Reciprocating semi-hermetic compatible with frequency drive

[marinechiller]

I have a 35 ton water to water heat pump that we built on site using a Carlyle 06Ez499 compressor. We sized it to handle peak loads which it does an admirable job of (heating in winter and cooling in summer). the challenge we face is control during shoulder seasons (about 4 months of the year). In the spring, we solved this by pumping extra water in order to get the temperature we needed (5°C, 11°C, 30°C). We do have unloader control on one of the 3 cylinder banks. Even unloaded, the heat pump is way too big for shoulder season.

I am considering two options.
1) ponying on a 10ton compressor for these situations.
2) adding a vfd to dial back the compressor speed (and therefore refrigerant flow rate through the system)

My questions are all to do with the VFD as I have never tried this before:
1) What danger might this pose to the compressor? oil issues? overheating? anything else?
2) Any suggestions on how to establish a base speed?
3) Any suggestions on controls?

A little more background. I am a P.Eng, not a refrigeration engineer. My fridgie and I have a lot of fun building systems for aquaculture and lobster pounds and have built reversible heat pumps for the federal government (5-30tons) with COP cooling 4+ and heating 5+. The vfd idea is more of an experiment that we want to do to see what the limits and capabilities are. Just can't afford to blow anything up:)

[Magoo]

google Carlyle compressors, their web site has alot of useful info.

[marinechiller]

google Carlyle compressors, their web site has alot of useful info.

Thanks, yes I have checked out their site. They have the smart unloader which looks like a good option. But no info on frequency drives that I (or their search function) could find. Essentially isn't the effect of slowing a positive displacement compressor such as this the same thing as unloading some cylinders?

[moideen]

You can use VFD for Carlyle 06E compressors. But there are limitations between 30HZ and 60 HZ. Below 30HZ may result in nuisance oil safety switch trip. Above 60 HZ may result in increased oil circulation and compressor valve failure.Moideen-Dubai

[THE DUDE]

I service Several Carlyle Compressors on a VFD. 30HZ is the minimum speed and the Oil Pressure will be ok on a GOOD compressor. I would reccomend a Bypass Contactor set up in the event of a Fault. Also when you will be running @ 60hz for a extended time you can use the Bypass to Run a straight 60HZ, this removes the Parasitic Loss from the drive outputting 60hz for long peroids.

[Coorsman777]

I Order all new racks with Carlyle 06D/E comps with VFD's and run the according to Carlyle specs. Also Have retrofitted several locations with Copeland semi-hermetic, both discuss and standard reed with some comps being 20+ years old and have had no issues with any of these. The only lesson learned wad that a VFD is only as good as the control algorithm you have running it. Emerson E2 Enhanced suction works wonders but has to be fine tuned for each situation. I retrofitted a store that was 25 years old with 4 homemade racks with a comp cycle total for all racks around 1200 to 1500 cycles per day and went to around 75 per day by adding the VFD's.