PDA

View Full Version : An issue about Digital Scroll Compressors







Icewine66
16-11-2010, 04:41 AM
Hello Everyone,

I have a question about Digital Scroll Compressors. As we know that a Digital Scroll operates in two stages - the "loaded state", when the solenoid valve is normally closed and "unloaded state", when the solenoid valve is open. During the loaded state the compressor operates like a standard Scroll and delivers full capacity and mass flow. However, during the unloaded state, there is no capacity and no mass flow through the compressor. The capacity reduction is achieved by modulating the solenoid with a PWM controller. The PWM cycle is 20 seconds. In other words, if we want to have 50% capacity, in each cycle, the compressor will be in loaded state for 10 seconds and in unloaded state for the next 10 seconds. As the compressor is on/off in every 10 seconds, I wonder that both the discharge pressure and the suction pressure will fluctuate greatly. The system always does not work in a stable condition and it is almost impossible for the EXV to maintain a stable superheat. Is this the way a Digital Compressor works? Are there any ways to make the system pressures stable?

Any comments and suggestions are welcome. Thanks.

NoNickName
16-11-2010, 10:41 AM
I've always been against the digital scroll, which is an extremely poor technology.

Don't use it, and divide the capacity over two smaller compressors.

sendhilkmar
16-11-2010, 11:39 AM
I have seen 21HP System whereas 3 Scrolls of (1 Digital+2 regular) were working . The pressures at the out let of liquid receiver and compressor suction line seems to be almost stable. I welcome any comments

Icewine66
16-11-2010, 12:23 PM
Hello NoNickName,
Thank you for your input. I used to discuss this issue with a Copeland engineer. They developed digital scrolls for supermarket applications. It works well in parallel systems with multi-compressors. Each compressor only has 10% to 20% contribution to the total mass flow. So the system pressure variation is not significant when one compressor is modulating. My question is: is this a good option for capacity control when there is only one compressor in the system? Please share experiences with me if anyone successfully designed a system with a single digital scroll for capacity control.
Thanks!

Icewine66
16-11-2010, 12:36 PM
Hello sendkilkmar,
What you talked (1 Digital + several regular) is a typical way to use digital scrolls. As the digital only generates 33% of the total mass flow, the pressure does not change significantly when the digital is modulating. But I am afraid it is a totally different story if there is only one compressor in the system.

NoNickName
16-11-2010, 03:04 PM
My question is: is this a good option for capacity control when there is only one compressor in the system?

Liebert used to install Digital scrolls on close control units, together with electronic expansion valves, one compressor per circuit.
Besides, server rooms have a relatively stable heat load all over the year and along the day, and their heat load matches the CCU capacity for the greatest part.
In my view, it was more of a marketing hype, than actually an energy saving device.

Kalmar
31-12-2010, 03:50 PM
Hello Everyone,

.... As the compressor is on/off in every 10 seconds, I wonder that both the discharge pressure and the suction pressure will fluctuate greatly. The system always does not work in a stable condition and it is almost impossible for the EXV to maintain a stable superheat. Is this the way a Digital Compressor works? Are there any ways to make the system pressures stable?

Any comments and suggestions are welcome. Thanks.




Suction pressure is fluctuating, of course, grade of fluctuation depends on system size (charge amount, load fluctuation, etc.)
Standard TXV's are not suitable for use, especially standard models, although some Alco valves are approved for use with Digital. Critical use of TXV is with PHE with small system charge (EXV is recomended).

We have a great experience (up to 12 evaps. on one compressor, restaurant application and it works fine).

If You have fast load changes, You can decrease modulation time to less than 20 seconds and than compressor will react faster to load changes, as simple as that...

Kalmar
31-12-2010, 04:14 PM
I've always been against the digital scroll, which is an extremely poor technology.

Don't use it, and divide the capacity over two smaller compressors.

...and enjoy poor modulation and short cycling :D

NoNickName
31-12-2010, 05:26 PM
Poor modulation and short cycling are well known issues that have already been addressed and solved at a varying degree of success.
Despite, IMO digital scroll technology doesn't appear to have been widely accepted nor any proven energy saving figure was published to sustain it. The rapid gain of acceptance of inverter driven compressors quickly displaced the digital scroll.

al
31-12-2010, 05:46 PM
NNN

Rather than the digital scroll would a VSD work with a scroll compressor?

alec

Kalmar
31-12-2010, 07:34 PM
You said:
"Poor modulation and short cycling are well known issues that have already been addressed and solved at a varying degree of success. "

Two compressors 50%+50% against Digital? How? Give some details...

"Despite, IMO digital scroll technology doesn't appear to have been widely accepted nor any proven energy saving figure was published to sustain it."

What about energy efficiency of inverter over the whole modulation range, AFAIK not more than 0,92 in whole range of modulation. Don't forget that Digital works with full COP in loaded state (no losses at all). In unloaded state consumption is aprox. 8% of nominal, so if load is in range 50-100% of nominal, Digital is more efficient. This figure proves it, doesn't it?
Inverter goes to aprox. 35% of nominal capacity (not below), but Digital goes to 10% what gives much wider modulation range (OK, with low efficiency, but no short cycling and with better evap. pressure control in low load conditions).

BTW, what about first investment cost, reliability and simplicity of VSD vs. Digital?

NoNickName
01-01-2011, 01:37 AM
Two compressors 50%+50% against Digital? How? Give some details...

That is one of the ways. There are others, like asymmetric capacity 33%+66% or multiscroll.



This figure proves it, doesn't it?


No it doesn't. If anything it disproves it.
First of all the capital costs is increased by the pratically compulsory use of a EXV, otherwise superheating is uncontrolled.
In an uncontrolled SSH situation, the energy consumption and the risk of slugging are high.
And whenever SSH is controlled, the condenser may or may not be able to control subcooling properly while compressor is modulating.
Additionally, costly solid state relays are required to be able to reach the million-high cycles of on/off. So far, few microprocessors have SSR onboard.

Again, it was a good idea, poorly engineered, and it failed miserably. Copeland itself is no longer marketing as aggressively as it did few years ago.

Kalmar
01-01-2011, 10:36 AM
That is one of the ways. There are others, like asymmetric capacity 33%+66% or multiscroll.

Yeah, right, three steps instead of two...much better modulation :)

Did You ever looked on gauges (Power Pack vs. Digital) and compared evap. pressure fluctuations?

Most of Power Pack controllers are more costly than digital controllers AND with multiscroll You need oil management (passive or active) AND You have bigger footprint, more material needed for frame, and, and, and...Rubish arguments.

No it doesn't. If anything it disproves it.

You don't understand or You don't want to understand...prove that I am wrong!

First of all the capital costs is increased by the pratically compulsory use of a EXV, otherwise superheating is uncontrolled.

Again, it is not compulsory, but there are limits! We don't use EEV's on larger systems with numerous evaps...there is NO need for EEV's, unless system is with low charge (in REF, very rare situation). What is wrong to use EEV? EEV has his own advantages vs. TXV (with or without Digital, doesn't matter) and gives energy savings (ask Danfoss guys for figures from field testing).

In an uncontrolled SSH situation, the energy consumption and the risk of slugging are high.
And whenever SSH is controlled, the condenser may or may not be able to control subcooling properly while compressor is modulating.

Completely wrong!
1.) Ideal system is with 0K (ZERO KELVIN) SSH. Better use of evaporator surface, lower discharge temperature, etc.
2.) Compared to piston/screw compressors, Scroll has no problems with liquid slugging (at least no mechanical damage caused by liquid refrigerant, although lubrification problem may occur if slugging is constant and intensive)
3.) First of all, condenser is not controlling anything! Second, subcooling on condenser is NOT controlled. Condensing pressure is controlled, by external device (only in winter to keep it on minimum level, required by EXV technology). Othervise, keep it as low as possible...

Rule of the thumb: 1K lower condensing pressure gives 3-4% better COP (same is with evap. pressure). E.g. take Solkane software (it is free), put conditions and You will see, or take basics of thermodinamics...

Additionally, costly solid state relays are required to be able to reach the million-high cycles of on/off. So far, few microprocessors have SSR onboard.

You need only one SSR (and I estimate additional cost on 80-100 EUR, lot of money, compared to price of Inverter?). Just look on new controllers, presented in last months (Wurm, Dixell, Carel, etc.). You have wide choice! For Your proposal You need Power Pack controller + Inverter. More costly for sure!

Again, it was a good idea, poorly engineered, and it failed miserably. Copeland itself is no longer marketing as aggressively as it did few years ago.

Again wrong. I have seen announcements (on Chillventa) for new models (both for REF and AC applications), works fine and what is important for us: price competitive & reliable & simple. We are satisfied and selling more and more. No need for marketing costs anymore, people understands, at least majority ;)

What is Your experience? How many comparisons You have made? I did my own calculations (technical and commercial) and I proved it on the field.

Happy New Year to all of You!

NoNickName
01-01-2011, 11:07 AM
I'm happy you are happy. I'm not trying to convince you. I'm already convinced.
As I'm already convinced that I will never buy a microprocessor from a company who started up producing electric saws (Wurm) and turned to electronic engineering.

ArupMajumdar
17-01-2011, 09:01 AM
There have been several questions raised in the forum thread but let me first try to answer the first question on "fluctuation in suction/discharge pressue during loading/unloading". There is a fluctuation in both pressures and it is primarily dependent on: amount of refrigerant charge in the system and cycle time. More refrigerant charge = more thermal inertia = less fluctuation. Shorter cycle time = less fluctuation. There is a range of cycle time that is used (from 12 seconds to 20 seconds) depending on the % modulation to optimize EER performance. Also, most manufacturers use liquid receivers that stablize this fluctuation and so the liquid entry to the expansion device (EXV or TXV) is at a fairly constant pressure.

Third_techtrio
22-01-2011, 01:25 PM
Hello Everyone,

I have a question about Digital Scroll Compressors. As we know that a Digital Scroll operates in two stages - the "loaded state", when the solenoid valve is normally closed and "unloaded state", when the solenoid valve is open. During the loaded state the compressor operates like a standard Scroll and delivers full capacity and mass flow. However, during the unloaded state, there is no capacity and no mass flow through the compressor. The capacity reduction is achieved by modulating the solenoid with a PWM controller. The PWM cycle is 20 seconds. In other words, if we want to have 50% capacity, in each cycle, the compressor will be in loaded state for 10 seconds and in unloaded state for the next 10 seconds. As the compressor is on/off in every 10 seconds, I wonder that both the discharge pressure and the suction pressure will fluctuate greatly. The system always does not work in a stable condition and it is almost impossible for the EXV to maintain a stable superheat. Is this the way a Digital Compressor works? Are there any ways to make the system pressures stable?

Any comments and suggestions are welcome. Thanks.



it would be best to understand first how the digital scroll accomplish the unloading and loading stages. as i recall, the compressor doesn't operate at unloading stage for long period of time so it does not create a problem on its operation such as the mass flow, super heat, txv and etc.. i think but not sure i remember it well that the unloading stage happen in the span of 10seconds, for example at 50% load, the solenoid that modulates the unloading closes at 1 second and opens after a second and this repeats at 10 seconds span.. another example if the load is 80%, then the solenoid on loaded operation for 4seconds and unload 1second, load again for 4seconds and 1second unload.. so for the span of 10seconds the digital scroll operates at 80% load..
pls, correct if i'm wrong

Kalmar
22-01-2011, 03:51 PM
.. i think but not sure i remember it well that the unloading stage happen in the span of 10seconds, for example at 50% load, the solenoid that modulates the unloading closes at 1 second and opens after a second and this repeats at 10 seconds span.. another example if the load is 80%, then the solenoid on loaded operation for 4seconds and unload 1second, load again for 4seconds and 1second unload.. so for the span of 10seconds the digital scroll operates at 80% load..
pls, correct if i'm wrong

Wrong.

See parameter list of one of controllers available on the market, e.g. Alco Controls EC2-552 (document is available on Web, sorry I am not yet allowed to send links:mad:), I do believe other controllers are working on simmilar principle:

...page 3, parameters:

Parameter F2: minimum output value 10-100% (You are setting minimum output, can be any value between 10%, and 100%). 100% means that compressor will operate as standard Scroll (no modulation at all), 10% means widest range of modulation (10-100%).

Parameter F3: PWM rate: 10-20 sec - fixed period of time for modulation, standard setting is 20 seconds. If You need faster reaction, set should go towards 10 sec.

Controller reads suction pressure and determins loaded state for next period of PWM rate, according to suction pressure value and trend.

Over the PWM rate there is only one loaded and one unloaded state.

Examples:
1.) F3 PWM rate: 20sec
If loaded state (over last 20sec) was 13sec, then capacity was 13/20=65% of nominal capacity

2.) F3 PWM rate: 15sec (for faster reaction)
If loaded state (over last 15sec) was 13sec, then capacity was 13/15=86,7% of nominal capacity

Life expectancy is 40 Mio cycles, what makes aprox. 25 years (if PW rate is 20 sec) being in constant operation.

This is valid for Ref applications. I have no proven experience with A/C applications.

ArupMajumdar
24-01-2011, 05:21 AM
Kalmar has explained the concept of the cycle time nicely. Just to reiterate, let us assume that the cycle time is 20 seconds. If the demand requirement is 20% capacity - the compressor will load for 4 seconds (work like a regular fixed capacity compressor) and unload for 16 seconds (the scrolls will disengage and there will be no mass flow through scrolls). At the end of the 16 seconds, the scrolls will load again and pumping will resume for 4 seconds.

In simple a/c applications, pressure transducers are not used. A thermistor is used to sense suction temperatures which in turn does simple superheat control and cooling demand calculation.

Asian OEMs use Digital Scroll in large numbers for VRF multi evaporator applications. If anyone has any questions on such applications, please let me know. Thanks.

Super mario
13-02-2011, 08:39 PM
Dear Guys,

I have just read all the comments about digital scroll, the good and the bad. I have worked directly with copeland about digital scroll into vrv systems. Now the most logicall and best concept of a digital scroll system in a vrv unit is that when you have 130% of indoor capacity working off a 100% capacity Condensor Unit it is a lot, but when you are using only 10% of the outdoor capacity on the indoors it is only consuming 10% of capacity, not like a vrv DC Inverter. whether you are using 10 % it is still consuming 35% of the actual outdoor unit capacity. And in my experience if the installation has been done properly by the book, you wouldn't even need to worry about the super heat and the evaportation of the liquid. That is for the Inventor to worry about, if you are an engineer or a sales guy all you need to worry about that the system is working fine and it will give a good 10 years trouble free. It is not dependent on any 5 different pcb one controlling the other and depending one each other and when one goes you might need to change nearly half of them or all or sometimes it would be cheaper to change the whole unit after not more than 4 years. And believe me I know what I am talking about , and explain to the customer that his investment that cost him thousands only lasted 4 to 5 years. because if you never have met this case before I have met tham nearly day in day out on all brands starting from Sanyo, to Daikin to Mitsubishi Heavy etc.. Lets get real and admit that dc inverter System apart that they are expensive they are incontrollable when a pcb goes out , that might even lead to a burnt compressor.

littleyapper
15-02-2011, 06:41 PM
heres my few thoughts on above :
digital scroll(even though pure mechanical) are fine in a multi comp system they help fill in the steps quite smoothly.and should maintain suction press pretty stable
over rated and over hyped:
have been told that they are not designed for a one to one system:
have been told also that they work better with a stepper eev as against a pulse eev... i think this is internal sales /marketing.
a scroll with an inverter is not much better as you are limited to top and bottom hz..

al
15-02-2011, 07:08 PM
So from a retrofit point of view, 2 compressor "pack" with prepacks on EEV's (pulse), is it worth exploring replacing one compressor with a digital unit? Is it possible to adapt a standard scroll for this purpose?

Alec

littleyapper
25-02-2011, 03:06 PM
sorry al not possible to retro fit a standard scroll into a digital....
yes it is possible to put a digital instead of 2 comps but i would stick to two comp....if nothing else a little piece of mind