Quote:
Originally posted by StirCrazy:
you stated that a centrifical pump will trip out at 0 head. you have me totaly baffeled with this statment
I have one in my fresh water tank and it never trips out.. I don't understand why you think it would.. I am sure lots of people have power heads going in there tanks constantly with out them tripping out all the time.. heck my power filter is 250 gph it is running at 0 head all the time and it never trips out.
..but there is still confusion on this 0 head thing, I have had lots of power heads over they yease and they all ran in the tank fine.. Steve[/QB]
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<font size="2" face="Verdana, Helvetica, sans-serif">Well, here we go. First I stated that *most* will trip out at 0 head and we were discussing your tub pump at the time.
Lets look at the power head first. I will give the disclaimer that I have never played with a powerhead but have other similar stuff. This is a *very* educated conjecture. First, the powerhead is operating at a very fixed/specific design point and is thus specifically built for the task (and is fairly atypical as far as centrif's go) Second, the discharge nozzle is sized to restrict the flow to certain rate (artificial head) - a flow rate that the motor is capable of handling. Should some poor misguided soul decide to tweak the powerhead a bit and drill out the nozzle (to reduce the back pressure so the pump can work easier!!!) - the flow rate will increase and overload the motor. Very certain on that.
The original start of the 0 head conversation was regarding the specs on your hot tub pump. Darren has confirmed for you my low head high amps theory. So as the head decreases, the flow increases, and the amps increase. If you look at the specs on your tub pump, at 30'/127gpm the pump will be working the hardest and drawing very close to the name plate amps. I draw this conclusion because that is where the manufacturer stops providing performance info as you stated. They dont stop publishing data because the pump will never encounter that situation (because believe me they eventually will in some application) Its because the manuf doesnt want you to f'in run it below this head. Why? Because as the head is reduced past 30', the gpm will increase, and remember our rule, the amps will increase and overload the motor. If the head is reduced further the pump will start cavitating like hell. This is not conjecture.
So now you're asking why dont they make a hot tub pump that can run down to 0 head ? Its all about design point and efficiencies. At the specific window/range of flow rate & head that your tub needs to run properly requires 1.5hp to spin the impeller. This window is usually in the pumps best efficiency range. Unfortunately that same impeller requires 2.5 hp to operate over its *entire* head range. Now do you put the extra hp on to cover a head range that shouldnt ever be encountered? Hell no. Just cut the curve off early and say dont run the pump here. That is why most centrifugals will overload at 0 or very low heads. The motor is sized to operate the pump in the effficientcy range and not over the entire head range.
With say the ampmaster and power filters they are designed to operate at 0/low heads so one wouldnt expect them to act up in this range. But dont expect all centrifugals to behave similarly as they are atypical in the wide world of pumps.
As far as the other stuff in your post the I dont quite agree with - well its not overly misinformed or misleading or dangerous and my typing totally sucks so.....
Talk to ya later.
Jim