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Bluenoser337
13-04-2004, 04:05 PM
Operating several screw type ammonia compressors that are PLC controlled. The loading/unloading controls are monitoring the position of the sliding valve for the PI controller. Sensors on the sliding valves are not totally accurate or reliable. Wondering if the motor amps could be used instead of the valve position input (if scaled accordingly). Anyone seen this done...or have any other suggestions? Many thanks!

Peter_1
13-04-2004, 05:06 PM
Wont work.

AMP's are not related to a stable suction pressure.

AMPs are in relaton with the pumped mass volume.

Slide valve is positioned so that the screw capacity is equal to the actual needed capacity. But when in partial load, motor is seriously underloaded, so there is no relation.

AMP signal should even be to rough to regulate, what if bearing wears, if oil becomes thicker then usua, AMP's goes up but has nothing to do with suction pressure.

We did it once with a Danfoss frequency inverter on Bitzer screws. The inverters where commanded by a suction pressure signal.

Bluenoser337
13-04-2004, 05:21 PM
Thanks Peter_1...
I'll put those thoughts right out of my head!

Andy
13-04-2004, 06:14 PM
Hi Bluenoser
Danfoss used to make a very nice, simple stage controller an EKC30 + EKC31.
The 31 part was used in conjunction with the 30 which was pressure or temperature set point with a neutral zone and adjustable pots for pulse up and down. The 31 part backed up the 30 and measured compressor amps, too many amps and the compressor was off loaded until the dead band was overcome, them loading started. Mean't that the motor didn't overlaod even with peak pulldown loads or say a high head situation.
That is the only way amps monitoring will help with loading. Better calibrate or change out the position transducer system.
It is entirly possible that a position input is not required, just min and max cams. Load it up acheive sett point, unload it and stop. This is the most effecient way to control. If you need small part load steps, fit a reciprocating compressor to handle this type of load.
Kind Regards. Andy. :)

anand
12-04-2006, 09:57 AM
Danfoss EKC30 + EKC31 was excellent controller. However, it is now out of production. But you can get a pin to pin replacement for the same from India. It is called FKD40A + FKD41A. visit www.manikengineers.com for details

guapo
12-04-2006, 01:53 PM
Hi bluenoser,
How accurate you need for the display of your capacity in the compressor? Some of the display is 10..20..30..100%. There is also 5..10..15..20.. 100%. May be you are looking for 1..2...3...100%.
In my equipment I do calibration every 3 months.
Is this realy matter for the very small diffirence of your equipment?

Guapo

latent heat
13-04-2006, 02:11 PM
If the slide valve readings are jumping all over the place and it seems as though the compressor is hunting to find the right load position I would first check the connections from the potentiometer to the analog board and then check the potentiometer itself and make sure it is calibrated correctly

US Iceman
13-04-2006, 02:32 PM
Well, it looks like an old thread has been resurrected.

In addition to the other comments, if the slide valve is moving around a lot you can also check the load/unload solenoids and the adjustment valves to make sure the valves are properly set.

The timer settings for the load/unload can also be another source of similar problems.

NoNickName
13-04-2006, 03:44 PM
Also I would like to highlight that the slider speed is not a constant figure, but depends on the difference between Pc and Pe.

amarjeet
14-04-2006, 12:03 PM
hi,
your solution i have that is www.nutronicsindia.com

send me mail to info@nutronicsindia.com

US Iceman
14-04-2006, 07:09 PM
amarjeet,

If you are providing a source of information for review of the RE members that is OK.

If you are selling these devices and recruiting sales, then we would ask that you refrain from advertising please.

Please mend your post accordingly.

Andy
14-04-2006, 07:14 PM
hi,
your solution i have that is www.nutronicsindia.com

send me mail to info@nutronicsindia.com
Hi amarjeet:)

If you would like to pay for an advertisement, please feel free to contact Webram.
Otherwise do not use the industrial forum for free advertisement:)

Normal control for this type of application would be PLC, with analog input channels from a 4-20ma source. The source is then buffered to steady the signal, with delays added in to improve the display resolution.

Kind Regards. Andy:)

Josip
25-04-2006, 02:18 AM
Hello, Bluenoser337

EKC 30/31 were very nice compressor capacity control devices but not available today. Another link www.nutronicsindia.com (http://www.nutronicsindia.com/) is not available (code 509 Bandwidth Limit Exceeded?) to see that offer?

Remake or copy of EKC 30/31, coming from India (FKD-40A+41A) are ok, but as I remember, it is not possible to control several compressors, connected in parallel at the same system, in a good way.

Each compressor is equipped with his own instruments and all running compressors at the same system will simuntaneously change capacity depending on the incoming signal. That kind of regulation is not good for any compressor and especially not for screw compressors.

When you have several compressors running in parallel at the same system you can control only last one (other compressors must be shut down or at 100% capacity) in starting sequence.

First control of that kind was introduced by Stal Astra with their STALECTRONIC 300 (SR - Sequence regulator) around 1975 and later with some other.

With one STALECTRONIC 300 was possible to control a group of 3 compressors connected in parallel or up to 9 compressors connected in parallel to 3 SR. In that time that was very inovative and powerful Compressor Control and Capacity Regulation System.

Today UNISAB II is offering all of that and much more. In my opinion, it is the most powerful PLC designed only for refrigeration compressors. It is very easy to connect up to 14 compressors together and all of them to SCADA systems using Sabvisual if you want. You can sit at your home and control all of them remotely by your PC or if you like they can call you to your mobile...;)

Here you can find more about:
http://www.yorkref.com/

then klick to:

controls- automation systems- sabvisual- download documents - download TD: sabvisual

JCI-YORK-SABROE as you like, today offers upgrade kit to install UNISAB II to any screw or reciprocating compressor. It is not cheap but....

Best regards,

Josip

US Iceman
25-04-2006, 03:49 PM
Josip brings up some very good points on screw compressor capacity control. Most of the compressor packages have some type of independent controller for a single compressor.

When multiple compressors are installed in parallel each controller will monitor a specific compressor. In this situation the suction pressure is controlled from each compressor.

At any time multiple compressors could be operating at partial capacity. This is very expensive as the part load performance of the screw compressors is not very good.

The key to compressor control is to maintain the suction pressure at the desired value and at the same time keeping a compressor(s) as fully loaded as possible.

The attached file shows the impact of part load operation in twin screws. This effect does vary quite bit. Low discharge pressure operation will provide some improvements.

Andy
25-04-2006, 05:17 PM
Today UNISAB II is offering all of that and much more. In my opinion, it is the most powerful PLC designed only for refrigeration compressors. It is very easy to connect up to 14 compressors together and all of them to SCADA systems using Sabvisual if you want. You can sit at your home and control all of them remotely by your PC or if you like they can call you to your mobile...;)

Here you can find more about:
http://www.yorkref.com/

then klick to:

controls- automation systems- sabvisual- download documents - download TD: sabvisual

JCI-YORK-SABROE as you like, today offers upgrade kit to install UNISAB II to any screw or reciprocating compressor. It is not cheap but....

Best regards,

Josip

Yes but in Multisab mode it runs each compressor to the max, so if you have a requirement for two compressors it will run both of them, even down to 30% I have seen two compressors running where one would have been enough.
Grasso GSC, with a little SCADA is the best option, but unisab/multi sab is easier and cheaper to fit, and easier to support.

Kind Regards. Andy:)

Josip
26-04-2006, 02:24 PM
Hello Andy,

Don't know what you mean with Grasso GSC (compressors, monitron or PLC 1502/2502).

Definately compressors are ok, Monitron seems also ok without too much expirence, PLC 1502 very bad expirence (or I'm so stupid :confused: becuse cannot undertand it's logic, PLC 2502 no expirence, hope no need if this younger brother is similar to older one ;)

Speaking about capacity control is not easy. Speaking only about compressor's capacity control in parallel connection to the same system;

we have to start from complete plant design like:

-total Q {kW} of plant/no. of compressors (big compressors less price and vice versa depend on customer request)
-partial load increase/decrease (only cold rooms or tunnels or big heat exchangers) to start 600 kW compressor on demand of small cold room of 25 kW :mad:
-response time to adjust timers or by regulating valves for capacity decrease/increase
-minimum load to stop compressor by PLC or auxiliary switch
-control by suction pressure or brine temp with set point, neutral zone, proportional band...
-oil temperature and presure control with set points
-Vi regulation (automatic :) or manual :(
-condensing pressure control

and so on....

Of coures there must be someone to conduct (set up) all those cooks to make a nice cakes at low cost :rolleyes: free if possible

My opinion is:
when we have nice balanced (designed) refrigeration plant it is very easy to have a good capacity control even without sophisticated equipment..

in another hand, having all sophisticated equipment installed on unbalanced plant cannot help you to have a good capacity control (I use to say it is not possible to make cakes of sh't)

Finally, having very sofisticated or old fashion controls without proper adjustment.... :(

Moral of this story is: nothing without good chef :)

Best regards,

Josip

US Iceman
26-04-2006, 03:04 PM
Moral of this story is: nothing without good chef :)

A very good way of making a point. There are of course many factors that go into the system design. Even the best control system will not allow a poorly designed refrigeration system to operate well.

Josip has a similar attitude as mine. The system should be properly designed to provide low energy use and maintain the desired temperatures. The control system provides the ability to maintain the low energy use.

The biggest mistakes I see are very bad part load capacity control and keeping the discharge pressure too high.

If the system has to operate in the winter, it should be designed for this. We normally find the system was designed to operate in the summer, and then all sorts of fixes are made to keep the system running as if it was summer (head pressure controls :mad: ).

A cold storage facility may require a different refrigeration system design than a food processing system. Both have different requirements, but I see a lot of systems that are designed the same to fit all sizes and requirements.

To sum this up, the systems should be designed to provide the desired temperatures and cooling capacity during the summer or winter, and, full load or part load conditions while using the lowest energy input.

If this is done, the control system then controls the system though this range of operation.

As Josip mentioned, there is more to this than just control systems.

The most difficult problem is not spending any of the owners money to do this.:D

Tycho
26-04-2006, 05:07 PM
Remake or copy of EKC 30/31, coming from India (FKD-40A+41A) are ok, but as I remember, it is not possible to control several compressors, connected in parallel at the same system, in a good way.


That is infact very simple :)

on a screw compressor you will have either a 4-20ma or microswitches on the 0 and 100% reading of the capacity.

you can connect as many compressors you want, using the 100% indicator of the preceding compressor as a run signal for the next.

same with the minimum signal, use it as a stop signal.


Most compressors I have seen on a single EKC 30/31 is three, but I dont see any problem in adding more than that :)

Josip
26-04-2006, 06:36 PM
That is infact very simple :)

on a screw compressor you will have either a 4-20ma or microswitches on the 0 and 100% reading of the capacity.

you can connect as many compressors you want, using the 100% indicator of the preceding compressor as a run signal for the next.

same with the minimum signal, use it as a stop signal.


Most compressors I have seen on a single EKC 30/31 is three, but I dont see any problem in adding more than that :)

Hello Tycho,

agree with you :) in fact, I like those instruments becuse it was very easy to regulate compressor's capacity either by suction pressure, brine temp or by power consumption.

To add additional timers or limit switches is quite simple, only to explain to customer the benefits :) and to convince them to open their wallets little more ;)

Bet regards

Josip

Andy
26-04-2006, 09:39 PM
Hello Andy,

Don't know what you mean with Grasso GSC (compressors, monitron or PLC 1502/2502).






Best regards,

Josip

GSC (Grasso Screw Control) a new generation of controller they use, first ones I seen are 2002. Based on a Siemens S7 plc with written software. Can be used on all Screw and recips, like unisab. More plc in nature, but with good network compability to SCADA. The best industrial control available today.

We have retrofitted it on Howdens and Halls Screws, with very little problems.

Kind Regards Andy:)

Tycho
26-04-2006, 10:15 PM
Hello Tycho,

agree with you :) in fact, I like those instruments becuse it was very easy to regulate compressor's capacity either by suction pressure, brine temp or by power consumption.

To add additional timers or limit switches is quite simple, only to explain to customer the benefits :) and to convince them to open their wallets little more ;)

Bet regards

Josip

This one is from back in the days, just saying it can be done, and I bet it's cheaper to do it this way than to install unisab's :D *poke poke*

We build our own standard PLC based control system now, it's a "one size fits all" and can be used for for any compressor type and any mode of operation.
Options are touch screen displays, GSM remote operation/sms alarm.

Chopper
28-04-2006, 12:28 AM
Grasso GSC, with a little SCADA is the best option, but unisab/multi sab is easier and cheaper to fit, and easier to support.


GSC (Grasso Screw Control) a new generation of controller they use, first ones I seen are 2002. Based on a Siemens S7 plc with written software. Can be used on all Screw and recips, like unisab. More plc in nature, but with good network compability to SCADA. The best industrial control available today.

We have retrofitted it on Howdens and Halls Screws, with very little problems.

Excuse my ignorance a wee bit Andy:) but what makes GSC a better option than the unisab?
What can it do that the unisab can't?
I thought that Sabroe had it covered with the unisab.

I have installed one GSC, it was on a brand new grasso screw skid, been in a year and no problems, what a difference to the monitron!!!!:D

Kind regards

Chopper

Julian
28-04-2006, 05:05 AM
What make of screw compressor & what are you using to sense the slide position micro switches or a potentiometer?

Josip
28-04-2006, 04:39 PM
Hello Andy, Chopper:)


Excuse my ignorance a wee bit Andy:) but what makes GSC a better option than the unisab?
What can it do that the unisab can't?
I thought that Sabroe had it covered with the unisab.

I have installed one GSC, it was on a brand new grasso screw skid, been in a year and no problems, what a difference to the monitron!!!!:D

Kind regards

Chopper

Don't want to argue about (I'm an oldfashioned RE) but in my opinion Unisab is tailored PLC exactly for reffrigeration compressors and many other PLCs (for general use) you can and you must programed and sometimes that is done by programers (worked with them) knowing little or nothing (my excuses to exceptions :) ) about regulation of NZ, P-band, time delay up-down, PI, PID and so on...

Siemens is a good manufacturer but they produce PLC for almost everythig in another hand Sabroe is manufacturer of only refrigeration equipment for decades and I believe they must be more familiar how to handle it.


What make of screw compressor & what are you using to sense the slide position micro switches or a potentiometer?

Hello Julian

Micro switches you can use only for one signal i.e. compressor is in min pos or in maximum to give a signal to this compressor to stop after time delay (without any demand to increase capacity), or to give a signal to another one to start after time delay also without any demand to decrease capacity. If we have any of those demands time delay starts to count again but only when compressor is in min or max position.

But there we still have 90% of capacity (slide position) where we must run our compressor before stop it or start another one.

This capacity we can only regulate by some incoming signal (pressure or temperature converted to some regulating signal (Voltage -10V to 0V, 0V to +10V by Pt100/Pt 1000 ohms) or current 0-24 mA or 4-24mA.

Depending on PLC and sensors we use (presssure or temp) we have our input signal or we can use some other device to transform this signal into signal we need for our PLC.

Potentiometer we use to give us a momentary information (analog or digital) where we are with our capacity (slide position) to calculate output signal according to input signal i.e. how far and fast to push up or down our slide. For this we have P-band and I-time in connection with SP (set point) and NZ (neutral zone). Without this you can have some kind of capacity regulation we call "hunting" i.e moving slide constantly from 10% to 100% and vice versa, what is not acceptable.

Best regards,

Josip

Andy
29-04-2006, 03:10 PM
Hello Andy, Chopper:)



Don't want to argue about (I'm an oldfashioned RE) but in my opinion Unisab is tailored PLC exactly for reffrigeration compressors and many other PLCs (for general use) you can and you must programed and sometimes that is done by programers (worked with them) knowing little or nothing (my excuses to exceptions :) ) about regulation of NZ, P-band, time delay up-down, PI, PID and so on...

Siemens is a good manufacturer but they produce PLC for almost everythig in another hand Sabroe is manufacturer of only refrigeration equipment for decades and I believe they must be more familiar how to handle it.



Best regards,

Josip

GSC is by grasso engineers they use a C7 (sorry I said S7 earlier), with distributed fieldbuss (the relays are separate from the plc, the plc is on the door and is also the HMI)

Unisab is a cut down version of the Prosab controller, no longer made (a real shame as it even gave you a possible cause if an alarm or warning came up)

Yes Unisab and multisab will do most things, but not all. The control is too basic.

If you want real deal, a plc and SCADA is the only answer, specifically written for that plant.

General control on a budget Unisab is good.

The next level, GSC with a SCADA set up and a central plc.

GSC has 11 modes and counting, ie auto, manual and network control. In network the GSC will accept control inputs from a remote process thru profibus (network protcol)


Josip,
an interesting debate, and very topical for our time. I know Sabroe is adding to the Unisab as we speak, with some added sofware a condenser pressure optomiser has been added.:)

All these functions and more will be needed for the future, especially with the price of crude oil rising all the time:(

Kind Regards. Andy:)

US Iceman
29-04-2006, 05:10 PM
Josip raises a very good comment on control systems. A lot of companies make PLC's that are very good. The difference between a refrigeration control system and a machine tool is the logic and I/O.

Logic for refrigeration system control should be done with an experienced RE describing how the control system should work, not how the programmer thinks it should.

Some programmers are fascinated by speed and reaction time. It is my opinion the refrigeration system should only be operated as fast as the system will tolerate.

Sure you can load a screw compressor in 10 seconds, but what happens in the liquid separators when you do this?

Another aspect of control systems is concerned with minimizing the energy use. This the area Josip is expressing for screw compressor control and part load efficiency.

One more idea is to use the control system to select the best point of operation to minimize energy and demand charges from the electric utility.

Josip
29-04-2006, 05:18 PM
GSC is by grasso engineers they use a C7 (sorry I said S7 earlier), with distributed fieldbuss (the relays are separate from the plc, the plc is on the door and is also the HMI)

Unisab is a cut down version of the Prosab controller, no longer made (a real shame as it even gave you a possible cause if an alarm or warning came up)

Yes Unisab and multisab will do most things, but not all. The control is too basic.

If you want real deal, a plc and SCADA is the only answer, specifically written for that plant.

General control on a budget Unisab is good.

The next level, GSC with a SCADA set up and a central plc.

GSC has 11 modes and counting, ie auto, manual and network control. In network the GSC will accept control inputs from a remote process thru profibus (network protcol)


Josip,
an interesting debate, and very topical for our time. I know Sabroe is adding to the Unisab as we speak, with some added sofware a condenser pressure optomiser has been added.:)

All these functions and more will be needed for the future, especially with the price of crude oil rising all the time:(

Kind Regards. Andy:)


Hello Andy,

Definitely we are driving on the same road, same direction but maybe in different lines ;)

You have a lot of experience with Siemens ...and I don't..:)

My experience with some other PLCs is quite small and sometimes not nice, but with UNISAB I'm a longer time.

Speaking about UNISAB II, are you sure that is "too basic" or full compressor control, pls check attachment :)

Coming to SCADA please use the link and see by yourself:

http://www.yorkref.com/upload/downloads/(UK)%20TD,%20Unisab%20+%20Sabvisual%20(964%20KB).pdf

To download DirCalc 1.14 please, visit next link:

http://www.danfoss.com/Asean/BusinessAreas/Refrigeration+and+Air+Conditioning/Product+Selection+Tools+Details/DIRcalc.htm

In one previous post you wrote that you are waiting for new copy (hope this one is the last one) if not replay with new link :)

Best regards,

Josip

BTW, are you still looking for RE, we are 3 here ;) not young but experienced and maybe useful, still ;)

Andy
29-04-2006, 06:05 PM
Josip:)

thank you for the DirCalc link:)

The Sabvision looks very very interesting, I am afraid Sabroe UK has not given me the correct information on this product.:(

I am looking after three 163 Mk4 female drive units, which are fitted to a nice plant, but the plant is over sized and the most in efficent I have ever seen.

I have given a price for a small plc to switch on and of the compressors and pumps as required (trying to keep the compressors either loaded or off)

I will be phoning Sabroe on Tuesday to discuss Sabvision (for this site and another with Grasso screws, not GSC, which I have considered putting Danfoss controls on also)

My company is still looking for engineers, our most urgent need is for commercial engineers.

My boss has told me to look for another industrial engineer, but I think we haven't enough work yet:)

Kind Regards. Andy:)

SteveDixey
01-05-2006, 10:39 PM
If the slide valve readings are jumping all over the place and it seems as though the compressor is hunting to find the right load position I would first check the connections from the potentiometer to the analog board and then check the potentiometer itself and make sure it is calibrated correctly

I had Tempress potentiometers on a couple of Sabroe screws running to Mitsubishi A\D convertors that gave endless problems with varying outputs, then not zeroing or spanning correctly. Ended up dumping the lot and just using two proximity sensors to detect 0% and 100%. On the plc getting the 100% signal, it pulled in the other recip compressors as needed. Seeing as the screws ran best at 100%, it seemed the easiest (and cheapest) route.

Steve

Chopper
02-05-2006, 05:07 PM
The Sabvision looks very very interesting, I am afraid Sabroe UK has not given me the correct information on this product.



Hi Andy,
With regards to your Sabvisual query, it is best to go to Aarhaus with that one as the UK is more spares and retrofit biased.:)
I have quite a few sites with the various different sabvisual packages on them and its very good, the only problem, which i am sure all sites are the same with any kind of SCADA package is that the site personnel dont use it to its full potential.:rolleyes:

Regards

Chopper

Andy
02-05-2006, 06:02 PM
I had Tempress potentiometers on a couple of Sabroe screws running to Mitsubishi A\D convertors that gave endless problems with varying outputs, then not zeroing or spanning correctly. Ended up dumping the lot and just using two proximity sensors to detect 0% and 100%. On the plc getting the 100% signal, it pulled in the other recip compressors as needed. Seeing as the screws ran best at 100%, it seemed the easiest (and cheapest) route.

Steve

I have just completed a site and all we did was have an input for 0 and 100%, using a danfoss ak2 pack controller feeding the start/loading requirements to a siemens PLC.

Have to say it works brillant:)

Kind Regards. Andy:)

Andy
02-05-2006, 06:03 PM
Hi Andy,
With regards to your Sabvisual query, it is best to go to Aarhaus with that one as the UK is more spares and retrofit biased.:)
I have quite a few sites with the various different sabvisual packages on them and its very good, the only problem, which i am sure all sites are the same with any kind of SCADA package is that the site personnel dont use it to its full potential.:rolleyes:

Regards

Chopper

Chopper,
thank you for this information, much appreciated.

Kind Regards. Andy:)