Cooling mixing tanks with a chiller, selecting chiller size

[HollyBoni]

At our company we make various cosmetics products like creams, gels, ointments. We use jacketed mixing machines to mix these products. Usually the products have a water and oil phase. We put the water phase in the mixing machine, heat up the water in the jacket with the built in heating elements, which heats the product inside the tank. We heat the oil phase seperately. When everything is up to temp, we mix the two phases, and we cool the product to around 25-30C.
So far we used tap water to cool these machines, but this is a huge waste, and our tap water is very hard, which ruins everything.
I'm looking for a chiller to cool the jacket of these mixing machines. I contacted a few different companies, but my issue is that a lot of them usually work in HVAC and don't seem to understand what we're doing. I've had companies recommending chillers anywhere from 15 to 150kw.

To give you some numbers, we have a 150l mixing machine for example. We usually mix 120-130l of product in it. The volume of the jacket is 40-50l. I built a cooling/heating system for this machine that could be used with a chiller in the future. It has a circulation pump on the jacket side, plate heat exchanger, PID controller which controls the heating elements, and controls a motorized ball valve which lets tap water flow through the other side of the HX.
We usually heat the jacket water and product inside the tank to 75-80c, then we cool the product to around 25-30c. Currently if the jacket and product temp is at 75-80c and I set 20c (temp of the jacket water) on the PID to turn on cooling, the jacket water reaches 20c in around 13-15 minutes. Tap water is usually 13C and flow is 10-15lpm.
After the jacket water cooled down to 20c, the PID lets it get up to 22c, then turns on cooling again. This happens every few minutes (like 5) as the product cools down. I measured last week, and cooling 120kg of product inside the mixing machine from 75C to 28C took around 40 minutes from the moment I turned on cooling on the PID controller.

I contacted Trane, their representative came to our factory and they gave me an Excel calculator made for mixing vessels. You put in some numbers like mass off product, mass of vessel, start temp, desired product temp etc. then it gives you a "duty kW" in kW/hr at the end.

My problem is with cool down period. If I set 15 minutes (this is how long it takes for the jacket water to cool down from 75c to 20c, which is fine, i'd like to keep that) I get 14kw. But the 120kg of cream can't physically cool down in 15 minutes, due to the slower heat transfer between the jacket and the cream. If I set 40 minutes for cool down time, I get 5kw.

So i'm a bit lost on how to size the chiller for this application. It needs to be able to handle multiple machines. We have this 150l machine, there is a 75l machine on the way, and we're also planning another machine, but the size of that is not known yet.

I'm wondering if any of you has experience with this who could help me in sizing a chiller?

 

[Latexman]

Do you have Kern’s Process Heat Transfer? His book shows the derivation of several equations for the differential equations of unsteady-state heat transfer. IIRC, your problem is documented there. They are also in Perry’s Handbook; the equations are presented here, not derived. Maybe if you get intimately aware of the base theory, it’ll help you.

 

[LittleInch]

To replicate what you have, try measuring your waterflow accurately over time and the inlet AND outlet temps. This will give you the energy absorbed by the tap water which you are hoping to replicate by the chiller.

Then plot this over time.

Your error is I think looking at this as a steady state process when it isnt. As the contents cool down, the heat flow from contents to the water jacket slows down.

You probably need something like a 20kW chiller but one which can modulate down which not many of that size do.

Or prob better to buy three or four 5kW machines and bring them on and off as required controlled by probably exit temp of the jacket water.

What is your ambient air temp? Could you do this with a more simple fin fan cooler with modulating guide vanes or fan speed?

Your 7 C variance between tap water and jacket water is probably about right as the same would apply to an air cooled HX.

 

[HollyBoni]

To replicate what you have, try measuring your waterflow accurately over time and the inlet AND outlet temps. This will give you the energy absorbed by the tap water which you are hoping to replicate by the chiller.

Then plot this over time.

Your error is I think looking at this as a steady state process when it isnt. As the contents cool down, the heat flow from contents to the water jacket slows down.

You probably need something like a 20kW chiller but one which can modulate down which not many of that size do.

Or prob better to buy three or four 5kW machines and bring them on and off as required controlled by probably exit temp of the jacket water.

What is your ambient air temp? Could you do this with a more simple fin fan cooler with modulating guide vanes or fan speed?

Your 7 C variance between tap water and jacket water is probably about right as the same would apply to an air cooled HX.

I have 3 temp sensors in this loop:
"Cold" side HX inlet
"Cold" side HX outlet
"Hot" side HX inlet, after the water went through the jacket

What can I do with this info exactly?

Unfortunately no flow meters, but I did measure the tap water flow coming out of the HX at around 13-15lpm with the stopwatch and bucket method.

Yes, I know this is not a steady state process, which makes things a bit harder. I contact any company dealing with chillers saying I need to cool down 75-80C water, and they freak out immediately.

We're in central Europe. Our summers are hot, winters can be cold. It's going to be 35-36C next week for example. In the winter it can get into the minuses during the day, but it fluctuates. So in the summer i'll need water that's lower than ambient temp. I thought about using a dry cooler in the colder months, maybe a shared loop with the chiller.

 

[Latexman]

I have 3 temp sensors in this loop:
"Cold" side HX inlet
"Cold" side HX outlet
"Hot" side HX inlet, after the water went through the jacket

What can I do with this info exactly?

If you have the analytical solution to the differential equation for your mixer (Kern, Perry’s), you can probably rearrange the equation (algebra) and plot the data (temperatures) versus time and obtain the actual U for your equipment.

I did that long ago and included a least squares fit in a spreadsheet to calculate U.

 

[georgeverghese]

Though there are expressions in Perry and Kern for this unsteady state heat transfer operation during cooldown, the difficulty here in using these expressions is the estimation for the overall U, which consists of subcomponents as follows from what I see, given your description:
a) the heat transfer coeff between bulk cream and the jacket inside surface - hi
b) the heat transfer coeff between jacket and recirculating jacket water - ho

Though I'm not an expert on calculating (b), process engineers working in plants with jacketed vessels can estimate this. The complication is with (a) - the cream- water mix may be non Newtonian, and viscosity may vary considerably as cream cools down from 75degC to 20degC. hi will also depend on agitator speed if this agitator is kept ON during cooldown. You may have to work this hi out by extrapolation from actual operation if this is the case.

If your tap water is scaling / fouling up the plate HX and is not suitable due to high dissolved Ca/ Mg, then may be see if you can set up a closed loop pumped demin water cooling circuit, with demin water cooled at an aerial fin fan cooler

 

[pierreick]

Hi,
Agree with proposals above,
But if your main issue is the quality of water not the supply, I will consider treatment like Resins. Nowadays you can buy no intrusive meter to measure flow rate (Based on Doppler effect).
For the chiller option, I will select multi units to adjust the cooling capacity .
Good luck
Pierre

 

[georgeverghese]

Also hi depends on cream - water ratio

 

[LittleInch]

I have 3 temp sensors in this loop:
"Cold" side HX inlet
"Cold" side HX outlet
"Hot" side HX inlet, after the water went through the jacket

What can I do with this info exactly?

Unfortunately no flow meters, but I did measure the tap water flow coming out of the HX at around 13-15lpm with the stopwatch and bucket method.

Yes, I know this is not a steady state process, which makes things a bit harder. I contact any company dealing with chillers saying I need to cool down 75-80C water, and they freak out immediately.

We're in central Europe. Our summers are hot, winters can be cold. It's going to be 35-36C next week for example. In the winter it can get into the minuses during the day, but it fluctuates. So in the summer i'll need water that's lower than ambient temp. I thought about using a dry cooler in the colder months, maybe a shared loop with the chiller.

What you can do with that is estimate the chilling power of the water.

Let's assume 15lpm, which is 0.25kg/sec. Heat capacity of water is 4,200 J/kg/ C

So the energy being exchanged is 0.25 x 4 200 x DT (Cold out T minus cold in) in W.So plot that over time e and it starts to tell you and us what sort of chilling "power" you need to replicate what you have.

When you contact vendors just tell them you need 10 C water at 15 l/ min with a return temp of ??(your initial cold outlet temp) reducing to ?? (your lowest temp at the end of your cooling period. Reduce your temps by 3C to allow for the chilled water of 10C not 13C.

They don't need to know why you need it.

 

[HollyBoni]

Thanks for the replies everyone. These calculations etc. are a bit over my head, but i'll try to educate myself a bit.

georgeverghese made a good point as well. At this point our company probably makes over 50+ different creams. Some might be 70% water, some might be 70% oil. If I change the product specific heat in the calculator that Trane gave me, I get wildly different results for cooling energy needed.

We may just get a ~15kw unit and see what happens. As we upscale our mixing machine sizes in the future, we can add more chillers later. Having multiple small machines instead of one huge machine is probably a good idea for redundancy as well, and our current infrastructure can't really handle a massive chiller.
I'll also explore options for dry coolers. When it's under ~10c outside, a dry cooler alone could probably handle a few mixing machines.

Our tap water here is brutally hard. A ball valve can develop pitting and leaks in half a year. I just pulled out a bucket of limescale from a 200l water heater, and it was cleaned out half a year ago.
I thought about adding water softeners etc. and staying with tap water cooling. But it's such a huge waste. For this 150l mixing machine, we use about 300-400l of tap water to cool a batch of product down, and that water goes down the drain. Maybe we could recirculate, reuse this somehow, but a closed loop cooling system with proper coolant which has corrosion inhibitors etc. seems like a much better solution.

 

[pierreick]

Can you share with us a typical and complete analysis of your water? this will help..
Pierre