterior, which itself is usually an environment with a relatively stable temperature such as a factory building.
Greater care is required when the environmental conditions are more challenging, involving protection against frost and condensation, extreme cold or heat or requiring temperature regulation.
Few off-the-shelf metal 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. Any metal connection between outer and inner shell provides a thermal short-cut. With metal construction, it is almost impossible to avoid metal parts in some design elements (such as the door frame, door leaf, window, wall penetrations for cables and tubing etc) because the stability of this type of housing is based on bent sheet metal, and insulation materials are typically soft. 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.
Moreover, designers almost invariably need to customize enclosures by cutting access holes, changing 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 is either an absence of insulation, or ad-hoc insulation arrangements that are often then incorporated in attempts to maintain a level of thermal insulation. 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 (Figure 1).
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, for example, in daytime desert conditions.
Starting a field protection application with the right kind of enclosure materials and construction techniques makes a big difference to the efficiency of protection. Intertec uses GRP (glass fibre reinforced polyester) materials as the starting point. Enclosure walls are usually 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 (Figure 2).
Insulation makes a 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.
Well-insulated enclosures with marginal thermal leakage play the key role in delivering a highly stable and slowly changing operating environment for sensitive field equipment. This helps to optimize the performance of the electronics or instrumentation system, while also minimizing energy consumption.
What is happening to the environment inside the enclosure? For an enclosure that is heated—to protect it from a cold external environment—there is a flow of thermal energy from the heater to the enclosed internal space, and from there to the external ambient via losses though the structural fabric of the enclosure, and any thermal short cuts (Figure 3).
In the steady state at constant temperature, these flow rates are constant. Although a majority of outdoor enclosures usually require some form of heating to protect against frost or harsher low temperatures, advanced insulation is equally applicable for enclosures that require cooling to protect equipment in hot climates.