Page 404 - Boiler_Operators_Handbook,_Second_Edition

Controls 389

corrode them. A normal low limit for natural gas is fice buildings, etc. simply can’t justify a system without
170°F, fuel oils used to cause corrosion at all tempera- pumps so they all include pumps to move the fluid
tures below the maximum operating temperature for around between boiler and heat user.
heating boilers (250°F) so operation at 240°F was recom- Unlike steam boilers where load is balanced by the
mended. If you fire oil most of the time you should ask flow of steam from its source of generation to the load,
your supplier for the normal acid dewpoint temperature hot water boilers cannot function with a plant master
of the oil and try to keep your water temperature above that controls the firing rate of all the boilers. Some sys-
that. The lower the start temperature the less loss due to tems have a master temperature controller but it doesn’t
cycling so review the section on steam pressure mainte- control the firing rate of each boiler; more on that in a
nance to get an understanding of how to set proportion- minute. There have been attempts to produce common
al fluid temperature controls. Also review the discussion control by operating the boilers in series (water flows
on thermal shock. through one boiler then the next and so on) but I have
For multiple boiler systems and large facilities the yet to see one that works well.
setting of hot water controllers is a little different be- When fluid heating systems become so large that
cause the pressure maintained in a steam boiler pushes the volume of fluid in the boiler is a small part of the
the heat out to the facility; in fluid systems the heat is entire system, control of the water temperature becomes
transferred by other means. There are basically two difficult. Another factor is the volume of water in the
methods for transferring the heat and both rely on mov- boiler; fire tube boilers contain a large volume of water
ing the heated fluid out of the boiler to the heat using and can have long residence times (how long the water
equipment and returning the fluid, after it has given up stays in the boiler) but water tube boilers can hold so
some of that heat, to the boiler to pick up more heat. little water that it’s replaced every few seconds.
The simplest method is gravity and it relies on the Boilers like that (with short residence time) can
difference in density of the fluid as it is heated. Most have a problem because the temperature of the water at
fluids expand when heated. They take up more space. the sensor is not the same as the average temperature
The density of the fluid (number of pounds per cubic of the water in the boiler. Temperature maintenance of
foot) decreases. The hotter fluid tends to float up in any those units can get erratic so another control method
pool of colder fluid just like a block of wood floats to the is required. The controls are very typical of high tem-
top of water because it is lighter than the water. A boiler perature hot water boilers (HTHW) which operate at
system with a proper piping arrangement can use this temperatures over 250°F and pressures over 160 psig.
to force the heated fluid in a boiler to flow up through For boilers heating water the method is easy to un-
the pipes to radiators on the upper floors because the derstand, you’re adding Btus to the water so the energy
fluid cooled in the radiators fills the return lines to the required is equal to the pounds of water going through
bottom of the boiler. The colder water is heavier than the the boiler and the temperature difference. The actual
lighter, hotter water producing a thermal siphon. We call heating load is determined by multiplying the pounds
it natural circulation. of water flowing through the boiler by the difference in
Only simple small systems use natural circula- inlet and outlet temperatures. For other fluids all you
tion. Even most small residential systems use an electric need do is multiply by the average specific heat of the
pump to circulate the water. The pump can produce liquid. Control logic that performs that calculation pro-
far more force to circulate the water than the thermal vides a very responsive control because any change of
siphon effect so pipes can be smaller and the system inlet temperature or fluid flow rate produces a change
costs less to install. If you’re buying a hot water system in the control signal, increasing or decreasing the firing
for your house you may want to think about that; the rate of the boiler to compensate. Since multipliers were
initial cost savings achieved by installing the pump is a problem in early controls most plants relied on a con-
rather quickly eaten up by the cost of electricity to run stant fluid flow so only the temperature difference was
that pump. A system designed for pumping won’t work needed to develop the control logic.
well on gravity when you need heat, the power is out, These systems cannot operate on that logic alone
and you try burning some wood in your furnace. Some because there’s no way to correct for changes in boiler
increase in initial cost may save a considerable amount efficiency or small errors in flow and temperature mea-
on electric bills and ensure the ability to get heat if the surement that would produce an imbalance between the
pump or power fails. actual load and the firing rate. A temperature controller
Large hydronic heating systems for schools, of- is used in these systems to provide a means of correct-