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

 

[LittleInch]

You're still doing about 20 to 25mm/sec flow velocity in the heater which if its say 3300mm long is still only about 12 seconds residence time.

sounds about right to me.

you're still getting a lot of flow over the elements.

But the missing point here is that for that level of scaling, you're really going to need to look at altering chemical properties of the water by installing a water softener and filter to remove or displace the ions which are causing you the issue.

There are loads of those little electric ones which no one can be sure they will work or the ones using different salts which you need to replace on a regular basis. Trying to solve this by reducing the surface temp and heat flux of your heater element is, IMHO, the wrong approach.

Think of the other elements you can't change in your washing machine or dishwasher??

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

 

[georgeverghese]

It is almost certain you'll need a recirculation pump / recycle loop to keep velocities high enough to get good U values, else elements will scale up. Take a bleed stream off the recycle loop as the exit hot water stream. If you are serious about preventing scale, you'll have to get off the well beaten track cut throat manufacturers travel on.

 

[Jack Benson]

I have already developed the design route to solution and informed the OP.

Who is the OP?

 

[zeusfaber]

Who is the OP?

You are. (I think it stands for Original Poster - certainly what it means)

 

[Heaviside1925]

I have already developed the design route to solution and informed the OP.

Brand new account and first post. Unless you know this person already, like any public forum, spam, trolls and posers... Some folks use a web link to a page with their contact information on it, but it doesn't appear that Jack Benson provided that, so I have no idea how adThp thinks they informed anyone.

 

[Heaviside1925]

Also, zeusfaber

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

Thank you for the clarification, I found the owner's manual online after I first posted, and it appears to function as you described. I forgot to give you credit for that earlier.

 

[Jack Benson]

Brand new account and first post. Unless you know this person already, like any public forum, spam, trolls and posers... Some folks use a web link to a page with their contact information on it, but it doesn't appear that Jack Benson provided that, so I have no idea how adThp thinks they informed anyone.

I didn’t get a message from them

 

[3DDave]

This forum doesn't have direct messages and discourages posting email addresses as that attracts harvester bots, so no surprise about not getting any messages.

 

[Heaviside1925]

adThp I meant no disrespect. It was the way your post, that was deleted, read, that thew me off. You indeed have been a member here much longer than I have. From a quick read of your 11 points you posted, you seem very knowledgeable in this subject matter.

Forums are a lovely place to 'shoot the breeze', but, sadly, not a place to attempt to engineer a new product - especially given the lime constraints. For this, for goodness sake, go to an expert, pay your money, and reap the rewards for a long time. The value of $0 advice is precisely that - $0.

I fully agree with this for the most part. I do feel that this forum can serve a purpose in terms of making up the gap between knowledge vs. application. Sometimes that gap may be filled with a couple paragraph explanation from someone who applies this knowledge professionally or pointing to a standard. Other times in involves educating the OP that even though they may have the knowledge, that there is years practical understanding that is required in order to apply that knowledge and that this forum would not be the appropriate place to provide that.
As for this poster, they appear to have a working prototype, just are having an issue with lime scaling. As a solution they are attempting to design a new heating element and are asking for advice as to how to empirically validate their design. I understand this is likely not the route that a subject matter expert would recommend for proper product design, but consideration may still be provided concerning heat transfer and fluid dynamics concerning the design. In this sense, results may trump theory and the poster's design may function as they intended, even though it's efficiency may be poor. I would agree that the best route is likely to engage someone like yourself in a professional manner.
As an aside. I know some professionals on this forum provide a link to their own page in their signature since there is no method for direct contact sharing. I would be interested in learning about your designs if such a link was provided on your signature.

 

[waross]

I have suffered showers from those instant units from Canada to Chile and points in between.
Many electric, some propane heated.
Never saw a thermostat or mixing valve.
A flow switch turned the element on, full.
Temperature was controlled with the valve setting;
More flow, lower temperature.
Some were somewhat under powered.
If you wanted warmer water you turned the valve to slow the flow.
Turn the valve a little, still to cool.
Turn the valve a little further, a little warmer but still not warm enough.
Turn the valve a little further and OOPS TOO MUCH.
The flow switch just cut off and YOU JUST GOT HIT WITH A BLAST OF COLD WATER.
I hate them.
A design common in the third world is an open nichrome coil with the water flowing directly over the energized element.
Thee water then passes through a grounded screen.
In the third world the screen is not always grounded properly.
These instant shower heaters are common at diving resorts.
A much travelled recreational diver shared a secret with my;
"Always grab an empty styrofoam cut from the dining room and take it to your room.
Use the styrofoam cup to adjust the water valve and you won't get a shock in the shower.
I hate them.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Jack Benson]

respectfully, you do not know the design of our system.

I am well aware of the cheap dangerous water heaters you are talking about, and our system is not like this.

We can give a constant in shower water temp of 42C with a 3.6 kW heating element at a flow rate of 6 lpm when the cold water is 7C.

in shower there is a standard or thermostatic mixer tap - nothing electric. We meet CE standards and localised standards regarding connecting systems to mains water supplies.


i was asking a very specific questions about energy density on a heating element and how to manage limescale.

 

[Jack Benson]

this now makes sense.

i have spoken to adThp on fiverr.com and he did some heat loss calculations for me.

i dont know why you (adThp ) did not mention this is who you are in this post or on the messages you sent me to be on fiverr recently.

the entire system is correctly earthed. the PCB has a fuse and the system must be installed on an RCD protected circuit.

we take our responsibilities for electrical safety seriously.

I am struggling to understand how this thread has got so twisted.

this thread is about energy density and limescale formation - not about electrical safety based on what you think you know about our system.

 

[LittleInch]

I am struggling to understand how this thread has got so twisted.

Well that's what happens sometimes during the course of a thread when people don't have all the information supplied or it arrives in bits and pieces...

People generally mean well by questioning things not specifically asked for to provide their experience on the issues which arise during the discussion.

So you can take these or leave them, i's up to you.

We don't know you, your experience, your product or who else you've been talking to and we get plenty on these boards who attempt to do things way out of their knowledge envelope with potentially high risk consequences.

Looking back at your picture of the limescaled elements, they do seem to be quite close together and with a relatively low velocity in the heater with scaling starting at the "hot" end that you're getting hot spots on the elements. Maybe give the elements more space? or cause the flow in the heater to swirl more and create less of a uniform flow down the heater. Maybe?

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

 

[Jack Benson]

It is surface temperature that affects limescale deposition. Energy density is not your challenge. You simply are not sufficiently qualified to draw this conclusion. Heater companies have been selling units into lime-challenged regions for years.

yes of course - and when i approached you to quote for helping me i was asking you to prepare an Excel file that took into account the inlet water temperature and the flow m/s of water over the heating element as well as many other factors. i know that energy density is not the deciding factor, but given many other variables are relatively fixed in my device its one parameter i can play with.

People generally mean well by questioning things not specifically asked for to provide their experience on the issues which arise during the discussion. So you can take these or leave them, i's up to you.

i appreciate everyone's help and input. I know you are doing this in your own free time and expect nothing for it. I value this forum and it has been very helpful. so thank-you to everyone who has helped on this thread and others.