Combined heat exchangers

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The combined heat exchangers combine the advantages of tubular heat exchangers and electrical heating. If the heating fluid is not yet available (e.g. boiler fired by heavy oil), electrical preheating is effected. The tubular heating surface transfers the thermal energy of one medium to the other. The fluids involved are mainly liquid but can also be gaseous. The type and properties of the fluids, pressures and temperatures as well as the desired operating points in the process are important for the design.

One fluid in the combined heat exchangers dissipates the energy, and this energy is transferred through a pipe wall to the other fluid. Here, it is important that the design be matched to the general conditions, for each fluid has its specific properties.

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Design models

Combined heat exchangers are mainly used for heating heavy-oil storage tanks. Of course, they can be used for all fluids which can be used for tubular heat exchangers and electrical flow-type heaters.

I. Water

  • Drinking water, max. surface load depending on the water hardness 4 – 6 W/cm²
  • Circulating and/or heating water, max. surface load approx. 10 W/cm²
  • Softened water; observe the maximally admissible chloride content, max. surface load approx. 19.0 W/cm²
  • Ultra-pure water; here, a low-pocket or pocket-free design with a defined surface quality is useful in most cases.
  • Fully desalted water; here, non-ferrous heavy metals should not be used, maximum surface load approx. 10 W/cm²

II. Oil

  • Heavy oil, not pumpable in a cold condition, maximum surface load between 1 and 2 W/cm² depending on the quality
  • Hydraulics oil, maximum surface load approx. 0.6 – 1.2 W/cm²
  • Lubricating oil, steam turbine oil, max. surface load approx. 1 W/cm²
  • Insulating oil, maximum surface load approx. 0.3 – 0.6 W/cm²
  • Heat transfer oil, film temperature calculation acc. to DIN 4754 required, maximum surface load approx. 10 W/cm² depending on the flow velocity and oil
  • Fuel oil, diesel, heating to max. 40 °C, max. surface load approx. 4 W/cm²

III. Gases

  • Air
  • Natural gas
  • Flue gas
  • Nitrogen

he fluid to be heated and the application temperature mainly define the materials which can be used for the unheated and/or heated surfaces. Otherwise, corrosion may quickly result in a failure of the combined heat exchanger, for example.

Materials of the wetted and unheated components

  • Carbon steel
  • Corrosion-resistant stainless steel
  • Heat-resistant stainless steel
  • Titanium, Hastelloy, special materials
  • Brass

Materials of the heating surface:

  • Carbon steel
  • Corrosion-resistant stainless steel
  • Heat-resistant stainless steel
  • Titanium, Hastelloy, special materials

Miscellaneous heating elements can be used for combined heat exchangers. Depending on the application, they will be selected by us.

I. Heating elements

  • Tubular heaters, diameter 8.5 or 16 mm
  • Cartridge-type heaters, diameter 16, 18 or 25 mm
  • Exchangeable heating elements, including a protective tube, diameter 25, 42 or 65 mm

We are prepared to agree with you on the type and position of the fluid connections. The following connections are available:

I. Fluid connections

  • Standard flanges (DIN, ASME, SAE etc.)
  • Female or male thread connections
  • Clamp flanges
  • Sterile flanges
  • Dairy-type pipe connections

Available design models for electrical equipment and controlling of the combined heat exchanger are:

I. Design models

  • Indoor or outdoor installation
  • Hazardous area (zone 1 or 2, 21 or 22)
  • Non-harzardous area
  • Type of protection IP54 or better
  • Switchgear cabinet for wall or floor mounting
  • Steel sheet connection housing, powder-coated or stainless steel
  • Electronic or electromechanical control
  • Communication with higher-level control system by floating contacts, standardized signal or bus
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