Page 141 - Boiler plant and distribution system optimization manual

126 Boiler Plant and Distribution System Optimization Manual

tions it can save a great deal of money. These are Other conditions poor control system linear-
sensitive instruments that require special atten- ity may cause, are that a boiler may only operate
tion in maintenance and calibration. The CO trim between certain restricted loads, like from 40% to
system delivers its best performance and payback 65% or from 80% to 100%. This is often a hidden
when all other parts of the combustion process error which makes smooth boiler control almost
are working perfectly. impossible and it takes a good analysis to uncover.
This is one of the many good reasons for
equipping boilers with good instrumentation, in-
4. CONTROL SYSTEM LINEARITY cluding fuel meters. This condition will cause a
boiler to cycle on and off or it may cause a boiler
Cost to operate below rated capacity making it neces-
Low sary for additional boilers to carry the load. Both
conditions waste fuel and make it very difficult to
Potential Savings do a good tuneup.
High When a boiler is first put into service, con-
trol system response should be checked to insure
Description that its operation is linear. Many boilers have this
When control systems have not been de- hidden problem which interferes with efficient
signed or calibrated for linearity they do not re- operation and makes life very frustrating for op-
spond properly especially when programmed to erators.
work in coordination with other boilers. If you
could compare a boiler’s control system to an au- Advantages/Disadvantages
tomobile’s accelerator, linearity error is like just It takes time and additional expense to in-
resting your foot on the gas pedal and having it sure that a boiler’s firing system is linearized and
accelerate to full speed with very little foot mo- operating in its rated operating range. It’s one of
tion. In another case, having your car not respond those things taken for granted when equipment
to large change in position of the gas pedal and is new. Unless this problem is resolved, it may be
only getting a response on the last sixteenth of an difficult to optimize the performance of a boiler to
inch of travel (Figure 10.22). the fullest extent.

Figure 10.22—Control system linearity. Non-linear response can cause control problems
and inefficient operation