Hydor External Heaters

[cybergibbons]

I would suggest 2 reasons that you want the heater on the return leg rather than the inlet. First the inlet is nothing but a siphon and it has very little ability to overcome any flow restrictions. Second, the warmer water is harder for the impeller in the filter to pump. It is almost like not having the filter far enough below the tank water level when the water is hotter. The impeller needs a certain amount of suction pressure to pump properly and that pressure is the amount that the impeller sees over and above the boiling point of water at that pressure. As the water gets warmer, the distance to the boiling point at reduced pressures can get too small and cause the impeller to cavitate. Warmer water and a suction flow restriction will put a double effect on the impeller, not a great way to go unless the filter is well lower than the minimum suggested by the manufacturer to make up for the loss caused by a heater in the inlet. For my canister, they recommend at least 2 feet from the water line to the top of the filter but allow up to 3 1/2 feet of height difference. I would no way go less than 3 feet for my filter in a suction heater situation. The extra foot of suction head would probably be enough to overcome that double whammy on suction pressure.

Hmm. I work with pumps quite a lot, and I really don't think all of this is true.

Most aquarium pumps seem to use a simple open vane radial impellers, with large clearances. They are horrendously inefficient, deal with particulates OK, are cheap to make and quiet to run.

The efficiency of the pump will drop massively if there is any restriction on the suction side. This is the reason why most flow restriction devices are on the outlet of pumps.

However, the cavitation argument in relation to temperature... even if the heater was raising the temperature from 20C to 30C, the increase in cavitation will be negligible. The pump simply isn't being pushed hard enough to cause cavitation in the first place, and this small change in temperature isn't going to make it worse.

 

[OldMan47]

I have no idea what the design parameters are for NSPHr for these impellers, do you have that data Cybergibbons? I must assume that the manufacturers took a typical tank temperature, filter media plugging and inlet hose cleanliness into account when designing these impellers. If you then decide to cheat the numbers a bit when installing your filter, you might well end up at a very narrow margin to the actual design parameters assumed by the tank manufacturers. The fact that they specify minimum tank water levels above the filter housing means to me that they are running closer to the NPSHr than we might expect from such crude pumps. I am not kidding when I say that an inlet heater may well put you over the edge with respect to design parameters. While we may speculate about the impeller's design, it must be said that going near the minimum recommended suction head while restricting flow on that side and also heating the water is a prescription for a nice cavitation situation that will destroy the impeller in record time. I also work with pumps as a part of my everyday life and know of no faster way to destroy an impeller than to operate with inadequate NPSHa.
The heater manufacturers are certainly protecting themselves by saying discharge only for their heaters. They have come to much the same conclusion that I have and want to avoid the civil law suits that operating with the uncertainty might expose them to.

 

[cybergibbons]

The impeller is quite a clever design with regards to NPSHr. With a lot of pumps that operate at low NPSHr (or when it varys), you provide a bypass from the discharge to suction side and this helps limit the pressure difference which can cause the cavitation in the first place. Having the massive clearances results in an automatic self-bypass which really helps limit the chance for cavitation to occur. The problem is that the impeller just churns water - the efficiency drops to almost zero. Like you say, a restriction on the inlet would likely cause this situation, and I'm not sure how these heaters deal with low or zero flow.

I don't think vapour pressure of water between 20 and 30 degrees will make any difference, as it's very very small, especially compared to atmospheric.

Quite a lot of the pumps have a minimum water line because they rely on water for both cooling and lubrication.

 

[AdamM]

thanks!
yeah but my thinking is that if i have a heater on the filter inlet, the heated water will cool down whilst in the filter then the heater on the outlet will 're-heat' the water before it enters the tank,making the heater on the inlet insignificant, in my opinion giving the heating power of just the single heater on the outlet.

In that case, if the water comes out too cold, turn the heater up a bit to compensate???

 

[Sanj]

One thing i will say, I have two 300w Hydor external heaters on my 8foot and they are well worth in my experiance. These have been running nearly two years and unless you have a sump are really much better than having the eysore of internal heaters. Good move.

Intlet and outlet, not sure why one is recommended and not the other.

 

[waterdrop]

Yeah, I have a 200w Hydor that's been running for over 2 years and has worked like a dream. Incredibly fast heating up cold water and rock solid thermostat control. I noticed one thing wrong in kiriyama's ETH link: I'm pretty sure when I got mine the 200w could be had in both the 12 and 26mm width.

All this is fine but isn't there some better way to heat a really large tank? Perhaps run a small sump and place some traditional heaters in it, or aren't there completely different things that are done with really large tanks?

~~waterdrop~~

 

[T1KARMANN]

i just sold 2 brand new 300w hydos on ebay for £50

i will have 2 used 300w for sale at the start of march if any one is intrested £30 for both will seal the deal

the reason they go on the return pipe is so you only get clean water going in them and you wont get over hot water going into your filter messing up the rubber parts

they are great heaters and move warm water around the tank much faster after a water change

great heaters for a ray tank

PM me if you are intrested in my 2 for sale