Insulation & Heat Loss

The insulation in a furnace is extremely important for a number of reasons. The insulation provides a means of keeping the heat contained in the heater where it can be absorbed by the heater tubes, resulting in higher overall efficiencies. The insulation also keeps the external shell cooler making it safe for operating and maintenance personnel to safely work around the furnace. This cooler casing temperature also results in the structural stability of the overall structure.

Twenty years ago, most of the insulation used in fired heaters was the gunned or cast refractory. This material often, was mixed on site at the heater manufacturer's shop, and thus frequently varied in insulating properties. The more popularly used mixes like 1:2:4 LHV and others became standard and over time the insulating properties became very predictable. This was improved upon by the offering of proprietary mixes, by a number of companies, which were packaged in controlled environments and were thus more predictable in their application.

During the early eighties, ceramic fibers became accepted in the heater industry and since they are a much better insulator, they quickly caused a decline in the use of refractory. In general, 3 inches of ceramic fiber blanket could do a better job than 6 inches of refractory and weighed much less. As an example, if we have a hot face temperature of 1200 °F and an air temperature of 70 °F on a vertical wall with no wind blowing, 6" 1:2:4 LHV gunned has a cold face temperature of 205.5 °F where 3" 8# 2300°F ceramic fiber would have a cold face temperature of 161.2 °F. And this is with a weight less than 10% of the gunned refractory, which reduces freight cost. Furthermore, the ceramic fiber blanket does not require "drying" in the field as would be required with the refractory.

Refractory is still used in special furnaces and in areas where it is more durable or easier to install. The floor of a furnace which must be walked on during maintenance and inspection, may use castable refractory or brick, or both because it is more durable. End tube sheets, when they have multiple tube penetrations such as in an end supported tube convection may utilize gunned refractory because it is easier(less costly) to apply between the openings for the tubes then ceramic fiber blanket.

Heat Loss Through Insulation:

The heat loss due to radiation may be calculated using the Stefan-Boltzman formula.

hr = 17.4*10-10*e*(T14 - T24)

Where,
hr = Heat loss by radiation, Btu/hr-ft2
e = Emisivity of surface, assumed at 0.95
T1 = Temperature of surface, °R
T2 = Temperature of surroundings,°R

The heat loss due to free convection may be calculated using the following method.

hc = 0.53*C*(1/Tavg)0.18*(T1 - T2)1.27

Where,
hc = Heat loss by convection, Btu/hr-ft2
C = A constant, assumed at :
1.79 for an arch or roof
1.39 for a wall
0.92 for a floor
Tavg = Average temperature of wall and surroundings,°R

The heat loss due to forced convection, where the air velocity is greater than zero, may be calculated using the following method.

hfc = (1 + 0.225 * V)*(T1 - T2)

Where,
hfc = Heat loss by forced convection, Btu/hr-ft2
V = Velocity of air across surface, ft/sec

To visualize the differences in the various materials, used for insulation, the following calculator can be used to run calculations for some of these materials under different conditions.

Thickness Of Layer, in: Insulation Material:
Hot Face Temperature, °F: Air Temperature, °F:
Air Velocity, ft/sec: Surface Type:

Cold Face Temperature, °F: Heat Loss, Btu/hr-ft2:
Thermal Conductivity, Btu-in/hr-ft2-F: