The heating of viscous or fibrous slurries has always been the most difficult heating application process.  Plugging, fouling, and inconsistent heating make the use of conventional indirect heat exchangers unrealistic.  They often choose Direct Steam Injection for these applications because of its energy efficiency and ease of use.

What is a Direct Steam Injection Heater?

A direct steam injection heater provides an expressly cost-effective method to heat water and other fluids by injecting steam directly into the fluid.  They design it to handle a wide range of flow rates and deliver hot fluids at precise temperatures with 20-35% less energy than an indirect heat exchanger.

Energy Available in Steam

The tremendous amount of energy available in the steam makes it imperative that the energy is dissipated quickly into the fluid in order to maintain stability.  Failure to dissipate and condense the steam quickly can lead to operational problems such as inconsistent temperatures, product burn on, and potentially severe steam hammer.

The dissipating of the energy during steam injection is dependent on several factors.  These factors are primarily the temperature differential between steam and fluid.  The relative velocities of the steam and liquid, the surface area of contact between the steam and liquid, and the fluid viscosity and surface tension.  As the viscosity of the fiber content increases, the job of heating the fluid becomes more difficult.

Challenges Throughout the Years

A number of methods have been employed over the years.  In an effort to use steam to heat the vast variety of fluids that are in the process industries.  Fluids like tomato paste, paper stock, and ore slurries have been especially hard to heat with steam.  Because of the viscous nature of the fluids.

The Advantages of Direct Steam Injection

Direct Steam Injection has a proven track record in challenging slurry applications.  Steam is readily available and can be inexpensive to produce.  Scaling from small to large flows with steam is effective and reliable.  A number of methods of direct steam injection can be considered.

For continuous fiber slurry heating processes an inline direct steam injection is the solution.  Inline direct steam injection heaters are capable of the high-temperature rise and in a multi-stage layout.  This is to allow for precise temperature control and smooth operation.  They also have a low-pressure drop across the heater which minimizes energy demand on the slurry pumps and limits flow disruption to the slurry.

Factors for Successful Direct Steam Injection

One of the key factors to notably successful steam injection is maintaining high steam velocity for effective mixing and condensation of the steam into the fiber slurry.  Internal modulation allows steam injection at sonic velocity to achieve choked flow.  Choked flow is the occurrence of accelerating a vapor to sonic velocity.  Thus, by creating a pressure differential through an engineered nozzle.  By creating choked flow and metering the steam mass flow it can precisely control the heating of the slurry.  Thus producing predictable results based on the position of the stem plug.  Through a variable-area steam diffuser, it meters steam flow at the point where steam and liquid first contact and mix.  It is important to realize, this method eliminates the need for an external steam control valve or downstream mechanical mixing devices.