When considering using direct-contact steam injection (DSI) for a process heating application, it is important to first understand the two basic techniques -- direct and indirect transfer -- used to transfer heat into fluids for process and utility heating.
Indirect contact heat exchangers such as shell-and-tube, plate-and-frame or scraped-surface designs have two or more fluid flow paths to prevent the fluids from mixing. The heat is transferred from one fluid to another across a thermally conducting but otherwise impermeable barrier, such as a tube wall or plate.
Direct-contact heat exchangers transfer heat by injecting steam directly into the process fluid (hence the name). This results in rapid heat transfer and often is more efficient than indirect heat exchangers. Direct-contact steam heating provides 100 percent thermal efficiency because both the sensible and latent energy in the steam are used. Energy savings can be considerable, with reductions in energy usages of 20 percent to 25 percent. Direct-steam injection heaters may be the best option for hard-to-heat fluids such as slurries or highly viscous materials that tend to clog; fluids requiring nearly instantaneous heating such as some food products; and products that would bake onto heat exchanger surfaces. The heaters themselves are installed inline with the process piping and usually are more compact than indirect heat exchangers.
Direct-contact steam heating is suitable for many applications, but of course, it cannot be used when the process fluid cannot tolerate steam addition. Where the addition of steam is acceptable, however, direct-contact steam heating provides energy savings and process efficiencies. Other advantages and limitations of direct-contact steam heating depend on the method and heater type used.
Types of direct-contact steam injection heaters include tank spargers, inline spargers, mixing tees and internally modulated steam-injection heaters. Direct-contact steam heaters can be classified as either externally modulated or internally modulated, which refers to how the steam injected into the process fluid is controlled.
External ModulationAn externally modulated steam heater uses a steam-pressure control valve to regulate the amount of steam injected into the process. The valve is located in the supply steam piping. At times, steam velocity and the volumes of steam and fluid mixing can vary widely over the operating range. Vibration, poor temperature control and inefficiency result at some operating conditions. Tank spargers, which use external modulation, can experience steam hammer and vibration damage to tanks. Energy inefficiencies also can occur with this heating method when the steam does not completely mix and escapes from the tank.
Mixing tees also are externally modulated devices. They are available in manual, semi-automatic and automated systems to mix steam and cold water. Each style achieves this mixing with varying degrees of accuracy. Both semi-automatic and manual mixing tees experience difficulty in compensating for changes in steam pressure, water pressure or hot water demand. Proper operational guidelines and maintenance procedures must be followed closely to avoid unstable operation and poor temperature control.
More sophisticated externally modulated devices control steam flow through the use of multiple orifices in the heater. When properly instrumented, this type of device provides improved temperature control and good turndown capabilities. As with other externally modulated devices, steam pressure at the point of injection can vary widely, resulting in poor steam/fluid mixing. These heaters may require regular monitoring and maintenance. In applications where hard water, slurries or high-viscosity liquids are heated, routine disassembly for cleaning plugged orifices is required.
Internal ModulationWith the internally modulated direct-steam injection heater, steam flow is controlled through a stem-plug assembly inside the heater. Changing the steam discharge area of the nozzle varies the amount of steam passing through the nozzle, and maintains good mixing characteristics (figure 1). Internal modulation eliminates the need for an external steam control valve.
Internally modulated direct-contact steam heaters inject metered amounts of steam into the process fluid through a variable-area steam nozzle. The nozzle design ensures constant steam pressure and velocity at the point where steam contacts the liquid or slurry, eliminating the potential for pressure upsets and ensuring smooth heater operation. Internally modulated direct-steam injection heaters are cleaned by their own turbulent mixing action (usually sonic velocity steam), so they do not encounter fouling or scale buildup. Because of this cleaning and mixing action, they are able to heat slurries containing a high concentration of solids or non-Newtonian liquids.
When operating internally modulated direct-steam injection systems, the differential between the steam pressure and process pressure at the discharge of the heater must be maintained to ensure proper operation. The units typically are controlled automatically through a loop controller based on discharge temperature.
Is It Right for Your Application?To take advantage of direct-steam injection heating, the fluid to be heated must be water miscible (that is, the fluid and water must mix). In this situation, direct-steam injection heaters can be used for a range of process and utility heating applications. From the simplest water-heating application to more complicated processes such as heating medium-consistency stock for paper production, corn mash for ethanol production, sewage for anaerobic digestion, or tomato paste for food production, direct-steam injection is a versatile, effective heating method. Examples of other typical applications are shown in table 1.
Several properties of the process fluid must be considered when determining whether direct-steam injection can be used. These include the fluid’s specific heat, density and viscosity. Specific heat determines the energy input needed to heat the substance. In English units, specific heat is the quantity of heat in BTUs needed to raise the temperature of 1 lb of material by 1°F. Density is the mass per unit volume of the substance, and the conversion between mass flow rate (for example, pounds per hour) and volumetric flow rate (for example, gallons per minute). Viscosity often is thought of as the substance’s “thickness” or resistance to flow, and most commonly is expressed in centipoise (cP). Some common liquids and their viscosity values are listed in table 2.
Energy SavingsIn many applications, direct-contact steam injection heaters show energy savings over indirect heat exchangers and spargers while also improving the bottom line and product quality.
More than 20 percent of steam’s energy can be present in sensible energy. By using both the latent and sensible energy of the steam, a direct-steam heater will require less steam flow for a given process when compared to indirect heating methods. Condensate return is not needed as all of the steam’s energy is transferred to the process. In effect, all of the condensate energy is recovered. Estimating the energy savings of replacing a heat exchanger can be calculated using basic fluid heating assumptions. An example for water heating is shown in figure 2. At these process conditions, energy savings would pay for the unit in less than two months.
If a user knows certain process conditions such as flow rate, incoming water temperature, required output temperature, and steam pressure, the energy usage can be calculated. The needed energy to use a heat exchanger can be compared the amount needed to use a direct-steam injection heater; comparing these two values will provide an estimated energy savings.
Other advantages of direct-steam injection heating include reduced maintenance in the form of less required heat exchanger cleaning. Also, at times, other pieces of equipment such as mixers can be eliminated because the heater provides turbulent action and thorough mixing. The small size of direct-steam injection heaters also can reduce floor space requirements. Precise temperature control saves energy by heating fluids to precise temperatures without overshooting temperature setpoint. Space savings also can be realized in better use of plant layout.
One company that saved money by taking advantage of a direct-steam injection heater is a major steel mill in West Virginia. Early in 2008, the mill replaced an aging water-heating tank used during the annealing phase with a direct-steam heater. Three internally modulated direct-contact steam units were installed to heat water for the wash-down stage of the process. Energy usage before and after the changes was recorded and documented as part of an energy savings assessment program with the United States Department of Energy (DOE). Preliminary readings show a sizeable decrease of 11,900 lb/hr of steam flow to perform the same water heating. This translates to savings of approximately $59,000 per month and a return on investment of only three months. The mill is looking at other areas of their process where direct-steam injection heaters can replace other heat exchangers or spargers.
Sidebar:Specifying direct-steam injection heaters and properly sizing the equipment for an application entails an examination of process requirements, general process information and the fluid characteristics. Properties such as specific heat, density, solids content and viscosity, and whether any abrasive or corrosive products are present need to be identified.
Putting It to Work
Application conditions such as flow rate (normal, minimum and maximum), inlet and discharge temperatures, pressure, and pipe sizes must be examined. Users should determine whether manual or automatic temperature control is desired, and whether continuous, intermittent or variable operation will be the norm. Properly sizing the heater determines maximum performance and benefit -- both short term and long range.
It also is important to remember that direct-contact steam injection is suitable for only water miscible liquids.
Finally, remember that when specifying and sizing direct-steam injection heaters, the advice of an engineer specializing in direct-steam injection is advised. In addition to knowing several fluid and process properties to determine the proper heater for the process and for maximum performance, he or she can advise you on about concerns specific to your process.