Process Heating by Direct Steam Injection

Review the development of this versatile heating method and learn how to use it in your application.

Figure 1. The oldest direct steam-injection method of heating, the sparger incorporates tubes at the bottom of the vessel that inject steam into the fluid to be heated.

Two heat processing methods use steam to heat liquids, solutions and slurries: direct, using direct steam-injection (DSI) heaters, and indirect, employing heat exchangers. This article describes methods of direct steam-injection heating and some common chemical processing applications for DSI heaters. Unquestionably, the pulp/paper segment has the most applications, with food processing following closely behind. Other segments -- plastics, chemical and synthetic chemical processing, pharmaceuticals and textiles -- also have uses for well-engineered, properly instrumented DSI devices; in fact, DSI units may be preferred to shell and tube or plate-type heat exchangers in many applications. Including both latent and sensible heat, 100 psig saturated steam contains 1,189 BTUs per pound. For the same heat input, compare a DSI device to a heat exchanger. A DSI:

• Absorbs 880 BTUs (74% efficient).
• Has an integral steam modulation valve.
• Has a broad operating range (20:1 turndown).
• Has little to no pressure drop.
• Fits into existing piping and space.
• Uses any clean steam supply.
• Does not experience scale buildup.
• Does not require a condensate return.
• Can be less expensive to install and operate.

By contrast, a heat exchanger used in a steam heating application:

• Combines all 1,189 BTUs.
• Requires a separate steam control valve.
• Offers little or no operational turndown.
• Requires a water pump with a higher horsepower rating.
• Occupies more floor space.
• Requires higher pressure steam.
• May experience calcium and magnesium scale buildup.
• May require a condensate return system.
• Can be more expensive to install.

Figure 2. Mixing Ts combine separate streams of steam and cold water to produce heated water. Because accurate temperature control is difficult to maintain with this method, mixing Ts are not the best choice for process fluids.

Scale buildup on heat transfer surfaces can cause both shell and tube and plate-type heat exchangers to fail.¹ Filtering the fluid to be heated prior to its entry at the heat exchanger can reduce operating costs; nonetheless, maintenance costs quickly can exceed thousands of dollars.¹ Therefore, if the process can be heated using a DSI unit, savings may be realized. Direct steam-injection heating of process flows and vessels of material has been practiced for more than a century. After all, directing a steam pipe into a tank of water and turning on the steam appears to be a fairly simple heating method. Much equipment employing steam as the heat source has been developed as users gain a better understanding of steam and how it heats, its process controls and their functions, and the appropriate mechanical construction. The DSI method offers advantages:

• Provides energy efficient heating.
• Provides accurate, dependable temperature control.
• Occupies less space and needs less auxiliary equipment.
• Costs less to purchase, install and maintain.

One type of DSI device that is difficult to define is the homemade unit such as an in-pipe sparger. Such on-the-job-engineered devices come in all sizes and configurations and have one common characteristic: They usually perform as conceived under tight operating parameters, yet fail whenever the operational envelope is exceeded. To satisfy applications demanding a more forgiving, flexible DSI heater, seven basic heater configurations have been developed: spargers, mixing Ts, venturi injectors, modulatable sparger tubes, mixing-jet cookers, multiple-orifice arrays and a steam injection static mixer.
Sparger. Sparging is the oldest, simplest and least complicated technique for mixing steam with liquid or slurry to effect heating (figure 1). Though considered simple and inexpensive, spargers are inefficient devices whose operation invariably results in any or all of the following:

• Poor heat injection economics due to BTUs escaping from vessels.
• Equipment failure (both vessel and sparger pipes) due to the vibration associated with steam hammer when not operated within their narrow design envelope.
• Usually less than satisfactory on/off process control. A sparger is the least modulatable DSI heater.
• Higher system maintenance costs for tanks, sensors and piping if the equipment operates outside the design parameters.

Figure 3. The venturi injects steam directly into he cold liquid or slurry feed to heat the product.

Mixing Ts. The mixing T is the simplest commercially available apparatus: It mixes steam and water to provide hot water with temperature control (figure 2). While commercially available, many mixing Ts are home-engineered or job-made. Because any change in water/steam flow or pressure results in a temperature change and, in many instances, produces steam hammer, this type of DSI device is best used to provide hot wash water rather than controlled process flow heating. Venturi Injectors. The first engineered DSI heater, the venturi injector was used to preheat and inject feed water into steam engine boilers (figure 3).2 As the first improvement over the sparger for process flow steam heating, the venturi injector was a significant development. When the steam flow pressure is sufficiently in excess of water pressure, operation proceeds quietly with no steam hammer. However, like spargers, they have a limited flow-through operational turndown: +/-5% of the unit's designed throughput rate. Also, they required a well-trapped, condensate-free, constant-pressure steam supply, and water pressure and flow rate must be controlled within design parameters to provide smooth, efficient operation.
Mixing-Jet Cookers. In 1938, a patent application was filed defining a steam injector for pasting starch (figure 4).3 This DSI was used as the first step in a starch liquefaction process producing sugar to ferment into alcohol. The device performed as a continuous pressure cooker and operated at temperatures from 200 to 320^oF (93 to 160^oC), depending on the saturated steam pressure available.
During the 1940s, the Sirod Co., Milwaukee, WI, began to manufacture this device, which it called the Hydroheater, for use in paper mills to heat white water, raw mill water and other process liquids or to cook starch. Initially a manually operated device -- requiring hand adjustment of the steam-valve-stem-plug travel and mixing-tube-to-steam-orifice gap via a wheel -- eventually, a pneumatic diaphragm actuator replaced the hand wheel adjustment of the steam orifice plug. As commercial mixing jet cookers evolved, to reduce the unit's manufacturing cost, the combining tube length was shortened. Also, the hand-wheel-combining-tube-to-steam-orifice-adjuster was replaced with a jacking bolt and slide plate.
Modulatable Sparger Tube Heater. The modulatable sparger tube (MST) heater -- first patented4 in 1948 with later improvements5 in the 1970s -- consists of a variable-injection sparger tube inside a cast process flow body. In response to a temperature controller, the injection tube modulates steam flow/injection via a flow control valve and spring-loaded piston (figure 5). MST heaters work well on clear liquids and some low solids (0.1 to 1.5%) solutions but are subject to severe clogging and steam hammer when applied to moderate concentration solutions and all slurries. Even at the lowest (0.25%) consistencies, they cannot be used for pulp slurry due to the sparger tube's fine steam orifice holes and its moving, spring-loaded piston assembly.
In 1984, a sanitary steam injection heater was developed that satisfied the mechanical and configuration requirements for processing liquids under sanitary conditions.6 Other than the sanitary construction, little change occurred in DSI process heater design until 1988. A steam-injection water heater using a baffled mixing T and integral differential pressure sensor to allow pressure balancing within the tube was developed.7 However, this device is a water heater and is not suited for processing slurries or solutions.

Figure 4. Mixing jet cookers operate as a continuous pressure cooker, heating white water, raw mill water or other process fluids in paper mills.

Improved Mixing-Jet-Cooker. Little change occurred in the mixing-jet cooker's configuration after its transition to the short combining tube and slide-plate until 1989. One exception was an attempt8 to duplicate the original mixing-jet cooker performance. No longer manufactured, a few of these jet valves may still be found in food processing plants and paper mills for pasting starch. In 1990, a mixing-jet cooker improvement9 was introduced. In this design, a precision micrometer gap adjuster enables recoverable setting of the mixing-tube-to-steam-nozzle opening. Adjustable during operation, this device also may incorporate a pneumatic actuator for hands-free operation. In the operation of a mixing-jet cooker, adhesive granules combine with saturated steam and accelerate to sonic velocity. As the granules enter the venturi mixing tube, they spin, expand and begin to disperse as they become hydrated. These swollen granule-gels pass through the mixing tube body, where high shear mixing disperses them into hydrated polymer chains. The smaller the mixing tube's inside diameter, the higher the shear rate within, and the greater the resulting polymer dispersion -- up to and including polymer chain length reduction.10
Steam Injection Static Mixer. In this DSI heater design, the fluid or slurry enters and departs the heater in a straight line.11 Steam is introduced via a side port and impinges an inlet cone, then enters six peripheral holes incorporating helical mixing elements rotating in the same direction. As the steam and fluid or slurry mix, the steam condenses and the product is heated. This DSI design is used to heat many paper and pulp mill process flows. But, as solids concentration -- or consistency, as wood pulp slurry concentration is called -- increases beyond the low (2.5 to 4%) range, this device can clog at the mixing elements.

Enlarge this picture Incorporating a variable injection sparger tube, the modulatable sparger tube adjusts steam flow and injection in response to a temperature controller.

Micro-Jet Array Process Heater. Regardless of the many applications for mixing-jet cookers, far more heating operations require a DSI heater that immediately condenses the steam and incorporates modulatable turndown with process control. The micro-jet array process heater satisfies these needs: Within its 90^o elbow body, steam condenses as it travels in the first one-and-one-half pipe diameters downstream from the heater's multiple nozzle orifice plate (figure 7). Its valve head and nozzle plate assembly divide the steam flow into several small-diameter, equal-pressure short jets rather than one large-diameter long jet, so the steam is exposed to more cold liquid, creating a much greater condensation impetus. This design offers less operating noise, high throughput turndown, low pressure drop and quick startups with precise output temperature control. While larger-diameter heaters result, they are shorter in length and weigh less per performance requirement.

Enlarge this picture Figure 6. The improved design of the mixing jet cooker provides precise, recoverable mixing-tube-to-steam-nozzle-gap adjustability.

Industrial applications for direct steam injection devices are numerous: pulp and paper, food and grain processing, chemicals and plastics processing, pharmaceuticals, textile manufacturing, and mining and metals processing. Specific processing applications may be useful in one or more chemical processing segments, and new applications become apparent as equipment capabilities evolve and process application opportunities arise.

Enlarge this picture Figure 7. The micro-jet array process heater incorporates a valve head and nozzle plate assembly that divides the steam flow into several small-diameter, equal-pressure short jets rather than one large-diameter long jet.

Admittedly, the preceding is an attempt to provide a document containing some history and resource references for others interested in the expanding field of direct steam injection. Without question, there is generous opportunity for applications expansion and new product development.

Author's Note: In preparing this article, I found the most informative source was the U.S. Patent Files. Many more patents exist and are worthy of any investigator's study. Those defined above will provide a good foundation for future work.