Is it sufficient to mount an SSR to 20x60 Alluminium extrusions for passive cooling, or is a heat sink a must? It will be running the big bed which is a 550W 24V DC setup.
Is that running off 24V?
Which SSR? The datasheet on it should tell you how much power it can handle without additional cooling.
This is the SSR
http://m.ebay.com.au/itm/191490047810?_mwBanner=1
I’ll try see if i can find A data sheet, hoping it’s an original…
And this is the datasheet from their website
I would suspect it’s a counterfeit: http://canada.ul.com/safetyalerts/ul-warns-of-solid-state-relay-with-counterfeit-ul-recognition-mark-release-13pn-52/
The pictured label on the ebay listing looks correct, or closer to correct, but the case design still has squared off corners in the inset compared to the one beveled corner on the legitimate one.
It may work, but I would test it at very low power to start with and see if it heats up. I also probably wouldn’t trust it above 50% rated load, unless you slowly ramp your way up to that in testing and it shows no signs of heating or failing.
550W 24V means 23A which is a fair amount of current. 20x60, bolted to the 60 face might be OK as a heat sink but the normal heat sink isn’t expensive that I don’t think it’s worth the risk unless you just don’t have the room to spare for the tall heat sink.
The idea was to bolt it directly to my frame, but now it’s a question of safety since it seems it might be a fake 
I’ll have to see if i can get to the TRIAC and see the rated current…
Assuming the SSR turns out ok to use, what size heatsink do i need? Mounting it to the frame will be interesting too…
If the SSR checks out, then maybe try it and see. Check to see how warm the extrusion gets every fifteen or thirty minutes.
There’s only one size heat sink I know about for that shape of SSR. It’s about 50mm tall.
I’m considering this heatsink
http://www.ebay.co.uk/itm/121775074607
did someone removed my comment?
Guess what? 24 volts or 24000 volts aren’t making any difference if the energy dissipated is always 550 watts…
Any heatsink for a 550 watts device is going to be good.
Conclusion:
Is this heatsink ok?
It isn’t dissipating 550 watts, it’s passing it. In the process some power will be lost to resistance and will turn to heat, but it will be far short of 550 watts.
If that’s a genuine SSR it’s rated for up to 40A I believe. You’d be pushing about 23A through it.
If it’s genuine it shouldn’t heat up much, and no heatsink may be needed, but if one is needed the extrusion will likely be enough. If it’s not genuine it will heat up significantly and you’re better off replacing it with a legitimate one rather than trying to make the counterfeit work with significant heatsinking.
Even if it was genuine, I wouldn’t run 24A through it without mounting it to something metal. Even reputable SSRs in that class can have substantial heat sinking. For example, this one is 30A and two thirds of the body is heat sink. http://www.crydom.com/en/products/catalog/c_km.pdf
The ones that don’t have heat sinks built-in are meant to be bolted to something that will draw out heat.
I wouldn’t run a known counterfeit though. Some are just “uprated” with a sticker.