Determining the Best Outside Blanket Touch Temperature

Firwin FAQ – How does one know what the outside blanket touch temperature will be for an application?

Users of removable insulation blankets frequently would like to know what the outside, or cold surface, temperature will be after installing the insulation blankets on their system. This is especially true where personnel safety is of concern, where the surface temperature of the blankets must be lower than a particular level in order to be safe for personnel working in the area.

Background

A typical insulation blanket is made up of three sections:

An inner (hot) surface: Usually a stainless steel mesh, the inner liner rests directly on the hot component. Its function is to keep the insulation material in place.

Insulation material: The middle layer of an insulation blanket is considered to be the actual insulation media. Usually fiberglass, but other materials can be employed, mainly in extremely high temperature applications (>1200 °F).

Outer (cold) surface: The outer cover is used for protecting the insulation from being damaged. There are different materials that can be used, but silicone impregnated fiberglass is most common.

‘Touch temperature’ refers to the temperature of the outer protective cover. While no particular standard for safe touch temperatures is available, the UL2200 specification for stationary engine generator assemblies lists safe temperature limits for both non-metallic and metallic surfaces.

The above table demonstrates that non-metallic surfaces, such as standard insulation blankets, can reach temperatures as high as 95 °C (203 °F) and still be considered safe for casual contact.

What Determines the Outer Surface Touch Temperature?

While there exists a huge number of variables to be taken into account when calculating the outside temperature of an insulation blanket, the following are the most critical and quantifiable:

Insulation Material: The efficiency of an insulation blanket is, of course, majorly determined by the type of insulation material employed. Some materials are high-quality insulators, and thus the touch temperature will be lower for those materials for a specified exhaust temperature.

Insulation thickness: The thickness of insulation material employed has possibly the most profound effect on the resultant touch temperature of an insulation blanket. Naturally, the thicker the insulation, the more effective it will be and the lower the outside surface touch temperature. It should be noted, however, that insulation thickness is one of diminishing returns - as one increases the insulation thickness, the added insulation value reduces. In other words, the insulation value will not get doubled by going from 1” thick to 2” thick insulation.

Ambient temperature: The temperature in the area where the insulation resides, called the ambient temperature, will also disturb the temperature of the outer cover. The temperature of the outer blanket is higher when the ambient temperature is higher.

Air flow (wind speed): Air flow cools off the blanket surface temperature by elevating the rate of heat escape from the surface. Thus, a faster air flow will decrease the touch temperature.

Outer cover material (emittance): The emittance of a material refers to its potential to release absorbed heat. Outer materials with a high emissivity value will discharge heat away from it back into the environment, and thus generate a lower temperature than materials with a low emissivity value. For instance, a silicone impregnated fiberglass cover has a higher emissivity and will produce a lower surface temperature than a mirrored aluminum fiberglass cover, whose lower emissivity leads it to retain more heat.

After being equipped with this information, the Engineers can quickly calculate what outside surface touch temperature users can expect for their specific application. Conversely, if a customer has a target touch temperature in mind, it will be possible for Firwin Engineers to determine what insulation material and thickness will be needed in order to achieve this goal.

Customers should realize that these results are approximations, as actual site conditions will vary. For most cases, using these variables suffice to give the customer a good estimate of what outer temperature he can expect. However, if a more precise analysis is desired, Firwin can perform a more sophisticated and time-consuming heat flow analysis. (For more information, see our article in an earlier newsletter titled “Heat Flow Analysis”.)

Brett Herman, Vice President of Sales & Customer Service, Firwin

This information has been sourced, reviewed and adapted from materials provided by Firwin Corporation.

For more information on this source, please visit Firwin Corporation.