Cookies help us deliver our services. By using our services, you agree to our use of cookies.

11. Temperature control systems

Chemical reactions are often very sensitive to temperature changes. The ability to influence the temperature in a reactor is therefore most important for the chemical industry. 

Chemical reactions are often very sensitive to temperature changes. The ability to influence the temperature in a reactor is therefore most important for the chemical industry. Depending on the kind of chemical reaction taking place, adding or rejecting heat using some sort of heating or cooling medium is necessary. Temperature control units are used to control temperature precisely. These compact units compare actual temperatures with the desired temperature and combine a heating unit and a cooling unit for water and/or oil. Temperature control units are used mostly for controlling the temperature of molds, tanks, rollers or cylinders. Often, quite complex systems are needed for tempering a reactor. The more straightforward its layout, the easier the system is to maintain.

To control process temperature accurately without extensive operator involvement, a temperature control system relies on a controller that accepts a temperature sensor such as a thermocouple as input. It compares the actual temperature with the desired control temperature, or set point, and provides an output to a control element.

The output of a temperature controller is connected to the injection mold, extruder, roller, storage tank, etc. The fluid running through the temperature control unit circuits is treated (purified) in a separate circuit (see Water as a heat transfer fluid --> Purification Processes and Water Types). Oil or raw water is used on the primary side of the BPHE.

There are three basic types of controllers: on/off, proportional and PID. On/off controllers are the simplest form of temperature control units. The output from the device is either on or off, with no middle state. An on/off controller will switch the output only when the temperature crosses the set point. On/off control is usually used where precise control is not necessary, in systems that cannot tolerate the energy being turned on and off frequently, where the mass of the system is so great that temperatures change extremely slowly, or in temperature alarms.

Proportional controllers reduce the average power supplied to the heater as the temperature approaches the set point. This reduces the heater’s output to avoid overshooting the set point, but allows the set point to be approached, thereby maintaining a stable temperature. The temperature can be controlled by turning the output on and off for short time intervals to vary the ratio of “on” time to “off” time to control the temperature, a process known as time proportioning. The proportioning action occurs within a proportional band around the set point temperature. Outside this band, the controller functions as an on/off unit, with the output either fully on (below the band) or fully off (above the band).

The third controller type combines proportional control with integral and derivative control, and is known by the abbreviation PID. This controller combines proportional control with additional adjustments that help the unit to compensate automatically for changes in the system. The PID controller provides the most accurate and stable control of the three controller types, and is best used in systems with a relatively small mass and those that react quickly to changes in the amount of energy added to the process. A PID controller is recommended in systems where the load changes frequently and the controller is expected to compensate automatically for frequent changes in the set point, the amount of energy available or the mass to be controlled. Figure 11.1 shows examples of different compact temperature control units.

11.1Figure 11.1 Temperature control units for water and oil (Courtesy of Regloplas)


Figure 11.2 below shows the principal components of a controller for water up to 90 ºC and oil up to 150 ºC. The BPHE (1) acts as a cooler. After being cooled, the fluid flows into a vessel (2), which is controlled by a level controller (3), a valve for automatic water refill (4), a safety thermostat (5), a heater (6) and a temperature sensor (7). If the temperature in the vessel exceeds the set point, the heater turns off and cold fluid flows through a valve (4) into the tank. If the temperature measured by the temperature sensor (7) is then still too high, a bypass (9), consisting of a safety valve, opens and the fluid is regulated until it reaches its desired temperature. The temperature range for these systems is calibrated by an attached controller.

11.2

Figure 11.2


SWEP solutions for TCUs

Some 90% of temperature control units are found in the plastic injection-molding industry. This is a single-phase application, but with its strong connection to thermo and circulating chillers it is normally produced by the same companies. The two main markets are pressurized water or thermal oil (up to 160 °C) and non-pressurized water (up to 90 °C). SWEP BPHEs are used for heaters and coolers. We also offer a unique BPHE solution, the T-Reg, for injection-molding applications in the plastics industry. 

The purpose of the T-Reg is to provide accurate and fast temperature control to the plastic molding process:

  • Heat is provided to give the plastic paste the right viscosity to be transported into the plastic mold.
  • Cooling is then provided to the mold to harden the plastic.
  • Accurate and fast temperature control is of the utmost importance in this application in order to be able to reach a high productivity level with excellent quality. 

This picture shows an example of a plastic molding process.
The T-Reg is situated within the temperature control unit (TCU) provided by our customer.

treg

In the T-Reg, several parts of the old TCU system have been integrated into one unit.

This means the cooling heat exchanger, the electrical heater, the piping between the two and the air ventilation have all been combined into one compact unit.
The T-Reg consists of a cast frame brazed together with a 24-plate E5T BPHE.

The process water flow enters the top of the unit and passes through the heater, where it is heated to a specified temperature.
If a high temperature is desired, the process water exits the T-Reg unit in the bottom connection on the frame (P4).
If cooling is desired, the heater is turned off and the process water flows though the heat exchanger package, where it is cooled with cooling water before exiting the unit in the F2 port.

terg inlet outlet

 

More performance from less material
The innovative T-Reg integrates the electrical heater and the BPHE, saving piping and space and minimizing the risk of leakage between the heat exchanger and the heater.

Better performance from a smaller volume
The system must be compact. Combining the electrical heater and the heat exchanger gives a compact unit with a volume at least half that of the current system set up.

Higher temperatures can be reached
The T-Reg improves the TCU’s performance by allowing higher temperatures. The high temperatures can be achieved using a pressurized system. Up to 160 ºC can be reached at a pressure of 16 bar.

Better temperature accuracy
The T-Reg can handle very high and low temperatures, and offer the possibility of mixing both so that different temperature levels can be reached, giving better temperature accuracy.

Flexibility
The T-Reg can be used with both open-loop and direct systems. The open-loop system’s risk of scaling is avoided effectively in the T-Reg because of its ability to split the water flows of the hot and the cold side internally in the heat exchanger. This allows constant full flow for the cooling media, keeping wall temperatures below the critical temperature and minimizing the scaling effect.

A reactor is cooled or heated by an ethylene glycol/water circuit, which flows from various sources into a reactor jacket. If the cooling and heating fluid entered the jacket directly, without flowing through heat exchangers first, the jacket would have to be emptied of one fluid before the next could be let in to achieve the correct temperature. The system would therefore respond very slowly. Furthermore, there is the potential for error by filling the jacket with the wrong fluid. Having intermediate heat exchangers for the different flows solves these problems. In the layout shown in Figure 11.3, four flows, containing water and ethylene glycol at different temperatures, can be mixed to supply the jacket around the reactor with liquid at the desired temperature.

11.3
Figure 11.3 Reactor temperature control system including steam unit with four BPHEs in parallel.

The different temperatures of the four flows are obtained by exchanging heat in four different heat exchanger units. The first heat exchanger unit cools the fluid to -20 ºC using another ethylene glycol circuit from a chiller unit. The second heat exchanger cools the fluid to 10 ºC with city water. Water from a cooling tower is used in the third BPHE to produce a liquid temperature of 30 ºC. The last unit consists of four BPHEs in parallel with steam connections on the front and water/ethylene glycol connections on the back. The liquid is heated to 138 ºC using steam at 143 ºC. The temperature of the flow entering the jacket is controlled by mixing it with other flows (e.g. the cooling tower water). Gasketed heat exchangers are not recommended, because the gaskets would melt due to the high steam pressure. Figure 11.4 shows the four parallel-mounted BPHEs for the steam application.

11.4Figure 11.4 BPHE type: B50x80