Not electronics so thought should be in off topic,
I'm building a glycol snap chiller for a off grid robotic milking setup.
I'd like to be able to over chill the glycol to make best use of the PV (most power is required when milking so I can use the excess when the robots are idle to chill the glycol to near it's freezing point so the compressor won't need to run while milking)
To this end I need a thermostatic mixing valve that works around 0*C I can separate the glycol tank into a warm tank from the return and really cold so the mixing valve pulls from both.

Does such a valve exist? I just end up with lots of domestic valves on a Google search.

There's lots of clever folk on here so I hope someone may have ideas!

Thanks Geoff

 

Does such a valve exist?

Don't understand why you think the valve needs to be special or different from the values you have found(?).

What exactly does the valve(s) need to do, and what will determine whether they are on or off?

 

Does such a valve exist? I just end up with lots of domestic valves on a Google search.

We used motorized mixing valves which used a 4 to 20 mA loop. The loop current came from a process controller measuring outlet temperature. A Google of "motor driven mixing valve" should get you started. They are also frequently called a three way valve. We wanted outlet water temperature at a pre setpoint. We just mixed hot and cold similar to how many home bath and shower valves work. About like using two valves to proportionally mix hot and cold.

Ron

 

Thanks,
I was hoping to find a mixing valve that works just like the domestic ones, just with a 0.5*C output, rather than the 50*C most of them are
I understand that the expansion of wax is much lower in that temperature range so may have to use closed loop electronics but was hoping for a mechanical solution to keep it simple.
Cheers

 

This seems like a "one-off" application, where your desire is to find a "Custom-Mixing-Valve".
If this is not a common application, I'm sure that there are manufacturers who will
design a Custom-Valve for some ridiculously exorbitant price.

My suggestion, ( based on almost no information ),
would be to install a very large, well insulated, Storage-Tank,
and simply keep, at least the bottom half of the Tank,
at or around the desired Temperature, instead of mixing Cold and Warm after the fact.

This would work exactly like a Heat-Pump-Powered Domestic-Hot-Water-Tank, but in reverse.

It sounds like the Tank that You may have now is simply inadequately sized to provide
the volume of Coolant required to complete a full Cycle of operation.

How precise does the Temperature control need to be ?

Can the Temperature control be achieved by controlling the Coolant-Pump speed ?

A small recirculating-Pump,
pumping Coolant in a short-circuit-loop around the device being cooled,
in combination with,
a speed-controlled Pump supplying Coolant to the loop,
( Coolant that is substantially below the required Temperature ),
could provide very tight, and even, Temperature control throughout the device being cooled,
rather than the device being too cold at the Coolant-Inlet, and too warm at the Coolant-Outlet.
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.

 

The tank I have should be large enough, what I'd like to do is have the option of cooling it much lower than my target output temperature.

I'm cooling milk, using plate and tube heat exchangers the milk needs to be as close to 0*C as I can get without freezing it as its going into a insulated storage tank with no cooling on it

The whole setup is going to run off grid (PV, batteries and a generator), the problem being that the power demand for the milking robot and cooling system will be more or less at the same time.

When the milking robots are idle power demand is low so if there is excess available I could run the cooling compressor then and chill the glycol to near it's freezing point, -15*C roughly depending on dilution.

I then hopefully wouldn't need to cooling compressor running when the next cow is being milked

Hence the need for a mixing valve, I have also been looking at having a warm pump and a cold pump varying speed with pwm to achieve the same end but if a valve exists and isn't to dear it's a much simpler system.

The glycol needs to enter the system around 0*C I think maybe a little less?

Thanks for the replies

 

At what exact Temperature does the Milk freeze ?
Knowing this value,
the Temperature of the Milk leaving the Heat-Exchanger should be continuously monitored.

The Coolant-Temperature must be substantially lower than this "Milk-Freezing-Value"
in order for Heat-Exchange to occur at an adequate rate.
Of course this will depend upon the efficiency of the Heat-Exchanger, and
the Rates of Flow of the Coolant, and Milk, and their Temperature-Delta.

But this all seems to be a moot-point if the cold Milk is then
allowed to rise to Ambient-Temperature after cooling.

Ongoing Chemical-Reactions in the Milk are never actually
stopped completely without Freezing the Milk,
and will continue at a rate determined by the Temperature of the Milk.

"Pasteurizing" destroys some of the desirable qualities of the Milk,
by stopping certain ongoing natural-processes in the Milk with Heat.
Freezing also stops these processes, but supposedly doesn't "damage" any desirable qualities.

The "Recirculating-Pump + Supply-Pump" setup, described previously,
will have superior performance and Temperature consistency when compared to
using a "Warm-Pump and Cold-Pump" mixing arrangement.

2- Low-Frequency-PWM-Solenoid-Valves can easily replace all Motor-Speed-Control ideas.
1-Valve would be the "below-freezing" supply to the Coolant-Loop around the Heat-Exchanger,
and the other Valve would control the return of Coolant from
the outlet of the Supply-Pump back to the Storage-Tank.
When 1-Valve is Off, the other Valve is On, and vice-versa.
1-Cycle-per-Second should be a more than adequate PWM-Frequency for the Valves.

This setup can probably hold the Milk-Outlet-Temperature exiting
the Heat-Exchanger to within 2-degrees of the desired Temperature-Set-Point, possibly tighter control,
and easily allow the use of extremely cold Coolant-Temperatures,
effectively increasing the Coolant-Tank "Energy-Storage-Capacity",
which will insure that the Refrigerating-System will not need
to "Short-Cycle" to maintain a higher Temperature-Set-Point.

The Coolant-Temperature should be maintained at the coldest Temperature practical,
then the Refrigeration-System can be powered-Off for the entire Milking-Cycle without
fear of loosing adequate Temperature-Control-Capacity.

Colder-Temperatures = Higher-Energy-Storage-Capacity in your existing Storage-Tank.
The Storage-Tank needs to be really well insulated to prevent Energy-loss.
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Thanks for the input, the freezing point of the milk does vary according to butterfat/protein levels but is around -0.5C

I have 2 robots to deal with the milk from, each has a small storage tank which is pumped out when each cow leaves.

I've fitted variable speed milk pumps so I can pump the milk as slow as possible through the tube coolers, then in my current set up they share a plate exchanger before the bulk tank.

They won't pump at the same time.

I know I'll need to monitor the output after each cooling stage and will have a separate pump for each cooling stage.

I like the idea of 2 pwm solenoids. I do wonder if a motorized mixing valve might be the best way if I can't find a valve.

But the question was is there a mechanical valve suitable, I can't be the first to consider doing it this way with a glycol chilling setup.

Thanks Geoff

Ps, the bulk tank is well insulated and the end target is to keep the milk in the tank below 5*C

 

Just found a 3 port motorised valve I had completely forgotten about
The motor fitted is either at one end of its travel or the other.

It wouldn't take much imagination to fit a servo motor to it and then it would do what I want (with a micro and a few other bits chucked at it)

Plus there cheap

Thanks