THERMOSTATS, LIMITERS, SUPERVISING, ALARM
The market of today offers a large number of thermostat/regulator types and computer based control systems.
This gives possibilities to exquisite functions for high and low alarms, temperature readings, logging of temperatures and ratings and control with miscellaneous control curves and soft starts with learning functions which neutralises overshoot at start-up. Different applications require different accuracy and by that suitable equipment is chosen.
Ex-areas requires increased safety and high sealing classes.
Thermostat/limiter used in hazardous areas can be designed in different ways:
- Mechanical thermostats
- Electronical thermostats
- Mechanical thermostats
Mechanical thermostats use the expansion in fluid included in a bulb, which is transferred via a capillary tube to a membrane, which influence the electrical contact of the thermostat. The capillary tube has often a max. length of approx. 3 meter.
- Electronical thermostats
An elecronical circuit influence a relay which control the heating cable directly or via a contactor (at loads over 2 kW, a contactor is normally required). The electronic circuit is connected to a temperature sensor (NTC, PT-100 or thermocouple) in a low-tension connection (as a rule 6-12V) and to these there is zener barriers available when the transmitter is placed in a ex-area.The advantages with these thermostats compared to capillary thermostats are partly that on large distances and those electronical thermostats most often has a larger precision.
Electronical thermostats can be obtained with a couple of extra refinements such as alarm, adjustable after-effects, temperature reading etc.
The regulators are the most advanced form of control system. They are built upon the same principals as the electronical thermostats but here you can choose time proportional control via solid-state-relays which is of advantage in soft starts etc. Memory based functions to avoid overshoot at start etc.Regulators is also available with several channels and with external connection to a computer for example, where reading and set-up can be managed from optional place on earth (where a operating thread/mobile telephone is available) via a modem.
Such a function can give great benefits at for example un unmanned depot for thick oil where the heat on loading shall be connected a couple of days before loading from incoming boat can be done. Here is often possibilities to connection of all types of transmitters which can be of a great advantage at reparation of old applications where existing transmitters are difficult to reach.
VÄRMEKABELTEKNIK have a complete program from Velox with advanced products for the industry.
CONTROL HEATING CABLES
Parallel resistive cable
The output of the parallel resistive cables are independent of the temperature, they can of that reason not be temperature rated. Parallel resistive cables shall therefore always be equipped with a overheating protection, for example a thermostat which in the same time can control the operating temperature. Each circuit is supplied with at least one thermostat.
Parallel resistive cables have no general approval for ex-surfaces and exemption must be applied for each application.
Self-limiting heating cables
The self-limiting cables have in the ex-approval been supplied with a T-class, which states the maximum sheath temperature the cable can accept during all circumstances.
Cables with a semiconductor core reduces its output with rising temperature but this does not mean that the cable control an application temperature. Heating cables never comes down to an output of 0W and this means to be able to maintain a required temperature on a piping, a control device should be supplied.
The control device can consist of a thermostat or a regulator with a temperature transmitter mounted against the heated object.
Series resistive heating cables
All series resistive cables require a thermostat. The need is equal with parallel resistive cables.
PLACEMENT OF THE THERMOSTAT TRANSMITTER
Thermostats can be used in different ways, as already been determined:
- To maintain a pre-set temperature.
- Switch on/off frost protection functions.
- As overheating protection (limit the sheath temperature on a cable in ex-applications or to protect the cable against overheating).
- To protect applications (plastic material).
- To protect temperature sensitive products.
The process of flow the pipe system, lengths with in/outdoor length and high risings with chimney effect, which as consequence is important details to determine the placement of the temperature transmitter.
Example 1 – placing thermostat transmitter
Heating cable application with maintenance temperature of 75°C on pipe
Direction of flow
In an application as in the sketch above, the transmitter shall be mounted near the heated tank in order to break from feeding to the heating cable at the percolation from the tank to protect against overheating.
PLAN THE APPLICATION
In at system with varying ambient temperatures, large risings and many branchings or if the temperature is close the upper limit for the max. temperature of the cable, it can be suitable to divide the application with separate cables and temperature transmitters for each reference area, respectively.
When designing a heating cable application one must consider to some parts of the pipe system such as expansion vessels, fillings and drainage’s not have part in the normal circulation in the system. This brings you to supply these branchings with a separate circuit with a separate temperature control.
The reason is when operating the other system, heat is spread with the circulating fluid, while the above mentioned parts has a not moving content and is not influenced by supplementary heat from processes etc.
VÄRMEKABELTEKNIK offers a large assortment of control cabinets in standard designs. We construct and also build special cabinets after your requirements and specifications.
The standard cabinets are available with electronic thermostat and ice-/snow alarms for roofs and ground surfaces.
Special cabinets can be supplied with communication possibilities for DUC, alarm senders, alarm functions for earth faults, released fuse current sensing alarm or another special function. Control can be done with thermostat, simple or with multi-channel regulator with or without telecommunication via PC.
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