Refrigerent-Cooled VFD's

[peebee]

I'm hearing from one of the major chiller vendors that the only VFD's they offer are refrigerent cooled. I've never heard of this before, but he's saying they've been standard for this vendor since '98 or '99. (as an aside -- he did also state that all his competitors offer only water cooled VFD's, which I know to be false as I recently installed three forced-air-cooled VFD's by one of his competitors).

I'm certainly familiar with refrigerent-cooled motors, but I'm comfortable with them because they are sealed inside the chiller; they're at no more risk than the rest of the chiller is. But these VFD's are piped up with tubing, tubing which snakes through the VFD -- lot's of places to leak and break and fail and overheat and suck up time to fix. . . . So I have some significant concerns about reliability, downtime for repair, newness of design, etc.

Any comments on this? Should I put up and shut up, or should I scream for an air-cooled VFD? Any experience with their reliability record? Any idea how long these things have really been offered? Any idea if air-cooled really is an obsolete design?

 

[peebee]

Update -- they've been standard for 3 years, not since '98 as I stated above.

 

[ozmosis]

I spoke with one of the bigger chiller companies a few years ago (about 5yrs ago) about this but there were some distinct restrictions on cooling the IGBT's at initial start-up prior to the refrigerant coming on line. These were drives 150kW~300kW region. I must say I've been out of this side of the business since then but it sounds like things have moved on since I was in discussion.
It does make sense though, the VFD needs to be cooled, costs of the big lumps of aluminium heatsinks are high and if there's plenty of cooling available then why not use it.
I think A-B had this technology coming through back then but couldn't say if it was made commercially viable as you could only use this VFD for this type of application so not sure if the volumes would make it worthwhile.
Interesting topic though.

 

[DRWeig]

Hiya Peebee,

I would be OK with it technically, but I sure do prefer being able to bid the drive separately from the chiller, and I'd want to be able to service or replace the drive completely without breaking the integrity of the refrigerant circuit....

Just IMHO that is.

Old Dave

 

[mc5w]

You might also need 2 compressors in parallel - 1 off of the drive and 1 that runs directly off of the line. This way you can establish cooling before starting the drive. Your refrigeration system is going to have a minimum load and making half the capacity fixed speed and the other half variable speed will cut cown on drive cost.

However, I would more inclined to use a forced air cooled VFD as this has the least number of things that can break. Running chilled water or refrigerant tubing can also cause water to condense on the drive which could damage the drive. Water and heat are the 2 biggest enemies of electrical equipment - I would think very seriously before using water to get rid of heat.

 

[peebee]

I'm surprised that there wasn't a stronger response to this question. Does that mean that most people are
1) OK with liquid-cooled VFD's, or
2) unfamiliar with liquid-cooled VFD's?

Also, another small update, it turns out the the VFD's are Reliance LiquiFlo II drives.

 

[jraef]

peebee,
I have seen this done on large MV VFDs, but the drives were cooled with a separate heat exchanger. So technically the VFD heat sinks were still air cooled, just with cold air from the refrigerant cooled exchanger. We have done 1600A+ soft starters where we had to use water cooled heat sinks. There is a risk of leakage as there always is, but that was the only way to get that much power through the system.

Not being aware of what size chiller you were discussing, I would venture to say your chiller vendor was speaking of a relatively small unit design that may have the VFD built in to the motor / chiller housing. Take a look at this presentation on new(er) chiller designs, specifically at the slide titled "New Compressor Tehnologies".

http://www.energy2003.ee.doe.gov/presentations/new-tech/1-Ivanovich2.pdf

That seems to show the VFD being integral to the motor, which in a sealed compressor motor would make it cooled by the refrigerant. If that is the case, they would have needed to take all the above issues into account in the initial design, and if properly tested could be very viable. Maintenance is another issue. I for one would prefer to see them separate, but most of these chiller manufacturers provide PM service anyway so the issue may be moot to the user.

"Venditori de oleum-vipera non vigere excordis populi"

 

[peebee]

Interesting, jraef. This site,

http://www.turbocor.com/literature/pdfs/articles/frictionless.pdf

, confirms your suspicion that the VFD on the Turbocor is integral to the chiller (and also adds that it uses a brushless permanent magnet synchronous motor).

We're looking at (roughly) 500-ton units. And our VFD's are skid-mounted but external to the chillers.

 

[jraef]

Seems rather unlikely that a 500 ton chiller could have the VFD integral to the motor then. Are you sure the vendor rep wasn't just blowing smoke, maybe combining what this company is doing with his own thoughts about how HE would do it for bigger units? It has been known to happen. I have encountered several "experts" in one field or another who make up stories like that so that they can appear important, when in fact it is based upon something they heard from someone else who read it in an email telling them about what a friend saw on a website somewhere etc. etc.

The additional cost to do it would need to be outweighed by some other relatively extreme savings. Real estate is often important in mechanical rooms, but how much can you really save? Sure the heat sinks can be smaller, but then you add back in the extra valving, tubing and protection systems.

In my opinion; mediocre concept, unlikely reality.

"Venditori de oleum-vipera non vigere excordis populi"

 

[peebee]

This seems to be the reality -- we've already received the submittals for 3 of these chillers on this project, that's when this first all came up. They are adamant that they cannot provide air-cooled VFD's.