Oil temperature control units with forced circulation
Principle of operation
The oil temperature control unit consists of a heater with precisely defined flow characteristics (forced circulation heater) which, when properly configured, prevents thermal overload of the heat transfer fluid. In the following example, the heater consists of a tube with a built-in heating element with either 20 or 40 kW capacity. The thermal oil flows around the heating elements, which are equipped with vanes to guide the flow. This configuration exhibits a low degree of thermal inertia (low thermal storage capacity of the heater) which leads to good control action. The precise flow conditions in the heater are defined, i.e. the pump capacity (flow rate) and the heating capacity (specific heat load), are coordinated in such a way that the temperature in the heater never exceeds the maximum permissible film temperature specified by the thermal oil manufacturer.
As a rule, this critical condition can be fulfilled only by the use of a forced flow heater with defined flow conditions. The heater configuration is therefore a decisive factor in the temperature control.
Unit‘s operational safety and reliability, as well as in the service life of the thermal oil. The heater configuration is determined by the following three factors:
- Pump capacity (flow rate)
- Heating capacity (specific heat load,W/cm2)
The relationship between these factors is as follows:
If the flow rate is high, the specific heat load in the heater can also be relatively high, which in turn makes it possible to keep the dimensions of the heater, and thus the temperature control unit itself, relatively small.
High capacity pumps, however, are more expensive.
If the flow rate is low, the specific heat load in the heater must also be low, which implies either low heating capacity or correspondingly large unit dimensions. However, a comparably inexpensive pump can be used.
Regloplas recommends forced flow heating for thermal oil above 150 °C for reasons of safety as well as for applications requiring primarily heating.
In addition to the components already mentioned, such as level control, safety thermostat, safety cut-outs and thermal relays for the pump motor, safety considerations require that temperature control units with forced flow be equipped with an expansion vessel, a bypass and a flow monitor.
The expansion vessel keeps the circulating hot oil separated from the atmosphere. This effectively eliminates the disadvantage associated with bath heating – accelerated ageing of the oil due to oxidation – as the circulating hot oil is separated from the atmosphere by the stationary oil in the expansion vessel, which also prevents flammable oil vapour from escaping into the atmosphere. At very high outlet temperatures, i.e. above the boiling point of the oil, or to reduce oxidation, a nitrogen blanket can be used in the expansion vessel.
The bypass is connected in parallel to the consumer and has the following functions:
- Protect the pump motor from overload
- Maintain a minimum rate of flow in the unit should flow in the consumer be insufficient, to ensure that the maximum film temperature is not exceeded in the heater and that the thermal oil is not overloaded
- Protect the consumer from excessive pressure (e. g. with double-walled tanks)
- Adjust the flow rate in the consumer (primarily for pumps with high capacities)
The flow monitor, like the bypass, monitors circulation. When the flow rate falls below the factory-set minimum pressure (indirect measurement of the flow rate), the heater is switched off. The flow monitor is thus triggered upon separation of flow in the temperature control circuit, for any cause. A flow monitor offers more complete protection against dry running of the pump and against unit overheating. The safety features designed into the temperature control unit – forced flow, expansion vessel, bypass, flow monitor – provide the following advantages:
- Minimum coking, a high degree of oxidation resistance and thus significantly longer service life for the oil
- Lower operating costs and greater operational reliability of the temperature control system
The cooler may consist of a bank of tubes, with cooling water running through and thermal oil flowing around them.
The one-way check valve in the cooling water outlet reduces scaling in the cooler that could result from water flowing back into the cooler.
The pump is most often a centrifugal pump with a mechanical seal or a magnetic drive. To reduce the thermal load on the cooler and to improve control during cooling, Regloplas temperature control units for temperatures above 300 °C are equipped with a cooler bypass circuit.
The cooler bypass circuit functions as follows:
- Heating: The three-way valve is closed between 12 and 2, i.e. the cooler (1) is shunted out.
- Cooling: The three-way valve is open between 12 and 1, i.e. the thermal oil flows through the cooler and directly over the heater.
The temperature controller in the unit operates the three-way valve via the solenoid valve.
The bypass circuit provides the following important advantages:
- Significantly better temperature profiles (no significant undershooting of temperature as a result of cooling capacity)
- Reduced thermal shock to the heat exchanger during cooling; reliable operation
- Reduced tendency to scaling
- No vapour release at beginning and end of cooling period.
The maximum outlet temperature in temperature control units without nitrogen blankets is approximately 350 °C.
The maximum outlet temperature is a function of:
- maximal permissible film and outlet temperature of the thermal oil used
- the configuration of the heater, i.e. the rise in temperature of the oil at the heating surface. This rise in temperature is in turn a function of the specific heat load and the flow rate
- Maximum outlet temperature 350 °C and higher
- Low oxidation rate of the oil: long service life
- Defined flow conditions: long service life and minimal coking of the oil
- No odour
- Flammable oil vapour cannot escape
- High degree of operational reliability