Seawater sample temperature control – heat exchanger design

[thereddevil]

I’m designing a system where a sea water sample must have its temperature controlled between near its freezing point (-2°C) and +30°C to simulate sea temperatures. To control it, the sample will be in contact with a block, which will be controlled by an external circulating water bath. The image should be useful.I’m new to the world of temperature control and heat exchange, so please correct me here.MATERIAL SELECTION:

- I believe thermal conductivity to be the most influential factor, but have read up on heat capacity and a combination of the two – thermal resistivity. Are these other two worth considering here?

- Sea water, therefore, corrosive environment

- SELECTION: It seems aluminiums and coppers are typical used in water cooling system, but the sea water element leads me to the route of bronze, copper-nickel and titanium (based on wiki page, which I now can’t locate). Any experience of material selection?DESIGN:

- I understand that it would be ideal to have as much of the sample in contact with the heat exchange block, but I must stick with a system using a glass tube as indicated.

- Air-cooled heat sinks have fins – do I want to go down a similar route of maximising surface area of block with water bath flow? I could add a number of drilled holes in that section.

- Should the volume of the water bath chamber be maximised?

- The block itself may be both smaller and larger in terms of volume of material. What pros/cons do they have? A large block would take longer to change temperature surely, meaning I want the block to be as small in volume.

- Worth going down the route of simulations?Thank you in advance!

 

[jrmichler]

Does your system need to heat/cool a sample of sea water to a specified temperature from a specified temperature in a specified time? Or does it merely need to maintain the sample at a specified temperature?

The sample is apparently not stirred, so heat transfer between the sample and the block will be relatively low. That means that the thermal conductivity of the block is not a primary factor in the total heat transfer. Use any alloy that is resistant to sea water corrosion.

At the lower temperatures, you will get condensation on the outside of the block. Condensation will pull enough heat from the block that your design will have difficulty maintaining sample temperature. Put insulation around the block or place it in a thermos bottle.

 

[anorlunda]

Since your range of temperature is both above and below ambient, you need a way to add heat or remove heat from the system. You did not describe that other than mentioning air cooling.

 

[thereddevil]

"Does your system need to heat/cool a sample of sea water to a specified temperature from a specified temperature in a specified time? Or does it merely need to maintain the sample at a specified temperature?"

Assume that the room is at ~20°C, it would need to be maintain the sample anywhere between -2°C and +30°C. Chances are the sample can be kept at the target temperature before entering the glass tube, but it would be preferable that the heat exchange system can alter the sample rapidly."The sample is apparently not stirred, so heat transfer between the sample and the block will be relatively low. That means that the thermal conductivity of the block is not a primary factor in the total heat transfer. Use any alloy that is resistant to sea water corrosion."

Good point on the stirring! So you're saying that a bigger factor would be volume of the sample since the heat transfer would be primarily within the sample? Less volume is better? The glass tube will be ~Ø25mm ID.
Titanium vs. bronze vs. copper-nickel - does it matter much?"At the lower temperatures, you will get condensation on the outside of the block. Condensation will pull enough heat from the block that your design will have difficulty maintaining sample temperature. Put insulation around the block or place it in a thermos bottle."

The final assembly will have the block inside a plastic block, so that will be covered. But you raise a good point with when I prototype this - I'll construct an insulative layer to mimic end product.

 

[thereddevil]

"Since your range of temperature is both above and below ambient, you need a way to add heat or remove heat from the system. You did not describe that other than mentioning air cooling."

The external water bath that is connected to the block will provide this?

 

[anorlunda]

"Since your range of temperature is both above and below ambient, you need a way to add heat or remove heat from the system. You did not describe that other than mentioning air cooling."

The external water bath that is connected to the block will provide this?

That doesn't make sense unless the external bath varies between -2 and 30C.

If I read your original question correctly, you want to vary the seawater sample temperature from -2 to 30C and then hold it there. A constant temperature bath won't do that unless the bath temperature changes.

By the way, you need to learn to use the quote facility to quote someone else when posting a reply To quote a reply, click the "QUOTE" button on the bottom of the reply. Or highlight a sentence and a "QUOTE" popup will appear for you to click on. Then in your reply window a "INSERT QUOTE" button will appear. Click that and it will appear as the text I quoted above. You can quote multiple replies at the same time.

 

[thereddevil]

I'll learn to quote, but can't find it on mobile!

Yes, the water bath can be set to a range of temperatures, with the flow of water passing through the section on the right being at set temperature. The challenge is to get the sea water sample on the left to a desired temperature, and hold it at that temperature for the duration of the experiment.

 

[anorlunda]

I'll learn to quote, but can't find it on mobile!

Ah, you're right. It doesn't work on phones.

Yes, the water bath can be set to a range of temperatures, with the flow of water passing through the section on the right being at set temperature. The challenge is to get the sea water sample on the left to a desired temperature, and hold it at that temperature for the duration of the experiment.

OK, that's a bit clearer. The material does not need to be metallic. A thin membrane would do, for example putting the sample in a balloon and immersing that in the constant temperature bath. It will come to thermal equilibrium with the bath (more quickly with stirring and thinner walls and more conductive materials), and remain so unless there is a net flow of heat in or out of the sample from an external source.

Is your sample immersed in the bath? You mention bath on the right and sample on the left. Does that mean that the sample is in contact with the sink on only one side so that heat can leak to ambient on the other side? A sketch would be very helpful.

 

[thereddevil]

OK, that's a bit clearer. The material does not need to be metallic. A thin membrane would do, for example putting the sample in a balloon and immersing that in the constant temperature bath. It will come to thermal equilibrium with the bath (more quickly with stirring and thinner walls and more conductive materials), and remain so unless there is a net flow of heat in or out of the sample from an external source.

Is your sample immersed in the bath? You mention bath on the right and sample on the left. Does that mean that the sample is in contact with the sink on only one side so that heat can leak to ambient on the other side? A sketch would be very helpful.

Sorry, the design was a simplification. The reason it is a tube is that at various angles in the block, there will be bores machines with light emitting into the glass tube, therefore, the block cannot be in a membrane or balloon. For this example, the seawater sample is only in the position shown. The water bath can only be accessed and fitted to the block on the side shown (on the right). I tried to attach an image - can you access this on the original post?

You mention a few things:

Stirring - unfortunately to test the sample, no stirring can be carried out.

Thinner walls - using the design in the diagram attached, would it be advantageous to make the volume of the block as small as possible, to reduce the time to change sample temperature?

More conductive materials - as mentioned, it must not corrode in seawater. Any ideas? I feel that a Cu-Ni or Ti would be best.

 

[thereddevil]

My apologies, my initial diagram was an early concept simplification - see attached for a better reason for having a glass tube. Essentially, all four bores are for optical purposes, with LEDs behind them. We had a previous product using the glass tube, but without the need for thermal control. Since this glass tube worked previously, we'd like to continue with this path. I've proposed an alternative, using windows - see image. There are optical issues with the windows concept (sample chamber is square in cross section), therefore, it is preferable to go with the glass tube.

 

[anorlunda]

Yes, the better diagram helps, but an important point is still not clear. Is the highlighted portion of the following what you are calling "the block"?

I’m designing a system where a sea water sample must have its temperature controlled between near its freezing point (-2°C) and +30°C to simulate sea temperatures. To control it, the sample will be in contact with a block, which will be controlled by an external circulating water bath.

Do you want to hold the seawater at a constant temperature, or are you ramping the temperature between -2 and 30?

- I understand that it would be ideal to have as much of the sample in contact with the heat exchange block, but I must stick with a system using a glass tube as indicated.

Yes, as much surface area as possible. But maximum area would be to eliminate the block and fill the whole chamber with bath water. For example, In the image below, yellow would be circulating bath water and blue is the seawater sample.

If the bath water circulates, then it is inherently stirred. Without stirring, there will be temperature gradients in the sea water. For example, the temperature near the windows will be different because of heat leakage. Is your measurement of temperature in the sample taken at only one place?

A large block would take longer to change temperature surely, meaning I want the block to be as small in volume.

The bigger the heat sink, the more closely you can hold temperature constant, but the more slowly you can change temperature. Only you can decide on the trade-off. You need to state your requirements in numbers, not words. It is also not clear how you change temperature. Are you controlling the temperature of the bath water? Does your question on temperature control relate to how to change the bath water temperature?

 

[thereddevil]

Yes, the better diagram helps, but an important point is still not clear. Is the highlighted portion of the following what you are calling "the block"?
View attachment 226438

Do you want to hold the seawater at a constant temperature, or are you ramping the temperature between -2 and 30?Yes, as much surface area as possible. But maximum area would be to eliminate the block and fill the whole chamber with bath water. For example, In the image below, yellow would be circulating bath water and blue is the seawater sample.
View attachment 226439

If the bath water circulates, then it is inherently stirred. Without stirring, there will be temperature gradients in the sea water. For example, the temperature near the windows will be different because of heat leakage. Is your measurement of temperature in the sample taken at only one place?
The bigger the heat sink, the more closely you can hold temperature constant, but the more slowly you can change temperature. Only you can decide on the trade-off. You need to state your requirements in numbers, not words. It is also not clear how you change temperature. Are you controlling the temperature of the bath water? Does your question on temperature control relate to how to change the bath water temperature?

When I say block, I mean the entire metal section. The yellow section you've highlighted - is this distance something I should reduce in order to get faster temperature change?

I want to hold the temperature constant and then run experiments on the sample at that temperature. When the temp is changed, I will run the water bath at required temp and the sample will be held in this water bath before hand to help get to the required temp. Make sense?

I will try and maximise area and try and get the water bath to flow as much around as possible - good recommendation.

I agree with the temperature gradient, it's something we'll have to assess and perhaps incorporate some stirring of the sample to prevent such a gradient. Unsure about the temperature measurement method at present.

I'm controlling the water bath temperature externally, it's the seawater sample I want to control using the water bath flow into the system. I think in this case since the water bath fluid will be flowing into the system continually, I'll aim for a faster rate of change, rather than a larger heat sink to keep constant.

Thank you for your help, been invaluable!

 

[thereddevil]

Any further input? Thanks in advance!