Page 400 - Boiler_Operators_Handbook,_Second_Edition

Controls 385

wise operator should know that he can improve the above that setting.
quality of his operation by adjusting that switch. It has Before we get off the subject of setting the pressure
two adjustments; one is the pressure at which it opens control switch there’s a question of where we set the
its contacts as the pressure increases to stop operation. stop pressure, the main setting of the pressure control
The other adjustment is the differential which is the switch. Many operators are instructed to set it as low as
difference between the contact opening pressure and possible because that makes the steam and water tem-
the pressure when the contacts will close again. Well, perature lower to cool flue gases more and reduce stack
to be honest the setting could be the pressure it closes losses. I will contradict that theory because the small
at and the differential determines when it opens; there savings in lower stack temperature will be lost with
are both types. more frequent cycling of the boiler. Set the switch as
Set (stop) pressure less differential equals start high as it can go and still prevent operation of the high
pressure. Many operators think they should set the dif- steam pressure switch (see burner management) and it’s
ferential as low as possible so the pressure won’t swing for two reasons. One, the larger the spread the longer the
as much. The result is an increased cycling of the boiler run time for a boiler when it’s cycling and two, the more
and lower efficiency (see cycling efficiency). To get the room you have for continuous operation.
best performance out of your boiler you should establish Now we can talk about modulating controls and
the widest possible operating range. the most common of those is a simple electrical propor-
For a simple on-off boiler operation your operating tional control system. A pressuretrol (trademarked name
range is the differential setting of that switch and the dif- of Honeywell) connects to the steam space in the boiler
ferential should be set as large as you can tolerate. You and consists of a diaphragm or bellows connected to me-
should find that you can set it larger in the summer than chanical linkage that adjusts the position of a wiper on
you can in the winter. The boiler will not start as often. a coil of wire. The coil has a constant electrical voltage
It will run longer on each operation but that reduces across it supplied by a transformer. Voltage at any point
the frequency of starts so there’s less of them for higher on the coil is proportional to the position along the coil
overall operating efficiency and less wear and tear. because the wire has a constant resistance. A matching
The next obvious question is “how do you know coil is provided in the modulating motor that changes
how low you can go?” You need enough pressure so all the firing rate of the boiler.
the heating equipment in the facility your boiler is serv- The wipers are not exactly like an automobile
ing operates properly. Frequently it’s the one that’s the windshield wiper but they operate similarly, swinging
longest piping distance from the boiler but sometimes so they touch the coil at any point from one end to the
it’s equipment at a shorter piping distance but the pres- other. A schematic of the system is shown in Figure 11-
sure drop to that one is higher or it is not as oversized 19. When the steam pressure changes it moves the wiper
as everything else. The best way to determine it is to along the coil in the pressuretrol. The voltage between
gradually drop the lowest pressure (increase switch dif- ground and the wiper in the pressuretrol will change
ferential) until someone complains then raise it a bit. which produces an electrical current through the wiper
If you can wander the facility you can read pres- to the balancing relay and the wiper on the coil in the
sure gauges and find it. Unless the equipment operates modulating motor. The balancing relay is upset by the
at full capacity summer and winter and it has its own current when the two coils don’t match so it makes one
steam piping from the boiler plant you can do the same of the electrical contacts which drives the modulating
thing in the summer. Summer loads are usually lower motor.
than winter loads so piping pressure drops are less and The direction of the motor is determined by the
steam demand on the equipment is less so you can drop voltage imbalance so it runs in a direction that moves
the pressure a little more at the boiler. its wiper until it is at the same position as the wiper in
A typical heating plant with a switch setting of the pressuretrol. When the two wipers are in the same
12 psig can usually operate well in the summer with position the voltage is the same and no current will flow
pressures lower than the maximum differential adjust- through the balancing relay so it centers to stop the mo-
ment of the switch. I’ve seen plants that operate as low tor operation. The system rotates the modulating motor
as 2 psig; however, they had to install a special switch proportional to steam pressure so it is basically a pro-
arrangement to get that spread. There’s a caution here portional controller.
that is covered more later. Don’t allow the starting The pressuretrol has two settings just like the
pressure to go so low that the boiler will modulate pressure control switch. One establishes the center of