Energy density of a 7-10.5 kW instant electric shower heating element 4

[Jack Benson]

Hello,

in the UK, Ireland and several other EU countries, instant electric showers that heat cold water at the point of use are common.

in the shower unit is a heating element that is between 7 to 10.5 kW (230v 30-45 amps)

Normally a coil heating element is used like this:

does anyone know the typical energy density (W / cm2) of the heating element?

thank-you

 

[Heaviside1925]

It varies depending on the rated flow rate, temperature rise and design. The energy density / heat flux is simply the rated KW divided by the surface area. Are you inquiring about information concerning the one in the pictures? If so, the electrical diagram, specifications, flow diagram would be helpful along with the exact information you are seeking to determine.

 

[Jack Benson]

i am trying to find out what is considered acceptable for an instant electric shower's heating element

the power range is 7 - 10.5 kW

the flow rates are between 3-6 lpm - higher in summer and lower in winter - all depending on the temperature of the incoming cold feed.

I do not know the surface area of the heating element which is why i am asking

the manufacturers do not publish data:

https://www.mirashowers.co.uk/p/par...sport-thermostatic-heater-tank-9-0kw-41563532

 

[Heaviside1925]

Understood. This may be difficult without further information because the design is an important factor. The two designs I am thinking about are a one pass type and slip stream or mixing type. The one pass has a thermostat for set point and simply the water enters, passes over the heating element and exits, with a thermostat switches the element on and off depending on the thermostatic set point. Downside is when flow rate and incoming water temp changes, you may not have a consistent water outlet temp. Then there is the slip stream type, where water is heated much higher than usage temp and a thermostatically controlled mixing valve mixes the incoming cold water with the very hot water, so the outlet water is at the desired temperature. This allows for a buffering effect, so the unit can adjust to changes in incoming flow rate and outlet temperature requirements better than the one pass. Overall KW of both systems may be the same but one heating element may have a higher heat flux than the other depending on the requirements of the system.

 

[Jack Benson]

these heating elements are sold by the millions for these types of instant electric showers.

i have ordered an electric shower to disassemble so i can work out the surface area of the heating element

someone must know what is the acceptable range of energy densities for these widely available products

 

[LittleInch]

A quick google found a 220V 9kW shower heating element, 8.5mm diam, 29cm long.

Actually on further review I think that's the length of the elements and there are 6 of them.

So in W/cm2 that's 116 w/cm2 or maybe (probably) 20W/cm2. that seems to equate better with a max for water of 140W/in^2 listed in a few online guides. 20 W/cm2 is 130W/in^2

Why do you want to know?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Chomalox and Watlow are two major manufacturers of sheath heating elements. Look in their catalogs. They make several different standard Watt densities and sheath materials. The highest density is for water. lower densities are for oil or air.

 

[Jack Benson]

they are spiral inside

at that density they will be destroyed with limestone very quickly

this is our current heating element which is 27W/cm2 (3600W/135cm2=27W/cm2) - this is after 6 weeks in a hard water area!

with this design we get to 8w/cm2 ( 3.14*10*1360mm = 427.04cm². 3.5 kW / 427 = 8.19)

 

[zeusfaber]

Heaviside, I suspect the design pictured is the third of the two types you describe. A variant of your one-pass design.

There's no thermostat, only the safety thermal cutout on the top of the element. A flow switch (top right-hand) turns the element on (coarse control comes from selecting whether you energise one element, the other, or both). Once switched on, the element delivers the selected power level continuously. Fine temperature control is achieved by adjusting the set point of the constant flow valve in the middle right of the unit(which guards against fluctuations in inlet pressure) and by assuming that the inlet temperature will stay essentially constant for the duration of your shower. Overall on/off control comes from the solenoid valve in the bottom right hand corner.

A.

 

[Jack Benson]

So in W/cm2 that's 116 w/cm2 or maybe (probably) 20W/cm2. that seems to equate better with a max for water of 140W/in^2 listed in a few online guides. 20 W/cm2 is 130W/in^2

can you share the links to the guidelines that you found

 

[LittleInch]

https://blog.belilove.com/2016/02/suggested-watt-densities-for-electric.html

Others say up to 300 w/in2, or <60 w/in2

The shower ones are probably higher because they need to be small, but mainly because they have forced flowing water running past them.

Immersion heaters which your picture seems to show rely on convection currents.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack Benson]

the one in my picture is from our version of an instant water heater - so water flowing past the heating element

 

[LittleInch]

Still not sure why you need to know though??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack Benson]

I need to know what energy density to target when I redesigned my heating element for our instant electric shower. Heating element is 3.5 kW, approximately 20 cm in length.

We can’t have this limescale problem

 

[3DDave]

Lime scale is from plating out the lime due to the temperature increase causing a decrease in solubility. It's the temperature change that is the cause, not the energy density.

 

[Jack Benson]

Depending on the energy density, will depend on the temperature of the water that is right next to the heating element. This will depend on different things such as the starting temperature of the water and the flow rate.

If the water that touches the heating element raises above 60 or 65° C, you start to have the buildup of limescale

As long as we can avoid reaching these temperatures, we shouldn’t have an issue with limescale

 

[TugboatEng]

From my experience operating evaporators you need to keep heat transfer surface temperatures below 170°F as at higher temperatures scale formation can become a severe issue.

 

[Jack Benson]

TugboatEng - I agree

I don’t know how to do the calculation to work out the maximum energy density we can have based on:
- the flow rate and
- surface area and
- inlet water temperature

 

[Heaviside1925]

OP,
I wish you had started with this last post. You now have one design constraint, being the water temperature cannot exceed 60-65 C. What is the max flow, max outlet temp and minimum inlet temp?

 

[Jack Benson]

Sorry, you are quite right. I should have realised this.


Inlet temp is up to 40C with a flow rate of 6 lpm

At 6 lpm, with a 3.5 kW heating element the max temp rise is 8C so the max outlet temp is 48C