Heating of viscous slurries with direct steam injection, Solutions!, Online Exclusives, November 2003

By Bruce Cincotta, chief technical officer, Hydro-Thermal Corporation

How can mill managers insure that they’re choosing a direct steam injection (DSI) system that will deliver reliable, consistent heat transfer and trouble free operation in difficult applications? We’ll explore the options.

Heating of viscous or fibrous slurries has long been among the most difficult heating applications in the process world. Plugging, fouling, and inconsistent heating make the use of conventional indirect heat exchangers impractical in most cases. Direct steam injection (DSI) is often selected for these difficult applications because of its energy efficiency and general ease of use compared to indirect heating methods. However, the tremendous amount of energy available in the steam makes it imperative that the energy be 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 dissipation of the energy during steam injection is dependant on a number of factors. These factors are primarily the temperature differential between the 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. In general, as the viscosity or fiber content increases the job of heating the fluid becomes more difficult.

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 found in the process industries. Fluids such as paper stock, tomato paste and ore slurries have been especially difficult to heat with steam because of the viscous nature of the fluids. While more traditional direct steam injection (DSI) heaters have been successful on water-like or free-flowing slurries, the challenges posed by the more difficult fluids have thus far proven insurmountable without major process limitations.

Two major classes of DSI heaters have evolved over the years to meet the large variety of heating needs in the process world. They are spargers and high velocity heaters.

Spargers
The first type of steam heater is the “sparging” style (see Figure 1). This type attempts to solve the problem of mixing the steam and fluid by breaking up the large steam volume into a plethora of smaller steam sources. The theory behind these devices is that the area of contact between the steam and fluid must be maximized in order to condense the steam in a rapid manner.

Figure 1. Typical Sparging Heater

In a sparger, the steam is directed from a central source to an array of holes. An external steam valve controls the amount of steam flow injected into the fluid. This modulating control valve reduces the pressure of the steam in the sparger in order to reduce the flow of steam into the product fluid. Spargers are widely used on water-like, or free-flowing slurries, but have two major shortcomings that limit their use on more difficult applications:

A “Steam Tree” is the second style of sparging DSI heater and is more commonly found on applications where there is an acceptance of the potential for plugging in the application and the need for a simple design that minimizes the potential for plugging. (see Figure 2.) Applications such as tomato paste or paper pulp are common examples.

Figure 2. Steam Tree Heater

This type of heater uses an array of steam injection points dispersed around the periphery of a standard pipe. The holes are relatively small and are sometimes individually controlled via a small solenoid valve. An external steam control valve controls the overall steam flow into the product. Steam trees have shown the ability to heat very viscous materials successfully, but suffer from some significant problems that limit their successful use.

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The external steam valve also makes this device susceptible to problems as the steam valve is throttled to reduce the amount of steam flow. Hammer, plugging, or startup problems are the common result. It is imperative that these types of devices be properly sized to avoid running at low steam or liquid flows where the holes may see limited steam pressures, or may be turned off periodically, leading to plugging. On many applications such as tomato paste heating, the steam holes tend to plug when not in use and cannot be cleared without manual cleaning of the device. Generally, these devices are purchased in pairs, so they may be swapped out periodically for cleaning, reducing the down time.

High Velocity Heaters
This class of heater employs the motive force of the steam to disperse the steam into the fluid to be heated. Venturi, steam jet, or static mixer technology is employed to disperse the steam into the fluid to be heated. In all cases, the primary agent for mixing is the high velocity of the steam.

Figure 3. Venturi Heater

          Venturi style heaters use the steam as the motive force to draw in the liquid. The liquid comes in direct contact with the steam as it is drawn into the unit (see Figure 3). An external steam valve controls the amount of steam added. Large steam or water fluctuations lead to problems as the external steam valve modulates the pressure to the device. As the pressure differential between the steam and fluid drops, the steam velocity and momentum drops as well. Because these heaters rely on the steam momentum as a motive force, they are generally limited in application to water-like fluids and relatively small turndown of liquid flow.

Figure 4. Static Mixer Heater

 Static mixer technology combines the high velocity steam jet concept with downstream static mixing vanes in an attempt to further mix the steam and liquid without relying on the steam velocity alone. Flow through the unit is generally straight, which is desirable for the more fibrous or viscous slurries (see Figure 4). These devices employ an upstream steam control valve in order to modulate the rate of steam injection into the fluid. In practice, the static mixers downstream of the inject point have a minimal positive effect on steam condensation rates. The degree of mixing required to achieve stable condenssation is at a scale much smaller than the mixing vanes create. As a result, they have almost no positive effect on the heating process. The mixing vanes do however have a tendency to fail relatively quickly because they are exposed to the steam as it is being condensed. The condensing steam may cause a cavitation-like wear on the vanes. If that occurs, the mixing vanes eventually will break and be discharged into the fluid stream.

          As was the case with the sparging type heaters, an external steam control valve is used to control the steam flow through the heater. The steam control valve creates a number of operational problems since it generally reduces the steam velocity during injection. The degree of mixing is often sharply reduced as the steam velocity is - 4 - reduced. Hammer, cavitation and poor operation are generally the result. For these reasons static mixing devices have had limited successful use on relatively small heating load applications.

          Steam jet technology has become the standard for most slurry heating or high steam turndown applications because of its stable and generally trouble free operation. These devices utilize internal control of steam through a constant pressure choked flow nozzle/ plug arrangement (see Figure 5). Choked flow nozzles are designed to take a critical pressure drop and discharge steam at sonic (>1300fps) velocities. An integral stem/plug internally modulates the steam flow without any further reduction in steam pressure. This plug/nozzle combination provides constant, high velocity steam throughout its full heating range, with none of the ill effects of the external steam control valve. In addition, the high velocity present in the steam heaters and the lack of hot product contact surfaces prevents buildup of product inside the heater. This ensures consistent performance throughout its life.

Figure 5. Steam Jet Heater

The major drawback to this design is the relatively complex flow path for the product through the heater and the single steam injection point into the product. While this combination is generally very effective on products with unheated viscosities of up to 5,000 cp, they have proven ineffective on more viscous slurries or products. The momentum from the high velocity steam jet is insufficient to disperse the steam into the product without encountering vibration from the remaining steam bubbles.

Mechanical Mixers

Figure 6. Peg Mixer Heater

On highly viscous material such as paper stock, mechanical mixing devices have become common ways of injecting and mixing steam into the product. These devices, sometimes called “peg mixers,” have an array of steam injection points around a large housing (see Figure 6). The product to be heated flows through this housing and steam is introduced. To facilitate the mixing of the fluid and steam, mixing elements inside the housing are rotated by means of a large external motor. This mixing action helps disperse the steam into the product and aid condensation. This style of heater has the ability to heat virtually any high viscosity fluid that can be pumped. These heaters have been able to heat paper stock to concentrations as high as 15%. Because of their very large internal volume, pressure drop is minimal.

Surprisingly, despite their success on high viscosity fluids, these heaters often have more trouble with low viscosity applications. This is due to the fact that the high viscosity fluids tend to damp out the vibrations from the imploding steam bubbles as they condense. The vibrations created by this process are contained within the device and not transferred to the surrounding piping. On less viscous fluids, there is no such damping and the condensation can create a good deal of shaking or hammering in the piping or support structure.

The heating with these devices tends to be inconsistent. There are relatively few injection points in the heater and the material closest to these injection points tends to be overheated for a period of time until the mechanical mixing action can disperse the heat. Depending on the goal of the heating, this may result in additional chemical usage, scorched product, or overall inconsistent quality of the resulting product.

The mixer and external motor are generally quite large. As a result, installation costs tend to be high and the options for installation are reduced because of the need to mount the unit on a level base. In addition, these units generally use the motors to help the flow of product and therefore the motor must remain running even if no heating is desired. The costs of running the mixing motor can be high, and are a source of energy inefficiency.

The mechanical nature of these devices makes them prone to mechanical failures. As noted earlier, the injection points are few and relatively large. This tends to facilitate the formation of large steam bubbles that must be condensed inside the unit. As these condense, the structures in contact with the steam tend to be exposed to cavitation wear. This, in addition to the general wear associated with the rotating parts, makes these devices maintenance intensive.

Mach diffuser technology
A new style of device takes the advantages of the above devices and incorporates them into one device. It combines a straight-through design, multiple injection points, and constant, high velocity injection to provide stable operation on fluids from water through difficult fluids like tomato paste or paper stock.

Figure 7. Mach Diffuser Heater

The major innovation in this patented device is the introduction of a rotary choked flow steam diffuser (see Figure 7). Steam is selectively introduced to the diffuser as the integral actuator rotates. A large number of holes provide a high degree of surface contact between the steam and fluid. That, coupled with the choked flow steam conditions, allow for very rapid condensation of the steam into the product. This rapid condensation results in virtually instantaneous condensation of all the steam into the product without the vibration or noise that is found with less sophisticated designs. The device also provides a straight-through flow path. Pressure drop of the product is minimal and there are no surfaces on which fibrous slurries could plug.

Internal modulation, or control, of the steam eliminates the problem of poor operation at low steam flows. The velocity of the steam is constant from 0-100% of steam flow. In addition to the benefits this brings to the condensation process, the availability of full steam pressure allows the unit to self-clean any plugging of the holes that may occur during down time when no steam is flowing through the holes.

Summary
Each style of direct steam injection heating offers advantages and problems when used in heavy slurry or viscous applications. The ideal selection is made based on the injection method that offers the least problems and the highest reliability during service. Few or no moving parts and a clean flow path are desirable because of the nature of the product being heated. Mach diffuser technology combines these traits with the advantage of constant steam pressure and velocity at the injection point. The result is a reliable consistent heat transfer and trouble free operation in these difficult applications.

About the author: Bruce Cincotta is chief technical officer for Hydro-Thermal Corporation, Waukesha, WI. For more information, contact Mark Boeder at (262) 548-8900 ext. 116, or email mboeder@hydro-thermal.com.