Author Topic: Pumps just on/off?  (Read 4277 times)

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Offline orion242

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Pumps just on/off?
« on: December 06, 2013, 01:47:45 am »
Sorry I had to do a face palm when I heard Dave say this...

VFDs are about as common as candy corn these days in commercial HVAC systems here in the states.  Almost every building built in the last ~10 years will have them on pumps and fans larger than a few HP.  Onehunglo is likely churning them out at this point...

Power consumption varies as a cube of the speed on a typical centrifugal pump / fan.  So small reduction in speed is a huge savings.  They also reduce the ware and tare on belts and linkages, PFC, etc.

In simple terms...

Back in the day they designed systems that were constant flow.  If nothing was needed in the space, they simply would bypass this flow and the motor would simply move fluid needlessly around a mile of pipe, or the pump / fan would deadhead and build pressure.

These days the bypasses are gone and the motors only move what is required.  VFDs slow them down as needed to meet the buildings demand.  Advanced control systems can analyze the building and constantly adjust  system pressures and pump / fan speeds.
« Last Edit: December 06, 2013, 01:53:36 am by orion242 »
 

Offline strangelovemd12

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Re: Pumps just on/off?
« Reply #1 on: December 06, 2013, 02:01:37 am »
He didn't sound facetious, but I had a chuckle either way.   There was another thing later that made me think "he's definitely an EE, not a wrench," but I've forgotten it now.  The two hosts seem to be a pretty good combo though, I just wish the iTunes catalog went further back.
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Offline orion242

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Re: Pumps just on/off?
« Reply #2 on: December 06, 2013, 02:08:32 am »
I think he is just too digital these days to get in touch with his analog roots... LOL

No offense meant in any of my posts.  They both create a great podcast.
« Last Edit: December 06, 2013, 03:33:12 am by orion242 »
 

Online NiHaoMike

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Re: Pumps just on/off?
« Reply #3 on: December 06, 2013, 02:26:37 am »
In residential HVAC equipment (at least in the US), variable speed fans are only used in high end systems and variable speed compressors are very rare. It's a real shame indeed, as it's an easy way to boost energy efficiency (and cut peak demand) and deliver many other benefits. Main barrier is the cost of variable speed compressors. There is a DSP algorithm out there (Shannon Liu quadrature drive) that can effectively vary the speed of an ordinary reciprocating refrigeration compressor (as is commonly used in residential and small commercial refrigerators), but it relies on the torque ripple (as the crankshaft sees the most load when the piston is going into the cylinder) to generate the feedback signal so it won't work with a scroll compressor.

And while centrifugal pumps/compressors and fans follow the cube law, positive displacement compressors (used in most refrigeration and air conditioning applications) are near linear with respect to power vs. speed. (That ignores the decrease in operating pressure at lower speeds, but it's pretty small in a well designed system.)
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Offline orion242

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Re: Pumps just on/off?
« Reply #4 on: December 06, 2013, 02:39:17 am »
>In residential HVAC equipment (at least in the US), variable speed fans are only used in high end systems and variable speed compressors are very rare.

This is changing fast.

Typical 95%eff furnace sold today has ECM motor variable speed fan and 2 stage gas valve.  These are about the same cost as a single speed 90%eff.

Typical resi scroll compressors are still single stage today.  Right now the "digital" scrolls are all over low end commercial.  These are nothing more than a constant volume pump, with a bypass solenoid + PWM.  I suspect these will hit the resi market soon.  Outside of that, the dirt cheap speed varying techniques have not been able to handle these applications from what I have seen.  Even commercially, refrigeration compressors on VFDs is not nearly as common as hydronic pumps or fans.
« Last Edit: December 06, 2013, 04:24:11 am by orion242 »
 

Offline ChrisGammell

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Re: Pumps just on/off?
« Reply #5 on: December 06, 2013, 03:12:02 am »
He didn't sound facetious, but I had a chuckle either way.   There was another thing later that made me think "he's definitely an EE, not a wrench," but I've forgotten it now.  The two hosts seem to be a pretty good combo though, I just wish the iTunes catalog went further back.

It's not iTunes, but the whole catalog is easily viewed here: http://theamphour.libsyn.com/

We tried listing everything on the feed one time, it was memory limited and we ended up not showing up at all when we overloaded the feed. Yipes!
 

Offline SeanB

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Re: Pumps just on/off?
« Reply #6 on: December 06, 2013, 04:47:50 am »
VFD's are a lot more common now, they are used instead of gearboxes or special ratio boxes when you want a drive that is not a standard speed. Often as a bonus you get slow start, regenerative braking (and with that you can share the main rail across a few drives and use the energy generated or you will need a chopper resistor to dump it) and ramp up and down along with a DC brake to stop the motor and load when commanded. The drive inverters can be cheaper than the control gear otherwise required.

Attached a motor at work, 7.5kW with associated gearbox. Funny enough I was checking it while listening to the Amp Hour podcast, and while Chris was saying how big his 180W unit was. This one has only a soft start, a inverter for it would have been about as big as the motor gearbox again, instead of a small sealed box on the wall with 2 contactors, a soft starter along with motor overload and a single 5A fuse all on a strip of symmetrical DIN rail. I use a chain hoist to work on it, as the motor is 78kg and the gearbox is 110kg, so not exactly hand holdable, Biggest one I used was 22kW, that one would happily destroy the load if things went wrong, 10cm output shaft and a lightweight frame so it only weighed 200kg.
 

Offline ChrisGammell

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Re: Pumps just on/off?
« Reply #7 on: December 06, 2013, 04:54:58 am »
Well, most like other things I say on the show, they should be taken with a grain of salt. This time especially, since I'm just starting to do anything in the field.
 

Offline Frenchie

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Re: Pumps just on/off?
« Reply #8 on: December 06, 2013, 05:11:22 am »
Yeah, I facepalmed  ;D, if it wasn't work I'd post a picture of one of our substations with three of its four walls covered in VFDs. I work in a relatively large plant and approximately 60-70% of our drives have them fitted. A lot are only configured to soft start drives to improve mechanical wear and tear. The drives that don't have them are typically doing trivially simple stuff.

They're substantial pieces of kit when you start looking at multi MW drives. Our biggest is on a 5MW fan.
 

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Re: Pumps just on/off?
« Reply #9 on: December 06, 2013, 07:41:40 pm »
Typical resi scroll compressors are still single stage today.  Right now the "digital" scrolls are all over low end commercial.  These are nothing more than a constant volume pump, with a bypass solenoid + PWM.  I suspect these will hit the resi market soon.  Outside of that, the dirt cheap speed varying techniques have not been able to handle these applications from what I have seen.  Even commercially, refrigeration compressors on VFDs is not nearly as common as hydronic pumps or fans.
Their (Copeland's) marketing tactic is backfiring on them. While it's technically (one bit) digital, so are the conventional compressors, they just work at a higher (but still very slow) modulation frequency. Basically a scroll version of the Brysch modulating compressor. The pressure cycling tended to cause leaks and made it difficult to get proper superheat/subcooling readings.

Contrast that to the digital scroll used in some high end Gree mini splits which uses a DSP driven inverter. Superheat and subcooling measurements are just as easy as on a conventional system.

BTW, some time back, I actually got to compare a (small) lab chiller with an inverter drive to an older version with a Brysch modulating compressor. They both regulate the temperature equally well (no surprise - the coolant has a lot of thermal inertia) but the inverter version just buzzes away in the background while the modulating version made its presence known by cycling between a soft hum and a noisy buzzing every minute or so. Pretty much everyone liked the inverter units more...
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Offline strangelovemd12

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Re: Pumps just on/off?
« Reply #10 on: December 07, 2013, 01:44:49 am »
It's not iTunes, but the whole catalog is easily viewed here: http://theamphour.libsyn.com/

We tried listing everything on the feed one time, it was memory limited and we ended up not showing up at all when we overloaded the feed. Yipes!
Thanks for the link, listened to a couple episodes last night at work.
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