BIZOMAT MAKEOS · Materials
Electric Tank Heater Sizing (Heat-up, Losses and Watt Density)
How much heater power will bring this tank up to temperature in time, and is the installed heater’s surface loading safe for the liquid?
Sizing of an electric immersion heater for a tank: energy to heat the liquid and the vessel (as the heating energy tool), average surface loss during heat-up and holding loss at temperature from an overall loss coefficient, the power needed for the heat-up time with a margin, the heat-up time with the installed heater, holding duty and the element surface watt density against your limit for the liquid.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Tank and liquid | ||
| Liquid volume | L | Volume of liquid heated. |
| Liquid density | kg/m³ | Density of the liquid. |
| Liquid specific heat | kJ/kg·K | Specific heat of the liquid (4.18 water, about 1.9–2.1 oils). |
| Vessel mass heated | kg | Mass of tank steel that heats up with the liquid. |
| Vessel specific heat | kJ/kg·K | Specific heat of the vessel material (about 0.5 steel). |
| Starting temperature | °C | Temperature at the start of heat-up. |
| Set temperature | °C | Working temperature. |
| Ambient temperature | °C | Surrounding air temperature. |
| Tank surface area losing heat | m² | Outside area of walls, bottom and liquid surface that lose heat. |
| Overall loss coefficient | W/m²·K | Overall loss coefficient (about 1–2 insulated, 5–10 bare steel in still air; an open liquid surface loses far more). |
| Heat-up time wanted | h | Time allowed to reach the set temperature. |
| Power margin | % | Margin on the power found. |
| Heater | ||
| Installed heater power | kW | Rated power of the heater fitted or considered. |
| Number of elements | Number of heater elements. | |
| Heated surface per element | cm² | Heated (sheath) surface of one element. |
| Largest watt density for the liquid | W/cm² | Highest surface watt density the liquid accepts (your value from the heater maker; much lower for oils and viscous liquids than for water). |
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.
Tank and liquid
What this means: Liquid volume
- What it is
- The volume of liquid to be heated in the tank at its working level.
- Units and format
- Enter a number in L: litres. The form accepts 0.000001 L to 10000000 L. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 L as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Heat-up energy, power needed and heat-up time rise with it.
- Get it right
- Use the working fill volume, not the tank’s brim capacity, if it runs part full.
What this means: Liquid density
- What it is
- The mass of one cubic metre of the liquid, which turns the volume into a mass to be heated.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 3000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Liquid heat-up energy is directly proportional to density.
- Get it right
- Enter kg/m³, not kg/L; 1 kg/L is 1000 kg/m³.
What this means: Liquid specific heat
- What it is
- The heat needed to warm one kilogram of the liquid by one kelvin, from the liquid’s data sheet.
- Units and format
- Enter a number in kJ/kg·K: kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/kg·K to 10 kJ/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.18 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
- Liquid heat-up energy is directly proportional to it.
- Get it right
- Check the unit is kJ per kg·K, not J per kg·K.
What this means: Vessel mass heated
- What it is
- The mass of tank shell, internals and fittings that warm up along with the liquid and absorb part of the heat.
- 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 200 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds to the heat-up energy in proportion to mass times its specific heat.
- Get it right
- Count the metal in contact with the liquid; external frames that stay cool can be left out.
What this means: Vessel specific heat
- What it is
- The heat needed to warm one kilogram of the vessel material by one kelvin.
- 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 0.5 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
- Vessel heat-up energy is directly proportional to it.
- Get it right
- Use the value for the vessel material, in kJ per kg·K.
What this means: Starting temperature
- What it is
- The temperature of the liquid and vessel when heat-up starts, such as after a cold start or a weekend shutdown.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 400 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A colder start widens the temperature rise, raising energy and heat-up time.
- Get it right
- Use the coldest realistic start, for example in winter, if the heat-up time must always be met.
What this means: Set temperature
- What it is
- The working temperature the liquid must reach and then be held at.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 400 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher set temperature raises heat-up energy and both heat losses.
- Get it right
- It must be above the starting temperature.
What this means: Ambient temperature
- What it is
- The temperature of the air around the tank, which the tank loses heat to.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 60 °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.
- Effect on the result
- A cooler ambient raises both heat losses.
- Get it right
- Use the lowest ambient expected when the heater must perform, not an annual average.
What this means: Tank surface area losing heat
- What it is
- The outside area of the tank that loses heat: walls, bottom and the liquid surface or lid.
- Units and format
- Enter a number in m²: square metres. The form accepts 0 m² to 10000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both heat losses are directly proportional to area.
- Get it right
- Include the liquid surface or lid; an open liquid surface also loses heat by evaporation, which this coefficient does not cover.
What this means: Overall loss coefficient
- What it is
- How many watts each square metre of tank surface loses for each kelvin between liquid and air, combining insulation, wall and air film.
- Units and format
- Enter a number in W/m²·K: watts per square metre per kelvin. The form accepts 0 W/m²·K to 100 W/m²·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 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
- Both heat losses are directly proportional to it.
- Get it right
- Use a value for the actual insulation and exposure; bare and insulated surfaces differ greatly, so area-weight a mixed tank.
What this means: Heat-up time wanted
- What it is
- The time you allow for the tank to go from the starting temperature to the set temperature.
- Units and format
- Enter a number in h: hours. The form accepts 0.000001 h to 100 h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A shorter time raises the power needed and makes the installed-heater check harder to pass.
- Get it right
- Enter hours, not minutes.
What this means: Power margin
- What it is
- An extra allowance added to the calculated heater power for uncertainty in the losses and supply voltage.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises only the power needed figure; the checks on the installed heater do not use it.
- Get it right
- Type the percentage itself, not a fraction.
Heater
What this means: Installed heater power
- What it is
- The total rated power of the heater fitted or being considered, across all its elements.
- 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 21 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More power shortens heat-up time and raises the watt density on each element.
- Get it right
- Use the power at your actual supply voltage; heater output falls with the square of voltage.
What this means: Number of elements
- What it is
- How many heater elements share the installed power.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More elements lower the watt density for the same installed power.
- Get it right
- Count individual heated elements, not heater assemblies or terminal boxes.
What this means: Heated surface per element
- What it is
- The heated sheath surface of one element in contact with the liquid, excluding cold ends and unheated lengths.
- Units and format
- Enter a number in cm²: square centimetres. The form accepts 0.000001 cm² to 1000000 cm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 cm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Watt density is inversely proportional to the heated surface.
- Get it right
- Use the heated length only; the cold zone at the terminal end does not transfer heat.
What this means: Largest watt density for the liquid
- What it is
- The highest heat flow per square centimetre of element surface the liquid can accept without scorching, boiling at the sheath or degrading.
- Units and format
- Enter a number in W/cm²: W/cm². The form accepts 0.000001 W/cm² to 50 W/cm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 W/cm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the watt density, only the check.
- Get it right
- Take it from the heater maker for this liquid; oils and viscous liquids need far lower limits than water.
Method and formulas
- Heat-up energy Q = (m c + m_v c_v)(T_s − T₀), with m = V ρ.
- Loss during heat-up ≈ U A [(T₀ + T_s) ÷ 2 − T_amb]; holding loss = U A (T_s − T_amb).
- Power needed = (Q ÷ t + heat-up loss) × (1 + margin); heat-up time with the installed heater = Q ÷ (P − heat-up loss).
- Watt density = installed power ÷ (elements × heated surface per element).
- Example: heating 1000 L from 15 to 60 °C in 3 h needs 19.848 kW; the 21 kW heater does it in 2.5734 h.
Limitations
- Uniform liquid temperature; poorly stirred tanks stratify.
- Evaporation from an open hot surface can dominate the losses and is not included in a simple U value.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a heater for a wash, plating or oil tank
- Check an existing heater’s heat-up time
- Check watt density so oil or solutions do not burn on the elements
Who it is for
- Plant engineers
- Maintenance engineers
- Process engineers
What you get
- Heat-up energy and losses
- Power needed and actual heat-up time
- Holding duty and watt density
Interpreting the result
- If the holding duty is very low, a smaller heater with a longer heat-up time may be acceptable.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 371–378. Topic reference; no source tables or recommended settings are reproduced.
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Open the Electric Tank Heater Sizing (Heat-up, Losses and Watt Density) calculator