dome ( Head ) design

Dome (Head) Design of Electric Water Heater Tanks

Dome ( Head ) design is a critical aspect of electric water heater tank engineering for two main reasons:

1 — Pressure Resistance and Enamel Protection

The dome (tank head) must withstand the internal water pressure while maintaining structural stability.
A well-designed dome distributes stress evenly across the surface, preventing localized stress concentrations that could lead to steel deformation or enamel cracking.

Since the internal surface of the tank is coated with glass enamel , excessive deformation or sharp stress zones can cause micro-cracks in the enamel layer, which may eventually lead to corrosion of the steel tank.

2 — Thermal Insulation Cost (PU Foam Volume)

The geometry of the tank head also affects the volume of polyurethane (PU) insulation between the tank and the outer shell.

• A hemispherical head provides the best pressure resistance because it distributes stress uniformly.

• However, this shape increases the gap between the tank and the outer casing, which significantly increases PU foam consumption and manufacturing cost, as illustrated in Figure 1.

Practical Industrial Solution

For domestic electric water heaters, manufacturers typically use standardized head geometries that provide a balance between mechanical strength and insulation cost.

The most commonly used shapes are:

• Ellipsoidal Head — DIN 28011

• Torispherical Head — DIN 28013

These head types offer good pressure resistance while reducing the insulation volume compared to a hemispherical head, as shown in Figure 2.

Engineering Calculation

The required dome geometry and dimensions can be calculated using our engineering tool android app : Pressure Vessel Volume Calculator  as the following example

Standards:-

   cr = 0.8 * d0

   kr = 0.154 * d0

   thi = sf + dh

 Example :-

D0  = 400.00 mm : external head diameter

Di  = 396.00 mm : internal head diameter

CR  = 320.00  mm : crown radius

KR  = 61.60  mm : knuckle radius

SF  = 15.00  mm : straight flange height

DH  = 100.53  mm : depth of dishing

THi = 115.53  mm : total internal head height

   t   = 2.00  mm : thickness

____________

Head Volume

   = 9.98  Liter

   = 2.64  US Gallon

   = 2.19  UK Gallon

   = 0.06 BBL

Blank Diameter: 525.69 mm2

____________

This example represents a typical electric water heater tank configuration.

In this case, each dome (tank head) has an approximate volume of 10 liters, and the forming blank size for the dome is 525 mm diameter × 2 mm thickness. Because the dome volume is known, it becomes straightforward to determine the required volume of the cylindrical section in order to achieve the total tank capacity.

For a 50-liter tank, the volume distribution can be estimated as follows:

• • • Dome 1 ≈ 10 L

    • Dome 2 ≈ 10 L

    • Cylindrical body ≈ 30 L

The volume of the cylindrical part of the tank is calculated using the cylinder volume equation:

V = (π × D² × L) / 4 

L = (4 × V) / (π × D²) 

Where:

• • • V = cylinder volume

    • D = tank diameter

    • L = cylinder length

Assuming:

• • • Cylinder volume V = 30 L = 30,000 cm³

    • Tank diameter D = 40 cm

L= 4×30000 / ( π×40 * 40 )  ≈ 23.9 cm

Adding 10 mm as overlap with upper and lower dome 5 each dome

L = 23.9 + 1 = 24.9 cm ≈ 25 cm 

Therefore, the cylindrical body length is approximately 250 mm.

This approach allows designers to easily adjust tank capacity by modifying the cylindrical length while keeping the dome geometry constant, which simplifies manufacturing and tooling.