Fired Equipment – Energy efficiency improvement of existing fired equipment is usually very lucrative. In many cases, fired heater duties have been reduced or even eliminated entirely by changing process operation or utilizing waste heat recovered from other equipment in its place and operating only equipment which is absolutely required for the process, safety, or continuity of operation. Where possible thermal duty should be shifted to efficient equipment and the less efficient shut down. Shutting down equipment is preferable to idling, since it also saves the non-production casing heat loss. Energy conservation can be achieved by operations, maintenance, or capital investment.
Energy Efficiency Guidelines for Heater and Fired Equipment’s
Evaluation of Potential Energy Saving
Heater efficiency is primarily dependent on two variables:
Stack gas temperature (TS)
Flue gas oxygen content (O2)
The efficiency can be read directly from Figure below once the %O2 in flue gas and the stack gas temperature are known. The overall heater efficiency obtained is the lower heating value (LHV) efficiency corrected for 2% casing (setting) losses.
To determine the potential energy and cost saving by reducing 02 and/or stack gas temperature the following procedure will be used:
Determine present efficiency (EFF).
Determine new efficiency (EFF1) with new O2 content and gas stack temperature.
Obtain present fuel usage rate (SCFH).
Obtain lower heating value (LHV) of fuel (BTU/SCF).
Obtain value of fuel ($/MMBTU).
Determine hours/year operation time for heater (HRS/YR).
Calculate Energy Saving (MMBTU/YR):
Calculate cost saving ($/YR).
Saving ($/YR) = (MMBTU/YR) x ($/MMBTU)
Potential Energy Cost Saving Example
Consider the following example of a 10 MMBTU/HR heater:
OVERALL HEATER EFFICIENCY
1. BASIS 1000 BTU/SCF GAS
2. 2% SETTING LOSSES INC’L
3. BASIS LHV
Heater Efficiency Enhancement through Operational Adjustments
There are two areas where furnace operations can be improved, resulting in savings:
(1) Draft Control
Most heaters are equipped with a stack gas damper to regulate the furnace draft at the arch. Lower than atmospheric pressure (0.05 inch to 0.15 inch H2O draft) IS maintained to avoid structural damage from hot combustion gas out-leakage. However, excessive draft results in increased ambient air infiltration which increases flue O2 content and lowers efficiency. Draft of 0.1 inch H2O at the arch should be used as the target for establishing efficiency operation. The only operational efficiency control for those heaters not equipped with stack dampers is through burner register adjustment.
(2) Burner Operation
Burner adjustments are mainly for control of combustion excess air. Also, burner adjustments affect flame shape and heat flux distribution in the heater radiant section and can have an effect on heater tube life.
Proper register adjustment is interrelated and dependent on furnace draft conditions. Adjustments of burner registers or stack gas damper will affect the proper setting of the other. Typically, small adjustments in both burner registers and stack gas dampers are made alternately, back and forth, to safely achieve the optimum setting for both. The optimum burner register setting will be different for each heater depending on design of heater and burners, fuel fired, %burner heat liberation, number of burners operating, and acceptable flame conditions.
Too much combustion air is inefficient because its heat content in flue gas is lost. Also, lack of combustion excess air is inefficient, because unburned or partially combusted fuel in the flue gas is lost.
Visual inspections of burner flame conditions are important to ensure proper overall burner operation. However, excess air measurement is generally required to establish efficient combustion excess air levels. This is particularly important when firing gas, since a wide variation in excess air result in visually acceptable flame conditions. However, 10% excess air should be used as the target goal for efficient gas fired heater.
In summary, we should inspect burners, draft reading, and %O2 in flue gas.
Heater Efficiency Enhancement through Capital Improvements
Even though a heater is operating at its greatest efficiency, further efficiency improvement may be practical by additional capital improvements. Adding heat traps is the most common method to improve the efficiency of fired equipment. As well, forced draft burners and use of gas turbine exhaust for fired heater combustion air also offers significant improvement over natural draft heater operation.