Electric heater and electric heating system

An electric heater for a fluid, the electric heater comprising:

• a plurality of electrically conductive tubes (4) arranged in a bundle for resistive heating having an axial extension, electrical connectors arranged between the tubes of the plurality of electrically conductive tubes, and electrical conductors configured for connecting the plurality of tubes with an external electric power supply,
• wherein the electric heater is configured for the fluid to flow through the bundle in parallel with the axial extension and in direct contact with inner and outer surfaces of the tubes,
• wherein each of the tubes has an outer diameter within a range of 6-40 mm and an outer diameter to wall thickness ratio within a range of 5-15 or within a range of 7-12, and
• wherein in a cross section of the bundle perpendicularly to the axial extension, a ratio between a total cross-sectional surface area in between the tubes and a total cross-sectional area inside the tubes lies within a range of −10% to +30% or within a range of −5% to +25% of a ratio between the outer diameter and an inner diameter of one of the tubes.