Thursday, October 4, 2007

Portable water-cooled chillers

Skippy: Hey Buzz - had this question come up concerning 'chilling' capacity - "Can anyone tell me what difference a more powerful process pump makes in a portable water-cooled chiller (25 or 30 ton) for injection molding processing? I have two competing quotes for a machine - one new with a 5 hp process pump and one used (much cheaper) with a 3 hp process pump. Otherwise, they appear to be approximately similar in specs. Can anyone help?"

Buzz: Hmm, depending on the sizing of the pump head, and the tubing connections, it may be that one pump is moving more gpm (Flow) or one was sized to make more efficient use of power. The short course is that the differences are probably in net cooling capacity and the more 'correct' unit more closely matches either your current cooling needs or perhaps your future needs (hopefully someone ran the analysis.)

Skippy: So that’s it?

Buzz: Unfortunately, no. Since these units are likely to work on one or more machines, and or one or more molds, the real answer might also be that what is really needed is two smaller units with them working in tandem on different areas of molds. Or they may need to be 'lag loaded' together to match the demand needs in the plant environment with the various size molds and cooling requirements one might encounter depending on production rates and the temperature of the plant process water (depends on whether is recirculated or not).

Skippy: What’s the difference in flow got to do with cooling – if the water available is the same temperature?

Buzz: To show a rough idea on the change in cooling capacity based on 'flow', try this simple experiment at your local gas station (assuming you are allowed to pump your own gasoline):

On a typical warm day, dispense the gasoline at a steady, slow to moderate rate into your tank, paying attention to the temperature of the gas nozzle assembly in the palm of your hand. Assuming the gas is stored in the ground it is likely that it is cooler than the ambient air (at around 56 degrees or so in the tank in the ground) the nozzle in your hand should feel 'cooler' in short order. This is because the cooler gasoline coming up from the ground as it flows by that point is pulling heat from the metal nozzle. Stop the process. After waiting a few seconds for the nozzle temperature to normalize again, dispense the gasoline at a much higher rate - and the nozzle should get colder quicker than in the first instance. The temperature of the gas from underground didn't change (still 56 degrees), but the amount of gasoline flowing by the point is pulling heat from the nozzle at the higher rate.

Skippy: ok, here are some typical sites for some clarification on 'flow' versus cooling capacity and the importance of 'right sizing'; along with some pros and cons in general to keep in mind about portable chillers:

Buzz: Now you're 'cookin with gas' . . . er uh, no; different topic. Remember, in the system, the questioner is looking for someone to clarify the inferred 'cooling capacity' as a function of this pump size business. Hopefully, the system as a whole has been analyzed rather than get caught up in just one generic item aka 'process pumps'.

Another topic to keep in mind is the fact that 'turbulent flow' is desirable (think and inquire about cooling channel design and layout.) And since we're starting to dig a little deeper, if you are thinking about cooling on injection molds versus part weight distribution in general, another interesting topic to look into is 'melt flipping' which will create a more balanced mold to be 'cooled' in the first place -

Of course these topics would be good for a later session -

Have a good day -

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