Boiler Controls: Low-Cost/No-Cost Energy Conservation
EP Editorial Staff | January 12, 2010
Want to get the most out of your fired equipment? Consider these suggestions for improved control settings.
So your budget is tight and you’re pressured to make some energy savings somewhere? Why don’t you try some control adjustments? “Like that will make any major change,” you say. That depends on how your boilers and processes are set up right now. Usually, as long as the boilers work, who really cares? This is precisely why your controls deserve a closer look!
Achieving the highest efficiency possible
Experts in the energy-conservation field typically say that the highest efficiency you can hope to achieve will come as a result of the boiler continuously producing what the load demands—and changing only slightly with each slight change in the load. This requires flame modulation. Most medium- to large-size boilers will have modulation control, in addition to on/off controls. Take a few moments to learn where your boiler turns on and off and where your modulation controls operate—and jot this information down.
Most boiler installations are set up to operate as follows…
- The safety valve is set to blow off at 125 psi and reseat at a few pounds below that.
- The high-limit control is set to cut out the boiler at say 120 psi and requires a manual reset once it has tripped off.
- The boiler regularly shuts down at say 115 psi and restarts at 100 psi.
- The programmer will start up and run through the startup cycle at 100 psi.
- The modulation control will be at high-fire at 103 psi and at low-fire at 112 psi.
For the installer, this mode of operation is great. It shows that the controls operate at their minimum and maximum settings, and function during a demonstration for the owner/operator customer at startup. The installer then gets the final payment approved and is out of there. As far as energy savings are concerned, however, this way of operating is the absolute pits. That’s because owner/operators rarely changes those original settings later on.
How this control set-up usually works
- Pressure builds up and the boiler shuts down at 115 psi.
- In order to start up again, the pressure must drop to 100 psi; the programmer starts up at that point.
- The pre-purge sequence, which usually takes about 30 seconds, starts as the burner damper is moved to the full-open or high-fire position.
- Next, there is a purge hold for, perhaps, another 30 seconds. At the end of this hold, another 30 seconds is required as the burner damper moves to the low-fire position.
- Once the damper proves it is at the low-fire position by tripping the low-fire switch, the pilot solenoid is energized and the pilot is lit. (There is usually about a 10-second trial to make sure the pilot will stay lit.)
- The main burner gas valves open up and the main flame is lit. It, too, has a trial period of about 10 seconds to make sure it will stay lit.
- The programmer hits RUN and the main flame is released to start modulating.
- At 103 psi, the modulation control starts to move the damper motor to turn down the fire. In about 30 seconds, it will reach its low-fire position, but the heat and pressure is way ahead of the damper’s movement. In light of the hot setting, it overshoots and shuts down the boiler. Pressure, though, continues to rise to 118 psi.
- The programmer, by now, is well into its post-purge and modulation is headed back to its high-fire position, never really reaching the low-fire position as it shuts off.
- With the burner fully shut down, the pressure starts to drop again.
- At this point, the entire cycle is about to repeat itself.
The deficiency is…
There is barely enough time in this cycle to establish internal boiler circulation before the burner is off. Once established, internal boiler circulation is important in that it aids in the removal of energy out of the hot flue-gas and into the boiler water. That tends to lower the stack temperature.
High stack temperatures (200 F degrees hotter than the steam produced) usually indicate considerable energy loss right past the heat-exchange surfaces and up the stack—off to the four winds, never ever to be recovered. Under light or heavy loads, this performance changes very little. With a light load, the overshoot almost trips the high limit. Under a heavy load, the boiler pressure may become extremely low before the programmer’s lighting sequence releases the main burner and allows modulation. The boiler pressure curves always have a very cyclical action and are far from that steady ideal discussed earlier.
Make a difference with new, more efficient settings
- The safety valve remains, as before, at 125 psi.
- The high limit remains unchanged, at 120 psi.
- The burner off point also remains the same, at 115 psi.
Here is where the change takes a major departure from the original setup!
- Readjust the burner ON point to as close as you can get to the burner OFF setting. You may find, at this point, that your boiler was fitted out with an inexpensive pressure control that may not have a very close differential capability. Here you may need to replace the control with one that has a separate adjustable differential screw with a minimum 2 or 3 psi setting.
- The modulation control may have to be readjusted a bit to get it to function in the proper place.
How the new, more efficient settings should respond
We will go through the same sequence as before.
- Burner shuts off at 115 psi and pressure begins to drop.
- The operating control starts the programmer at 113 psi.
- The damper moves to high-fire and holds there for 30 seconds.
- Damper then moves to low; it takes 30 seconds to reach low-fire.
- Low-fire switch then makes and the pilot is lit, followed by a 10-second pilot trial.
- The main flame lights, followed by another 10-second trial.
- The programmer goes to RUN, and the main flame is released to modulate.
Where the pressure drops with these new settings
- On extremely light loads, you will see the damper at low-fire and the boiler will be shutting down and starting up because the load is below the low-firing rate
- On light loads, expect the modulation to be on low- or ¼-fire with a continuous flame.
- On heavy loads, the pressure would drop faster to a ½- or ¾-fire position initially and still operate with a continuous flame.
- Only on cold startup will you see modulation at high-fire for very long.
With this continuous flame, you will see the modulation and the burner damper stay in one position for several minutes and jerk slightly to match any small change in load. A large change in load will cause the burner damper to move in a series of tiny jerks.
The boiler now has now achieved that steady state of load matching which the experts say is the most economical fuel use possible. Internal boiler circulation is established and remains steady. This lowers the stack temperature by extracting the most heat from the flue gas. Running at the lowest firing rate you can get makes the heat exchange surfaces seem larger to the flame. You achieve the lowest BTU input per square foot of boiler heating surface possible and still maintain proper operating pressure.
The burners of older boilers were only capable of a 3:1 turndown ratio. What does that mean? Well, at full- or high-fire, the boiler will operate at the nameplate rating of BTU/hour input or output. A 3:1 turndown ratio simply means the boiler is capable of operating at one-third of the nameplate rating. Put another way, a 6 million BTU/hour boiler will operate down as low as 2 million BTU/hour on low-fire before it cycles on and off.
Late-model burners, though, have been designed to operate at a 5:1 turndown ratio. This higher turndown ratio will allow your boiler to operate with a continuous flame down to one-fifth of the nameplate rating. That means our 6 million BTU/hour boiler can now operate down to the 1.2 million BTU/hour level on low-fire with a continuous flame before it has to cycle on and off.
Many boiler manufacturers offer upgrade packages to give your older boilers higher turndown ratios. It may be worthwhile to consider that type of improvement for your equipment. Remember, however, if the controls are not set to operate more efficiently, there’s just no way you are ever going to get the most from your fired equipment. MT
From Canadian Oxy’s Durez Plastics Division to Stevenson Memorial Hospital, industry veteran Gary Burger is now on the Board of Directors of the HVACR Heritage Centre Canada. Log on to: hvacrheritagecentre.ca