Electric cabinet heaters

Hi all,
Just a quick question. Does anyone here have experience of using those little extruded aluminium electric cabinet heaters? They are about the
shape of half a cube and come in low power ratings like 20 W and 40 W. I think they're primarily intended for preventing condensation.
The reason I ask is because this week I had rather a lucky score. I got a power supply from a school physics laboratory. It supplies AC or DC at something like 0-25 V and 15 A. I'm planning to use it for electrolytic de-rusting. This means it will have to live outside in the shed. I've had a few problems with non-oily things corroding in the shed. I need to do a bit of rewiring inside the power supply, so I was wondering about adding a small heater. What do people think? Will it help prevent corrosion or are they a waste of money?
Here's a picture of the power supply for those who are curious:
http://www.mythic-beasts.com/~cdt22/lab_psu.jpg
Best wishes,
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

No, but I have heared that people put a blank over the object and put a lamp under it (of course, you pay attention that nothing will start burning). Guess it is much cheaper than a heater.
HTH, Nick
--
The modular DRO
Available now in USA / Canada
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I can send you a few feet of Raychem heating cable, it is self limiting (I assume it prevents overheating and fire) and produces 5w per foot. Rated for 208-240 volts, so I assume it is OK for the UK.
Your power supply looks very nice.
i
On Sat, 12 Aug 2006 20:42:57 +0000, Christopher Tidy

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus1648 wrote:

Sorry I haven't got back to you about this sooner. I've decided I'm going to try the hygrostat approach as I've seen some cheap on eBay. I figure that a 60 W heater and hygrostat should give the protection when it's needed, and hopefully not consume too much electricity in the process. Thanks very much for the offer though. I'll post some pictures when I've got the de-rusting setup working.
Best wishes,
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Simple 60 watt light bulb will provide that heat easily, and cheaply! An empty tin can and a ceramic holder can also prevent light from getting to your cabinet if required. Heating level can be changed easy as changing bulb wattage.
Hope this helps, Peter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 31 Aug 2006 22:10:24 +0000, Christopher Tidy

Sounds good. Derusting will be easy with your power supply.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If you get a book on vacuum systems, you will find they talk a lot about baking systems to get the water vapor off the surface where it has condensed.
On storage, you might think about an airtight container and some decicant. A ziplock bag would be better than nothing, but some plastics are not really good at stopping water vapor from going through.
I think part of your problem might be the change in temperature in the shed. There might not be any condensation that you can see, but there still might be some. So another approach could be to use timer to turn the heater on for a few hours late at night/early morning.
Dan
Christopher Tidy wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
They are a standard item for pianos, keeps the humidity at bay. They work by raising the temp a few degrees, just enough to move the cabinet temp above the dewpoint. They are most effective for rising temps experienced in the morning when the shop is colder than the outside. They do consume a lot of electicity: 8760 hours per year times 40 watts is 350 kwh. (!!!) I pay something under $.10 per kwh so $35 a year for me.
Christopher Tidy wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Christopher Tidy wrote:

How about a plastic bag and jar of silica gel inside, with the powersupply.. Plastic bag works as a moisture barrier and silica gel absorbs the moisture inside the bag after reclosing it after using the powersupply.. Sure, it won't look good, but it'll work and consumes no electricity, and will keep the powersupply safe from the local small animals looking for a warm place to live.
Kristian Ukkonen.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I use two 25W 110VAC light bulbs wired in series to keep my welding rods dry & keep rust out of a metal cabinet - works here on the rust coase - east Florida....Joel ==========
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
My apologies for dampening (ugh!) the enthusiasm for using heaters to combat workshop humidity, but I think that applying a heater to the rear shear of one's lathe is a bad idea, particularly one that may reach 68C thereby creating a significant temperature gradient and stress within the casting. Although a contact surface heater will increase the temperature locally, I doubt if sufficient heat will be transmitted to the front shear, and remote parts such as the chuck or the tailstock are most unlikely to benefit. All unheated bare steel surfaces in the workshop will derive no protection from the moisture-laden air: heating one part of one machine above the dew point merely protects that area. I believe that heating costs would be substantial to achieve an acceptable overall result in protecting tools and machines.
I apologise for dampening (pun intended) the enthusiasm for using heaters to combat workshop humidity. Heating one part of one machine above the dew point merely protects that area, whilst all unheated bare steel surfaces in the workshop will derive no protection from the moisture-laden air. I believe that heating costs would be substantial to achieve an acceptable overall result in protecting tools and machines. In my opinion, it is better to deal with the cause at source by installing a dehumidifier, and I heat my workshop only when needed for my own comfort. For 28 years, a 170 Watt domestic dehumidifier has resided in my workshop where it diligently maintains the humidity at around 65-70% according to two cheap hygrometers. I know that it runs for far less time than it is idle (automatically controlled by a hygrostat) though I have not recorded its running time. In cold dry weather, when heating elements would be working hard, the dehumidifier does not run at all: relative humidity is low and there is no chance of condensation or rusting if the workshop is unoccupied. My workshop is built from concrete blocks with no cavity, dry lined on battens and has a concrete floor and insulated plasterboard ceiling but no specific vapour barrier other than the painted plasterboard; I am diligent in keeping the draught proofed door closed. Those who install a vapour barrier in their workshops will benefit from lower operating cost. I have had no problem with rust despite living beside a marsh and within half a mile of the sea. Regards Ian
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dehumidifiers depend on an ambient temp above about 60F (15C). Lower than than and the coils will freeze. Your choice of solution is dictated by ambinet temp range. Unheated shops in my area will drop to around 0F (-18C) in my area. That is not damaging, it is the F to +60F swings in spring and fall that do the damage.
Heaters work fine if the entire machine is tented or covered with a tarp, the heater resides in the lower part somewhere. Tenting material needs to be reasonably air tight but NOT water tight like a plastic tarp. Clamping a heater onto one section of the machine will indeed introduce stresses unless you are dealing with something like an engine block (internal heaters used all the time)
snipped-for-privacy@jerseymail.co.uk wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hello everyone,
Thanks for all the replies. Sorry I've taken a few days to follow up my question. I had a hectic weekend.
I'm also a little puzzled about the mechanism of rusting. Every chemistry book and article I can find on the subject refers to rusting occurring when there is liquid water on the surface of the iron. But I don't think condensation occurs very often in our shed. Sometimes I have been out there at dawn and I've never noticed any condensation as the air temperature rises. So I figure that water vapour in the air must be able to cause rusting. But heating seems to stop the rusting. I have had a piece of machined steel (a part with a broken tap stuck in it) sitting on my desk in the house for the last two years. It hasn't rusted.
Suppose we take some damp air and heat it, but keep the pressure constant. For the pressure to be the same, the water molecules in the warm air collide with my iron object less often, but with greater momentum than the water molecules in the cold air. Maybe this is the key? Water molecules in warm air collide with the iron object less often, so the rate of corrosion is lower. But if that was the case I would expect iron objects in the house to corrode slowly. They do not appear to corrode at all. Are there any chemists here who can provide me with a simple explanation for why heating prevents rusting?
Iggy, thanks very much for the offer of the heating cable. For a while I wondered what it was, because I thought that as the cable length is increased, the resistance and so the power output must fall. But I figure that it must consist of two wires with some kind of resistive material between then. Is that right? I will have a think about the best way to protect this power supply and get back to you. One website publishes a formula for calculating a heater size, and it recommends a wattage which would be unrealistic with 5 W/ft tape. I have seen some hygrostats on eBay, so I might invest in one of those and connect it to a higher wattage heater.
It is a nice power supply. It's built like a tank and built to be maintained. I was lucky to get it actually. Last Monday I stopped by a laboratory which I had helped clear out a couple of weeks before (I took their unwanted gear, some to keep and some for eBay). There was no one there but the power supply was left for the trash. A couple of days later the whole building had been demolished. It's amazing how quick buildings get demolished now.
Best wishes,
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Christopher Tidy wrote:
<snip>

<snip>
Should have been "because I thought that as the cable length is increased, the resistance must increase and so the power output must fall".
You probably figured it out anyway.
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 16 Aug 2006 00:44:23 +0000, Christopher Tidy

I think so. It is supposed to be self limiting. According to my seller, it would even make 5w/ft even at 120 v. (it was a local $5 ebay purchase, a lot of cable, so he had little reason to lie at pickup time)

would that be a heater in close contact with the machine, like that cable? I think that a cable wrapped around the machine, would transfer more heat to it.

Yep. I had a few supplies PP-1104C/G built like that. Very nice, with good regulation also. I did not need them, though, because I have some smaller supplies for small work and I have my welder for big power.
i

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus15692 wrote:

All the Raychem cables I can find online seem to be rated for one voltage only (see http://www.accontrols.com/acatalog/Self_Regulating_Cable.html ). If they are able to maintain a constant power output across such a wide range of voltages, I'd be very interested to know how they achieve this.

It would be bolted to something, most likely the transformer as this is fairly central and low down in the unit. There are so many things inside this power supply (transformer, variac, selenium rectifier, rotary switches, panel meters, fuse holders, etc.) that I think I will just have to aim for heating the whole cabinet. I am thinking that perhaps a powerful heater (perhaps even a light bulb) and hygrostat might provide more effective protection than a low wattage heater which runs all the time. But I'm still not sure at what humidity corrosion becomes a problem. I am going to e-mail a chemist friend of mine and see if he knows.

I suspect the regulation on this is pretty good also. It has a 15 A output, but a battery can be added to give a boost up to 45 A. This makes sense as it supplied a teaching laboratory with about 45 outputs for experiments. I have my welder for big stuff, but the welding output is AC only. It has a 75 A battery charger but this broken, and I am not especially keen to open my welder's oil-filled tank to try and fix it. I also wonder if using my welder on a low setting for de-rusting will waste a lot of power. I'm not sure about it but I think it might as I'd expect the machine to be most efficient at its maximum current.
I'll let you know what I decide.
Best wishes,
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.