Energy Savings >> Ancillary Equipment Integral to Energy Conservation

Most boilers we encounter are designed to have a pressurized combustion chamber. Sadly, the vast majority of boilers that we audit are operating at negative draft over fire conditions. The energy losses associated with poor draft control are many, including:

• Poor combustion and burners that are "over fired."
  Most burners are equipped with a retention head that requires a specific differential pressure measured across a pressure plate/diffuser. On the high-pressure side is the forced combustion air, on the low-pressure side is where the fuel and air are mixed. If this differential pressure is too great, as is the case when the combustion chamber pressure is less than the design calls for, poor mixing of fuel and air results. This is called loss of retention and leads to increased excess air that result in significant losses through the stack (chimney). The rated manifold pressures of burners are often dependent on positive over-fire pressures. If the burner has been adjusted to a manufacturer’s specified manifold pressure, but the chamber pressure is lower than specified, the fuel input will automatically be greater than what the manufacturer calls for, resulting in an over-fired condition with associated inefficiencies.
  
  
• Poor thermal transfer within the combustion chamber.
  If the boiler is operating at designed chamber pressures, the transfer of heat within the combustion chamber is uniform and the maximum available surface area is being used. However, if the draft is not regulated and the chamber is at lower than designed operating pressures, the chamber temperature is not uniform, having higher temperatures concentrated towards the rear of the chamber. This results in one portion of the chamber being over heated and the other portion being under heated. Besides the obvious inefficiencies, different rates of expansion within the combustion chamber result in distortions of the shell material which ultimately will lead to costly tube or section seal failures.
  
  
• Standby losses.
  Without efficient draft control, excess draft will draw cool air through the burner housing and lead to stack (chimney) losses while the plant is in standby mode. These losses are often extreme within New York City due to the increased stack heights found in multi-floor buildings. TEC can provide draft control strategies that keep stack losses to a minimum.

Water Feed Systems

Keeping the water level at the exact designed steam drum level with respect to steam boilers is important for the following reasons.

• Too much water can reduce your steam production because the water surface area (steam producing area) within the steam drum is significantly reduced by an excessive water level. Also, there is more water to boil.
• If the water feed system cannot keep up with the load, the boiler will cycle off on a "low water" condition. This "short cycling" leads to the losses discussed in the section on PID controls together with the cooling of the boiler in the post and pre purge cycle.

TEC incorporates proportional water feed systems as a energy conservation strategy.

Another way of increasing efficiency is by raising the water temperature being fed into the boiler. Below is a graph showing the relationship between feed-water temperature and additional BTUs required to heat cooler water.

See System consultation to see how you can optimize efficiency by increasing feed water temperatures.

Oxygen Trim

To use an analogy, a linkaged burner is to a linkageless system what a carburettor is to fuel injection engine on a car. To further the analogy, oxygen (O2) trim would represent electronically controlled fuel injection (EFI) that is found in today's modern fuel-efficient automobiles complete with oxygen sensors.

Essentially, O2 trim is a stand-alone analyzer that constantly samples exhaust gas emissions and sends a trim signal to the burner's combustion control to automatically adjust the fuel, air or both. This constant "trimming" of the fuel mixture can optimize fuel efficiency by removing the buffer zone usually created by the person adjusting the burner.

Usually an extra percentage point of excess O2 is desired when adjusting a burner. This eliminates the possibility of building elevated levels of carbon monoxide (CO) under changing extraneous conditions. Changes that can affect excess O2 readings include ambient air temperature, over-fire draft, or variations in fuel supply pressure. (This is especially noticeable where dynamic gas pressures changes relative to demand in multi-boiler plants.)

O2 trim systems can be integral to the combustion control system or a stand alone system that can operate independently. O2 trim can significantly reduce excess O2 and maximize efficiency. TEC typically adds a 1% to 3% (site specific) gain in combustion efficiency with the addition of O2 trim in our ROI calculations.