Watts density explained

Informational content provided by Jamie at immersionheaters.uk  Call on 01827 215684.

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.

WATT DENSITIES FOR TYPICAL APPLICATIONS

8 watts/in2 (1.3 watts/cm2)

Typical Applications: Fuel Oils, Heavy Oils.

16 watts/in2 (2.5 watts/cm2)

Typical Applications: Medium Weight Oils ● Heat Transfer Oils ● Liquid Paraffin

23 watts/in2 (3.6 watts/cm2)

Typical Applications: Lightweight Oils ● Degreasing Solutions ● Heat Transfer Oils ● Forced Air & Gases ● Caustic Solutions

48 watts/in2 (7.5 watts/cm2)

Typical Applications: Process Water

60 watts/in2 (9.3 watts/cm2)

Typical Applications: Clean Water ● Deionized Water ● Demineralized Water

Thanks for visiting, if you now want to buy something to actually do some heating, you have options.

All content provided by Jamie, who is Immersion Heaters UK Ltd, BreweryHeaters.co.uk, Heating Elements.co.uk & FlangedImmersionHeaters.co.uk. And we do Vat Heaters as well, if you wanna go “Over the Side.”

Or just call01827 215684 and have a chat. 

How to choose a brewery immersion heater

Jamie at immersionheaters.uk 07897 246779 loves breweries and brewers, who provide happiness and hangovers to us all. Could live without the hangovers, mind you! The older I get, the longer they seem to last.

Remember all of the stuff that applies to all water immersion heaters, plus the key consideration for breweries is ease of cleaning.

Our existing ranges of immersion heater had limitations when used in brewery applications.

The II Range has many benefits, including bespoke screwplug size, immersed length, double stat pockets, etc, all with a 2 week leadtime, BUT as you can see from the images (particularly the 5th) our aim of maximising the length of 8mm diameter element, hence double looping rather than merely U bending, leads to a fair old ‘clump’ of elements.

Now, we are fully aware that all of you happy brewers are only boiling clean water (ha, ha) and are fastidious in cleaning thoroughly with a jet wash between every brew (chuckle, chuckle), but still, this ‘clump’ led to build up of ‘mystery stuff’ over time, leading to hot spots, element splitting and heater death.

So, in conjunction with one of our Italian suppliers, we designed the BI Range. These are a 10mm, U bent element, which has masses of gap between the elements (see pictures 2, 4 & 6) to enable efficient jet washing of the element to clean off any deposits. We will never be able to deviate from the specs on the website, as we have to buy in bulk with horrible lead times. But thankfully, we seem to have pitched the immersed lengths just right, and the watts density at around the 60w/in² mark, seems to be just right.

Even with the antique gear that some of you chaps are recycling for 21st century brewing, no one has needed any screwplug size other than 2.25″, tho I suppose we could throw on an adaptor to make them 2.5″ if needed.

So if you fancy buying an immersion heater that was actually designed for brewers, give me a call on 07897 246779. Feedback from new and existing customers is greatly appreciated and collated at G##gle & FB.

Thanks for visiting.

How to choose an oil immersion heater

Informational content provided by Jamie at ImmersionHeaters.uk

Call on 07897 246779.

Remember all of the stuff that applies to all water immersion heaters, plus for oil heaters our target is to get the watts density down to 12w/in² or below for heavy, industrial oils, and 25w/in² or below for lighter cooking oils.

Oil Imersion Heater

The II Range has many benefits, including bespoke screwplug size, immersed length, double stat pockets, etc, all with a 2 week leadtime. The 8mm elements can be double looped to fill your maximum available immersed length with a large overall length of element. As can be seen in the fifth image on our PII web page, the elements drop from the screwplug to your maximum available immersed length, before looping back up to within a few inches of the screwplug, before dropping back to full immersed length, then returning back up to the screwplug. This allows us to fit nearly twice the length of element possible in a plain U bent element. For example, in 24″ of maximum immersed length, we could use an 84″ element double looped.

if you have a very large tank with a massive maximum available immersed length, you may choose not to double loop, but to have plain U bent elements over a meter in length.

Informational content provided by Jamie at ImmersionHeaters.uk …who would love you to leave a comment.