Criteria for Testing your Furnace


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Document Category Engineering
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Criteria for Testing your Furnace

  • Excessive smoke from stack.
  • Three months since last efficiency check
  • High flue gas temperature reading from furnace log or indicator in stack
  • No change in operating condition with change of burner firing rate
  • Physical appearance poor, with badly sealed doors and flames escaping from furnace, poor
  • Insulation and refractory(s)
  • Little or no fitted instrumentation in working order
  • Manually adjusted burners
  • Large fluctuations in furnace draft readings
  • Installed flue gas analyzer indicating very high percentage of carbon dioxide, very low
  • Percentage of oxygen (less than 2%), or carbon monoxide in excess of 20 PPM
  • Produce quality problems

It is usually possible to identify some energy conservation opportunities based on the quick audit of your furnace. These opportunities are:

  • Reducing losses from openings and leaks
  • Reducing losses during recycling and idle equipment shutdown

Heat losses through openings can be estimated from the size of the opening. The amount of heat loss is linearly proportional to the area of the opening, and is approximately proportional to the fourth power of the absolute temperature. Chain curtains can be used to reduce the loss by 80%; the operator/energy surveyor can estimate the savings using the following steps:

1. Estimate or measure area of opening
2. Determine heat loss per unit area for furnace operating temperature

Annual heat loss = (heat loss/unit area) (area of opening) (hours)
Losses resulting from cold air leakage into the furnace are serious. The “chimney effect” of a furnace and flue can cause a slight negative pressure at the bottom of the furnace combustion chamber. The pressure is lowest at the furnace floor and rises to the local barometric pressure where the flue exhausts to the atmosphere. The difference in pressure is related to the furnace temperature, the height of the stack above the furnace floor, and the size of the opening through which the cold air is infiltrating.

The cold air may be infiltrating because of poorly adjusted dampers or door seals in need of repair. Ninety percent of the heat loss can be prevented through modification when the seals are very poor. If the seals are good, 50% of the loss can be prevented.

The operator/energy surveyor can estimate the savings using the following steps:

  • Calculate area of opening
  • Estimate or measure flue exhaust height above furnace floor
  • For furnace operating temperature and flue height, obtain heat loss from the given table above
  • Heat loss per year is calculated = (area of opening)*(heat loss/unit area)*(operating hours/year) = watts/year
    Savings = (percent saved)*(yearly heat consumption)
  • The operator/energy surveyor may be able to identify opportunities for reducing losses during recycling and idle equipment shutdown by reviewing operating practices.
    In the absence of specific data, it is very difficult to quantify savings associated with these measures.

The plant operator needs to know such information as:

  • Time needed to cool furnace to room temperature and several intermediate temperatures with burners off
  • Rate of fuel use when idling furnace at operating and intermediate temperatures
  • Reheat times from room temperature and intermediate temperatures
  • Fuel needed to reheat from intermediate temperatures
  • Maximum rate of temperature change that will not result in equipment damage
  • More involved furnace energy audit includes the following activities:

Prepare for audit with some essential steps to be taken as follows:

  • Repair/install instrumentation as necessary
  • Identify testing positions
  • Select appropriate test day
  • Perform test
  • Testing procedures and techniques
  • Data collection
  • Evaluate data
  • Calculate efficiency
  • Determine improvements

Identify conservation opportunities such as;

  • Tune-up
  • Improved control
  • Heat recovery
  • Alternative fuels

Types of furnace tests include the following:

  • Full commissioning test
  • Efficiency test to standards as laid down by bodies such as ASME and British Standards
  • Efficiency test by loss method
  • Combustion test only

The most common test is the efficiency test by the loss method. The other methods are used less frequently and are variations of the loss method. Although they can be more accurate, the extra cost and time required to carry out the other tests are often not justified.

The simplified heat balance method does not take into account the heat content of combustion air, and it determines wall losses by subtraction. The calculations are few in numbers and are not complex. In estimating an energy balance; there are three different energy sources that must be considered:

  • Heat of combustion
  • Flue gas Losses
  • Sensible heat
  • Process heat
  • Energy input( in kJ/hr )= (fuel use) x (gross heating value)
  • Flue gas losses

Process heat is defined, as the heat required to does physical or chemical change in the material. This information is normally found in standard references.

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