Cooling System for Machine Tool Neat Cutting Oil

[TomSign]

Hello everybody.

I'm new engineer and while lacking experience I'd like to have your opinion about a cooling system design.
Hopefully my English won't be a problem.
I have to design a centrifuge for the purpose of a machine tool. One of the requirements is to keep the neat cutting oil NCO (and coolant at the same time) at 25 +/-2 °C. I have available water at 14-15 °C, 3bar.
Volumetric flow rate of NCO can fluctuate: 30 - 90 l/min depending on the usage of the machine.
The NCO after the centrifuge enters a tank, (centrifuge exit is about 40-50cm above the tank upper surface), from where it's being pumped back to the machine (additional filtering before the machine and after the pump is possible). Abrasion particles aren't fully removed by the centrifuge (mainly ≤ 1µm in diameter).
The total heat addition to the NCO during a whole cycle is estimated to be a maximum of 2kW.
Room temperature 18 to 28 °C. Minimizing the noise the device produces is of high concern.
The heat exchanger has to be cheap and easy to clean. A customized device can be manufactured by my company.
The space for the cooler is limited to (400x400x700mm). Not to small for the low amount of heat to be extracted.

My proposal is to use a hybrid form of counter-flow tubular exchanger. Instead of one inner tube there will be used several smaller pipes (like shell-tube exchangers)in order to meet the limited space requirements. Water will pass through the pipes while the NCO will flow outside them under the minimum possible pressure loss.
Material of pipes is Aluminium, light, cheap and resistant to corrosion.
The outer tube will consist probably of two semi-cylindrical part so that it will me able to dismantle and clean the pipes.
The cooler will be placed half emerged or right above the fluid in the tank.

That's the rough idea. I need you opinion, problems you may see and your own proposals.
No need to calculate heat exchange area. Go for a rough estimation of 0.3 to 0.4 m²
Any enthusiasts?!

Kind regards

 

[EdStainless]

Don't split the outer tube, you will never keep ti from leaking.
Tube-in-tube heat exchangers are common in food processing, the sections are often fastened together with special flanges and clamps so that they can be disassembled for special cleaning. But they are inefficient at heat transfer.
A machine cooler would usually just be a small shell and tube HX with flanged ends on the shell. That way you could remove them and then just use a brush to clean out the tubes.
Filter, filter, filter,. I know that it will cause pressure drop but keeping your NCO as clean as possible is very important.

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P.E. Metallurgy, Plymouth Tube

 

[TomSign]

Thank you EdStainless very much for your reply!
E-mail from this thread landed in my spams :/

A shell and tube HX seems to be of the best solutions in order to avoid high pressure drops and noise. 70db for the whole apparatus is quite difficult to achieve, though.
Quick market search showed that it's not a standard manufactured type, but has to be specially ordered, at least in Europe. Objection? (please!)

Important question. How are pressure drop and heat exchange efficiency related to the operation costs? Would it be deterrent to go for a lower efficiency in favor of lower initial capital, lower noise level and cleaning simplicity?

Enjoy the day

 

[willard3]

It is your job to answer these important questions questions, you are the Engineer.

 

[TomSign]

Sure it is, but no one has ever born engineer. You have to get the knowledge somehow either the source is called professor, book, forum etc.
So, willard3, would you help me to obtain part of this knowledge that will make me a better engineer?
Thanks!

 

[EdStainless]

Why aren't you looking at packaged filtration/chiller units.
There are a number of them available commercially.
Take a look as see how they do it.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Your NCO flow fluctuates, but the rate of change of flow and/ or used NCO temp can be a problem if you are looking for +/-2degC max permissible fluctuation in NCO feed to the cutting tools. Do you know what the rate of change will be, at least roughly? A regular thermowell with TT to feed the control loop for CW supply to the HX will be too sluggish if the thermowell doesnt pick up the rate changes quickly enough. In most plant process applications, standard TIC loops are the slowest and the most problematic.

Why do you need such tight control for this application?

 

[TomSign]

There are 3 separate machines. Each machine requires a NCO flow rate of 30 l/min.
So depending on the workload of the client's company the flow rate can be 30, 60 or 90 l/min. So we have a discrete and not continues change of flow.
According to the client the processed materials are strongly hygroscopic and easily affected by temperature changes. He set so this temp. requirements.

So far, the company has used just a heat exchange plate or coils inside the NCO container, with CW coming from a cooler and the temperature was controlled at the inlet of the cooler.

 

[georgeverghese]

You'll need to find a way to smoothen out these NCO flows during flow ramp up and ramp down, else you can expect temp fluctations exceeding well beyond 2degC. Maybe install a pressurised buffer tank with cooled NCO, and let the flow drain down from this tank as required. And fill this tank on level control (between high and low settings) with the HX. Operate the HX on temp control with sensor in this pressurised tank. This will also mean that the flow out of the cutting tools would have to be collected in a collection surge tank, from which you would pump out to the HX on level signal from the chilled NCO tank.

Else, is it possible for the flow to be ramped up or down slowly ?

The HX you describe sounds like a Brown Fintube (fully countercurrent??) multitube HX assembly, which would be okay if you have to clean the CW side occasionally.

 

[MikeHalloran]

If your customer's machines are currently independent of each other, you are introducing a single point of failure by combining the NCO flows into a single separator.

It might be simpler and more robust overall to equip each customer machine with its own separator, NCO tank, panelcoil cooler, and temperature control loop.

OR, I am misunderstanding something, which has happened.


Mike Halloran
Pembroke Pines, FL, USA

 

[lilliput1]

Can you remote mount the heat exchanger so as not to be restricted to the size limitation indicated so you can use commercially available shell in tube or plate type heat exchangers? Manufacturer of each can make selection based on the flows piped counterflow.