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…
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
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
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