Open bath temperature control units

A temperature control unit, or heating/cooling unit, is a device used to bring the consumer to which it is connected to a specific operating temperature and hold that temperature constant by heating and/or cooling a circulating fluid heat transfer medium.

A pump forces the heat transfer fluid from a tank with built-in heater and cooler through the consumer, from which it flows back into the tank. A temperature sensor measures the temperature of the heat transfer fluid and transmits this value to the controller in the control section. The controller regulates the temperature of the heat transfer fluid and thus indirectly that of the consumer. If, during production, the temperature of the consumer rises above the controller‘s set-point value, the controller opens the solenoid valve to the cooling water mains and keeps it open until the temperature of the heat transfer fluid, and thus the consumer, drops back to the set-point temperature. If the consumer temperature falls below the set-point temperature, the controller triggers the heater.

Temperature control units are subdivided according to the heat transfer fluid used: water or thermal oil. Temperature control units using water usually have a maximum outlet temperature of 90 °C (approximately 160 °C for pressurised water units). Temperature control units using thermal oil without a pressure blanket have a maximum outlet temperature of 350 °C. 

2.1 Temperature control units with bath heating
2.1.1 Principle of operation

In bath heating, the circulating water or thermal oil is in direct contact with the oxygen in the air. The tank functions simultaneously as an expansion vessel.

2.1.2 Application
Bath heating is suitable for temperature control units using water and thermal oil at moderate outlet temperatures, i.e. up to 90 °C for water and 150 °C for thermal oil. At temperatures above 90 °C, water vaporates very quickly; therefore pressurised water units are used for higher temperatures. At temperatures above approximately 150 °C, thermal oil also tends to produce vapour. Oil vapour is malodorous and can be flammable under certain conditions. For more detailed information, see “Thermal oils”, on page 99. Temperature control units with bath heating are therefore not recommended for use at temperatures above 150 °C.

2.1.3 Configuration/method of operation 
Temperature control units are normally constructed as follows: An immersion pump (usually a centrifugal pump) is located on top of the tank. The advantage of mounting the pump vertically on the tank is that no leaks can develop, because the pump is completely immersed in the heat transfer fluid. An immersion heater is located in the tank with fluid flowing around the heater. When using thermal oil, the following points must be taken into consideration (not critical for water): 
The correct ratio between heating capacity (specific heat load) and pump capacity (flow rate at the heating surface) is an important factor in the service life of the oil. The greater the specific heat load and the lower the flow rate, the greater the thermal load on the thermal oil and thus the shorter its servicelife. Another factor in the length of the oil’s service life is the magnitude of the outlet temperature.An inappropriately configured temperature control unit can cause the thermal oil to break down within a short time, drastically shortening the service life of the oil.

The products of the oil‘s decomposition settle on the surface of the heating  elements, forming an insulating layer that will destroy the element over time. The film temperature is an important parameter in the selection of the appropriate heater. The film temperature is defined as the temperature of the heat transfer fluid in immediate contact with the heating surface.

To prevent thermal stress in the oil, the temperature within the unit (at the heater) shall not be permitted to exceed the maximum film temperature specified by the manufacturer of the thermal oil. Because the configuration of temperature control units with bath heating prevents the exact flow conditions at the heating surface from being known, the film temperature cannot be calculated. Therefore, the specific heat load must be kept to a minimum,  generally below 2.5 W/cm2. Although the specific heat load is not the only factor in determining the ideal configuration of the temperature control unit or in the length of the oil‘s service life, a low heat load is always preferable to a higher one in bath heating. The fact that flow conditions in bath heating are undefined is one reason some manufacturers of temperature control units, including Regloplas, recommend bath heating of thermal oil only for temperatures up to 150 °C. However, bath heating is comparably inexpensive and perfectly suitable for temperatures up to 150 °C.

The cooler is a spiral tube built into the tank. The cooling water circulates through the tube, and the heat transfer fluid flows around it. Units using water as their heat transfer fluid tend to use plate heat exchangers instead of cooling tubes. The advantages are a high cooling capacity at relatively small dimensionsas well as good exchangeability.

The solenoid valve, an on/off valve, opens when cooling is required. Custom units sometimes use proportional regulating valves, but these valves are both more demanding technically and considerably more expensive. The temperature sensor measures the temperature of the heat transfer fluid and transmits the value to the controller. This sensor is usually either a Pt100 resistance thermometer or a type J thermocouple (Fe-Cu Ni). The level control consists of a float inside the tank that actuates a switch when the fluid level drops too low, interrupting power to the pump and heater (protection against running dry). The safety thermostat interrupts power to the heater as soon as the maximum outlet temperature is reached. The control section contains the control and automatic monitoring elements, such as pushbuttons, contactors, thermal relays for the pump motor, etc., as well as the temperature controller itself. For safety reasons, the control section should be kept separate from the mechanical section (pump section). In addition, the electrical components must be protected from accidental contact from outside (minimum protection class IP 40).

The temperature controller is a three-position controller (heating/neutral/cooling) with, for example, PID or PD/PID control action. When used with semiconductor contactors, the safety cut-outs or fuses interrupt the flow of electricity to the component to be protected (heater, etc.) in the event of overload or short circuit.

2.1.4 Advantages
The advantages and disadvantages listed below are with respect to the methods described in sections 3.2 and 3.3 (forced flow and pressurised water) and are therefore relative. The decisive criterion for evaluating the advantages and disadvantages of any method must be the specific application.

Advantages

  • Low price
  • Simple design