Mixing valve 2

[itsmoked]

I'm tasked with building a programmable water dispenser for a bakery. I need to provide mixed hot/cold water at a set temperature between about 60F and 110F. My sources are city delivered 55~60F cold water and 140F hot. I need to deliver it in pounds. This happens in about 6 cycles of 4 minutes every 5 or 6 hours.

There don't seem to be a lot of controllable mixing valves out there.

I was looking at lots of motorized ball valves to use for mixing but was finding that they were limited in operation on threat of overheating the motors. Others state lifetimes of "200k cycles".

What's a "cycle" on a three-way valve? Stop to stop? I'd expect these valves to roam around the middle almost constantly while in operation, almost never going to either stop. Otherwise, I can't see these valves lasting a year.

Are there other types or ways of mixing water dynamically?

Should I use two valves and vary them both or just one three-way?

Ideas!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

Are you trying to mix water to maintain a pre-determined temperature setpoint which then remains at that value, or mix water to a temperature which can be modulated by some sort of control system? If the former then there are self-contained thermostatic mixing valves available - for example, here's a European one:

https://www.advancedwater.co.uk/STOMEMO/A227-373-0006/Honeywell TM300 Installation Guide.pdf.

If the latter then you're probably looking for a globe valve rather than a ball valve as the modulating element.

 

[itsmoked]

Hi Scotty.

I'll be mixing for a specific temperature UNTIL a specific amount (weight) of water has been delivered thru a hose into a 800lb mixing bowl. So two things are being controlled. I have a dump path available to spew out of, until the temperature requested by the user on a PLC HMI is met, then the flow will be diverted into the delivery hose.

I'm hoping to control the temp so swiftly I can nix the dump and the hassle it brings and possibly keep track of the 'early' out of spec water allowing me to compensate for the low temp early water by delivering slightly warmer water briefly following the 'early water'.

This will be touch-button so no OTS thermostatic valves can work. Nor will the early correction allow a mechanical thermostatic to do the trick.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Ahh, I knew there was something bigger hiding behind this question.

Many of those 3-port valves aren't designed for modulating duty in my experience - they're common on small domestic heating systems, and have a synchronous motor driving the valve in one direction and a spring return when the motor is de-energised. The mid-position is held by injecting DC into the motor winding via a microswitch and rectifier diode built into the housing. It's quite an elegant solution with minimal components; the microswitch might be the limiting factor in the life of the actuator.

I'm sure you've considered an in-line heater with a simple phase angle regulator to directly heat the water to the desired setpoint rather than mixing hot and cold? You could still provide the compensation for the early flow below setpoint, and with a sufficiently powerful heater cartridge the response could be rapid.

 

[itsmoked]

Funny Scotty! Two Three EEs, the only people carrying on a valve discussion in the M.E. section.

Have a LPS on me!


Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Can I jump in? You approximately know the cold and hot temperatures so why not 2 parallel paths for hot and cold? Measure the temperature and flow of each so you can fine adjust to get the proper final temperature mix. In other words, turn on both for a while (say to reach 1/2 the volume wanted) then calculate how much more of each is required to both reach final temperature and volume. That would let you use simple on-off valves.

 

[itsmoked]

Another EE!

I love it! Great idea Hutz.

I also asked them for how much of this temp controlled water they use a day; 865 gallons.
I have to figure out how much this would cost to electrically heat.

[0.0024444 Kwhr to raise 1 gallon of water 1°F]
Call the tap temp 58F call the controlled temp 100F

ΔT = 100-58F = 42F
865gal x 42F = 36,330 GallonDegrees a day.

0.0024444Kwhr x 36,330 = 89kwh/day
Around here call it $0.18/kWhr

89kWhr/day x $0.18/kWhr = $16/day ($5,850/year)

$16/day to use an electric inline temp booster avoiding all valve control forever. Hmmm.

Not sure what they pay a day for the gas heated water.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[HitokiriAoshi]

My recommendation is dumb down the control system and add in a well insulated mix tank (stainless steel 316 or better water tank, better yet just go full duplex ss and never worry about corrosion issue ever, consider cleaning needs) monitor this tank's temperature to the desire set temperature. buy two food grade ball valve with delrin seat, on off control system. it should last with minimal maintenance, and just keep a spare set of seats for emergency repair, hang said spare seats in a sealed bag by the valve. keep ball valve in full on/off operation only.


oh, and do your own actuation. it's easier than you think.

 

[itsmoked]

Hi xsnipersgox. Welcome to Eng-Tips!

This is for batches that change temp constantly. One batch might be 5 gallons at 65F and the next 20 gallons at 102F. I'm not sure if I'm understanding your suggestion fully though. Are you suggesting water batches that get made then delivered?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[btrueblood]

Keith,

I hate 3-way valves. I doubt you will find one with a quick enough actuation time to suit your controls. And they are a maintenance headache due to seal wear.

Pulse-modulated solenoids might make more sense, especially if you can track the mixer bowl contents temperature to close the control loop. If you can't track the bowl temperature, and literally are delivering the water through a hose, add an inline static mixer and put a temp. sensor downstream of same.

What happens when the mix temp. you give them cools by 5 degrees because the operator was taking a break, or the bowls were extra cold from being in a cold shop overnight?

 

[itsmoked]

Hi tru.
I'm realizing that, speed not withstanding, ball valves have shockingly short lives. I understand why butterfly valves get used for these type applications now, nothing to really wear out and fast as you can move them.

There is no way to measure the bowl temp, the water's just delivered down a 1/2" hose. The dozen ingredients, about 300lbs are dumped into the bins then the water is delivered. Within a minute or 3 the bins are moved into the mixers and mixed for about 15 minutes. I suspect a few minutes and even a degree or two doesn't materially effect the bread result. It will change the rising time a bit but the rising process is many hours. Once the big hopper has risen it's lifted by a crane and dumped onto a 4 x 10ft table. It's sliced into foot square blocks and dropped in bins to rise longer yet then being processed a bin at a time.

If the hot water is close (recirculated and it is) and control is fast I can see a static mixer and two feet of distance could get the water under control just into the big mixer bowls. Currently they bypass a hundred gallons a day into bins outdoors so they can use it for cleaning. A big hassle really, but they're being good stewards and not just shoving it down the drain.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[btrueblood]

Is water going into the bowl with dry ingredients (including yeast) already added? The yeast may have a do-not-exceed temp. or it can be killed (I think that's north of 120F but can't say for sure). No big deal, but my approach in lieu of an upper limit would be to turn the hot water on full and then pulse the cold to achieve desired temp.

Sounds like you have the problem in hand. Just be careful-ish about feed pressures and the valve flow coefficients so you can accurately predict the flow rates and thus mixed temperatures.

I'm no big fan of butterfly valves either, but if you realize that they really only on-off modulate (there is a huge hysteresis band in the Cv curve of a butterfly flap) you'll be ok. Seals will still wear over time, but they are generally cheaper than ball valves...but I just noticed the size you wanted, can you even get 1/2" butterflies? For 1/2" valves I'd think a standard solenoid-operated poppet valve (or solenoid-piloted diaphragm) type would be your best bet, these can go millions of cycles before seals need replacement.

 

[itsmoked]

Is water going into the bowl with dry ingredients (including yeast) already added?

Definitely. A huge bunch of multicolored piles with the water added last washing-machine-pump-out style.

The yeast may have a do-not-exceed temp. or it can be killed (I think that's north of 120F but can't say for sure).

True. The general rule is to not exceed 110F.

The hot, being the place is a food production facility, the hot is blazing hot. But watching the temp out of the static mixer I should be able to see the rising temp of the interface and to keep hose water always below 110F.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

I sure hate seeing water lines and valves inside of an electrical enclosure!

 

[itsmoked]

It's a Hobart! I'm less enthralled with them everyday.

Keith Cress
kcress -

http://www.flaminsystems.com