Overheating heating element

Question

I'm making a system that is going to heat water in a 25 liter tank to a given temperature and keep the water on this temperature with a precision of +/-0.5 °C. As I have done some research I have decided to control the heating element with a zero crossing solid state relay and control this relay with an Ardunio. The heating element is 230VAC, 3kW.

My question is, for how long can I run constant current through the heating element without overheating it? I mean, if I turn the hot plate on my stove on max, will it run constant current through the element or will this still be pulses and how to determine the pulse width and time between these pulses?

Answer 1

About 10 minutes.. till the water starts to boil.
25 liters = 25,000 cc of water. It takes 1 Joule to raise 1 cc 1 degree (C or K). Assuming the water starts at 25C that is,

75(deg)*25,000(J/deg.)/3kW = 625 seconds.

Answer 2

Roughly speaking, an electric heating element rises in temperature until it reaches a state where the power input (3kW in your case) is equal to the power output (heat into the water).

A heater designed for immersion in water is built so that water will keep it cool enough not to be damaged at its rated power. The water will usually be driven by convection to move constantly past the element, carrying away enough heat to prevent the element overheating. Once the water reaches boiling point, the temperature of the water stops rising, but the conversion of water into steam uses lots and lots of heat, which the element replaces. At this point the element will be significantly above the temperature of the water (it's this temperature difference which drives the heat out of the element into the water), but its temperature will be roughly constant.

Things will stay like this, until enough water boils away that the element starts to become exposed to air. The air is a much worse conductor of heat than the water was, so the element temperature needs to rise to re-establish the '3kW in, 3kW out' equilibrium.

An element designed solely for use in water will not be able to survive this new, much higher, temperature, and will be damaged very quickly.

So the short answer is, you can supply 3kW to a 3kW immersion heater for as long as you like as long as it is immersed and not a second longer.

The element on your electric stove is also designed to run continuously at its full rated power (at 'max', it will not be pulsed), but it does this by being longer than a typical immersion heater of the same power (so more surface area to lose heat from) and being built to run at a much higher temperature without failure.

Answer 3

While searching for heating element I found this one. Several years have passed, but I saw some problems in your setup:

Too much power 3000W in a small volume (25L), would need the water to be recirculated / stirred very strongly to avoid hot/cold spots, which could be much greater than 1°C.
Another point is about the controller: it should not be with and On-Off controller.
The recommended controller should be a PID one, which output would be configured varying its duty cycle (of a PWM) over the Zero-crossing solid-state relay you said.

As George’s answer calculated,heating range of 75°C is achieved is just 625s, for your 3000Watts & 25liters or about 1C in just 8 seconds. For +/- 0.5C the reaction time is just 4s!

Recommended setup:

Heater size - If it is not an aquarium with life inside, use a much smaller heating element (about 5~10W/L ~= 100~300W for 25L) and an aquarium pump (up to 40C it survives) for recirculating and water stirring; or use a stirrer/mixer for wider a temperature range.
Aquariums - Another reference: 1-2W/liter is the recommended power density for indoor freshwater aquariums. With careful heating-element positioning and proper recirculating, a temperature stability of 0.2C can be achieved.
About sensor: An NTC sensor is quite good for -20C to 100C temperature range.
Temperature controller types - Probably an On-Off controller as STC-1000 would be enough and uses an encapsulated (waterproof) NTC. An improvement (luxury if using a low-power heater) would use a PID controller, as this kit using PID40W, but the cost can be about 5x ~ 10x of an On-Off version.

Even if it doesn’t help the Original Poster (after 5+years), at least I hope this can guide other fellows here.