Parellel Condensing Unit

[JLI]

Hello. I am looking for assistance in a matter that just gets worse as time goes on. The unit is a parrellel compressor unit with 3- 7.5h.p. compressors and 1-10h.p. compressor. 4-3/4 h.p. condensing fans on Penn stats!, and a High Efficiency coil, supplying refrigerant for @ 43 evaps (mid-temp) with different btu loads.

The refrigerant is 404a, the compressors are the ZS series (mid-temp) Copeland Scrolls. It has a Headmaster control, 200 pound reciever, accumulator, with electronic control board. And a few hundred feet of tubing.
The intial charge was @ 240 pounds of 404a, with the suction pressure @ 28psig, the target high pressure of 225, and the high pressure limit of 326. After the unit was installed we starting noticing the evaps. were freezing up.

I went thru and checked the evaps and found the sensing bulbs for the TXV in each evap were either loose or not even fastened to the suction line at all. (poor workmanship) I also noticed the evaps were undersized for each refer they were installed in. This was confirmed by the installer.
I then installed the sensing bulbs in the appropiate positions, and rechecked the boxes, still freezing up. I readjusted the suction pressure up to @ 35 psig to give me a evap temp of @ 8 - 10 degrees and this helped out. I also installed the correct size Evaps in a few of the boxes and this made a big difference. I then started on the Condensing unit to set the unit for low ambient temps and was able to obtain a full site glass by cycling the Fans to control the head pressure and maintained the target pressure as well as maintaining a full site glass.

The compressors were being controlled by the circuit board to cycle at different times to prevent from becoming logged with oil. So far, so good. Now comes the problems. The pressure controls on the compressors were Johnson controls that were sticking, erratic and I asked to obtain some digital pressure controls to eliminate the inconsistant properties of the existing controls. Not working, sometimes working, etc.
I also wanted to eliminate the Penn stats and control the head pressure by running Digital pressure controls for the Fans. With one fan running at nite, (the unit is not under any great demand since some Kitchens are shut down), the head pressure is hard to maintain without controlling the Fan.

My question is, what should the suction pressure be at this point of the game. I have more to ask but this thread is already too long and I don't want to put anyone to sleep.

[taz24]

Hello.



Hello Back.




I have more to ask but this thread is already too long and I don't want to put anyone to sleep.

No probs:) .

Back pressure would be in the region of 3 to 3.5 bar (45 to 50 psi for those on the other side of the pond).

Look forward to hearing about this system.

I would like to know why so many miss matched comps have been used?

taz.

[JLI]

Thanx for reply Taz. The mis-matched compressors are the brainstorm of the Outfit who installed it. I set the suction pressure at 35psig due to: improper evap sizing, and to allow oil return to the oil tank since there is no oil return pump. I understand the maximum setting is @ 38psig suction pressure for this type of unit.

I did set the cut-in for 44 psig and the cut-out @ 35 psig. The real problem has been oil return, and logging of the compressors. This was created by some others who decided to disconnect the compressors from the circuit board thus causing the early demise of one of the compressors, then installing an udersize compressor from the ZR series of the copeland scroll family. This is where I have the "Problems" And just last week, another compressor was ran out of oil and FRIED! I have'nt been able to convince them to quit messing with it due to I work on Grave shift and they are on another shift.

I just rec'd three more Omni-Temp Evaps and was getting ready to install them, when this issue "Popped" up. Seems to be undermining my attempts to rectify the issue with the undersized evaps in the other boxes I haven't gotten to. The other compressor that replaced the previous ZS series, was a low temp Vapor injected model that had the injection port soldered shut. Brilliant, eh? It is a ZR24K when it IS supposed to be a ZS56K compressor.

I hope you can understand this Problem I am experiencing here. Any way, the unit supplies refrigerant to Walk-ins, Beverage coolers, deli-tops for food/condiments, under-range storage, and the temps are all the same,@ 38degrees.
My reason for the settings I have done to this unit is to keep it in a favorable range until i can replace the Evaps in the remaining units. I have inherited a mess.

[The MG Pony]

I'd try the crow bar methode, it works like this: Find the guy that screwes with the system and the incompitant morons that screwed the install, apply crow bar to knees liberaly, then fix the problem in a much better mood now that you shared the pain of fixing with the ones who cuased the problem to begin with!

[taz24]

improper evap sizing, and to allow oil return to the oil tank since there is no oil return pump.

The real problem has been oil return, and logging of the compressors. This was created by some others who decided to disconnect the compressors from the circuit board thus causing the early demise of one of the compressors, then installing an udersize compressor .


Sounds like you've got your work cut out then eh:)

Has the system got an oil seperator in the discharge line after the compressors in the common header?
The realyence on return oil sounds like a nightmare:D
Good luck.

The wrong comps that were fitted were either order'd wrong or very cheap:D .

Can you not connect the controler in the system again?

taz.

[JLI]

Yes, there is an oil seperator prior to dropping thru the roof down @ three stories. I will re-connect the control board after I use the Proper "Crowbar" to remove the "Monkeys with sticks",(seen this on National Geographic), or I took an ON-Line Course and I-R-Tech. mentality. The Original compressors as I said were 3-7.5 h.p. and 1-10h.p. @ 56k per compressor.

[JLI]

Taz, I am going to get the Controllers specs and model tonight when I go to work, for general reference. I stated earlier that the Target Presures were initially Suction-28psig, Head Pressure-225. Last look at the pressures were Suction-50-66 psig, with a head pressure of 350 psig. Oil tank overfull, and they also dumped in an extra 180 pounds of refrigerant.

I am afraid this unit is going to blow as a result of these pressures, overcharging, and incompentence others have placed upon this unit. The 10 h.p. is now placed on line, prior to this the oil tank was empty, and in a matter of a few minutes, it was full. Suspicious, isn't it.

I should really be investing money in Compressor oil stock at this time, I could be making a killing in it.

[lana]

Hi JLI,

What I understand is you have several compressors in parallel. (too much abbreviations :D :D ).

If my understanding is correct, then compressors must be equalized for oil and gas, otherwise you will have a problem with oil management.

I can not say more, otherwise I know the exact problem.

Good Luck.:)

[Peter_1]

Is there some sort of capacity control on the rack, VFD's or compressors shutting down when load decreases?

If there's a capacity control, how does the controller determines what compressor to shut down?
You have 7.5 + 7.5 + 7.5 + 10 HP.
So your smallest step is 7.5 HP, then 10 HP, then 15HP, then 17,5 HP, then 22.5 HP and so on. There must be some logic in it and without a PLC, this is very hard to accomplish this.

Is 7.5 HP no too big for the smallest load or are some cold rooms always heavy loaded?

A better solution could be replacing a 7.5 HP by a 3 HP so that you have a better capacity control. Or replace one with a digital scroll.

The fact that they're freezing up has nothing to do with the fact that the evaporators are undersized, that's anyhow my point of view.
This only means that you're evaporating far too low.

How is oil supply to each compressor done: oil equalizing line or oil floats? You're speaking about an oil tank, then I suppose they're oil float controlled. Is this correct?

Are suction lines inclined towards the compressor?

Installing a capacity control could possible solve something but for the moment, I have not enough to give you some other ideas.
A be
Are you able to make some pictures of the pack/rack?

What's the total evaporator capacity?
Try to translate your numbers also to SI units.

You understand, too much questions for the moment.

[Peter_1]

Re-reading, 28 psig is 1.9 bar, this seems very low for a noon freezer application. Why that low?
I should say 3.5 bar (50 psi)
One possible reason why the oil tank can float very heavy is that the suction line speed isn't correct for a partial small load.
Is there a suction accumulator? If so, is it installed in the right direction by these cowboys?

[JLI]

Hello Peter_1, I typed in a rather lenthy response your inquires yesturday, by I rec'd a Prompt that there was a server error. As to your questions, I will be brief this time. #1- Yes. It has a Com-Trol, 4500 CC Controller. #2- It determines when each compressor runs. It also cycles the compressors at staggard intervals to stop oil slugging. #3- The compressors run one at a time, unless there is a heavy demand on the unit then the 10 h.p. comes in and runs until the pressures have dropped down to the single compressor can keep up. The three 7.5 h.p compresors are cycled and different times to handle the load. At light load the compressors (one at a time) are basicly maintaining a suction pressure. The unit is a Coldzone ETHN-5, with 5 single speed fan motors controlled by Penn Stats. The purpose of the configuration of compressors is for Prolonged Life and Efficiency of the unit. There has been no problems with the Boxes that were put in by another contractor, just the as-builts supplied by the one. This is where the problems began. Yes the Evaps are incorrect for the as-builts, as to the aknowledgement by the contractor himself After a Year had passed. I'll write some more later as the Graveyard shift has tired me. Thanx

[momo]

After wading through twice:confused:, problem 1: consistent oil return(?): do you perhaps need an oil blow back cycle (as used when inverter ac's running slow at low demand need a short oil return boost cycle)
2: suction pressure stability etc see Peter_1' s check list + evaporation pressure valves on problem units ? (or one way valves :the evaps might be interfering with each other or with different flow constrictions) thus allowing a lower suction pressure on main pipes and lower P's all round;
3: meddling fingers... a consistent and tracked policy of modifications, controls and checks is vital, Japanese Katakana chop:mad: can help, but being politically incorrect - management diplomacy and group discussion is only nearest acceptable option.:(
good luck

[Peter_1]

1- Yes. It has a Com-Trol, 4500 CC Controller. #2- It determines when each compressor runs. It also cycles the compressors at staggard intervals to stop oil slugging.

This is not clear for me. Does the logic unit switch on at regular intervals all the evaporators at once so that max speed in the suction lines is achieved?


The purpose of the configuration of compressors is for Prolonged Life and Efficiency of the unit.
Unless I understand you not correct, the purpose should be: maitaining a stable suction pressure.

[Gary]

If I understand correctly, the main problem here is oil return. It isn't easy getting oil to climb up three stories of suction pipe and filling the condenser up with liquid refrigerant isn't the answer.

Describe the suction risers. Does each unit have it's own suction line or do they dump into a common suction line somewhere? Are there intermediate traps in the riser(s)? Are there dual risers?

How about temperature control? Does each unit have it's own thermostat and solenoid or are evaporator pressure regulator (EPR) valves used?

Too many questions and not enough answers at this point.

[JLI]

Peter_1- No the Evaps are all independent. Just like a single refer, with one to three evaps in each box, controlled by one liquid solenoid. These individual Boxes are all at a temp of 38 degrees.
Gary- I suggested the application of EPR's some time back. Each box has it's own stat. And they all run back in one common suction line and liquid line. This application is MID-TEMP. It uses the Copeland ZS56k series of compressor. The line sets are ran all thru the building with no traps.
Peter_1- According to the info. I was given, the configuration of the compressors was for energy efficiency, as not to run any one compressor too long, as well as maintaining a suction pressure. At low load during the night the compressors will cycle at staggard intervals to maintain this suction. Sounds confusing I know. I actually wrote down the cycle on the unit for others to see, but was lost on those who didn't get it.
MoMo- your right about the meddling.
Peter_1- I am trying to maintain an Evap temp of 15 Degrees. Most recently, I just installed another New 1700 btu evap by Omni-Temp and it has its own TXV, Stat. and it doesn't freeze up as the smaller Evaps do. I have noticed the other Omni-Temp evaps are forgiving as far as the issues at the compressor Rack. Anyway, the desired eavp temp is 15 degrees and the suction pressure should be @ 50 psig, correct? I need a 10 psig differential, so set it at 40 cut-out and 50 psig cut-in on the control board/pressure control, correct?
Hey guys, thanks for the input, welcome to my nightmare.

[Peter_1]

Just a quick reply after coming home from a marriage party (1 AM)
Isn't it possible that the evaporators are underfed due to:
lack of gass
too small orifice
to much flashgas (not enough subcool, too small liquid lines)
Haven't re-read the complete post, is SH within 5 to 7 K on each evaporator?
Any possibilty to take pictures of the lines, pack,...
Are the lines correct sloped toward the compressors?
As Gary already asked: correct installed and dimensioned risers (if any)
Are all the evaporators activated together at regular intervals to increase speed to its maximum in the suction line?

[lana]

Dear JLI,

Is it possible to attach a simple drawing which shows the system arrangement? I think this way, the problem will be solved very quickly.
Cheers:)

[JLI]

I just downloaded the driver for my scan/printer, seems the new windows vista wasn't compatible with my printer, and I will try to get a drawing for you. As far as pictures, there is a legal issue here to be careful of.

[Peter_1]

... As far as pictures, there is a legal issue here to be careful of.

What do you mean? That we may not copy your system with a lot of troubles :p :D

[JLI]

No. I work at a place that forbids the act of picture taking. I work at a Casino.

[lana]

No. I work at a place that forbids the act of picture taking. I work at a Casino.

Hi JLI,
I didn't mean that you take a picture :confused: . A simple drawing would help.:)
Cheers

[Sledge]

Hi

I have a couple of questions.

1./ freezing issue.

If the evap is undersized, is the TX sized to match evap? Are the lines (LL & Suct) sized to match the evap, or the evap that should have been? What is the defrost?

If the evap is undersized and the tx is sized to match the existing evap, and the piping is sized to match the evap that should have been, then I am thinking that the evap is not being starved.
This would mean that the lines are oversized compared to the evap. Oversized LL to evaps 30 ft below comps, means solid liquid column, possible hunting of TX but no starving. Oversized Suct compared to evap, means low return velocity, and oil slugging. Neither would cause freezing, I dont think.

If the evap is undersized while the tx and piping are sized to match the evap that should have been, then I am thinking that you would experience hunting and refrigerant slugging, but not freezing.

If the evap is undersized for the application, but the piping and TX are sized to match the evap, you would not be able to cool the space. In this scenario there would be too much load available also preventing freeze-up.

Need more info.

2./ Oil return;

Am I understanding that you have a 30 foot rise on your suction line, without trapping and double risers?

Is there an oil seperator at the compressors?

3./ Compressor run schedule.

I dont understand the concept of sharing the run time of the compressors. This method guarantees that when your equipment starts to experience failures due to age/hard duty/etc then you will be facing a total system failure, because the equipment will all be equally worn.
I believe it is much more effective for the long term ability of your system to provide reliable service, as well as for budgetting, if you subject one compressor to carry the hard duty/longer hours/etc. When that compressor wears out, replace it, and switch the hard duty to one of the older compressors. Run your system with a long term mindset, recognizing that failure is inevitable, and spread the impact of these failures over a longer time period.
The first method will give you a longer initial period of time without major failures, and then you will be ambushed by a series of major troubles.

4./ Slugging at compressors

Is there a suction accumulator.

I am confused about how the run schedule of the compressors will impact liquid slugging? I see these issues as being unrelated.



I agree with what someone else said; You have to have epr's, or you will always have uncertain performance, liquid slugging etc. You have to have some method to return oil at low loads.

[Peter_1]

No problem JLI, where here to help you.
Owww :eek: sorry, i thought you came back with an update for us after all our helping/advices.