Page 143 - Boiler_Operators_Handbook,_Second_Edition

128 Boiler Operator’s Handbook

delay in response to changes in the boiler firing rates.
Checking the temperatures and a little math will
allow you to determine what percentage of the water is
flowing through the operating boiler. When waters of
two different temperatures are mixed the resulting tem-
perature is dependent on the quantities of water at each
temperature. The percentage of water flowing through
a boiler will equal the difference between the mixed
water temperature (Tm) and the return temperature
(Tr) divided by the difference between the boiler outlet
temperature (Tb) and the return temperature times 100;
boiler water flow as % of total = (Tm-Tr) ÷ (Tb-Tr). This
formula comes in handy when you want to know how
much water is in each part of a mixture.
You can also use the basic formula for energy to
determine how much heat is lost in an idle boiler, the
temperature at the outlet will be lower than the temper-
ature at the inlet. As in all cases where you’re comparing
Figure 4-5. Hydronic boilers in series
differences in gauge or thermometer readings it’s a good
idea, where possible, to switch the devices so you have a
of arrangement put a cardboard blank over the com- different reading from the same instrument.
bustion air inlet to minimize the airflow due to draft. Series operation of hydronic plants requires the
The hot boiler will still waste heat to the boiler room as piping arrangement allow for total flow through each
radiant losses and some thermosyphoning of the air will boiler and means for isolating the boiler which requires
occur in the stack so it’s not the best solution. three valves, two valves to isolate the boilers and one for
Closing one of the valves (supply or return) on bypass as shown in Figure 4-5. The water is heated first
an idle boiler will eliminate the heat losses but it will in one boiler then its temperature is raised further in the
change system and boiler flows and those effects have second boiler. These systems commonly use a header
to be considered. Some boiler plants have a bypass line temperature controller to regulate the firing rate so the
between the supply and return headers that simulates two boilers fire at the same rate. When the boilers are
the pressure drop of one boiler so you can open it after controlled independently the modulating controller for
closing off a boiler to restore the flow rates in the operat- the first boiler has to be set lower than the second one so
ing boiler and system to normal. it doesn’t take all the load.
When operating with less than the full comple- Without the common controller you will find your-
ment of boilers on line and bypassing around or through self constantly adjusting the controller set points (or
others be aware that the system supply temperature will firing one boiler on hand) to fire the two boilers evenly.
be less than the boiler outlet temperature because it is An alternative to the common controls is using the po-
mixed with the return water flowing through the idle sition of the second boiler as a controller for the firing
boilers or bypass. Some plants use a header temperature rate of the first boiler, simply adding another rheostat
control so the idle boilers or bypasses doesn’t change to the modulating motor of the first boiler and installing
the hot water supply temperature. It will require higher a selector switch will allow both single and two boiler
temperatures in the operating boiler. operation.
If you have a common header temperature control
it should be on the return. These systems usually have a
proportional control so the firing rate of the boiler will be BOILER WATER CIRCULATING PUMPS
proportional to the difference between return tempera-
ture and the set point (desired return temperature). The Boiler water circulating pumps are common on hot
return temperature will be held near the set point but the water heating boilers and consist of a small pump that
supply water temperature will vary depending on the delivers a fraction of the total flow through the boiler. The
blend of firing and idle boilers or bypasses. It won’t hold purpose of the pump is to help eliminate thermal shock.
a constant return temperature either because there’s a They pump enough water from the boiler outlet back to