Most vacuum process heaters should include a de-superheater.

Vacuum process heaters frequently utilize boiler steam, either directly injected into the heater, or as the motive flow to a thermocompressor. In either case, the water vapor entering the heater will be superheated at the point where it enters the process heater.

Ideally, this vapor should be at the saturation temperature. The degrees of superheat (superheat temperature minus saturation temperature) is typically harmful as this excess temperature can scorch the product. Extensive fouling of the heat transfer tubes is another frequent result.

The simple solution is to install a de-superheater into the vapor stream, upstream of the heater. A steady stream of water is sprayed into the vapor stream, which subsequently evaporates and absorbs the excess heat represented by the superheat temperature. The vapor will be lowered to the saturation temperature.

While a de-superheater is quite simple in principle, the “devil is in the details.” A sufficient flow must be used with a reasonable excess, calculated from the thermodynamic properties of the steam and the water. The water must then be properly introduced into the steam/vapor stream, making sufficient contact to evaporate rapidly in a very short time. Finally, the de-superheating process should be automated, requiring no manual steps.

As noted, the key benefit of a de-superheater is to avoid overheating and scorching the product being heated, which simultaneously minimizes fouling of the heater. A vacuum process is being utilized in the first place to minimize heat damage. There are additional benefits as well. De-superheating reduces the temperature extremes that the heater will experience, which will extend the life of the heater. In addition, heat transfer is accelerated and stabilized by using saturated steam instead of superheated steam.

If a de-superheater appears to make sense for your vacuum process heater(s), it should be properly specified for your process conditions. Caloris can handle this and provide the correct equipment.

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Contact Caloris today if you have questions about your evaporator or other process equipment, give us a call at 410-822-6900 or send email to problem.solved@caloris.com.