Application of VFD for Screw Type Compressor

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mjain Active Contributor Join Date: Dec 2010 Posts: 23	Application of VFD for Screw Type Compressor 01/14/2013 10:36 AM
	Dear All, We are working on a project, the system has: 2 screw type ammonia compressors for refrigeration uptp -40degree C., these compressors are with 2 300HP motors. Our task is to monitor the temperature and accordingly, reduce the speed with vfd. My questions are: Will monitoring of the temp is enough to decide the set speed, when I went through available material on web it looks like it is not very simple. will using PID available with vfd be sufficient? We are planning to use ATV71 schneider VFD, we have no prior experience with this. Any experience would be helpful. If you have done similar job, the type of precautions we need to consider before starting this project. Thanks in Advance,
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Fredski Guru Join Date: Feb 2011 Posts: 7025 Good Answers: 207	#1 Re: Application of VFD for Screw Type Compressor 01/14/2013 11:05 AM
	do yourself a favor and get somr help onsite, this is NOT an on the job training project!
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Tornado Guru Join Date: May 2009 Location: Richland, WA, USA Posts: 21017 Good Answers: 795	#2 Re: Application of VFD for Screw Type Compressor 01/14/2013 2:28 PM
	Check if the oil system will work properly at less rpm. Some types will, and some won't. If the control response is too fast, there may be risk of liquid flooding in the compressor suction. __________________ In vino veritas; in cervisia carmen; in aqua E. coli.
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Who Wants 2 Know??? Associate Join Date: Jun 2008 Location: South Carolina, USA Posts: 46 Good Answers: 2	#3 Re: Application of VFD for Screw Type Compressor 01/14/2013 10:25 PM
	Can the speed of a screw compressor even be reduced? I was under the impression that this equipment needed to maintain a set speed to be efficient... __________________ Knowledge Gained is in Direct Proportion to Equipment Ruined... or, K(+) = E(-)
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MOURADIG Associate Join Date: Nov 2007 Location: Beirut Posts: 33 Good Answers: 2	#4 Re: Application of VFD for Screw Type Compressor 01/15/2013 1:50 AM
	Hi I have connected a honeywell vfd drive to a water pump for HVAC application . It worked ok . Just have one warning for you. Do not connect the motor to the vfd before you program it. You should set the motor power, the nominal working voltage the rpm the coefficient of power after which you can connect your motor. __________________ A man wants to know everything about everything, a woman wants to know everything about everybody
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Rockyscience Power-User Join Date: Sep 2011 Location: Near the New Madrid Fault. USA Posts: 269 Good Answers: 1	#5 Re: Application of VFD for Screw Type Compressor 01/15/2013 8:51 AM
	I think if you deviate much from running speed on a screw compressor for a refrigeration system the high (and low) pressures will change and if you do not have complex controls like thermostatic expansion valves, throttling suction and accumulator's the system will be all over the place on cooling capacity and efficiency. It may never settle down and be chasing the set-point all the time. It would be good to ramp the motors up to speed and protect them with a VFD, a soft-start will also do that with less money invested. Good Luck... __________________ It's not Rocket Science
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Anonymous Poster #1	#6 Re: Application of VFD for Screw Type Compressor 01/15/2013 9:56 AM
	FYI, we have used this energy efficiency equipment on constant speed screw compressors. It contains a soft start and also works on conveyors, hydraulic pumps, mixers, and similar applications. We are seeing a typical reduction of about 20% in kWh. http://thetechnologyconsortium.com/SPEEDhtml.html
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kra101 Active Contributor Join Date: May 2011 Posts: 12	#7 Re: Application of VFD for Screw Type Compressor 01/17/2013 6:51 AM
	Hi, Most screw compressor manufacturers supply PLC along with the compressor package which is required to control the compressor and the oil pump. There will be analogue out to VFD to control RPM and input from VFD to read the actual speed. Normally, screw compressor speed and capacities are controlled based on its suction pressure. Hope this helps.
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Anonymous Poster (1); Fredski (1); kra101 (1); MOURADIG (1); Rockyscience (1); Tornado (1); Who Wants 2 Know??? (1)

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