Process fluid heating with steam is a well-understood topic— or is it? When heating process fluids with steam, there are several options, using indirect or direct heating. When reviewing and selecting the best technology for your application, how do you choose a process heating system? For example, is indirect heating (heat exchangers) or direct heating (spargers and direct steam injection) the better alternative?
Regardless of your familiarity with each technology, heating with steam is an integral method that, if not fully optimized, will increase maintenance and operation costs while creating inefficient processing conditions. On the other hand, indirect heating systems are more widely known but much less efficient than direct heating systems, ultimately making a choice clear when deciding on the right heating technology to optimize your processing needs.
Direct steam options are essentially 100% energy efficient, utilizing both latent and sensible heat, eliminating the condensate energy loss.0
The Hydro-Thermal Advantage
Hydro-Thermal Direct steam injection heaters are designed to internally modulate the mass flow rate of steam at the injection point and utilize the principle of choked flow. A nozzle or a diffuser is generally used to modulate the steam flow and inject it at sonic velocity.
Choked Flow
Choked flow can be defined as a fluid dynamic condition. At a given pressure and temperature, a compressible fluid passes through a restriction (vena contracta) into a lower pressure region resulting in increased velocity. The rate increase occurs with the mass flow being unchanged, following conservation of mass. Therefore, the generation of choked flow can be identified as the operating point at which the ratio of the downstream pressure to the upstream pressure falls below the critical pressure ratio limit. While the choked flow condition is achieved, a change in the downstream pressure will not affect the flow velocity — if the pressure ratio is within the recommended critical pressure ratio limit.
A sonic velocity for steam injection can be achieved across a flow restriction such as a nozzle or orifice when the absolute pressure ratio between the process pressure (liquid) and the steam pressure is below 0.575 (i.e., downstream absolute pressure is 57.5% of the upstream absolute pressure). Using sonic velocity for steam injection uniformly disperses the steam into the process liquid, achieving a consistent temperature profile. Natural turbulence is introduced within the controlled volume.
Reduction of Energy Loss
The sonic velocity steam jet also provides mixing velocity to condense the steam vapor into the liquid instantaneously. This rapid condensation of the steam helps eliminate problems such as two-phase flow and the formation of bubbles. Internally modulated direct steam injection heaters achieve complete condensation of steam and uniform energy dissipation before the liquid exits the discharge of the heater.
Minimal Maintenance and Eliminating Steam Hammer
Sonic velocity steam also produces another positive outcome: a self-cleaning action. When properly sized for the process conditions and the application, the internally modulated DSI heater eliminates scaling and fouling, minimizing maintenance requirements common to other steam heating designs.
A direct steam injection heater with internal modulation can be installed inline or around a tank to prevent the possibility of having live uncondensed steam within the given system, eliminating the potential for steam hammer in pipes and tanks. Internally modulated heaters utilize a single nozzle that focuses the incoming steam and accelerates it to a very high velocity (usually sonic velocity) before contacting the process fluid. This high velocity helps atomize the steam vapor and shear the incoming process fluid, allowing for a more complete, efficient heat transfer. The result is no steam hammer, precise/uniform heating, and reduced maintenance.
Conclusion:
While direct heating cannot be used for every process heating application, it is a wise choice in many applications. The proven attributes of Hydro-Thermal heaters are the most attractive process heating option available compared to other heating systems. Maintenance requirements are reduced or eliminated, and the small size of DSI heating systems reduces floor space requirements, leading to a more efficient plant layout. In addition, precise temperature control saves energy by heating fluids to exact temperatures, lowering setpoints, and enabling operational consistency, resulting in improved product
