Electric Heating

Tank & Liquid Heating

Calculate electric heater kW requirements for heating liquids in tanks. Covers start-up heating, steady-state operating loads, and surface heat losses using industry-standard engineering methodology.

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Conversion factor

3,412 BTU/kWh

Electric heating is 100% efficient; all electrical energy converts to heat.

Water constant

8.345 lb/gal

Multiply by specific gravity for other liquids.

Safety factor

1.1 to 1.25 typical

Accounts for voltage variations, aging, and unforeseen losses.

Contents

Use this guide when you need to:

  • Size electric immersion heaters for tanks
  • Calculate heat-up time requirements
  • Determine surface loss reductions from insulation
  • Size heaters for makeup and work product loads

1. Heating Load Components

Tank heating applications require sizing heaters for multiple load components. The total heater capacity must handle both transient (start-up) and steady-state (operating) conditions. The governing case determines the final heater size.

Start-Up

QS = Initial heat-up

Heat required to bring cold liquid and tank from ambient to operating temperature.

Operating

QO = Steady-state

Continuous heat required to maintain temperature against losses and process loads.

Surface Losses

QLS = Heat loss

Continuous heat loss through tank walls to ambient; reduced by insulation.

Governing Case Selection

The heater must be sized for the larger of start-up or operating requirements:

Heater Sizing Rule: Installed kW = max(QS, QO) x Safety Factor Where: QS = Start-up kW requirement QO = Operating kW requirement Safety Factor = 1.1 to 1.25 (typical 1.2)
Batch vs. Continuous: Start-up typically governs for batch processes where the tank cools between cycles. Operating typically governs for continuous processes with high makeup or work product rates.

2. Start-Up Heating

Start-up heating must raise the liquid and tank from initial (cold) temperature to operating temperature within a specified time. This includes heating the liquid mass, the tank thermal mass, and compensating for surface losses during heat-up.

Heat to Raise Liquid Temperature (QA)

Liquid Heating Energy: QA = (V x 8.345 x SG x Cp x ΔT) / 3,412 Where: QA = Heat to liquid (kWh) V = Liquid volume (gallons) 8.345 = Water density (lb/gal) SG = Specific gravity at 60 F Cp = Specific heat (BTU/lb-F) ΔT = Temperature rise (F) 3,412 = BTU per kWh

Heat to Raise Tank Temperature (QC)

Tank/Container Heating Energy: QC = (Wtank x Cpmaterial x ΔT) / 3,412 Where: QC = Heat to container (kWh) Wtank = Empty tank weight (lbs) Cpmaterial = Material specific heat (BTU/lb-F) Steel = 0.12, Aluminum = 0.22

Start-Up Power Requirement

The start-up kW combines the energy requirements divided by heat-up time, plus surface losses at operating temperature:

Start-Up kW: QS = (QA + QC) / t + QLS Where: QS = Start-up power (kW) t = Start-up time (hours) QLS = Surface loss rate at operating temp (kW)
Heat-up time trade-off: Shorter heat-up times require larger heaters. A 2-hour heat-up requires twice the heater capacity of a 4-hour heat-up (excluding surface losses). Balance capital cost against operational requirements.

Worked Example: Start-Up Calculation

Given:

  • 500 gallons of water (SG=1.0, Cp=1.0)
  • Initial temp: 60 F, Target: 180 F (ΔT = 120 F)
  • Steel tank weighing 500 lbs (Cp = 0.12)
  • Desired heat-up time: 2 hours
  • Surface loss at operating: 5 kW

Solution:

QA = (500 x 8.345 x 1.0 x 1.0 x 120) / 3,412 = 146.7 kWh

QC = (500 x 0.12 x 120) / 3,412 = 2.1 kWh

QS = (146.7 + 2.1) / 2 + 5 = 74.4 + 5 = 79.4 kW

3. Operating Loads

Once at operating temperature, the heater must maintain temperature against continuous losses and process loads. Operating loads include surface heat losses, makeup liquid heating, and work product heating.

Makeup Liquid Heating (Qwo)

When cold makeup liquid enters the tank, it must be heated to operating temperature:

Makeup Heating Load: Qwo = (Fmakeup x 8.345 x SG x Cp x ΔT) / 3,412 Where: Qwo = Makeup heating (kW) Fmakeup = Makeup flow rate (gal/hr) ΔT = Operating temp - Makeup temp (F)

Work Product Heating (Qws)

In processes like plating, cleaning, or heat treating, cold parts are immersed and must be heated:

Work Product Load: Qws = (Wproduct x Cpproduct x ΔT) / 3,412 Where: Qws = Work product heating (kW) Wproduct = Product rate (lbs/hr) Cpproduct = Product specific heat (BTU/lb-F) Steel parts = 0.12 BTU/lb-F

Total Operating Load

Operating kW: QO = Qwo + Qws + QLS Where: QO = Total operating power (kW) QLS = Surface heat losses (kW)
Continuous processes: If makeup rate or work product rate is high, operating loads may exceed start-up requirements. Always calculate both cases and size for the larger.

4. Surface Heat Losses

Heat continuously escapes through tank surfaces to the surrounding environment. Surface losses depend on temperature difference, surface area, and insulation level. Insulation dramatically reduces these losses.

Surface Loss Calculation

Surface Heat Loss: QLS = (A x U x ΔT) / 3,412 Where: QLS = Surface loss rate (kW) A = Exposed surface area (ft2) U = Surface loss coefficient (BTU/hr-ft2-F) ΔT = Operating temp - Ambient temp (F)

Surface Loss Coefficients

Approximate U values for still air conditions (indoor):

Insulation Level U (BTU/hr-ft2-F) Relative Loss
Uninsulated (bare metal)2.5100%
1" mineral wool0.5020%
2" mineral wool0.3012%
3" mineral wool0.229%
4" mineral wool0.187%
Insulation ROI: Adding 2" of insulation reduces surface losses by ~88%. For high-temperature or large-surface tanks, insulation often pays back within months through reduced operating costs.

Wind and Location Effects

Outdoor installations with wind exposure have higher losses. Multiply the above U values by:

  • Still air (indoor): 1.0x (base case)
  • Light breeze (5 mph): 1.3x
  • Moderate wind (15 mph): 1.6x
  • High wind (25+ mph): 2.0x

Surface Area Estimation

For cylindrical tanks:

Cylindrical Tank Surface Area: A = pi x D x H + (pi x D^2 / 4) Where: A = Surface area (ft2) D = Tank diameter (ft) H = Tank height (ft) First term = cylinder wall Second term = bottom (top often open or air space)

5. Heater Sizing & Selection

Final heater sizing combines calculated loads with safety factors and practical considerations. Select heaters from standard sizes that meet or exceed the calculated requirement.

Safety Factor Application

Installed Heater Capacity: kWinstalled = max(QS, QO) x SF Where: SF = Safety factor (1.10 to 1.25) Recommended SF values: 1.10 = Stable voltage, well-characterized loads 1.15 = Normal industrial applications 1.20 = Standard recommendation (default) 1.25 = Variable conditions, cold climates, aging concerns

Standard Heater Sizes

Round up to next standard heater size:

  • Small (under 10 kW): 1, 2, 3, 4.5, 6, 7.5, 9 kW
  • Medium (10-50 kW): 10, 15, 20, 25, 30, 36, 45, 50 kW
  • Large (over 50 kW): 60, 75, 100, 125, 150, 200 kW

Heater Type Selection

Heater Type Application Considerations
Flanged Immersion Tanks, vessels, process fluids Direct contact, high efficiency, easy replacement
Screw Plug Small tanks, pipes, fittings Compact, NPT threaded connection
Over-the-Side Open tanks, portable heating No tank penetration, removable
Circulation Heater Flow-through applications External to tank, good for viscous fluids

Watt Density Considerations

Watt density (W/in2) must be appropriate for the fluid to prevent local overheating:

  • Water: Up to 80 W/in2
  • Light oils: 20-40 W/in2
  • Heavy oils: 10-20 W/in2
  • Caustics: 15-25 W/in2
  • Waxes/tars: 5-10 W/in2
Multiple heaters: For loads over 100 kW, consider multiple heater elements for redundancy. If one element fails, partial heating capacity remains. Also provides better temperature distribution.

Flow-Through Heating

For continuous flow heating (not batch tanks):

Flow-Through Heating: kW = (F x 8.345 x SG x Cp x ΔT x SF) / 3,412 Where: F = Flow rate (gal/hr) Other terms as defined above Alternative with mass flow: kW = (m x Cp x ΔT x SF) / 3,412 Where: m = Mass flow rate (lbs/hr)

Liquid Properties Reference

Liquid Cp (BTU/lb-F) SG @ 60F Max Watt Density (W/in2)
Water1.001.0080
50% Ethylene Glycol0.851.0760
Light Oil (SAE 10)0.500.8530
Medium Oil (SAE 30)0.480.8825
Heavy Oil (SAE 50)0.450.9220
Fuel Oil #20.470.8725
Fuel Oil #60.400.9512
Caustic (50% NaOH)0.801.5320
Sulfuric Acid (98%)0.351.8415
Paraffin Wax0.500.908

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