View Full Version : variable frequency drive

07-06-2003, 11:24 AM
has anybody tried variable frequency drive for resiprocating ammonia compressor ? does it save energy on resiprocating type compressor load ?

Peter Mitchell
16-10-2004, 01:43 PM
energy saving on resiprocating compressors is very limited as the duty/efficency of this type of compressor drops away very rapidly with speed. (inverse square) If you had screws then you can vairy the speed and this changes the duty in direct proportion. if you could increase the frequency both the resiprocating compressor and the screw compressors give a better output but there are issuse with voltage torque and heat disapation
would you save energy Yes but very littel, the cost of installation alone would make it not worth doing, if you include the cost of the drive the life time of the pack I dont think you would get pay back even over 15 years

16-10-2004, 07:53 PM
Hi gapatel :)
there would be some savings, but only on aplant that runs down to some very low loads in comparison to the 100% duty, typically below 25% (the smallest step available). I know of a plant with two 750kw piston compressors with variable speed drives, but this is to close control the water leaving temp on the chiller they are on. In this case energy is saved by having only evaporator pumps, and now service pumps or a buffer tank, hence the need for drives to close control and react to the load as it happens.
Kind Regards. Andy.

16-10-2004, 09:36 PM
I regard your comments highly, Andy. I suppose, when we are talking about ammonia plants, that the loading is not nearly as variable as we would see in smaller warehouses, supermarkets, etc.

But I have to think there are opportunities to design an ammonia plant to take significant advantage of variable speed drives, on lead compressors, condenser fans and perhaps even evaporators. I am not thinking in great detail, here, but more in general. If something saves energy here, then why can't it save energy there, sort of thinking.

Crazy idea, but a unitized compressor/condenser/evaporator penthouse arrangement that varies all speeds as a unit? Surely some engineers are thinking about things like this. Not a job for a field person, perhaps, but worthy of design people.

There is "off" and "on." The most energy-efficient position is for something to be "off." The best method of refrigeration is for something to be "on."

Somewhere in-between is the answer, and changing speeds appears to me to be a necessary part of an improvement. It makes for fun thinking.


Peter Mitchell
17-10-2004, 07:32 PM
I agree there is room for savings, but not using resirps

17-10-2004, 08:10 PM
Hi Peter
industry favours Screws; but recips are more effecient, especially at anything other than full load. You could fit a screw with an inverter drive and nearly match the effeciency of a screw, but not quite. Recips are cheaper to install, cheaper run where power consumption is concerned, slightly more expensive to mantain. Another plus is if you neglect to service they let you know, usual letting go with a piston or so, without doing any great damage. Forget to maintain a screw and they right themselves off.
Screws are the flavour of the month, but recips will make a come back.
Me I specialise in Screw compressor work, it pays me well, but in general recips have still a lot going for them.
Kind Regards, Andy.

Peter Mitchell
18-10-2004, 12:40 PM
Hi Andy
I agree with what you state.
The question is could you get energy savings if you all ready have recirps in place by using speed controle. i would say yes but it would save very little, better savings with scrols or screws. Then there is maintenance but that is a different matter.

Peter Mitchell
18-10-2004, 03:04 PM
Elm had put supermizers on packs and fans. they eventally gave up on packs and moved to only fitting to fans the savings on packs was limited and i was told that the efficency of the compressors drops off very quickly with speed. As i have no real evedence for this i will take what you say Marc on in good faith.

10-03-2005, 04:06 AM
The extra cost of the inverter duty motor and inverter are hard to recover for most refrigeration applications if the compressor has an internal unloading system such as a slide valve or cylinder unloading. Compressors also have a lower limit on the speed at which they can be run. Most of the screws I have worked with have a lower speed limit of 1500 rpm, so you are limited on how low you can go. Most manufacturers also show a reduction in efficiency at lower screw speed due to the lack of centrigugal force to keep the oil ot to the tips for sealing and they do not develope the oil wedge in the rotors, so blow-by is a larger issue.

Now on fans for air handlers and pumps can really benifit from inverter drives.


10-03-2005, 07:03 AM
Here in Belgium, they subsidise enormous VFD's to get the Kyoto requirements (one of the reasons) They're almost paid for half teh price by the government.
Pistons compressors (we tried it with Bitzer compressors and with an oil pump fitted) can go as low as 20 Hz.

10-03-2005, 11:48 PM
On a screw, blow by volume is considered proportional to the operating pressure ratio, regardless of rotor speed, by the same old square law. It's just that at lower speeds that constant blow by becomes too great a proportion in relation to the volume pumped.

This brings up a point that has intrigued me for years. Compressor efficiency. Volumetric efficiency in recips, for example, has pretty much been the bellwether for compressor efficiency in most things I have read over the years, yet scrolls and screws have no volumetric efficiency losses to speak of. But recips such as the discus compressor indicate higher efficiencies than rotating compressors when compared on an equal basis.

Is it this thing we call "blow-by" that causes the difference?

05-11-2005, 02:27 AM
When my friend Tiffany helped build a quad Britney simulation server for science, she actually used a digital control board to run the 1HP recriprocating compressor at something less than full capacity most of the time. It used large MOSFETs to vary the frequency of the power to the compressor. The actual control electronics included a lot of 74HC discrete logic along with a 40 pin Jessica Simpson micro (which also controlled the front panel LCD and the system power supply). I'm not too sure about the details but it interfaced to the motherboard through a RS232 connection. Using the right program, it's possible to display the approximate compressor RPMs and estimated high and low side pressures based on temperature. There were 4 TXVs, one for each evaporator, to ensure proper operation even with uneven loading. The main purpose is to keep liquid refrigerant out of the compressor.
One of the tricky things, though, is that the compressor must run at some minimum RPM or the oil bearings will not work properly (it relies on oil between the surfaces, and at low RPMs, there's not enough pressure to keep the oil in the bearings). I remember that a minimum RPM is hard coded into the firmware. (2900RPM?) But I suppose that running the oil pump on a separate motor that always runs at full speed will solve this. It would also reduce the stress on the parts from cold boots since the parts will be lubercated before they start moving.

BTW, one of the coolest things about that server is that it's possible to actually hear the compressor speed up at high load. It reminds me of a race car speeding back up to racing speeds after a caution.

17-07-2006, 06:32 PM
We have had great success with VFD's on our condensers and larger size evaporators, but have never ventured into putting them on recips, have thought about putting them on screws.

30-08-2006, 10:03 PM
We have put some on recips, but the one thing you have to watch out for is the minimum RPM that the compressor can run at. Most ammonia recips have internal oil pumps, they also have a minimum GPM of oil supply needed to keep bearings happy. So there is a limit to the minimum RPM on ours it is between 400 and 600 depending on the amount of cylinders. If you venture below that you can install an external oil pump.

31-10-2007, 09:43 PM
which its the capacity of your compresor