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Lower energy requirements and better temperature control make direct-contact steam injection attractive for heating many process fluids.
Precise temperature control, reduced energy consumption, and lower maintenance costs are the key benefits of direct-contact steam injection heating in chemical processing applications. Unfortunately, directcontact steam injection is often overlooked or not used to its full advantage by chemical engineers because they are unaware of its potential in chemical applications.
This article offers guidelines for the use of steam injection heating. It explains what steam injection heating is, describes the types of equipment used, outlines several common applications, and summarizes the information required for selecting and sizing a steam injection heater.
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There are two basic types of exchangers used to transfer heat between process fluids - direct and indirect. Generally, indirectcontact heat exchangers, such as shell-andtube, plate-and-frame, or scraped surface exchangers, have two or more fluid flow paths that do not allow for direct mixing of the fluids. They promote the transfer of heat 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 precisely metered amounts of steam from a plant's steam supply into the process fluid (liquid or slurry) that needs to be heated (Figure 1). Injecting steam directly into the process fluid results in more rapid heat transfer and more efficient energy usage than indirect heat exchangers. Direct-contact steam heating can provide 100% thermal efficiency, because both the sensible and the latent heat of the steam are used. Energy savings can be considerable -- reductions in the 2W25% range are not uncommon.
Additional benefits of direct-contact heating relate to plant layout and maintenance. Both equipment expense and space requirements can be trimmed: Direct-contact steam injection systems generally require less space than other methods of heat transfer, and they also eliminate the need for condensate return systems, which are often required for indirect heat exchangers. Additionally, maintenance costs may be significantly lower than those of other heating systems.
A potential limitation of direct steam injection heating is that the steam is injected directly into the process fluid. If the process fluid is sensitive and cannot tolerate this, steam injection cannot be used. The addition of...