BIZOMAT MAKEOS · Materials
Hot Surface Insulation Check (Bare Loss, Insulated Loss, Touch Temperature, Payback)
How much heat does this hot surface lose bare and with insulation, is the insulated surface safe to touch, and how quickly does the insulation pay for itself?
Check of insulation for a hot flat surface (tank wall, oven casing, duct): bare-surface loss by radiation (as the radiant heat loss tool) plus convection, insulated loss through the insulation and the outer surface film (as the layered wall heat flow tool), the outer surface temperature against your touch limit, the yearly energy saving and the payback of the insulation cost.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Surface area | m² | Area of the hot surface. |
| Hot surface temperature | °C | Temperature of the hot surface (the metal under any insulation). |
| Surroundings temperature | °C | Temperature of the surrounding air and surfaces. |
| Emissivity of the bare surface | Emissivity of the bare surface. | |
| Convection coefficient of the bare surface | W/(m²·K) | Natural or forced convection coefficient of the bare surface. |
| Insulation | ||
| Insulation thickness | mm | Thickness of the insulation. |
| Insulation conductivity | W/(m·K) | Thermal conductivity of the insulation at its mean temperature. |
| Outer surface film coefficient (convection + radiation) | W/(m²·K) | Combined convection and radiation film coefficient of the insulated outer surface (cladding). |
| Costs and limits | ||
| Hours hot per year | h | Hours per year the surface is hot. |
| Heat energy price | $/kWh | Cost of a kilowatt-hour of heat delivered. |
| Insulation cost per square metre | $/m² | Installed cost of the insulation and cladding per square metre. |
| Largest outer surface temperature | °C | Highest outer surface temperature allowed for people. |
| Longest payback accepted | years | Longest payback you accept. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Surface area
- What it is
- The area of the hot surface to be insulated, such as the outside of a tank, duct or pipe run.
- Units and format
- Enter a number in m²: square metres. The form accepts 0.000001 m² to 1000000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- All heat losses, savings and the insulation cost rise in proportion to area.
- Get it right
- For pipes, use the outside circumference times length.
What this means: Hot surface temperature
- What it is
- The temperature of the hot metal surface, which stays the same under the insulation.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -100 °C to 1500 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure the metal surface, not the fluid inside; it must be hotter than the surroundings.
What this means: Surroundings temperature
- What it is
- The temperature of the surrounding air and nearby surfaces the hot surface loses heat to.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -60 °C to 100 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Emissivity of the bare surface
- What it is
- The emissivity of the bare hot surface: how strongly it radiates heat compared with a perfect radiator, from a materials reference for the surface finish.
- Units and format
- Enter a plain number with no unit. The form accepts 0.01 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bare radiation loss rises in proportion to it.
- Get it right
- A dimensionless value between zero and one; oxidised or painted surfaces radiate far more than bright metal.
What this means: Convection coefficient of the bare surface
- What it is
- The convection heat transfer coefficient between the bare surface and the air, which depends on still or moving air and the surface orientation.
- Units and format
- Enter a number in W/(m²·K): watts per square metre per kelvin. The form accepts 0 W/(m²·K) to 1000 W/(m²·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 W/(m²·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bare convection loss rises in proportion to it.
Insulation
What this means: Insulation thickness
- What it is
- The thickness of the insulation layer over the hot surface.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Thicker insulation lowers the insulated loss and the outer surface temperature.
What this means: Insulation conductivity
- What it is
- The thermal conductivity of the insulation at its mean operating temperature, from the insulation maker’s data sheet.
- Units and format
- Enter a number in W/(m·K): W/(m·K). The form accepts 0.000001 W/(m·K) to 10 W/(m·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.04 W/(m·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher conductivity increases the insulated loss and the outer surface temperature.
- Get it right
- Conductivity rises with temperature; use the value at the mean of the hot and outer face temperatures.
What this means: Outer surface film coefficient (convection + ra…
- What it is
- The combined convection and radiation coefficient from the outer surface of the insulation or cladding to the surroundings.
- Units and format
- Enter a number in W/(m²·K): watts per square metre per kelvin. The form accepts 0.000001 W/(m²·K) to 1000 W/(m²·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 W/(m²·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower coefficient raises the outer surface temperature.
- Get it right
- Bright metal cladding radiates less than painted cladding, so it runs hotter to touch.
Costs and limits
What this means: Hours hot per year
- What it is
- The number of hours per year the surface is hot, used to turn the heat saved into yearly energy and money.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 8784 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6000 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The yearly saving rises in proportion to hours, so payback shortens.
What this means: Heat energy price
- What it is
- The cost of each kilowatt hour of heat delivered to the surface, including boiler or heater efficiency, from your energy costs.
- Units and format
- Enter a number in $/kWh: money per kilowatt-hour. The form accepts $0/kWh to $100/kWh. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $0.08/kWh as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The yearly saving rises in proportion to it.
- Get it right
- Use the cost of delivered heat, not the fuel price per unit of fuel energy.
What this means: Insulation cost per square metre
- What it is
- The installed cost of insulation and cladding for each square metre of surface, from a contractor’s quote.
- Units and format
- Enter a number in $/m²: money per m². The form accepts $0/m² to $100000/m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $60/m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Payback rises in proportion to it.
What this means: Largest outer surface temperature
- What it is
- The highest outer surface temperature you allow where people could touch it, from your workplace safety rules or the applicable standard.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0.000001 °C to 500 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 55 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Longest payback accepted
- What it is
- The longest payback period you accept for the insulation investment, from your organisation’s investment rules.
- Units and format
- Enter a number in years: years. The form accepts 0.1 years to 50 years. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 years as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter years; the result also shows payback in months.
Method and formulas
- Radiation = ε σ A (T_s⁴ − T_a⁴) (as the radiant heat loss tool); convection = h_c A (T_s − T_a).
- Insulated: U = 1 ÷ (t ÷ k + 1 ÷ h_out); loss = U A ΔT; outer surface = T_a + U ΔT ÷ h_out (as the layered wall heat flow tool).
- Saving = (bare − insulated) × hours × price; payback = insulation cost ÷ saving.
- Example: bare it loses 33.151 kW, insulated 1.2963 kW; the outer surface runs at 37.963 °C and the insulation pays back in 0.4709 months.
Limitations
- Flat surface; pipes need the cylindrical form.
- Film coefficients as entered; wind raises the bare loss considerably.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Justify insulating a tank, oven or duct
- Choose an insulation thickness for personnel protection
- Estimate energy saved by lagging
Who it is for
- Energy managers
- Maintenance engineers
- Plant engineers
What you get
- Bare loss by radiation and convection
- Insulated loss and outer surface temperature
- Saving and payback
Interpreting the result
- The first few millimetres of insulation save the most: extra thickness is often set by the touch temperature rather than by energy.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 371–378. Topic reference; no source tables or recommended settings are reproduced.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.