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Maintaining temperature control is imperative for many industrial processes. Consequently, it is one of the most difficult parameters to successfully control. To achieve operational efficiency and adequate temperature stability, digital control panels require careful consideration. They must withstand environmental conditions while responding to temperature differentials in a way that best suits both the instruments they interface with and the heated medium.

Signal Inputs

Reference signals supplied to the controller include thermistors, resistive temperature devices (RTD), thermocouples and analog inputs. The precision and stability of an RTD allows for unparalleled set-point accuracy, while the extended temperature range and durability of thermocouples make them most suitable for working in aggressive environments or where systems operate over an extended temperature range. Multi-stage control panels may also include analog inputs for monitoring flow rate, liquid level or for interfacing with signal conditioners and other ancillary devices.

Figure 1: On/off control panels (NEMA 4). Source: WattcoFigure 1: On/off control panels (NEMA 4). Source: WattcoDigital controllers from Wattco offer effective closed-loop control of industrial heaters. Process parameters dictate the type of sensor used; Wattco offers controllers with a range of A/D and D/A converters for the flexibility to handle a wide range of signal inputs, including:

  • RTDs — featuring long-term stability and high accuracy
  • Thermocouples — with extended temperature ranges, durability and fast response times
  • Current loop 4-20 mA, analog voltages (mV)

Controller Outputs

The controller can directly or indirectly interface with an industrial electric heater. Low voltage control circuit transformers energize contactors and relays that power external devices, while fused contactors and silicon controlled rectifier (SCR)-based controllers interface directly with industrial heaters.

In either case, it is important that careful consideration is given to VA ratings, voltage drops due to excessive wire leads and the secondary voltage that is present under no load. In compliance with NEMA Standard ICS 2-110, AC contactors close when line voltages exceed 85 percent of its rated value. Incorrect assessment of systems parameters can lead to switch failure, inadvertent cycling or premature device failure.

Wattco offers a wide selection of fused control transformers, fused magnetic contactors, solid-state relays and solid-state power switching devices that are available to handle a wide range of load requirements and switch rates. Fused control circuit transformers provide up to 120 V secondary control voltages, electromagnetic contactors that carry large loads up to 600 V/4,000 A and solid state relays that excel at increased switch rates. Each of these outputs is associated with the following:

Figure 2: SCR direct control panel. Source: WattcoFigure 2: SCR direct control panel. Source: Wattco-Fused control circuit transformers

  • Transformers are selected based on their load capacity
  • Powers external devices such as industrial heaters, contactors, relays and timers

-Fused contactors

  • Electromagnetic contactors carry larger loads up to 600 V/4,000 A
  • Minimum cycle time should be greater than 30 seconds

-Solid-state relays (SSRs)

  • Improved accuracy
  • Switching rates up to one second
  • Minimal noise reduces thermal shock/extends heater life

Wattco SCR-based digital controllers provide for smooth operation with minimal noise and excellent response times. The unprecedented accuracy and repeatability of SCR controllers is due to their ability to compensate for fluctuation in mains voltage and heating-element resistance by regulating the firing angle (phase angle), duty cycle (zero cross) and RMS output current (burst mode).

They are available in single-phase, three-phase and three-leg configurations to handle a wide range of applications from large tank heating to rapid heating of process fluids. These solid-state switching devices accommodate higher switching rates with an extended life expectancy and are ideal for high-voltage devices, rapid heating requirements or when precise control of process temperature is required.

Control Methods

Control circuitry is the brains of the process controller and Wattco’s range of programmable digital control panels provides the most efficient means of establishing closed-loop control of process temperature. Products range from simple on/off control panels that are the most cost-efficient solution when external influences are minimal and set-point accuracy is less critical, to fully functional multi-stage control panels with proportional, integral and derivative (PID) control circuity, step controls and highly-precise solid state switching devices.

On/Off Control

On/off control circuitry includes a programmable set-point and an adjustable hysteresis band. The simple circuitry provides for two-position control that results in a cyclic temperature profile. A narrow hysteresis band can be used to mitigate hunting at the expense of increased switch rates and continual cycling of the system.

Proportional, Integral and Derivative (PID) Control

Proportional control signals increase temperature stability by decreasing the system’s response as a set point is approached. A variable output helps alleviate hunting or cyclic temperature profiles, although systems that solely operate with a proportional control signal tend to stabilize at an offset temperature.

Integral control actions take into account the size and duration of the temperature offset. They force the temperature profile to achieve a target temperature by taking into account both the duration and magnitude of the temperature differential. Use of integral control actions when accompanied by a proportional output can provide for increased temperature stability and set-point accuracy.

Derivative control signals provide for additional control action when dealing with external influences. Constant temperature fluctuations complicate system responses, and use of a derivative control signal can help reduce the magnitude and duration of the temperature offshoot by analyzing the rate of change.

PID controllers are weighted based on system parameters to optimize stability, increase set-point accuracy and rapidly respond to external influences. These advanced control functions are ideal when interfacing with devices that support rapid switching rates like Wattco’s line of SCR direct control panels.

Wattco Multi-stage Control Panels

Wattco’s multi-stage control panels are designed for complex systems with isolated outputs and step-control functions. They are designed to handle multiple process loops with contactors and fuses for multiple load variations. Multi-stage control panels allow operators to control multiple heating elements and other system parameters, including flow rates and tank level. They are also optionally supplied with SCR solid state power control for fine-tuning process temperature.Figure 3: Explosion-proof temperature control panels. Source: WattcoFigure 3: Explosion-proof temperature control panels. Source: Wattco

Operating Conditions

All of Wattco’s digital control panels are housed in NEMA-rated enclosures. Whether operating in an indoor environment, outdoor environment or in a hazardous location, Wattco designs digital control panels to best suit customer needs.

  • NEMA 1 terminal boxes — 306 stainless-steel enclosures suitable for indoor, non-hazardous operation
  • NEMA 4 terminal boxes — (weather-resistant enclosure) aggressive environments and outdoor operation
  • NEMA 7 terminal boxes — explosion proof/hazardous locations

Digital Control Panel Solutions

Digital control panels are complex components. Proper selection requires in-depth knowledge of the application, environmental conditions as well as the associated instruments. Wattco digital control panels are fully customizable to meet the needs of a diverse range of industrial processes. Contact a Wattco representative today to determine how their solutions best fit your needs.