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 = Q_{a}/Q_{f}

Where:

Eff = Efficiency

Q_{a} = Heat absorbed by the process

Q_{f} = 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 = Q_{f} - Q_{l}/Q_{f}

Where:

Q_{l} = Fired Heater heat losses

There are two areas that determine the heat losses from a Fired Heater:

- Stack losses are related to the temperature of the flue gas and the amount of flue gas leaving the stack.
- The heat losses are also due to radiation from the Fired Heater casing.

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 H_{2}O, N_{2},and CO_{2},
we can find their enthalpy in terms of BTU/lb. For example, CO_{2}
at 600°F is about 120 BTU/lb while N_{2} is about 135 BTU/lb and O_{2}
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:

- 1. Measuring the temperature of the casing with a contact thermocouple or an infrared camera.
- Measuring or noting the air velocity.
- Referring to a Transmission Curve which plots the difference between the casing temperature and ambient temperature against the heat transmission rate as a function of the air velocity.
- Multiplying the transmission rate (BTU/ft2/Hr) by the surface area (ft2) produces the radiation loss (BTU/Hr).

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:

- For Fired Heaters designed to absorb more than 100 MBTU/hr, the calculated net fuel required is increased by 1% (i.e., multiplied by 1.01).
- For Fired Heaters designed to absorb between 15 and 100 MBTU/hr, the calculated net fuel required is increased by 2% (i.e., multiplied by 1.02).
- For Fired Heaters designed to absorb less than 15 MBTU/hr, the calculated net fuel required is increased by 3% (i.e., multiplied by 1.03).

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 O_{2}
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 O_{2}.

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.