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PeterDrawa
13-02-2013, 11:06 AM
Hello,

I would greatly appreciate any comments on the following issue:

I am trying to reduce forced convection within a climatic chamber which is used in testing electronic devices. My additional problem is a large temperature gradient within the chamber volume.

The chamber operates within 0 deg. to +50 deg. C range and is built around three air blowers that pump air into its volume via two 100mm by 750 mm inlets (please see fig 1.). Two temperature sensors are positioned right next to the chamber outlets (X = 500mm; Y = 740mm; Z1 = 0mm and Z2 = 750mm) to monitor the inside volume temperature.

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Figure 1 - Climate chamber

Air-speed for the two inlets is about 1.5-3 m/s, but there is a clear non-uniformity of the flow. (Please see fig. 2). To my understanding this is the main cause of temperature gradient within the chamber volume (see fig 3.), which I would hope to reduce.

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Figure 2

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Figure 3

Am I correct in thinking that I could reduce the temperature gradient within the chamber's volume by applying extra baffles to the inlets? My idea is to force the air to reach the bottom surface of the climatic chamber, before it has a chance to escape via the outlets.

Thank you very much for any comments or suggestions.

Peter Drawa

Brian_UK
13-02-2013, 11:23 PM
How well is the air straightened before it enters the chamber?

What sort of air terminal are you using?

Is it feasible to utilise additional fans within the chamber to 'stir'/mix the air within the chamber?

PeterDrawa
14-02-2013, 09:03 AM
Hello,

Thank you for your reply.

1) I doubt it is well straightened. The air is blown into the chamber by three fans (approx 20cm dia) located in a top rack above the climatic chamber. These fans push the air through two adjacent 100mm by 750mm inlets, both shielded with a metal mesh (perforated metal sheet). None of the inlets is directly in front of the fans, which means that the air flow is bent. Also, there is little space for the air to be straightened before entering the chamber. Please see the attached pdf file for some clarification.

2) The chamber is hooked up to a Silensys condensing unit (such as this one: http://www.tecumseh.com/en/corporate/silensys). The unit uses 3.5kg of R507 coolant.

3) I am open to any suggestions - if you say I need additional fans within the chamber, I will listen carefully.
We need the volume between Y = 0mm to Y = 500mm; X = 150mm to X = 880mm; Z = 0 to 750mm; for our application - so no additional equipment to be placed there.

I hope this answers your questions. Please let me know if you require any further info.

Thank you,
Peter

Tesla
14-02-2013, 11:38 AM
If you place some baffles/guide vanes on/in the outlets to balance the flow evenly this should help. You could also use socks to help disperse the air more evenly as it travels down the chamber wall. Any restriction of air flow will on the other hand increase the difference between the inlet and outlet temps, dependent on the load or deviation from set point.
Do you use it at a set temp (as shown around 35deg)? or several temps?

PeterDrawa
14-02-2013, 12:11 PM
Hello Tesla,

Yes, using baffles/guides was my initial idea too. Thank you.
My concern is the non-uniform temperature distribution within the chamber volume. I was hoping that his can be achieved by balancing the flow.

We use it at three temperatures: T1 = 14 deg. C; T2 = 25.5 deg. C; and T3 = 36 deg. C. When switching between these, we always wait for a minimum of 60 minutes for the device under test (inside of the chamber volume) to thermally stabilize.

Cheers,
Peter

Tesla
15-02-2013, 06:29 AM
Hi Peter
Cool so it's worth a try. Personally I would take 50% of the center of the flow, divide it into two with plastic pieces cut from a bucket or something thick then glue with epoxy. This would give four even flows from each outlet.
To help prevent mixing and air short cycling straight back into return air material socks from each segment about 500mm long. Initially you could use something like stockings for a day to test it out then have some socks manufactured to fit. They would need to be cleaned monthly (washing machine) so as they don't clog.
If there is too much restriction there may be an undesirable increase between the upper and lower parts of chamber.
Please let us know how you go and the results you get. I know these mods on chambers are not cheap but the above should give a good budget wise option.