lab scale cooling of a high shear mixer

Hi folks,

I'm a chemist making a foray into uncharted waters here, trying to achieve greater cooling of system composed of the following:


a half gallon reactor which drains into a high shear mixer (the source of heat generation) after which the low viscosty alcohol based material which I am mixing is recirculated into the half gallon reactor. Currently, I have cooling water from the tap being directed to the mechanical seals of the mixer, as well as the jacket of the reactor. Additionally I have a small chiller set to -5 deg centigrade that is connected to a small (about 4 inches) condenser type heat exchanger (anybody familiar with laboratory chemistry will know the type, just a straight galss tube with a cooling jacket) on the return line to the reactor. I have been able to cool to 23 deg centigrade and my goal is to get to 15 deg centigrade. Unfortunately I am not able to greatly increase the surface area of the return line condenser without reducing flow and therefore mixing speed. I need to look at other ways to cool the system. Any suggestions?


Keith

I'm thinking a picture would be very informative here.

If the chiller is chilling water, it should not be set below freezing temperature.

Does the chilled water flow through the heat exchanger in the same direction as the chemical being chilled (parallel flow), or do they flow in opposite directions (counterflow)? Counterflow is much more efficient.

What is the temperature of the chilled water entering and chilled water leaving the heat exchanger?

What is the temperature of the chemical entering and chemical leaving the heat exchanger?

Thanks gary for rsponding. Antifreeze is used with the chiller. Countercurrent cooling is used. In actuality there are three heat exchangers being used. The one that has the chiller hooked up to it has a small surface are, only about 45 square centimeters. I have ordered one with about 3 times the surface area. The other two are hooked up to tap water. Presumably tap water has infinite cooling capacity and surface area is not a problem here either.

I am unsure whether surface area is the issue or simply the cooling capacity of the chiller for the antifreeze cooled heat exchanger. Unfortunately, with my current setup, I am unable to measure temperatures at the inlets and outlets of the exchanger. Sorry, I realize that doesn't clarify the picture much. I think when I get the larger surface area condenser, I ought to have a better idea. I don't know what effect, if any, tripling the surface area will have.

If it doesn't work, I presume I will need to either

1. further increase the surface area with a more state of the art exchanger
2. buy a chiller with a larger capacity (expensive!)

or

3. Have somebody of infinitely greater wisdom bestow a great revelation upon me!


Keith

A thought. What sort of chiller is it? Can we reduce condensing pressure by applying a greater rate of heat transfer through it? Can we do something to increase the subcooling of the liquid leaving the condenser at this lower condensing pressure? Also, a different refrigerant might help.

As Gary says, a picture of this device, perhaps from the manufactuer's site would help us unchemists understand your system.

A fridge guy in strange waters talking to a chemist in strange waters. You have to love it!:)

I wonder if that is how HCFC's became the standard for small compression devices such as the one we are discussing, 1/2 of each the other does not understand.

Dan

No picture I'm afraid. I think the chiller settings are optomized already. Thanks for the input, I will post with the results of the new heat exchanger in a week or so.

keith

Hi Keith

You don't say wether the system has always been like this or not! Is there a fundamental problem with the refrigeration system (scale, crap, refrigerant charge etc... the list goes on and on) I suggest first of all that the system is checked. Only then when it has been given a clean bill of health can further investigations take place.

Maybe the chiller just isn't capable of provided the outlet temperature that you require. You don't state a flow rate of the fluid to be cooled...absolutely vital, cannot determine current system capacity without it...unless we get the PH charts out!

The formula for determining system capacity is

Q (kW) = M C Delta T

Where M = Fluid mass flow (kg/s)
C = specific heat capacity of the fluid (kJ/kg degrees k)
Delta T = Fluid temp drop across chiller (Degrees k)

I f any of the equation values change, then the other values will follow suit.

The point I'm trying to get across is that a) the flow rate may be reduced b) Is the fluid concentration correct (concentration effects specific heat capacity and viscosity of the fluid)

Purely increasing the condenser surface area of the condenser will only have a marginal effect on the performance of the system.

If you can get me any more info I'll try and take it further.

Frosty

Think about what happens when one medium cools another medium. The warmer medium becomes cooler, while the cooler medium becomes warmer. The two temperatures "approach" each other. The difference between the two is the approach differential.

The 4 temperatures I asked for would give us a pretty good idea of heat transfer. If the approach differential is small (good heat transfer), a small improvement will be achieved by increasing heat transfer surface area. If the approach differential is wide (poor heat transfer), the improvement will be substantial.

Then there are the 3 heat exchangers. Two of these are cooled by tap water. These should also be counterflow.

If the end product is warmer than the tap water then we might as well use tap water for all three heat exchangers.

The chemical to be cooled should be close to tap water temperature BEFORE it enters the chiller heat exchanger.

Here's the bottom line: Unless you have very warm tap water, you should be able to cool the chemical below the current 23C with tap water alone, and THEN send it through the chiller exchanger to achieve the target temperature.

I would put the new heat exchanger upstream from the chiller heat exchanger and run tap water through it (counterflow).

The more cooling the tap water does, the lighter the load on the chiller.

If the desired result still can't be achieved, enlarge the chiller exchanger.

If that doesn't do it, the chiller is (a)not working properly or (b)not big enough.

If flow restriction in the return line is a problem, you could increase the diameter of the return line (and/or heat exchangers) or have parallel return lines and/or parallel heat exchangers.

If these strategies exceed expectations, you may be able to set the chiller above freezing and do away with the anti-freeze.

Here's another thought:

Let's imagine that we have two heat exchangers in series in the return line. The first uses tap water and the second uses chilled water. Let's call these the pre-cooler and the post-cooler.

We could install yet another heat exchanger in the tap water line, such that the chiller is cooling the tap water before it reaches the pre-cooler. This would increase cooling effect in the pre-cooler without restricting the chemical flow. But in this case, the chiller MUST be set above freezing so as not to freeze the tap water.

Hmmmm...it would help if I understood the goals to be achieved here.

Would it be desirable to reduce the temperature throughout the process?

If this is the case, we could use tap water for all three heat exchangers, and install a large heat exchanger in the tap water line (cooled by the chiller) to reduce the tap water temperature to just above freezing. Then add parallel return line heat exchangers as needed to achieve the desired temperature without restricting the return line flow.

Alternatively, and more directly, we could simply use chilled water for all three heat exchangers, add parallel return line exchangers as needed, and dispense with the tap water altogether.

If the goal is to increase the cooling effect only in the return line, the strategies outlined above would be preferable.

To achieve the additional 8K cooling required one of two things must be done.
1) Increase the contact surface area to improve heat transfer, i.e. increase the mass flow, or
2) Decrease the cooling medium temperature in accelerate heat transfer
Nothing else will do it
frank

P.S.
Nice to see you posting Frosty!

That's true, Frank. But these two factors must be considered for each of the heat exchangers. If the incoming tap water is above 15C, the chiller doesn't stand a chance. In fact, to achieve the 15C target, the tap water should be 10C or less.