Known from the state of the art are field devices, which are used in industrial plants. In process automation technology, same as in the manufacturing automation technology, field devices are often applied. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. Thus, field devices are used for registering and/or influencing process variables. Serving for registering process variables are measuring devices and their sensors. These are used, for example, for pressure- and temperature measurement, conductivity measurement, flow measurement, fill level measurement, etc. and register the corresponding process variables, pressure, temperature, conductivity, pH-value, fill level, flow etc. Used for influencing process variables are actuators. These are, for example, pumps or valves, which can influence the flow of a liquid in a pipe or the fill level in a container. Besides the above mentioned measuring devices and actuators, also referred to as field devices are remote I/Os, radio adapters, and, in general, devices, which are arranged at the field level.
A large number of such field devices are produced and sold by the Endress+Hauser group of companies.
Many field devices for process automation are exposed to various environmental influences, depending on their location of use. In different branches of industry, different environmental conditions prevail, requiring appropriately different protective measures.
In process automation, especially in hygienic applications, processes are frequently run, in which high humidity is present. Steam deposits as condensate on surfaces of the process plant. This condensate can damage the plant and especially the field devices located therein. Field devices with metal housings can corrode from longer contact with water. If steam gets into the interior of the housing and condenses there, the functionality of the field device can be degraded. For example, condensate can accumulate on the electronics and cause defects and/or corrosion. Furthermore, short-circuits between the electronics and the housing, caused by the moisture, can arise and limit the ability of the field devices to function.
Pressure measuring devices, especially relative pressure measuring devices, require a reference air duct for correct operation. This is led into the housing and is in contact with the environment by means of a pressure equalizing element. If condensate deposits on this pressure equalizing element, then, in given cases, the air supply into the reference air duct is blocked. As consequence, defective pressure values are measured.
Condensate occurs when the temperature subceeds a threshold temperature, the so-called dewpoint temperature, also referred to as the dew point. The dewpoint temperature of a gas mixture is determined by the percentage of a condensable component. The gas mixture can be, for example, air, in the case of which the condensable component is water. The higher the percentage of the condensable component in the gas mixture, the lower is the dewpoint temperature.
If a process has one or more phases with low process temperatures, for example, cooling phases, then in these phases there is increased danger of condensation.
An opportunity for protecting field devices from condensation is to change the dimensions of the housing. With a greater housing length, sensitive components, such as, for example, the electronics or the pressure equalization element in the case of pressure measuring devices can be located removed from the process connection, so that low temperature does not reach these components. This is, however, associated with increased material costs. Moreover, space is limited in many process plants, so that the housing dimensions have to be as small as possible.
Another possibility is to decouple the process temperature from the sensitive components. This is done via so-called temperature decouplers. Temperature decouplers are housing sections with special forms, for example, ribs or constrictions. Disadvantages of this method are, depending on form of embodiment, a low efficiency and increased effort in the case of the housing development.
An effective technique for avoiding condensate on the housing is to keep the temperature of the housing higher than the dewpoint temperature.
DE102013108531A1 describes a field device with integrated heating element and temperature control loop. This field device is applied in the case of very low temperatures in the range of −40° C. to −60° C.; the heating element, in such case, protects temperature sensitive components, such as, for example, microcontrollers, from failing due to these lows temperatures. Disadvantages of this technique are in the increased effort involved with the control loop, the additional manufacturing costs and the increased electrical current consumption.