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CONTROLLING STEAM HEATERS

© Walter ********, P. Eng., 2000 May 20. walter(at)********(dot)ca

First published in Hydrocarbon Processing , November 1996.

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INTRODUCTION. Steam Heaters are simply heat exchangers in which one of the media is steam being condensed while the other is a process fluid being heated. In doing this, there is a phase change which puts special demands on the process control system. It is difficult to generalize about the various options for control. Special system requirements often put unexpected constraints on the process. Even the orientation of the exchanger can have peculiar and unexpected results.
[IMG]http://www.********.ca/ce4_sh/4-1.jpg[/IMG]A SIMPLE STEAM SPACE HEATER. Figure 4-1 shows a steam heater such as those used to heat a warehouse. This simple example demonstrates many of the characteristics of steam heaters of all sizes and applications. Steam enters the heater at the top. As the moving air draws away the heat, the steam condenses. The condensate flows down the tubes, through the steam trap, and into the condensate drain header.
The function of the steam trap is to prevent steam from blowing through into the condensate system. It is the one essential part of any steam heater and will receive further attention later. For now it is sufficient to say only that it passes condensate and blocks steam.
This system tends to be rather self-regulating. The moving air rises to some temperature approaching that of the steam and draws away as much heat as it can. Colder air will draw more heat, and warm air will draw less. The steam trap is essentially a level controller with a set point of zero.
This arrangement can be compared to a shell and tube exchanger where the room itself is the shell and the air is the process stream. The fan draws some of the air through the heater and then blends it with the remaining air in the room. The first level of control complexity is to add a thermostatic switch to control the fan. As with any exchanger on bypass control, the sensing element must be placed at a point where the two stream has mixed sufficiently to provide an representative temperature (not directly in front of the fan, as the drawing shows). When the temperature in the room reaches the setpoint, the fan will stop and the air immediately around the tubes will rise to the steam temperature. The heat withdrawn will be reduced until only a small amount of steam is condensed.
If it were practical to stop all air circulation and to fully insulate the heater so that no heat is transferred out of it, steam condensation would cease and no condensate would flow through the steam trap. This is not practical, so on a hot day any amount of steam that still is condensed by air convection is a complete waste. Furthermore it adds to the heat in the room. Thus the next level of complexity is to block the steam to the heater. When this is done, the steam already in the heater condenses, the temperature drops to room temperature and the pressure drops to the corresponding vapour pressure. Condensate will not flow through the trap once the pressure drops below that of the condensate header. Because of the higher density of water, a given volume of steam condenses to a much smaller volume of condensate. The final equilibrium is reached with a pressure of about 2.8 kPa_abs (0.4 psia), essentially full vacuum, and with the tubes about 0.15% full of water. (The steam supply in this example is assumed to be at 170 kPa_ga (25 psig), fully saturated.)
The simple system described above, minus the fan, is used for many non-process heating applications such as steam tracing or open tank heating.
STEAM TRAPS. As steam condenses, the resulting water drains downward. A steam trap is placed at the low point of the system. It is a valve that opens to allow the water to drain out into the condensate system but closes when all the water has been drained and steam tries to pass through. There are numerous varieties of steam traps operating on various principles. A detailed discussion of various types can be found in the article Steam Traps, Key to Process Heating¹ by Haas.
CONTROLLING A PROCESS HEATER. The parameter of interest in any process heater is the temperature of the process stream at some particular point in the process. There are essentially only three means of control:
· Bypass a fraction of the process stream around the exchanger and blend it with the fraction that has passed through.
· Vary the effective surface area of heat exchange. This is accomplished by restricting the outlet and partially flooding the exchanger with condensate.
· Vary the temperature of the heating medium. This is accomplished by throttling the steam and dropping the pressure of the steam in the exchanger.
Each of these is discussed in turn below.


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