Problems Recirculating During the Boil

[akmolloy]

Hi,

I've built Jamil's Whirlpool chiller, and used it for the first time today. this was also the first time I've used my March pump. Everything worked fine when the boil was off, it was during the boil that I had some problems.

I tried recirculating boiling wort the last 10 minutes to sanitize everything, but it appeared to be having some strange problems. It was almost like it was sucking the air created from the boil and wasn't able to get enough liquid. As soon as killed the flame, it immediately pumped at full volume. Before flameout I could see air moving in the line, although it was moving through much slower.

Does anyone else see this problem? Any ways around it? Am I hurting the pump with those bubbles in the line like that?

I also lost my dip tube, and I believe this happened when I initially started the recirculation. I recall hearing a pop, but I attributed it to an air bubble or something. Is there some weird physics happening here?

-Tony

 

[ClaudiusB]

It was almost like it was sucking the air created from the boil and wasn't able to get enough liquid. As soon as killed the flame, it immediately pumped at full volume.

I also lost my dip tube, and I believe this happened when I initially started the recirculation. I recall hearing a pop, but I attributed it to an air bubble or something. Is there some weird physics happening here?

All the symptoms sound like pump cavitation problems.
Noise is one indication, flow rate change, pump vibration, shock waves from the vapor bubbles, etc.

Preventing pump cavitation:
Lower the fluid temp, increasing the pressure at the suction side, check pumps data sheet and a GOOGLE search "Pump cavitation definition".
A pump is a very interesting subject.

Cheers,
ClaudiusB

 

[Catt22]

Yep, it's a cavitation problem. At or near boiling temperatures even a low degree of pump suction will cause vapor pockets to form. The bubbles in the line you observed were vapor pockets and the effect on the pump is the same as if it were sucking air. Part of the problem may be that the diameter of the dip tube is too small. This creates a restriction that will make the problem worse. Ideally, you want no part of the suction line side to be smaller than the pump inlet which is usually 1/2" ID. Many dip tubes are only 1/4" ID (3/8" OD) which can be a problem especially at a high flow rate. Cavitation can damage a pump if it continues long enough. Most likely no harm was done to your pump as it was for only a short duration. The pump should be positioned below the kettle outlet in order to maintain a positive suction head. IOW, the wort should be able to make it to the pump inlet by gravity alone. The lower the better under most circumstances. Most of us have our pumps arranged this way and you probably do too, so that shouldn't be a contributing factor.

 

[akmolloy]

Hmmm, interesting, I'll do some searching.

FWIW, my pump is lower than my kettle, and my dip tube is 1/2 inch, as is my valve and the tubing to the inlet on the pump. The only restriction would be the Quick Disconnect, which is called "1/2" inch, but the port is actually smaller. It's the standard one people seem to all order from McMaster for their march pump on this board, so I can't see it being the problem.

What I did to prime was to connect the 2 hoses, then (while holding the end of the outlet tubing near the ground) open the valve on the kettle and then slowly open the valve on the pump. Gravity sends liquid all the way to the end of the tubing this way. Then I close the valve on the pump, quickly connect the outlet tubing and turn on the pump and open the valve again.

Like I said, it works fine this way unless the water is boiling. I'll have to do some experimentation I guess.

Edit: I just did some searching and it looks like if I only slightly open the valve on the outlet side when starting up I might not see this problem. I believe I was opening it fully and quickly, which might explain things.