High temperature heating in hazardous areas
02 February 2009
Martin Hess of Intertec provides an overview of key points to consider when specifying or building heating equipment for process instrumentation. Hess explains why it is advisable to consider and specify the enclosure, the so-called 'hot box’, and the heating equipment as an inter-related system.
High temperature heating in hazardous areas
For instrumentation in chemical and petrochemical plants, and especially in sample conditioning systems for analysers, the maintenance of an elevated temperature is often essential in order to prevent the condensation and/or crystallisation of media.
Complete insulation of the measurement and analysis station is rarely possible due to the requirement for maintenance and calibration. Some users create ad hoc solutions in the form of a 'hot air shower' where a fan, or compressed air, warms the local environment. These can be very energy intensive and can be difficult to build for hazardous areas, and are not usually suitable for applications demanding closely regulated temperatures.
The optimal solution is a so-called 'hot box' consisting of an enclosure for the equipment in question, fitted with a heating element and controller. For best results when building a hot box, care needs to be taken with the design: choosing and considering the elements as an inter-related system, rather than simply assembling the box from a collection of discrete components.
Most media in chemical and petrochemical plants is explosive; consequently, heaters must be certified for use in hazardous areas according to the standards and laws of the country. Today, the rules that apply in most countries around the world are based on the IEC 60 079 standard, which has been developed from the European ATEX standard, EN 50 014 ff. Heaters for use in the USA and Canada must be certified according to different standards: NEC or CEC.
Regardless of what standards apply, explosion proof heaters are something of a paradox: they must heat to high temperatures, but they may not get hot. Surface temperatures may not exceed the temperature class. Internal temperatures may not exceed the rated material temperatures.
With the exception of self-limiting heaters — which in practice are limited to low power applications — every explosion proof heater must be fitted with a temperature limiter that may not be reset.
For safety, there is a margin between the temperature class and the switching temperature of the limiter, and there has to be a margin between the peak maximum temperature of the heater and the switching temperature.
If the heating system is not controlled well and/or laid out appropriately, there is a risk that the heater's internal temperature limiter will switch and the heater fails and must be replaced.
With any heating process, there is a constant flow of thermal energy from the heat source to the room that shall be heated, and from there to the ambient. In the steady state flow rates are constant.
The maximum temperature of the heater is determined by the temperature limit required, and the ambient temperature — which depends on the plant's location. The heater needs to be specified for the worst case. Thermal flow can be usefully considered as an electrical model, in a serial configuration with two temperature drops , with the heat output divided between them in the same way as a potentiometer divides a voltage. Equipment temperature depends on the relation of: heat transfer resistance between the heating element and the equipment that has to be maintained at temperature, usually the tubing and devices such as regulators that contain the medium, and heat transfer resistance between the equipment and the ambient.
The heat transfer ratios are what matters. If the enclosure is well insulated, with no large heat losses, then the heater output needs only to be in a certain power range to make precise temperature control easy. Over-specifying the heater power can cause problems.
Achieving a stable and well-managed thermal environment depends on a well-insulated enclosure with no large heat losses. The usual way of achieving this is to improve the insulation by adding a thick layer of material such as PUR foam. But, the better the insulation of the walls, the more important the effect of any heat sinks.
When designing a hot box, the construction materials and processes of an enclosure become very important. Steel enclosures are made of materials with high thermal conductivity. Any metal connection between outer and inner shell provides a thermal shortcut. And with metal constructions, it is almost impossible to avoid metal parts in some design elements because the stability of this type of housing is based on bent sheet metal, and insulation materials are typically soft.
An interesting solution to manufacturing enclosures without thermal sinks is a sandwich-construction housing based on glass-reinforced polyester (GRP) sheeting surrounding an inner layer of insulation. The use of long fibre reinforced GRP sheeting provides great structural strength, combined with high resistance to weather and the corrosive effects of aggressive chemicals. All the construction materials have a low thermal conductivity, and the sandwich construction means that enclosures can be made using bonding techniques, avoiding heat short cuts between inside and outside shells. The overall thermal insulation of GRP sandwich shelter is typically more than twice as good as a steel shelter with the same insulation thickness.
Heat transfer efficiency from the heater to the equipment should be as good as possible. The choices available are conduction, convection, or a combination.
Conduction is a very effective heat transfer method. As air surrounds the equipment inside the enclosure, it acts as a good insulator. As a rule of thumb, conduction is roughly five times as effective as convection, so conduction is usually the preferred method if it is possible to use it. Conduction heaters have a flat surface for good heat transfer. Therefore the equipment to which they are fastened must have good flat surfaces for proper heat transfer. Other equipment gets its heat by conduction through the fittings and tubing.
When positioning the conduction heating element(s) it is very important to get most heat to the equipment in the cabinet that acts as a heat sink, such as a regulator. For the effective heating of gas cylinders special conduction heaters with a cylindrical surface, called Cylindertherm, are available.
Often, the equipment that has to be heated has no flat surface, and is not suitable for conduction heating. Another problem can arise when a cabinet contains numerous items of equipment that need heating. In this case, the air has to be used as the transport medium for heat.
Natural convection is powered by difference in density: hot air is lighter than cold air, and therefore floats upwards. Care needs to be taken when using convection heating, to position the convection heater for optimum performance. Key factors include: installing the heater below the critical equipment; aligning heater fins vertically to optimise the convection process and choosing a cabinet with enough space, and positioning the heater and equipment to aid air flow.
In some situations, especially when equipment needs to be maintained at a temperature close to the level of the temperature limiter, it makes sense to combine both heating principles to provide an efficient heating cycle.
The temperature difference between the heater temperature and the temperature inside the cabinet, known as delta T, is finite and limited. The higher the temperature that needs be maintained in the enclosure, the closer that temperature is to the shutdown limiter level, and in these situations the efficiency of the controller becomes very important.
Thermostats are commonly used as a temperature control mechanism for lower temperatures, and where precise temperature control is not required. However, the way they operate imposes limitations when control at higher temperatures is required.
Speaking generally, thermostats are adequate for controlling temperatures up to around 40 degrees C. For higher temperature applications, a controller with a continuous action is the preferred solution. A 'smart heater' combines a PID controller with an electronic limiter for the heater temperature.
However, as discussed, such precise control also relies on the overall efficiency of the other elements of the complete hot box system: a well insulated enclosure without heat sinks, and a heater with good heat transfer. Considering these three elements, controller, enclosure and heat transfer, as an inter-related system will yield great benefits when specifying and building hot box systems for high temperature hazardous area applications.
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