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Special Systems                                                                                      131

               HTHW plants. I consider them to be far more dangerous
               than any other kind of boiler plant. The heated water con-
               tains a lot of energy and any rupture of a piping system
               or a piece of equipment will result in a steam explosion.
               The rupture of an HTHW pipe will discharge almost 100
               times as much steam as a steam pipe with steam at the
               same temperature. The number and location of exit doors
               from a HTHW boiler plant should greatly exceed those
               for a steam plant and any control room should have at
               least one exit that leads directly outdoors.
                                                                                  Figure 4-7. Fluid heater


               ORGANIC FLUID HEATERS AND VAPORIZERS                 ers will also have a steam boiler that must be in opera-
                                                                    tion in order for the organic device burners to function
                    Organic fluid is basically oil, hydrocarbons that  because the steam is used to quench any fire that might
               are used as heat transfer fluids because they have much  occur in the organic device.
               lower vapor pressures than water. What that means is     Normally a thermocouple in the outlet or stack
               they can be heated to higher temperatures before they  is  monitored  and  any  rapid  increase  in  temperature
               evaporate. Organic fluids are available that will remain  automatically results in burner shut down and opening
               a liquid and not evaporate at temperatures as high as  of the steam quench valves. A few small units are fitted
               800°F at atmospheric pressure. By and large these mate-  with compressed CO  extinguishing systems to avoid
                                                                                       2
               rials function the same as water and steam, they simply  the provision of a steam plant but it takes a lot to put out
               evaporate and pressurize at much higher temperatures.  an organic heater fire. Once it takes off, any leak adds
                    Organic fluids are used to produce high tempera-  enough fuel to melt more of the boiler metal to allow a
               tures without the expense of handling high pressure. A  bigger leak and bigger fire.
               system can be designed to operate at 500°F (a common     The higher temperature fluids tend to have high
               maximum operating temperature) and pressures not  pour points. That means they don’t flow well, if at all, at
               exceeding 30 psig where a steam or HTHW plant would  normal atmospheric temperatures and the system will
               have to operate at almost 900 psig. Both liquid and vapor  freeze up on shut down. Fluid systems for those high
               systems are considered high pressure plants because the  temperature fluids use steam tracing to warm up the or-
               temperature is always higher than 250°F. The boiler is a  ganic fluid enough that it can be circulated in the system
               power boiler even if the operating temperature is below  in order to get it started.
               15 psig. A fluid heater is basically the same as a hot water     One operator I know is very happy that he’s op-
               boiler and a vaporizer is very much like a steam boiler,  erating the fluid heaters at his plant. He told me he’s
               the principal difference is the operating temperature.  happy because “I don’t have to fool around with water
                    The typical fluid heater (Figure 4-7) looks a lot like  treatment.” While it’s true that organic fluids don’t need
               a common firetube boiler from the outside and many  the attention of a water plant, because the systems are
               operators confuse them with a firetube boiler. They’re  designed to retain the fluids and vapors so there’s no
               actually water tube boilers. What looks like an outer  to little makeup, the fluids do break down and regular
               shell is a casing. The tubes form one continuous coil sur-  sampling and chemical analysis is still required.
               rounding the furnace and in many cases are two coils     Over a period of time the fluid can break down and
               to produce a secondary pass surrounding the furnace  has to be replaced or reconditioned. Scale as we know it
               pass. Unlike a firetube boiler, flow has to be proven in  in water based systems isn’t a problem but carbon can
               these units before the burner is started and flow must be  build on the inside of tubes just like scale if the boiler
               maintained or the burner should be tripped.          is fired too hard, fluid flow is lost, or the fluid begins
                    Other significant differences between steam and  to break down, and that can eventually result in a tube
               organic  fluids  include  flammability,  especially  when  failure. A tube failure can result in the entire heater melt-
               they are heated to such high temperatures. If a water or  ing down so there is a concern for proper operation to
               steam boiler has a leak the tendency is to put the fire out.  prevent carbon formation just like there are concerns for
               If an organic heater or vaporizer has a leak the tendency  scale formation in a water boiler.
               is to add to the fire. Almost any plant with organic heat-     Monitoring the pressure drop across the liquid
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