Autoloading slide valve for Screw compressor

[screw_geek]

Hi All
This is my first post to this forum, i have been reading the articles here for quite sometime and found it really helpful especially newbies like me.

I have a MyCoom screw compressor 320VSD-HE and i need to program its cpacity control using two solenoids; Load and unload. I am quite new to screw compressors.

There are two solenoids to control a screw compressor slide valve; one is for Load and the other one is for Unload. I would like to control the capacity of the screw compressor using these solenoids. When the suction pressure is above the Auto Load Setpoint the Load Relay is energized. The relay is pulsed on/off according to Load On and Load Off timers. The unit will not auto load if the suction pressure is above the Auto Load Disable Setpoint.
Any ideas or suggestions how this can be accomplished in RSlogix5000 platform for Compact Logix controllers? Commenst will be appreciated.
Regards

[Andy]

Hi Screw Geek

you could use an analog input from a transducer and a look up table, but probably at this stage in your learning curve a dead band pressure switch with a very small differential would be the way to go. Basically you would be replacing the timers with the plc on input of load up and load down from the dead band pressure switch.

Kind Regards Andy

[screw_geek]

Hi Andy
Thanks for replying to me. There is no Deadband pressure switch on the unit. All i have, is the suction pressure coming in and then two setpoints; one for Auto load setpoint and other is Auto load disable setpoint, and i am comparing them against the suction pressure, to pulse the relay on and off based on Load On and Load Off timers. Hopefully this helps

Regards

[Andy]

Hi Andy
Thanks for replying to me. There is no Deadband pressure switch on the unit. All i have, is the suction pressure coming in and then two setpoints; one for Auto load setpoint and other is Auto load disable setpoint, and i am comparing them against the suction pressure, to pulse the relay on and off based on Load On and Load Off timers. Hopefully this helps

Regards

Hi Screw Geek
yes the suction pressure goes up but you will either be receiving a digital, on/off or analog, usually 0 to 10 volt or 4 to 20ma.
The dead band switch is your digital load and unload signal.

Kind Regards Andy

[NoNickName]

Controlling a heat load based on suction pressure makes no sense to me. Suction pressure corresponds to an evaporating temperature which in turns reacts to water or air temperature, not to heat load.

[Andy]

Controlling a heat load based on suction pressure makes no sense to me. Suction pressure corresponds to an evaporating temperature which in turns reacts to water or air temperature, not to heat load.

Hi NoNickName

what if you have multiple rooms on a system
suction pressure is the only way of controlling the compressors.

Kind Regards Andy

[NH3LVR]

Controlling a heat load based on suction pressure makes no sense to me. Suction pressure corresponds to an evaporating temperature which in turns reacts to water or air temperature, not to heat load.

Hi, NoNickName
Oftentimes there is no other option but to control Room Temp with Suction Pressure. It is inexact, but sometimes the only practical way. (Unless you have the luxury of a Temperature operated pressure regulator on each Evap)
I am think here of Cold Storages with large Flooded Evaps. The cycling of thermostats can cause large variations in Suction Pressure.

[screw_geek]

Hi Guys thanks a lot for your feedback. Just to let you guys know its an refrigeration application , compressor is working to compress Propane.
ANdy: You must be talking about the statuses coming back to plc panel from Load and unload solenoids as DI(digital input signals), i know that the start the load and unload solenoids i have two discrete outputs associated to them.

One more thing, when the slide valve is starting to load the unit, your suction pressure will go down...is it right? So by deadband means, the two setpoints for suction pressure (Load enable and load disable) or the two output signals? How can i be pulsing my load and unload solenoids based on load on and load off timers?

[NH3LVR]

Deadband is a range (usually a pound or two (1/2 to 1Kg)) where no load or unload signals are sent.
In other words if the Deadband is set at 1 Lb, no loading or unloading will occur until the suction pressure rises or falls by this amount.

The Suction pressure should not change when you load or unload when being automatically controlled. The Controller responds to changes in Suction Pressure and attempts to maintain the set pressure.

What kind of panel does this have now?
Electronic or Electromechanical?

[screw_geek]

Hi
It is a plc based control compressor. So you mean to say that Auto load setpoint would be a deadband for me for loading and Autoload disable setpoint would be a deadband for unloading. Lets say

if Autoload setpoint = 100Kpa
& Auto load Disable SP = 150 KPa

So if my suction is above 150Kpa then my loading would be disabled and if it is within 100 go 150 Kpa band, loading would be enabled?
i am really sorry for the confusion i am creating here...this is the first time i am doing screw compressors.

[NH3LVR]

We are getting there.
What you are calling the autoload disable is what I believe is a point at above which the Compressor will not load. This is prevent the compressor from overloading at high suction pressures. (The higher the Suction Pressure, the higher the load on the motor.)
The deadband setting is usually with 6-10 KPA.
If this is not a standard control panel made by a manufacturer, you may not have a deadband programed in.
Is this a custom panel, not by the Compressor Manufacturer?

[screw_geek]

Yes it is a custom control panel which is based on PLC, not build by the manufacturer. We have to control the slide valve for the screw based on those parameters i mentioned:

Suction pressure
Auto Load SP
Auto Load Disable SP
Load oN Timer
Load Off timer
Load solenoid (DO) (it needs to be pulsed On Load On and OFF timers)

[NH3LVR]

If I understand correctly, you want to replace the existing controls with Allen Bradley. Are the controls you have now operating satisfactorily?
There are many functions for both operating and safety in a control panel. All of them are important. My favorite panels are built by an American Company, FES. Others may be as good, but I am familiar with these.
If you would like I can send you some information on the various functions built into a panel. Send me a Private Message with your E-Mail.

[US Iceman]

Sounds like a ladder wiring diagram is needed to help write the program for the PLC. The manufacturer of the compressor probably has one from old control systems.

[screw_geek]

Hi
Its a brand new system, no retrofits.

Regards

[NoNickName]

Now, suppose one or more evaporators are clogging, and the net result is a lower suction pressure. The PLC will operate to reduce the load.
Not only the evaporator net heat exchange rate is lower, but the PLC will also let less refrigerant in it. This is a bad move, as the two effects will sum.
As a client, I would never accept this PLC.

[US Iceman]

Its a brand new system, no retrofits.

I'm confused.

If it is a new system, why is the control panel in question. The compressor package manufacturers usually provide a control panel (PLC, micro-controller, or electro-mechanical), so all you should have to do is inter-connect the I/O from the process to the compressor controller and the motor starter.

The load/unload controls (timers & solenoids for the slide valve) are almost always part of this control package.

Once the control and power wiring is completed, the only thing that needs to be performed is setting the operating values (including the timers) and some minor adjustments for oil injection, slide valve hydraulics, etc.

[NH3LVR]

I'm confused.

I am as well. I sent Screw_Geek some information on the Parameters which need to be controlled. The E-Mail Address was no good.
I am always concerned when people use other than standard controls. It can cause a real service problem.
I on rare occasions get called out to a Plant about 50 miles away, always in the dead of night, with a production Crew standing around with nothing to do.
The plant control system is custom using a PLC. Troubleshooting is difficult with my limited PLC background.
Among the oddities are the water level in the Condenser being controlled by the PLC, instead of a simple float valve. For some reason if the one of the Compressors shuts down on High Pressure, the Water Solenoid for the Condenser is cut off. The NH3 feed for the Low Pressure Accumulator is another strange one. The float send a signal back to the PLC which in turn send a signal back to the Solenoid. There is no information in the Plant on the control sequence. I had to rewire some of the controls as no one was available to locate the problem for several days.
I am not against Technology. I just do not feel that are Customers are served by installing systems that cannot be serviced by the Refrigeration Mechanic.
I guess the answer would be for all Technicians to be competent in PLCs and have the Software for all the PLCs their customers have. Unfortunately this may be beyond the abilities of a generalist like myself.

[US Iceman]

I'm with you...

Control guys like to control everything, and they like to make the controls fast.

Two problems with this...

Refrigeration systems do NOT like it when they are controlled quickly, AND, most control guys do NOT understand refrigeration.

Unfortunately, I suspect us refrigeration guys need to learn controls and programming so we can do what we want or need to. Either that or, have the programmer standing there so we can tell them what we want.

If the email address was bad, hopefully it was a typo rather than someone trying to break into the refrigeration market by selling "add on" control systems.

[Mike W]

After years of trying to teach programmers how I wanted the refrigeration plants to operate and each time I did a plant I would have to teach a new programmer, I learned how to program myself and since then we have had no problems. We have fitted AB plc,s to numerous screw compressors and plants which have operated for years without any problems. Best thing I ever did.
I make sure as much information as possible is available for servicemen to access through the screens as there is nothing more frustrating than not knowing why the plant has stopped. I work on the premise that the display should tell you what the plant is doing and if stopped, why it is stopped.


Mike W

[NH3LVR]

Mike W;
This sounds like a sane approach.
Perhaps I would be less skeptical of custom control systems if the installation include a dedicated Computer, loaded with the appropriate software, to facilitate trouble shooting.
I have had some experience with Johnson Controls Metasys. Although I did not modify or write the routines, we could change the parameters and effectively troubleshoot. It was also very convenient to have all the HVAC information available in one place, rather than having to travel from one place to another to take readings.

[US Iceman]

I make sure as much information as possible is available for servicemen to access through the screens as there is nothing more frustrating than not knowing why the plant has stopped. I work on the premise that the display should tell you what the plant is doing and if stopped, why it is stopped.

That's a very good approach to use. Controls are essentially black boxes, but the data is extremely helpful in understanding what is going within the system.

[Josip]

Hi, Screw_geek

Hi
It is a plc based control compressor. So you mean to say that Auto load setpoint would be a deadband for me for loading and Autoload disable setpoint would be a deadband for unloading. Lets say

if Autoload setpoint = 100Kpa
& Auto load Disable SP = 150 KPa

So if my suction is above 150Kpa then my loading would be disabled and if it is within 100 go 150 Kpa band, loading would be enabled?
i am really sorry for the confusion i am creating here...this is the first time i am doing screw compressors.

Can you explain in plain english what are your process there and what you have to control. Suction pressure/temperature or discharge pressure/temperature. Limits for both?

At what suction pressure you start your compressor, which one you have to obtain and which one or two (up and down) are limits?

To help we need as much as possible informations.

Best regards, Josip

[hendry]

Guys,

my newly acquired clients also have such problems. Packager from US and Europe love to chuck 'complex' PLC ctrl to Asia along with screw compressor.

it is sort of an 'in' thing which barely give reasonable performance. when being enquired, the packager will chicken out and will never responds to client's questions.

basically, you need someone with both knowledge in compressor and ctrls.

we may be able to help you if you wish in the future.

[nh3wizard]

we may be able to help you if you wish in the future.

I dont think so

[hendry]

I dont think so

only matching key or worm gear will click together...

[eskimko]

Hi all,

we are planning to build one new glycol chiller with J&E HSS 3220 compressor. One cooling circuit, air cooled condenser, S&T evaporator, SET Tglycol = - 7 c, R407 C. Challinge is that we for first time must apply capacity control slide valves. We planned to apply Eliwell ECH 400 SV (screw compressor controller) on which it can be setted up outputs for solenoid valve A (load) and solenoid valve B (unload). Unfortuanlly after evaluation of documentation, I wound that condition for starting up of Hall compressor is that compression chamber must be in minimum load condition, and Eliwell isn't compatible with this requirement. Also Hall is equiped with pressure probe 4-20 mA, which is planned to be used for slide valve position signalling and prevention of starting in load condition. Second thing is that on Eliwell ECH 400 SV there is two parameters related to: 1pening time of compression compartment, and 2 closing time of compression compartment.

Can some of you reccomend me how to overcome this issue.

Thanks in advance

[PaulZ]

Hi eskimko Have a look at the Danfoss EKC 30 controller I have seen it used on screw compressors for loading and unloading. I think they are still made. They have a pulse on loading setting, pulse on unloading and pulse interval time. You can set the required pressure and the can also be connected to a current limiter. They are very reliable. Paul

[eskimko]

I found that this Danfoss type has stoped with production. Any alternative? Do you have manuall for it?

[SteinarN]

I made a ground water heat pump, glycol/water to a puplic institution 2 years ago. It has the second largest Prestcold/Copeland Discus compressor with 50% 75% 100% capasity steps. I wrote a program for a Hitachi EH150 PLC in adition to configurating the operating panel. The PLC calculates the necessarry leaving hot water temperature dependent of the actual outside temperature. I wrote a number of functions like pump down stop, min stop and runtimes, controlfunctions to minimize liquid flodback to the compressor at start up capacity unload at low suction pressures, and so on.

I had no experience with PLC beforehand. Had to study the PLC manual which was some 500 pages as well as the manual on the programming software. It was a LOT of work.

However it is wery satisfactory knowing how to program this PLC and i look forward to the nex PLC project.

Anyone here with experience on Hitachi PLC?

[Josip]

Hi, Eskimko

Welcome to RE forums

Hi all,

we are planning to build one new glycol chiller with J&E HSS 3220 compressor. One cooling circuit, air cooled condenser, S&T evaporator, SET Tglycol = - 7 c, R407 C. Challinge is that we for first time must apply capacity control slide valves. We planned to apply Eliwell ECH 400 SV (screw compressor controller) on which it can be setted up outputs for solenoid valve A (load) and solenoid valve B (unload). Unfortuanlly after evaluation of documentation, I wound that condition for starting up of Hall compressor is that compression chamber must be in minimum load condition, and Eliwell isn't compatible with this requirement. Also Hall is equiped with pressure probe 4-20 mA, which is planned to be used for slide valve position signalling and prevention of starting in load condition. Second thing is that on Eliwell ECH 400 SV there is two parameters related to: 1pening time of compression compartment, and 2 closing time of compression compartment.

Can some of you reccomend me how to overcome this issue.

Thanks in advance

.....you can do that with your Eliwell controller but then you need to convert signal of 4-20mA to one readable for your Eliwell...0-10V/-10-0V...

......EKC 30/31 is out of production, but there are some very good Carel compressor controllers with input signal of 4-20mA....

or very expensive one Unisab II or Unisab III (Made by Sabroe)

Minimum position you can solve with some relays, timers, etc..quite simple

Best regards, Josip

[Casper Dube]

hie every body, i am new to the site and must say am already getting some useful technology here,
am in Zimbabwe and must say we seem to be in touch with the wourld from the questions i have seen.
thanks!!

[zolimer]

I read about a quarter of the thread, so if i am repeating anything, I apologize. plc supervisory programs have some
valuable time and money saving attributes! As far as trouble shooting, you have trending where you can see how any temps, psig's and levels ect.. were at when the system was running fine and look at what had changed since. You can actually view the ladder logic as the program is running and see what should be energized according to the program! As far as money saving, you have internal(software based) timers and counters. And writing the program is based on ladder diagram logic. I took a 60 hour plc course at the community collage and it helped me a lot!