Guide to Outside Blanket Touch Temperature

Firwin FAQ – How do I know what the outside blanket touch temperature will be for my application?

People who use removable insulation blankets would often prefer to know the temperature conditions on the outside as well as on a cold surface as soon as the blankets are installed on their system. In conditions where personnel safety is a major concern, the surface temperature of the blankets should be below a certain level to be considered safe for the personnel working in the area.

Background

A typical insulation consists of three sections:

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

Insulation material:
The middle layer of an insulation blanket represents the actual insulation media. Usually fiberglass is used, but other materials can also be used, mostly in very high temperature applications (>1200 °F).

Outer (cold) surface:
The outer cover protects the insulation from being damaged. While there are numerous materials that can be used, silicone impregnated fiberglass is most commonly used. ‘Touch temperature’ refers to the temperature of the outer protective cover. Although there is no definite standard for safe touch temperatures, the UL2200 specification for stationary engine generator assemblies specifies safe temperature limits for both non-metallic and metallic surfaces.

UL2200 Specification for Stationary Engine Generator Assemblies

Contact Surface Metallic Non-metallic
Handles or knobs grasped for holding 50° C
(122° F)
60° C
(140° F)
Handles or knobs that are contacted but do not involve holding; other surfaces subject to contact and user maintenance 60° C
(140° F)
85° C
(185° F)
Surfaces subject to casual contact 70° C
(158° F)
95° C
(203° F)

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

What Determines the Outer Surface Touch Temperature?

Although the outside temperature of an insulation blanket is influenced by a large number of variables, the following are the most quantifiable and crucial:

Insulation Material: The type of insulation material used can help determine the effectiveness of an insulation blanket. There are some materials that behave as superior insulators and therefore, the touch temperature will be lower for such materials for a given exhaust temperature.

Insulation thickness: The resultant touch temperature of an insulation blanket is considerably affected by the thickness of insulation material used. Naturally, the thicker the insulation, the more effective it will be and the lower the outside surface touch temperature.

However, it must be noted that insulation thickness is one of diminishing returns - when the insulation thickness is increased, the added insulation value gets lowered. In other words, the insulation value will not be doubled when the thickness is raised from 1” to 2”.

Ambient temperature: Ambient temperature is the temperature in the area where the insulation lies. It will also influence the temperature of the outer cover. When the ambient temperature is higher, the temperature of the outer blanket will also be higher.

Air flow (wind speed): Air flow increases the rate of heat escaping from the surface and cools off the surface temperature of the blanket. Thus, the touch temperature is lowered by a faster air flow.

Outer cover material (emittance): The emittance of a material refers to its capability to emit the absorbed heat. Outer materials with a high emissivity value will emit heat away from it back into the environment, producing a lower temperature when compared to materials with a low emissivity value. For instance, a silicone impregnated fiberglass cover possesses a higher emissivity and thus produces a lower surface temperature, whereas a mirrored aluminum fiberglass cover has lower emissivity and thus preserves more heat.

Once the engineers are furnished with this information, they can rapidly compute the expected outside touch temperature for a user’s specific application. However, if a customer has a target touch temperature in mind, engineers at Firwin Corporation will be able to find out the type of insulation material and thickness needed to attain this target.

Brett Herman, Firwin’s Vice President of Sales & Customer Service said that “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, Firwin can carry out a more refined and time-consuming heat flow analysis if a more precise analysis is required.

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

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

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