Joined: 18 January 2003
Over the holiday I have seen oil-filled electric heaters for domestic use, i.e. 230V, up to 3kW. I don't have one available to dismantle, to answer this question.
I have worked quite a lot with mineral oil and found that the thermal expansion can be significant. For example, filling at 20 degrees C and heating up to 100 degrees C causes about a 5% increase in volume.
There is no way that a commercial heater will withstand the resulting pressure increase if it is fully filled without some form of compensator.
Presumably there is at least one low-cost solution out there that can provide a service lifetime of several years and several hundred thermal cycles while keeping the casing stress to an acceptable level. Furthermore the internal pressure at max temperature would have to be extremely low to prevent the clear hazard of a high-pressure, high-temperature oil leak.
What are the typical solutions for this? Is it as simple as filling the radiator to a certain level to leave an air pocket? If so, would oxidation of the oil be actively prevented, and if so, how?
I have assumed that the heater casings are fully sealed to prevent leakage. Maybe this is not the case?
Edited: 06 January 2014 at 10:46 AM by Kermode
Joined: 15 January 2005
All the old Dimplex oil filled radiators used a special oil and refilled through the thermostat hole.
The instructions said to stand on edge opposite the thermostat (element replacement instructions) and top up/refill to appropriate mark on the aluminium gauge. This would leave enough air to allow for oil expansion.
If overfilled, oil would escape through the thermostat housing.
I trust this explains the oil gap reason.