BIZOMAT MAKEOS · Materials
Heat-Treatment Furnace Load Plan (Time, Energy, Cost)
How long will this load take to heat, how much energy will the cycle use, and what does it cost per kilogram?
Energy plan for a furnace heat-treatment cycle: energy to heat the load and its fixtures through the temperature range from mean specific heat (as the heating energy tool), heat-up time from the furnace power and the share reaching the load, standing losses during the soak, total energy, cost per load and cost per kilogram of work.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Work mass | kg | Mass of parts in the load. |
| Fixture and basket mass | kg | Mass of baskets, trays and fixtures heated with it. |
| Mean specific heat over the range | kJ/kg·K | Average specific heat between the two temperatures (about 0.6 kJ/kg·K for steel to 850 °C, including the transformation). |
| Start temperature | °C | Temperature of the load going in. |
| Set temperature | °C | Furnace set temperature. |
| Furnace input power | kW | Rated input of the furnace. |
| Share of input reaching the load while heating | % | Share of input power going into the load during heat-up (the rest heats the furnace and escapes). |
| Standing heat loss at temperature | kW | Power the furnace needs just to hold temperature empty. |
| Soak time | h | Time at temperature (from the heat-treatment specification for the section size). |
| Energy price | $/kWh | Price of electricity or fuel per kWh. |
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: Work mass
- What it is
- Mass of the parts being heat treated in one furnace load.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Cost per kilogram is based on this work mass only.
What this means: Fixture and basket mass
- What it is
- Mass of baskets, trays, grids and fixtures heated along with the work.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- These absorb heat and energy too, but they are not counted in the cost per kilogram of work.
What this means: Mean specific heat over the range
- What it is
- Average heat needed to warm one kilogram of the load by one kelvin over the whole range from start to set temperature, from a materials reference.
- Units and format
- Enter a number in kJ/kg·K: kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/kg·K to 5 kJ/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.6 kJ/kg·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Heat into the load and heat-up time rise in proportion.
- Get it right
- Use a mean over the range, including any phase change, not a room-temperature value.
What this means: Start temperature
- What it is
- Temperature of the load when it goes into the furnace.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 1500 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Set temperature
- What it is
- Furnace set temperature the load is heated to and soaked at.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0.000001 °C to 2000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 850 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Furnace input power
- What it is
- Rated input power of the furnace while it is heating, from its nameplate.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 100000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Heat-up time is inversely proportional to input power.
What this means: Share of input reaching the load while heating
- What it is
- Share of the input power that goes into the load during heat-up; the rest heats the furnace lining or escapes.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 5% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Heat-up time and heating energy are inversely proportional to this share.
What this means: Standing heat loss at temperature
- What it is
- Power the empty furnace needs just to hold set temperature, from the maker or a power meter reading.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 10000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds standing loss × soak time to the energy.
What this means: Soak time
- What it is
- Time the load is held at set temperature, from the heat-treatment specification for the section size.
- Units and format
- Enter a number in h: hours. The form accepts 0 h to 200 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Hours, not minutes: 30 minutes is 0.5 hours.
What this means: Energy price
- What it is
- Price of one kilowatt-hour of electricity or fuel energy delivered to the furnace.
- Units and format
- Enter a number in $/kWh: money per kilowatt-hour. The form accepts $0/kWh to $10/kWh. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with $0.25/kWh as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cost per load rises in direct proportion.
Method and formulas
- Heat Q = (m_work + m_fixture) c_p (T₁ − T₀).
- Heat-up time = Q ÷ (P × share).
- Energy = Q ÷ share + standing loss × soak time.
- Cost per load = energy × price; per kilogram = cost ÷ work mass.
- Example: 500 kg to 850 °C takes about 2.3056 h to heat and costs 7.2167 cents per kg.
Limitations
- Mean specific heat; the real heat-up slows as the load approaches set temperature.
- Soak time must come from the heat-treatment specification for the section size.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Quote heat treatment by energy cost
- Plan furnace loading for throughput
- Compare fuller loads against more cycles
Who it is for
- Heat treaters
- Production planners
- Estimators
What you get
- Heat into the load
- Heat-up time and total cycle
- Energy and cost per load and per kilogram
Interpreting the result
- Fixture mass is pure overhead: lighter baskets and fuller loads cut cost per kilogram.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 472–480, 372–376. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Where the heat goes: available heat and the furnace load
What it covers: Follow a gas furnace's fuel into the load, the walls and the flue, calculate available heat from flue temperature and excess air, and plan a load's fuel, time and cost.
Method explained here · BIZOMAT original book
Tempering after hardening
In Induction Hardening by the Numbers
What it covers: Why freshly hardened cases are tempered, how time and temperature trade off, planning the tempering oven's load, energy and throughput, and a weekend batch left untempered that cracked.
Method explained here · BIZOMAT original book
Heat treating castings
In Metal Casting by the Numbers
What it covers: Solution treating, quenching and ageing aluminium castings: what each step does, the furnace energy and cycle time per load, the quench tank's heat balance, and a tank too small for the load.
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