Watts density explained

Informational content provided by Jamie at immersionheaters.uk  Call on 07897 246779.

I was going to provide a link to the best definition of watts density available on the web, but they are in very short supply, so here goes.

Watts density is the amount of heating energy emanating from any given amount of surface area of the hot part of an element.

In Imperial Britain and the States, this is usually described in terms of watts per square inch, but could be expressed (to keep the Eurocrats happy) as watts per square cm, mm, or indeed, meter. But why bother?

For 8mm diameter elements, simply divide the heating power output (in watts) of the given element by the length of hot section, ie. subtracting the cold ends at either end of the element, which can be 2″, 4″, 6″ or any custom length.

So, for the benefit of a simple example, if we have a 1kW, 54″ element with 2″ cold sections, we first take 4″ (2 ends of 2″) of cold off the total length of the element to give our hot section, 50″. Then we divide the power output of 1000w by 50″ to give us a watts density of 20w/in².

Now imagine we have a 1kW, 104″ element with 2″ cold ends (c/e), giving a hot length of 100″, do the maths as above, and you should get a watts density of 10w/in². Now, picture an inch square in your mind, or draw it if you prefer. Now, fill the box with 10 or 20 little boxes, happy faces, “watt monsters”, whatever crumbles your cookie. The size of the square doesn’t change between the 2 examples, but the size of the contents do, more watts per square inch means more squashed, little watts trying to get out of the same space. Now draw a box, mentally or on paper, and squeeze 40 of your little watt icons in that box. While your at it, have think about how long a 1kW element with 2″ c/e, would need to be to give a watts density of 40w/in²?

For the less imaginative among you, somebody kind drew this –

Different watts densities are requires for different applications. Some are set in stone, for obvious reasons, such as oil heaters needing to be no more than 12w/in², others are a little more flexible and open to “interpretation”. ElementYoda always told me 5w/in² for elements in still air, but as can be seen below, grill elements can work at up to 42w/in². Fortunately, we don’t supply many of them, cos there wouldn’t be much of a guarantee at that top end. We are mainly suppliers of immersion heaters for water, with soft water being fine between 50-75w/in², but hard water being better suited to 40w/in² or below.

Our (Element Air) EA Range is rated at 27w/in², designed for forced air heating units, where the airflow needs to be 2m³/s. The simple way this is achieved is that every meter of element equates to 1kW, so if you want 3kW you have to bend/coil a 3m element into your available space.

Below is a detailed table of watts density in inches and equivalent cm, plus details of colour change and suitable applications.

W/in2	W/cm2
94	14.5	Immersed only
83	12.9	Immersed only
73	11.3	Immersed only
63	9.7	Immersed / High Arflow / Machined Fit
52	8.1	Immersed / High Arflow / Machined Fit
42	6.5	Equivalent to Grill / Radiant / Orange
31	4.8	Glow Red
21	3.2	Colour Change
10	1.6	Little or no Colour Change Still Air
9	1.5	Little or no Colour Change Still Air
8	13.0	Little or no Colour Change Still Air
7	1.1	Black Heat
6	1.0	Black Heat
5	0.8	Black Heat
4	0.6	Black Heat
3	0.5	Black Heat
2	0.3	Black Heat

And for the visual learners among you, here is that same info in colourful form that I “borrowed” from a friend –

To convert to other element diameters, please divide your solution as above by the following factors.  If you think of the 1kW, 54″ element with 2″ cold sections that we started with, only this one is 12mm in diameter, share our 20w/in² initial answer by the factor of 1.484, gives a watts density of 13.48w/in², which makes sense as a wider element will have more surface area for the 1000w to “escape” from.

mm diameter	FACTOR
8	0.99
8.5	1.05
9.5	1.175
10	1.24
11	1.36
11.5	1.42
12	1.484
12.7	1.57
13.4	1.657
16	1.98
20	2.474
50.8	6.28

When you understand all this, and realise that glowing ain’t good, you can see the sense in squeezing as much element into the space available, cos simply put, the lower the watt density, the longer it’ll last, whatever it is heating.

Thanks for visiting, please leave a comment.

Informational content provided by Jamie at ImmersionHeaters.UK or 07897 246779.

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How to choose a water immersion heater

Informational content provided by Jamie at immersionheaters.uk

Call on 07897 246779.

Remembering all of the aspects and considerations discussed on the “How to choose an immersion heater” page, water hardness must be taken into account when designing a water immersion heater.

A detailed description of water hardness can be found HERE, but for the purposes of this blog, we will say that hard water has a higher concentration of calcium and magnesium which leads to limescale, which we are all familiar with in kettles, etc, within the home. Whilst merely being an inconvenience in the home, (one that can be avoided by using one of these beauties which are awesome and well worth the money if like me you consume litres of water a day, highly recommend, no business interests or bias by bungs!) it can be a real issue in commercial and industrial applications.

The problems are caused by the limescale deposits building up on the hot part of the elements. Once this process starts, it becomes self perpetuating, and indeed self accelerating, if not removed by jet washing, disolving or other means. Basically, the limescale build up prevents the heat getting away from the element, acting like an unwanted insulating jacket, increasing the element surface temperature, which in turn, attracts more deposits. An example of which is shown below, courtesy of gimmecoffee.com, which would perhaps be better used as a christmas tree than for making coffee!

The easy way to maximise the working life of your immersion heaters, whether in a hard water area or not, is to minimize the Watts density. The article explains in detail a simple concept which we are using every day, maximise the element length THUS minimising the watts density THUS maximising the working life of the element, all other things being equal.

Informational content provided by Jamie at immersionheaters.uk Call on 07897 246779.

What is a heating element?

Informational content provided by Jamie at immersionheaters.uk

Call on 07897 246779.

For the full shilling version, see the web authority on everything, Wikipedia, or an informative, if uninspiring, site referenced on the wiki page.

The extremely abridged version – its an electrically powered rod which applies direct heat to liquids and gases.

And finally, the ImmersionHeaters.uk version. Here is one I chopped up earlier –

 

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Video for those that are allergic to reading.

The top end is the terminal, sometimes a spade connection, but more usually a thread, washer and bolt combination, geared at holding the electrical connection, ie, incoming wire, in place.

Below the nuts is a white plastic plug which serves as a seal against ingress of moisture and also to hold all of the important bits inside in the middle of the tube, away from the sheathing.

The sheathing material is the outer housing, the solid casing, usually made of a metal, in this case Incoloy 800, a stainless steel variant. This serves to encase and protect the important bits, protecting them from whatever is being heated, ie, oils, chemicals, etc.

The white compacted powder inside the tube is magnesium oxide powder, which serves as an insulator against electrical leakage, but as a conductor of the heat generated by that electrical current.  It serves to hold the filament wire centrally within the tube so that contact with the sheathing material is all but impossible.

Finally, the tiny wire seen protruding at the bottom of the picture is the filament wire which creates the heat in the core of the element.  Usually Nickel Chromium, this converts electricirty very efficiently into heat, with only a little wastage as light, as with the filament of an old light bulb.  Whilst it is straight in the picture above (due to being hacksawed), under normal circumstances it would be coiled centrally in the magnesium oxide, as shown below, in this very cool x-ray.

The picture above also helps to explain the terms cold end, cold section, dead length, etc.  The bit before the wiggly filament, that looks a bit like a hose pipe, is the other end of the terminal pin, the nuts & washers electrical connection as described above. Usually mild steel, this section is not going to create a great deal, if any heat, and so this is termed the cold section.  All elements have two terminals, one at either end, to complete an electrical circuit, and both ends will have a cold section of 2 – 6″ dependent on the application.

This length would have to be subtracted from the element length when calculating watts density, ie, a 48″ 1000W element with 6″ colds has only 36″ active length (48″ – 6″ – 6″ = 36″), so the watts density of this element is 1000 / 36 = 28w/in².

So that is a plain heating element, but what about finned heating elements?

Mainly for use in air duct heaters, finned heating elements are merely plain elements as detailed above, but with finning added by a very clever machine.

This allows for better dispersion of the heat, basically by creating more surface area of heated element in the same available space.

Edit – Oct 2021.

There is much debate as to how much finning increases the surface area of the element, thus reducing the watt density, thus allowing you to fit more power into smaller spaces. I am not clever enuf to disprove a claim by one supplier that it increases surface area by 11 times, but in consultations with another friend & supplier, we have come up with a clever possible alternative. A Medical Supplies customer is test driving this idea for us currently, and if successful, will lead to a whole new approach. And probably another blog post for the first time in many a long year.

Informational content provided by Jamie at immersionheaters.uk

How to calculate the kW required to heat a volume of water in a particular time.

Click HERE for our online water heat up time calculation page. Or here.

At ImmersionHeaters.UK, (Call Jamie on 07897 246779) we have 2 types of customer.  There are those who tell us what they want as they know their exact requirements.

Then there are others who need guidance and advice as to what they need.  One question which comes up time and again is “How many kW do I need to heat up my tank?”

If we can calculate the volume of water and the required temperature rise, we can answer this question.

The following formula is used to calculate the power of heating element needed to heat a specific volume of water by a given temperature rise in 1 hour.

volume in litres x 4  x temperature rise in degrees centigrade / 3412

(4 being a factor and 3412 being a given constant)

for example 100 litres of water, to be heated from 20ºC to 50ºC, giving a temperature rise of 30ºC would give –

100 x 4 x 30 / 3412 = 3.52

meaning that the water would be heated in 1 hour by 3.5kW of applied heat.

Also we can use this information to extrapolate both ways.  To heat the same water volume in half the time (30 minutes) would need twice the heating power, ie, 7kW.

Converesely, if we only use half the heating power, 1.75kW, it will take twice as long to heat up to desired temperature, ie, 2 hours.

If we only have a 1kW element available, we will expect a heat up time in excess of 3 hours.

Also we can use this formula as the basis of similar calculations for heating oil.  Generally, oil heats up in half the time of water, due to its density.  However, oil needs a much lower watts density element than water, as described in the “How to choose an oil heater” article.
 

Our updated website is up, running and ready for viewing.

Following a great deal of hard work and patience on the part of our Website Team at Counsell.com, our new WordPress website is ready to view at immersionheaters.uk

There is a lot more content on the site, much more product detail, and hopefully, all laid out in a way that a first time visitor in search of a UK based supplier of immersion heaters or heating elements, can drill down through the content to find information about exactly the product that they are after.

For example, someone in search of a 1kW finned heating element, may land on our website from natural listings or from adverts, on the ‘heating elements’ page, they would then select ‘Finned Heating Elements – PEF Range (250W – 7kW)’ where they would then find a list of the various lengths and kW ratings available as finned elements, from which they would select 1kW, taking them to the PEF Range – Finned Heating Elements 1000w / 1kW page, which lists the various lengths of element that are available with a 1kW rating (20″, 24″, 26″, 30″, 32″, 34″, 36″, 40″, 42″, 48″, 54″, 60″, 66″, 72″, 78″, 84″, 90″, 96″, 104″, 114″, 120″, 132″, 144″, 170″, 180″). They can then choose the length which best suits their application, then ring us on the phone numbers ever present in the right hand bar, to place their order.

Also new additions are many of the photographs, showing much greater detail of terminal boxes, element looping, element diameter, etc.

Again, these are for the benefit of new customers, to get a feeling for the dimensions, quality, etc, of the heaters they are considering purchasing.  These pictures are complemented on the specific Range pages, by tables showing all available kW ratings, plus sub pages for each individual heater in that range.  We hope that the intuitive nature of the information allows ‘viewers’ to navigate to exactly what they want, then ring us and tell us what they want, rather than ringing ‘blind’ and asking if we supply something suitable to their needs.

The ‘OddBins’ page is also new, and is my attempt to sell off some of the accumulated bits and bobs at bargain prices, just to get rid of it and reclaim some space.  These are items, sometimes ordered in error, sometimes sent as samples by prospective suppliers, but just oddments that may fit somebodies exact requirements.  I’d never even heard of a ‘Hopper Heater’ but now you can buy one from us!

The website will grow, as will this blog, over the coming months and years, and we welcome any feedback, input, guidance, etc, from anyone with the interest and motivation to give it.

Just to repeat our enormous gratitude to all at Counsell.com for teaching a man to fish. Cheers!

(and still fishing, all these years later!)