The most common method of transferring energy with steam is indirect heat exchange, which is used in familiar applications including process, plant sanitation and HVAC. Condensing the steam releases latent heat and a membrane, such as a tube or plate, transfers that heat into a fluid. The process generates a byproduct condensate that is discharged through a trap and returned to its source, typically a boiler, where it continues to produce steam.
This tried-and-true method, however, has a significant drawback. Due to the pressure drop as the condensate exits the trap, some portion is inevitably lost to flash evaporation. To keep the system functional, cold replacement water must be added. As the condensate is lost, system efficiency is impacted. The level of impact varies in accordance with the pressure of the steam supply — the higher the pressure, the less efficient the system.
Figure 1: In indirect heat exchange, a portion of the condensate is lost due to flashing and must be replaced with cold water. Flash losses vary with steam supply pressure. Source: Pick Heaters Inc.
Yet there is an alternative method that is ideal for high-pressure systems: Direct Steam Injection (DSI). Here, the steam is not held within a membrane to keep it separate from the process fluid, but rather blended directly into it. The need to recover condensate is thereby eliminated and instead of being lost to flashing, it is fully used. As a result, the system achieves 100% heat transfer efficiency.
Several advantages are realized by the DSI approach; chief among them is cost savings. The boiler used in a DSI system is fed by the same cold replacement water used in indirect heat exchangers and requires greater heat input to convert this water to steam. However, this is more than offset by reduction in steam demand at the use point, yielding a net reduction in fuel consumption and cost savings for the end user. A Pick DSI system can save up to 28% of the fuel required for indirect heat exchangers.
DSI also offers more precise temperature control thanks to its rapid-response adaptation to load changes. Condensate is not recovered, so there is no need for a flash tank or condensate return system. Finally, surface area is not required to effect heat transfer, making for a more compact device that is easier both to house and to maintain.
Figure 2: With Direct Steam Injection, steam is completely consumed and no condensate is returned. Flash losses are eliminated. Source: Pick Heaters Inc.
DSI systems are well-suited to a variety of industrial applications that can benefit from a steady supply of on-demand, precisely controlled hot water. Pick offers a Constant Flow Heater, for instance, that serves the cross-industry trend of shifting from steam to hot water for jacketed heating, eliminating the potential for hot spots, burn-on and thermal shock. Pick’s Variable Flow Heater allows for frequent start-stop applications, making it a natural fit for plant sanitation and clean up.
The food and beverage industry also has employed DSI systems for in-line product cooking, clean-in-place (CIP) heating and nitrogen gas injection. Pick’s Sanitary Jet Cooker can heat, cook or sterilize water and slurry-type food products on a continuous, straight-through basis. The device features low-velocity mixing and a non-shearing design that allows it to handle small food pieces without damage.
In the chemical processing industry, DSI supports automated systems with precise temperature control that ensures optimum effectiveness of jacketed reactors and eliminates the potential for destruction and waste of heat sensitive products. Other applications include charging reactor vessels, tank cleaning and CIP, and smooth blending of condensate streams.
Additional industries that have realized efficiency improvements through deployment of direct steam injection include pulp and paper, energy and power, pharmaceutical and other industrial markets.
Thinking past the traditional
Probably the biggest obstacle to wider DSI deployment is insufficient understanding of the technology. Process engineers have long been focused on strategies to minimize the condensate lost in indirect heat exchange systems. DSI removes condensate from the equation in a way that may seem “too good to be true.” As one of the primary manufacturers of DSI systems in the United States, Pick is able to provide data and case studies to back up its efficiency and cost-savings claims, and can conduct a customized energy comparison study.
Although there are a handful of applications in which DSI is not appropriate — low-pressure systems, for instance, or systems processing liquids that must be kept separate from steam — the technology represents a giant leap forward for a wide range of industry needs. To learn more, contact us.