Managing the Oil Fired Maple Arch
Managing the Oil Fired Maple Arch
Written by Brian Chabot and Stephen Childs, Cornell Maple Program with technical information provided by Gary Bouchard at the 2009 Northern New York Maple Expo, January 31, 2009 in Potsdam, NY
Managing your oil-fired arch involves a tradeoff between maximum throughput of sap and maximum efficiency of fuel use. Most equipment is set up to obtain a high throughput, the most sap boiled per hour possible. Few are set up for maximum energy efficiency. Improving fuel use efficiency requires finding a technician with the proper instruments. At the Northern NY Maple Expo, Gary Bouchard, an experienced service technician, shared the sequence of adjustments that your burner technician should make.
Adjusting the Draft
The draft, or amount of air exiting the chimney, is measured with an air flow meter through a small hole in the exhaust chimney below the damper. The meter measures air pressure drop due to airflow. Acceptable draft with a fully fired arch should be 0.02 inches of water. Draft is adjusted using the damper. Counterweights will be increased or reduced to get the draft in the proper range. Excessive airflow will move heat through the arch before it has a chance to deliver heat to the pan. The draft adjustment should be done first.
Smoke Test
Smoke, or unburned carbon, in flue gas indicates poor burner performance. The amount of smoke should be measured with an instrument that measures unburned carbon. There may be too much unburned carbon even when you cant see visible smoke. Poor combustion can be caused by:
soot formation on the heating surfaces where the flame pattern is interrupted
insufficient draft, incorrectly adjusted draft regulator, improper fan delivery
poor fuel supply, malfunctioning fuel pump
defective firebox
oil-burner nozzle defect, wrong pattern or wrong size
excessive air leaks in boiler or furnace
wrong fuel-air ratio
The technician should make adjustments to achieve a #1 smoke level.
Carbon Dioxide or Oxygen in the Exhaust
Low CO2 content of the flue gas (less than 8 percent) or O2 content that is too high (greater than 7%) indicates incomplete combustion. This means that unburned fuel may be going up the chimney. Instruments are used to measure either the CO2 or O2 content of the exhaust gasses. These two gasses are related, so only one is typically measured. Low carbon dioxide content may be caused by:
burner nozzle is too small
air leakage into the furnace or boiler
under-firing in the combustion chamber
wrong fuel to air ratio
The higher the CO2 content without smoke, the more efficient the burner combustion is. If the technician is measuring O2, he will aim at 7% O2 at low fire and 5% at high fire.
Stack Temperature
Stack temperatures are a quick way to assess the efficiency of heat transfer within the evaporator system. They should be measured below the damper, through the same hole used for the other measurements above. Generally stack temperatures should be about 100ΊF above the syrup temperature for greatest fuel efficiency. Typical values should be between 300 - 500oF. High stack temperatures may be caused by:
undersized firebox for burner size
excessive draft
over-fired burner
draft regulator improperly adjusted
soot formation on the heating surfaces
Do You Have the Right Fire?
The oil burner, fan adjustment and nozzle design determines the amount of fuel and air available for combustion. There are several basic nozzle designs that affect the size and spread of the flame from burning fuel. Through the above measurements, you can determine whether your burner is sized for high heat delivery and sap throughput or for fuel use efficiency. Smaller nozzles, along with the above adjustments, can improve fuel efficiency. This will save you money for fuel, but may require more time at the evaporator. High throughput is gained by more fuel, bigger flame, and more heat going up the chimney (higher stack temperature). Even if you want high throughput, it makes sense that your system is operating at peak combustion and heat transfer efficiency. Making changes to the fire requires that you go back and check and adjust the items suggested earlier in this article.
Efficiency through Evaporator Design and Maintenance
Evaporator manufacturers take a number of steps to maximize heat delivery to the boiling sap. Increasing surface area under the pans with deep flues, insulating the arch, and the shape of the combustion chamber all affect how effectively heat is delivered to the sap. Higher temperatures in the arch increase heat delivery, but much of that excess heat will go up the chimney. Soot, unburned carbon, on the underside of the pans is an insulator that reduces heat transfer from the flame to the sap. Soot should be brushed or washed from the pans when a build up is visible.
Pre-heaters and the Steam-away improve fuel efficiency through capturing heat from evaporating steam. These do not replace having a well-functioning combustion system.
Capturing Excess Stack Heat
Some of the excess heat going up the smoke stack can be captured and put to use for the maple operation. Using excess stack heat to warm sap can be risky. For instance some have experimented with putting a coil of copper tubing around or in the stack to pre-heat sap. Two problems can result from this. First when the sap runs out or is first added to hot coils sap sugar may burn on the inside of the coil adding burn flavors to sap passing through the coil. Second, if the flow through the coil does not match the available heat, sap may turn to steam in the coil blocking sap flow and even developing explosive pressures. Having a system for heating water or providing hot air to warm the sugarhouse should be safer and more practical.
Measuring System Efficiency
The actual efficiency of your entire system can be measured by recording the gallons of sap boiled, the gallons of syrup produced, and the amount of oil used. The gallons of water evaporated are the difference between sap going in and syrup coming out. The gallons of oil used times 138,000 BTU gives you the total BTUs of energy used. Divide the total BTUs used by the number of gallons evaporated to get the energy used per gallon. Divide 9396 BTU by your energy used per gallon and you should get a number between 0 and 1. Multiply this by 100 to make it a percentage % so it should now be between 0 and 100%. This number would reflect the percentage of the burned fuel that accomplishes the desired evaporation. The 9396 is the BTUs estimate it takes to boil away one gallon of water with 100% efficiency.
Managing fuel costs in maple production can be an important part of controlling costs and making the maple enterprise profitable.
