VFD on Compressor - how to control

[windpoint]

I have 4 10 ton package units with built-in VFD on in-door blowers. I think it's better to add VFDs on compressor> this will give the compressor the ability to unload when air flow reduced and heat load goes down.

Half of my Units have TXV on it and others come with fixed orifice.

• I understand VFD need a minimum speed to be safe, I will set it at 30HZ .
• I also plan to run full speed every certain period of time, to spread oil around.

My question is how should I control the speed? suction line temp or there is a better way.
Any advise will be appreciated.

[Gleng]

The Carrier 10 , 12 and 15 ton units that I have seen with VFD on them have two compressors in them.so it is a two stage unit.The VFD does a lower fan speed with one stage calling.When the second stage calls.the VFD increases the fan speed.I don't think it is variable, it is factory set aat a certain speed per stage.I believe the speed can be changed, but you have to have the VFD controller to reprogram the speed.I don't know why it would need to be reprogramed.However ,there are two different speeds for first and second stage.The older units with no VFd had only one speed, regardless of first or second stage calling.The individual compressors don't have partial loading or unloading,I don't think this is necessary.That is why there are 2 smaller compressors instead of one larger compressor.If only one larger compressor were used ,it would probably have unloaders.This is how the older units with one big semi hermetic were set up.

[stanbyyourword]

ramp fan and comp minimum/max speed together commission with professional HVACR technician

[numbawunfela]

I have 4 10 ton package units with built-in VFD on in-door blowers. I think it's better to add VFDs on compressor>
Half of my Units have TXV on it and others come with fixed orifice.

• I also plan to run full speed every certain period of time, to spread oil around.

Your post seems to say you have a VFD on the fan. But then the questions seem to ask about VFDs on compressors. Which situation do you have?
Are these single stage or 2 stage equipment?
Orifice says low tech/ low efficiency. VFD says High tech/ high efficiency. It is not usual to have both features in the same unit. Are these VFDs being applied as an add on? Or did the units come with these as part of the unit itself? The distinction is important, as I have seen these misapplied.
Also, why are there drives? Is it to accomodate zoning? Usually variable speed fans are not there only to make the compressors happy, they are usually there for some other reason.

[windpoint]

Thank you @gleng. They do have 2 compressors setup and single compressor also. The reason I dont like two stag setup is that the evaporator coil are split to two circuits.
when unit runs at 1 stage, you have half coil cold and other half warm. It just doesnt work good. We basically run two compressors all the time.

I think keep both compressor run at lower speed should be better. I have electrical background and have experience programming vfds. But I need some basically knowledge how those compressor should be controlled.

[windpoint]

@numbawunfela , sorry for the confusions.
The exiting system come with vfds on blowers. I am not sure aftermarket or original.

I want to add vfds to the compressor. We have both 2 compressors and single compressor setups

[stanbyyourword]

yes and warranty depending on age may come into question also

[windpoint]

yes and warranty depending on age may come into question also

5 years warranty on compressor only. That doesnt mean anything. We always have coils fail before that, which is not part of the warranty program.

[numbawunfela]

They do have 2 compressors setup and single compressor also.
when unit runs at 1 stage, you have half coil cold and other half warm.
I think keep both compressor run at lower speed should be better. But I need some basically knowledge how those compressor should be controlled.

The issue is not that there are 2 circuits, it is that your evap coil is split-faced, and not interlaced. You can have 2 circuits with the right evap, but you do not have it. This tells me that your VFDs are applied as an add on, not from the factory. My reccomendation is to leave it with the mechanical guy to fix it and stay away. They chose the wrong unit to apply this way.
Putting a drive on a compressor is ok with reciprocating and screw compressors. Scrolls do not like it. They have issues with oil getting pulled from the sump up to the scrolls. That is why variable capacity scrolls use valves and separating the scrolls to modulate, never a VFD. I am pretty sure you have scrolls, so a drive is out for you.

[MaxBurn]

Pretty much exactly what this company does: http://www.bes-tech.net/digi-rtu/

We looked at them but we are sort of sidelined at the moment as we take issue with voiding warranties like that, just superstitious.

Why are you having coils fail inside of 5 years?!

[stanbyyourword]

this isnt vav Im assuming? if ramping down on load that should be doable within limitations probably around 50-60% ramping blower and compressor together. needs "yessir OK" from someone somewhere, and yes the prolonged running at low load will very likely be oil return issues, good luck

[Spitz]

Run them off of suction pressure. Temperatures will lag

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[Spitz]

What type of compressor? A discus may be slowed to around 25hz. A basic scroll 45-50hz.

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[Cool breeze 38]

to answer the controls questions, ramp the compressors by space temp demand.

I think its a bad idea honestly.

[numbawunfela]

Even though you cannot use a drive on your compressors, you can still be the hero. Tell them to get the mechanical guy retrofit the thing with one of these:

https://rawal.com/products/apr-control-hot-gas-bypass

It has been a while, but on packaged systems they used to say they could unload down to 15-25% of total capacity. Then you can run your fan as low as you want. The system just mechanically unloads - nothing to control. Super easy. I installed one a long time ago, and it worked well. It was tough to braze though... not your problem I guess.

I also wanna know why you go through evaps so fast though....

Why are you having coils fail inside of 5 years?!

[Cool breeze 38]

it sounds like you have a Carrier unit with the evap description. I have never used it. but carrier makes a kit to replace the orifice header on the evap with a txv. kit includes a txv, distributor and cap tubes. if you want to go all mad scientist, you could put that in but split the coils with it, and do the same to the suction header. PITA I know.

why dont you look into making the refrigerant circuits a common circuit.

[windpoint]

Thank you for the link. It's very interesting. Just curious, how the by-pass method can save kwh, which is a major benefit to VFD the compressor. If the compresser runs at the full speed, the kwh usage should be about the same. Please correct me if wrong.

to answer @MaxBurn the question, the units are located where there are lots of lint. Those lint enter inside the condenser coil and make the system fail. We all know those coil can not be economically replaced. It cost more than half of the unit price. I cant hardly understand though.
I asked the question in another thread.
https://hvac-talk.com/vbb/showthread...8#post25759118

Even though you cannot use a drive on your compressors, you can still be the hero. Tell them to get the mechanical guy retrofit the thing with one of these:

https://rawal.com/products/apr-control-hot-gas-bypass

It has been a while, but on packaged systems they used to say they could unload down to 15-25% of total capacity. Then you can run your fan as low as you want. The system just mechanically unloads - nothing to control. Super easy. I installed one a long time ago, and it worked well. It was tough to braze though... not your problem I guess.

I also wanna know why you go through evaps so fast though....

[windpoint]

What type of compressor? A discus may be slowed to around 25hz. A basic scroll 45-50hz.

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2 5 ton compressor from carrier packaged unit. I guess they are scrolls. If I can unload it to 45hz, I will be very happy.

[numbawunfela]

Thank you for the link. It's very interesting. Just curious, how the by-pass method can save kwh, which is a major benefit to VFD the compressor. If the compresser runs at the full speed, the kwh usage should be about the same. Please correct me if wrong.

Work = amps. When you slow a motor using a VFD, it does less work, so it draws less amps. However, when you bypass from the discharge back into the suction side of a compressor (called hot gas bypass), you also do less work, and therefore draw less amps. There is no real work done, so your amps drop off, and therefore your power draw is less. It's like running a blower motor with no blower wheel, the amp draw is negligible, because no work is being done.

The problem is that it is possible to hot gas bypass a compressor to death. They are not made to suck hot gas, so when you get above a certain % of bypass, the oil breaks down, windings get hot, things go boom. So the APR does the HGBP into a chamber, and places a TXV on the other side of said chamber. The txv adds enough cooling capacity to cut the superheated gas down to a reasonable temp to save the compressor, making more unloading possible to do safely. This is a very simplified explaination, as I got work to do and you have a link. But it should be clear enough.

As a side note, I am a firm believer that if one cannot be nice they should be quiet.... but be careful what mechanical advice you take. If you decide to take some of the mechanical suggestions on this thread, run them through the pro's forum on commercial service first. Just be sure the poster has a firm grip on what he is talking about before you lay your reputation or a customer relationship on the line.

[numbawunfela]

I asked the question in another thread.
https://hvac-talk.com/vbb/showthread...8#post25759118

Just a heads up, your thread link above discusses Condenser coil failures. In this thread you mentioned Evaporaror coil failures. Maybe it was a slip of the keyboard?

Anyhow, I posted in your other thread.