Recirculation pump stops working when heat applied to MLT

[ASantiago]

Yesterday was my first time using a SS MLT with direct heating. I'm using a Top Tier brew stand and burner and a 15 Gal pot with a false bottom (all Blichmann). I'm also using a Chugger pump. The objective was to do step mashing for a 10 gal batch composed of 31 lbs of grist, which included 1 lb of rice hulls and <5% flaked wheat.

As long as I didn't apply heat to the mash tun, the pump would recirculate without any issues. However, shortly after turning on the propane burner, the pump would stop moving liquid. This was at around 150F (my estimate).

I'm totally puzzled by this. I tried a variety of heat level and pump rate combinations, but the result was always the same.

I can venture a guess here, but I really need some help. I'm guessing the heat is eventually turning into a boil, which is generating air bubbles which are then stalling the pump. But... how??? One of the many scenarios I tried was really low heat rate and rather high pump rate, which one would think doesn't result in boiling and in any case liquid is being moved out fairly fast.

Has anyone run into this before? Any suggestions, ideas? I know what I'm trying to do is done on a regular basis with the same equipment. What am I missing here???

 

[RM-MN]

I think you are right on about the wort boiling. If you had just water in the tun, the heat would distribute evenly but with the grain in the tun, you can quickly heat the bottom of the tun without even warming the top. I'd suggest you lower the heat and perhaps even stir while heating and recirculating to avoid this.

 

[ASantiago]

Thanks. I did stir while recirculating (sorry didn't specify in my original post), but mainly because I realized the temperature of the mash wasn't changing around the thermometer. I had heat going in, so obviously I expected the temp reading to increase, but it was stuck at the same temp. For a while I was pissed thinking I had a faulty thermometer. Then I started stirring and the temp reading updated. But liquid would stop flowing down to the pump.

What about mash thickness? Out of habit from mashing single infusion in a cooler, I went with 1.25 quart/gallon. Maybe that's too thick for a recirculating mash?

Recirculation is done routinely by many home brewers, some with much larger set ups and grain bills.

 

[acidrain]

There have been many discussions about this. Direct firing a mash tun will cause the pump to cavitate.
Moving the pump lower and throttling the pumps output will help, but not completely eliminate the problem.

 

[gandelf]

There have been many discussions about this. Direct firing a mash tun will cause the pump to cavitate.
Moving the pump lower and throttling the pumps output will help, but not completely eliminate the problem.

I have been direct firing my mashtun and continuously recirculating it for almost 10 years now. It does work and the sky doesn't fall; at least not in Wisconsin. I do use a false bottom to suspend the mash off the bottom of my 15 gallon Polarware kettle (scorching). I do throttle the pump output, because a moderate flow is all that is needed. I have a tee plumbed in on the kettle output for an RTD for my PID. The PID controls the furnace valve; which for my system results in a 0 to +1 temp deviation because of the dead band.

For the last 2 years, I have been doing BIAB in it.

 

[ASantiago]

I have been direct firing my mashtun and continuously recirculating it for almost 10 years now.

What mash thickness do you normally go with?

There is no question that this is being successfully done regularly by homebrewers all over the land. I want to be there too.

 

[kanzimonson]

I've had the same problem and never fixed it. Eventually I just said screw it. I now just try to hit my strike temp right on, and then let the temp fall as it will. If I want to do any step mashing I'll do it with multiple infusions or decoctions.

I partly came to this method after hearing so many brewers say the mash almost entirely converts in about 20 minutes. Plus, it sounds like most brewers are chasing a drier beer, so a little extra conversion doesn't bother me. If I want more body, I'll add more proteinaceous malts.

But I'd like to see if anybody has a solution.

 

[ASantiago]

I've had the same problem and never fixed it.

What is your typical mash thickness?

 

[ASantiago]

I've had the same problem and never fixed it.

What is your typical mash thickness?

 

[kanzimonson]

What is your typical mash thickness?

Usually 1.5qts/#, and I have a keggle MLT with a good 1-3 quarts below the false bottom (I can't remember). The dip tube draws from millimeters above the bottom so I imagined it was a similar thing with boiling within the diptube causing cavitation.

 

[pickles]

I have a RIMs system and mash at 1.75 - 2 qt/lb.

 

[ASantiago]

I have a RIMs system and mash at 1.75 - 2 qt/lb.

Are you having the same issue or is this working well for you?

 

[pickles]

I've never had that issue. My system works best when the mash is thin, there's evidence that a thinner mash improves wort quality. Beau kaiser has a thread on here somewhere that addresses thin mashes. My guess is that you're pump is cavitating. What brand pump are you using?

Edit: I also have a bottom draining mash tun.

 

[ASantiago]

I've never had that issue. My system works best when the mash is thin, there's evidence that a thinner mash improves wort quality. Beau kaiser has a thread on here somewhere that addresses thin mashes. My guess is that you're pump is cavitating. What brand pump are you using?

Edit: I also have a bottom draining mash tun.

I have a Chugger that I purchased a couple of months ago. Up to this past Saturday I used it only to drain the mash tun and transfer wort to the fermenter, but on Saturday I tried using it to recirculate the mash using a new SS pot I got for direct heating step mashes. The pot also drains from the bottom.

I agree the pump is cavitating. But why? What's introducing air into an existing flow with 10 gallons of water?

Another thread mentions possible issues with Chuggers. https://www.homebrewtalk.com/f11/chugger-pump-just-stops-during-recirculation-439549/

 

[pickles]

I'm sure you're issues are related to direct firing the tun and the bottom drain configuration. You're likely boiling the wort in the tubing.

 

[kanzimonson]

Well to add to that, I have the newer version of the March pump (supposed to be even better about cavitation issues) and it's happening to me.

But the strange thing is I'm also using my pump for whirlpool chilling. In that case, I'm pulling boiling wort out of the kettle and sending it back. I have to throttle back the pump's outflow or else it will cavitate and then the impeller gets off kilter with the magnet. But it works.

 

[kanzimonson]

*Addendum: I begin recirculating 15min from the end of the boil to sanitize, so there's a good 15min where I'm pumping boiling wort through.