By Lester W. Davis, Jr.
Increasing the thermal efficiency of a Fired Heater reduces the heater’s carbon footprint and operating costs. For example, assuming a Fired Heater with a heat release of 100 MBTU/hr, an increase in efficiency of 1% will result in a savings of $38K/yr at a fuel cost of $4 per MBTU.
The thermal efficiency of a Fired Heater is an indication of how much of the total heat fired is absorbed by the process, and can be defined by the following equation:
Eff = Qa/Qf
Eff = Efficiency
Qa = Heat absorbed by the process
Qf = Heat fired
The amount of heat fired may by readily obtained by measuring the flow rate of the fuel fired (lb/hr) and the fuel’s heating value (BTU/lb). The amount of heat absorbed by the process is not readily known. Therefore we turn our attention to the Fired Heater heat losses. By subtracting the heat losses from the heat fired, we can determine the heat absorbed by the process. The equation for efficiency then becomes:
Eff = Qf - Ql/Qf
Ql = Fired Heater heat losses
There are two areas that determine the heat losses from a Fired Heater:
As discussed, stack temperature of the flue gas at some temperature indicates the amount of heat lost to the atmosphere. By performing an analysis of the fuel fired we can determine the amount of constituents in the flue gas in terms of mole % or wt %. Knowing the flue gas constituent fractions, which are typically H2O, N2,and CO2, we can find their enthalpy in terms of BTU/lb. For example, CO2 at 600°F is about 120 BTU/lb while N2 is about 135 BTU/lb and O2 is about 123 BTU/lb. By summing up the constituent enthalpies we determine the BTU/lb of flue gas and heat leaving the stack. Most Thermodynamic and Combustion references have this information in table or graphic form. See Figures 1 and 2 (next page) taken from API Recommended Practice 532.
The heat loss from the Fired Heater casing can be determined using a rigorous analysis which involves:
In general, we find that the radiation losses are comparatively small. Therefore for design purposes, after calculating the fuel required to support process conditions, the following guidelines are normally used to cover radiation losses:
Excess air influences stack loss by decreasing or increasing the stack gas flow rate. To minimize the flue gas flow rate, the excess air to the burners should be minimized. This can be accomplished by first testing the Fired Heater to determine the minimum excess air level at which it can safely operate.
Fired Heaters typically operate at an internal pressure which is less than atmospheric or negative. Therefore, any openings in the Fired Heater will allow ambient air to be infiltrated or leaked into the box. The result of this leakage has the same effect on flue gas rates as operating at high excess air levels through the burners. To reduce the air infiltration, the openings should be sealed.
To determine the amount of air being leaked, a portable O2 analyzer should be used to measure the O2 entering and leaving the convection section. The openings typically occur at tube penetrations and header boxes which are mostly located in the convection section. A visual inspection should be made of these areas, and also of the radiant section around peep doors and outlet piping.
There will always be some air infiltration. The object is to minimize it to below 1% excess O2.
Increasing the thermal efficiency of a Fired Heater reduces the heater’s carbon footprint and operating costs. This article provided a simple definition of thermal efficiency, and identified the main factors that affect it. A focused audit done by an experienced fired equipment engineer can often quickly identify things that may easily be done to improve thermal efficiency without making any capital investments.