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...employed for heating process streams. Heat exchangers employed in this service are known as process heaters.
The stall chart, which compares steam pressure at various steam-heater turndown ratios with the system back pressure, provides a general overview of a given steam heating application. A basic understanding of this industry tool is necessary, and can be acquired from a variety of sources [1]. However, the basic stall chart assumes that the steam heater has not been oversized, which is usually not the case; heat exchanger area is generally over-surfaced for fouling factors and capacity considerations.
An effective alternative is the extended stall chart (ESC). Easy to employ, this plot readily accommodates oversizing.
Review of the stall chart
As the demand load varies in any heat exchanger using inlet-modulating steam control, the delivered steam-pressure profile will correspondingly change. As an example, consider the heat exchanger (process heater) shown in Figure 1. For this setup, the steam-supply pressure ([P.sub.1]) can be relatively constant at a typical value of 150 psig. But the pressure ([P.sub.2]) of the steam delivered directly to the heat exchanger by the control valve may vary greatly. That means the inlet pressure ([P.sub.3]) to the condensate-drainage solution (CDS) will also vary considerably. (For simplicity, it is assumed that no pressure drop occurs across the heat exchanger, so [P.sub.3] = [P.sub.2]).
[FIGURE 1 OMITTED]...
NOTE: All illustrations and photos
have been removed from this article.
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