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Peter_1
20-04-2009, 09:28 AM
Application: heatpump dryer.

Copeland tandemcompressor D88DJ-1200 X, cooling capacity 290 kW (989521 BTU/h or 82,460 TR), refrigerant R404a, 0°C evaporating and 45°C condensing. SH = 6K, SC = 10K

Evaporator divided in 2 sections each 145 kW (494761 BTU/h or 41,230 TR), common HP circuit.
Velocities of 8 m/s (1574 ft/min) for discharge , 0.8 m/s (157.48 ft/min) for liquid, 8 m/s for suction and discharge.

The common discharge must transport 380 kW (290 kW cooling + 90 kW absorbed) (380 kW = 180.051 TR)
This gives me 3 1/8. We now have 2 1/8, so 2.5 too small and 19.2 m/s ( 3779 ft/min)
We have now 3 m tube and 3 elbows which gives me a DP of 0.137 bar (2.01 psi) (for 2 1/8).

Liquid outlet of condenser going to subcooler outside the building is 1 3/8. In this tube, we have 2 phase flow because not everything is condensed yet (evaporator and condenser are behind each other inside the machine (+/- 2 x 2 m) with the compressor between it)
Tube from subcooler to receiver is 1 3/8.

A capacity of 290 kW must be transported in the liquid line gives me 2 5/8.
We now have 1 3/8 at a speed of 3.52 m/s. (692 feet/min)
In this common liquid line an Alco SV of 1 1/8.
Inlet at the receiver over a length of +/- 20 cm in 1 1/8.
3 m (10 inch) tube en 4 bends gives me 0.0838 bar (1.26 psi) drukval for 1 1/8 which is very small.

Suction is now 1 5/8 for a capacity of 145 kW (1 evaporator). Calculation on speed gives me 3 3/8. Speed is now 41 m/s. (8070 fpm) Even a 2 1/8 should still give me 23,4 m/s (4606 fpm)
2 m tube of 1 5/8 and 3 elbows gives me 0.228 bar DP. (3.35 psi)

Speed is extremely high but DP is within limits and the corresponding temperature drop in every line is never more than 1 K.

What do you think about this selection?

Another strange phenomena: suction accumulator of Heatcraft BT36 (standard inlet 2 1/8) in each suction line of 145 kW. A DP over it of 1.9 bar (28 psi) No oil was filled.

Peter_1
23-04-2009, 10:14 PM
Realy nobody has an opinion about this?

nike123
23-04-2009, 10:21 PM
Higher refrigerant speeds, if doesn't affect mechanically components in circle and doesn't produce noise above acceptable level and if pressure loss is in limits could only be beneficial because of oil transport.
I think if all above is acceptable, than your design is OK.

desA
24-04-2009, 03:42 AM
Perhaps an odd question - if this is still at design stage.

Have you thought to split the hp into two parallel hp's, connected in/out by a common manifold. Makes piping easier & is good for maintenance/reliability. If one goes down, then partial capacity is still available.

Peter_1
24-04-2009, 06:57 AM
Machine is running since 2.5 to 3 years and already 5 compressors (60 HP) broken.

desA, I think if one goes down, the common HP still can handle this partial load. Speed is anyhow high to very high

K.R.Iyer
24-04-2009, 10:58 AM
Peter_1,

what type of failures you saw in the semi hermetics? Is it related to absence of oil? Is that the reason you are reviewing tube sizing at this stage?

Peter_1
24-04-2009, 11:23 AM
I was called after the 4th compressor broke as an independent to supervise.

As far as I heard the story: first had a broken motor, second burned motor, relay contacts got welded together.
Then we had also another one with broken pistons. The 4th I saw, there the oil was completely thick black oil and windings were burnt.
The 5th now had an insulation fault to earth but we even didn't saw teh qpot on the windings where the fault was located.
Technician hadn't rinsed the system, no burn-out dryers.

There was originally an Alco valve installed with an orifice 11 (X9144-B11B) and a MOP point of -18°C (XC726 SW40-2B or a freezer application)
Bot were wrong.

Part winding motor, one OCR on the main, then divided to 2 relays, each going to their own PW motor. Motor's aren't protected this way.
So this is in my opinion also a concept fault.

Additional condenser/subcooler on the roof is controlled by a thermostat (bulb placed in the hair between evaporator and condenser) and not controlled by pressure. HP is going till 25 to 25.5 bar (363-370 psi) till fans engage.

For the Swiss manufacturer, this is normal.

K.R.Iyer
24-04-2009, 11:24 AM
Copeland AE bulletin AE1265-R4/1994 suggests to use an external float oil equalisation mechanism for parallel application of 8D model semi-hermetic, due to higher pressure drop across the 8D motor design than their other models.

desA
24-04-2009, 04:08 PM
For the Swiss manufacturer, this is normal.

5 compressors gone in 2-3 years. Certainly not normal.

I'd say that the line velocities are way, way too high, as you've pointed out. They've probably got all types of pressure waves bouncing around the system.

Does the TXV hunt? Surely it must, with all that going on?
:eek:

Peter_1
25-04-2009, 02:35 PM
... They've probably got all types of pressure waves bouncing around the system.

Does the TXV hunt? Surely it must, with all that going on?
:eek:
Once the system is running, the pressures are very stable but flows at very high velocities.
Danfoss recommends for their EEV's (AKV) max 0.5 m/s in the liquid lines bu this is more related to the PWM working principle of these valves. A thermostatic TEV opens more modulating and doesn't bother that much about speed in the liquid line.
Oil return is of course very easy at those speeds.
Is it then better to select your lines for both max speed and max temperature drop (1 or 0.5 K) and choose then between those 2? (choice between max energy efficiency and cheapest lines with an acceptable temperature drop?)

How do you do it in a practical application, for example a supermarket with long lines? There you will go over the 1K temp. drop rule.

desA
25-04-2009, 03:14 PM
^ If you sized according to the ASHRAE recommendations, or Dupont's guidelines, surely that would bring you into a safe(r) operating condition?

What's the cost comparison between adequately-sized piping, versus 5 compressors in 2-3 years?

Peter_1
25-04-2009, 03:24 PM
What's the cost comparison between adequately-sized piping, versus 5 compressors in 2-3 years?
Who's saying anyhow there's a relation between breaking of compressors and line speeds?
I doubt and I'm almost sure both are independent from each other.

It's just analyzing this machine and trying to be critical about the designing of every component in it.
I'm not searching to find faults, I'm just re-designing it somehow to see if everything matches.

The manufacturer doesn't help me at all. I asked what the pressure drop should be for the evaporator coil, the face velocity through the coil, ....

Finding hopefully the cause(s) of these problems must be finally the purpose of my job.

nike123
25-04-2009, 03:27 PM
What's the cost comparison between adequately-sized piping, versus 5 compressors in 2-3 years?

If that is the cause of subsequent compressor failure.
From described, I would not be so sure that high speed is only (or at all) cause of compressor failures.
I think that system should be thoroughly cleaned and new compressor than closely and prolonged monitored for insulation resistance and temperatures of casing and windings.

Peter_1
25-04-2009, 03:47 PM
Nike, our posts crossed each other but we seems to agree somehow about a doubtful relation between line speeds and breaking of compressors.

What I found the first time was a complete clogged evaporator (+/- 6 mm -1/4 inch cm dust on it, completely covered) They looked at the inside and thought it was clean. Because evaporating temperature was very low (no load) technician had replace both TEV's and opened both spindles to increase evaporating pressure. SH was something they never heard of. Opening the valve was for him increasing the evaporating pressure.

Flushing the system?? Never heard of? Burnout dryer?

I advised to install suction filter (with burn-out in it now) and 2 liquid separators. We still have the DP of almost 2 bar over these accumulators.

I also regulated the LP so that it shuts off the compressors at -7°C evaporating and on again at 5°C (air on is 45°C) in case it freezes up.
It acts now also as a protection for a clogged battery.

Advised also to install a DP measurement over the battery and filter but therefore I need to know the DP of a clean battery. It was cleaned after my first visit but I feel some area's with almost no airflow. I had no velocity meter with me when I was there but checked it with paper
The difference between the negative pressure of the 2 access doors (filter section and machine section) is enormous. If you're in the machine while running (machine section between evap. and cond.) , you can't open the door alone from the inside, even if you take a start from 2 m further away.

Advised to install V filters instead of flat filters like you find in kitchen hoods. Flat filters were originally installed. Suction is from (air on comes from sludge)


Short circuiting has in my opinion nothing to do with line speeds and/or clogged batteries or incorrect SH but are remanings of acid carbon of the previous compressor.

I also advised to take oil samples every day in the first week and change as soon it becomes acid. They did it every 2 weeks.

desA
25-04-2009, 04:08 PM
^ Sounds like you've been a real saint. :)

What was going through my mind in terms of the relation of high lines speeds & compressor failures is that as the speeds increase, so do things like gas/liquid waves in the refrigeration circuit. There could conceivably (not certainly) be some case of a wave bouncing around the refrigeration loop, causing internal instability.

I've seen wave effects in heat-pump trials, on moderate speeds, caused by the TXV stroking, for instance. I'd hate to know what happens at very high speeds.

Anyway, this was just a thought based on previous experiences in a different industry - it may, or may not apply here.

US Iceman
25-04-2009, 04:53 PM
Peter,

I have not spent a lot of time carefully reading these posts, but did want to comment on the pipe sizes. The way I look at line speeds is: How much speed (velocity) do I need to return or move the oil at any operating condition? Then, I look for the pressure loss to make sure the operating pressure loss is not greater than the design pressure loss used for selecting the equipment.

The only time I have seen line speed be a factor in breaking things is when the discharge lines of two parallel compressors are piped so that the discharge gas impacts the flow of the other compressor. That will break piping very fast!

Peter_1
25-04-2009, 06:34 PM
desA, everything can help while brainstorming on this subject. Even if you think that it doesn't matter, it can bring others on ideas which can help.
BTW, what do you mean with "Sounds like you've been a real saint"

USIceman, the setup is quite basic, on/off and load is always the same and very stable.
We had in the initial setup a suction only going downwards, no lines going up. And we're evaporating at 0°C (32°F) so oil return shouldn't be such a big problem I suppose .

What design pressure loss should you suggest? I used a pressure drop which gives me not more than 1K temperature drop.

But I got confused with this machine which is seen on speed side far beyond every usual speeds but far within the 1K rule.

As I understood between the lines USIceman, speed is for you not a problem besides the common discharge example and perhaps also noise.

I needed some advise from others so that I don't judge this case wrongly. I already adjusted my vision about the section of the selection of the lines. I will give a reviewed opinion about this.

US Iceman
25-04-2009, 07:10 PM
USIceman, the setup is quite basic, on/off and load is always the same and very stable.
We had in the initial setup a suction only going downwards, no lines going up. And we're evaporating at 0°C (32°F) so oil return shouldn't be such a big problem I suppose .


I agree. If you have vertical or long horizontal pipes then velocity is more important. However, this becomes an issue of balancing the pressure loss with the speed you need to lift or move the oil.



What design pressure loss should you suggest? I used a pressure drop which gives me not more than 1K temperature drop.


That is very reasonable. I almost always use a very low pressure loss for design.



But I got confused with this machine which is seen on speed side far beyond every usual speeds but far within the 1K rule.


Speed could be an issue if the line contains 2-phase liquid and vapor, but for vapor only I do not see a big problem if pressure loss is OK.

High speeds in liquid lines CAN be an issue for quick closing valves which can cause valves and pipes to break!



As I understood between the lines USIceman, speed is for you not a problem besides the common discharge example and perhaps also noise.


That is my opinion Peter based on the comments above.

K.R.Iyer
25-04-2009, 08:48 PM
Peter_1,

I am familiar with the 4 & 6 cylinder SH. Not yet used a 8 cylinder one. But the reasoning should apply here as well. I have few comments:-

1) Each evaporator of 145KW needs a 3 1/8" or 3 3/8" line. I checked with the compressor specs sheet. It matches with the suction size of each D8Dj600X compressor valve.

2) You mentioned the operating condition as 0C evp and 45C cond. Did you measure actual operating pressure of system and convert it to saturation temp (or) refering to design conditions?

3) The 2 1/8" accumulator pressure drop of 28 Psi is high. Could be due to choked oil return orifice inside. Does it have a filter for the oil return orifice? I have not seen Heatcraft brochures nor used one so far. I am wondering about the pressure drop, the compressor motor cap filter will be having, with the poor servicing practice that has been described by you. Did you get to measure the pressure at suction service valve and on the compressor body (suction side)? This DP indicates the status of motor cap filter. A choked filter can cause a havoc inside, without we knowing it, if the LP cutout is connected to suction service valve.

4) Does your D88DJ-1200 use capacity control modules on cylinders (suction blank-off pads and solenoids to be ordered separately and installed)? The compressor operating envelopes are limited at different part load operations. Please check.

5) I notice that the compressor is located between evaporator and condenser i.e. cold air stream. Is that the right location?? The compressor has already seen broken pistons.

6) Dividing from one incoming power supply MCB / switch to contactors is a normal Copeland way for part winding motor. The part wind stators have total 6 thermistors inside connected to an external motor module to protect. Do you know if the 6 thermistors have been used on the stators when re-wound or serviced? (I had a bad experience in a McDonald Mumbai site on a 6 cylinder model).

I feel you need to revamp the whole system including all refrigerant line sizing (check evaporator no. of circuits as well), compressor location, electricals etc. Inspecting and attending a complaint on a compressor located inside the unit this way is a hell of job…

US Iceman
26-04-2009, 12:48 AM
Each evaporator of 145KW needs a 3 1/8" or 3 3/8" line. I checked with the compressor specs sheet. It matches with the suction size of each D8Dj600X compressor valve.


If I might add a comment to this...

The compressor suction valve pipe size is NOT the size you want to use in all instances. The compressor manufacturers provide a pipe size on their valve based on some criteria that would fit most applications economically. The pipe size should be selected for the actual operating conditions and the desired pressure loss in the suction line (or discharge line for the discharge service valve). Then when the pipe approaches the compressor suction valve use a reducer fitting to match the pipe size installed to that of the compressor service valve.



[FONT=Verdana]The 2 1/8" accumulator pressure drop of 28 Psi is high.


28 psi pressure loss in the accumulator!!!!:eek::eek:

I just noticed this from your post and saw that Peter had mentioned this in his initial post. I have to apologize, I did not see that before.

That is too high of a pressure loss. If it has a suction screen on the inside tube connections, the screen is full of junk (or possibly solder/brazing rod melted into it:confused:).

If the evaporating temperature is supposed to 0°C and then you add the suction line pressure loss to the pressure loss of the accumulator the compression ratio on the compressor would be very high (and operating very hot).;)

That could explain the frequent compressor changes (besides any acid content that formed).

desA
26-04-2009, 01:00 AM
BTW, what do you mean with "Sounds like you've been a real saint"


Judging from what you wrote, I think that you've been incredibly helpful to your client. It appears that you've tried your very best to assist him to get to the root cause of the problem & have tried to pursue each & every angle possible.

This kind of service is a rare thing & you are to be commended for your diligence & work ethic.

Hence the choice of word - 'saint'. :D

Peter_1
26-04-2009, 08:06 AM
When I saw the machines the last time, the compressor were completely covered under ice, the complete tandem body and partially the cylinder heads.

I asked the technician if he hadn't noticed this. He regulated the SH on the TEV's to it's strict minimum in an effort to increase the suction pressure of 3.6 bar= -10°C or 52PSI and 14°F for an air on of 45°C/113°F
You feel on your elbow that there's something wrong.
He said that he still had 7K superheat, pressure at compressor (measured on the sump) was -10°C and just ice on the body was 5K for him.

I said he was measuring total SH and not evaporator SH.

I doubt the 1 element burnout filter for a capacity of 145 kW.

Fortunately, we had a schraeder valve at outlet of both evaporators on the valve before each suction accumulator (SA), on the valve after the SA, on the cover of the filterfolder and on the valve after it.
I measured the evaporating pressure of both evaporators (on the valves before SA) and measured 6bar/5°C or (87PSI/41°F) After the SA 4.3 bar/-5°C (62PSI/23°F), after the suction filter with a burnout in it 4.1 bar/-6°C (59PSI/21°F)

I asked him how much oil ha had charged and this was the correct needed quantity. Possibility was a SA full of oil.

What he did in my opinion was completely overfeeding both his evaporators (SH=0K) and filled up completely both its SA's. The injection of the TEV was just enough to let overflow the SA's and to provoke somehow a correct SH on his compressors.

He also told me that he has charged 155 kg(455pound) r404A and that this charge was in the past more than enough to fill the sightglass. No there were still huge gas bubbles in his sightglass but he had to get some more R404a bottles. So, there was just enough liquid coming to the TEV to ensure the SH he saw on that moment. I told him that if he should charge till his sightglass was full that both compressors should fail due to liquid hammer. He didn't understand it any longer.

So I advised him to isolate both SA's and recover them to a cylinder and weight how much he recovered out of each SA. If it was almost the content of the SA, then I was right, if it was far less, then there was a restriction. We asked Heatcraft and there are no mesh filters in the tubes nor somewhere inside the SA's.

Then I said that he needed to regulate for a proper SH of 6K on his evaporator whatever his suction pressure was. He should also check if all the circuits (12 on each coil) were fed correctly. I advised to do this with an IR camera to see if the coil was flooding equally in each circuit.
If they still didn't achieved an evaporating temperature of 0°C(32°F), then they needed to re-clean the coil or they have to consider a new coil.

A temporarily solution (machine can't be stopped for more than 1 day till the holiday's) to avoid costs could be in changing from R404a tot R134a. You only need to replace gas and TEV's. Evaporating pressure will then increase above freezing point of water and coil will not freeze up. Or let run only 1 compressor instead of 2 but they then have to hijack the PLC a little bit.

I have to call the back now how work progressed because they should have done this last Monday.

Peter_1
26-04-2009, 08:16 AM
]...
Each evaporator of 145KW needs a 3 1/8" or 3 3/8" line. I checked with the compressor specs sheet. It matches with the suction size of each D8Dj600X compressor valve.

K.R.Iyer, i follow here USIceman, the valve is for me never a guide to select my lines.


You mentioned the operating condition as 0C evp and 45C cond. Did you measure actual operating pressure of system and convert it to saturation temp (or) refering to design conditions?

I measured pressures and converted them for this forum to temperatures.


The 2 1/8" accumulator pressure drop of 28 Psi is high. Could be due to choked oil return orifice inside. Does it have a filter for the oil return orifice? ...
I am wondering about the pressure drop, the compressor motor cap filter will be having, with the poor servicing practice that has been described by you. Did you get to measure the pressure at suction service valve and on the compressor body (suction side)? This DP indicates the status of motor cap filter. ....

Or SA completely full of refrigerant (temperature on the outside was 5°C) and the small oil return hole is just enough to pass liquid refrigerant with the gass flow to provoke the needed SH on the compressors


Does your D88DJ-1200 use capacity control modules on cylinders (suction blank-off pads and solenoids to be ordered separately and installed)? The compressor operating envelopes are limited at different part load operations.

No cap. control nor part load conditions. Once it runs, it's a very stable process.


I notice that the compressor is located between evaporator and condenser i.e. cold air stream. Is that the right location?? The compressor has already seen broken pistons.

If evaporator SH and total SH is correct, then I don't see any problems in this setup. The airstream has a temperature of 45°C.


Dividing from one incoming power supply MCB / switch to contactors is a normal Copeland way for part winding motor. The part wind stators have total 6 thermistors inside connected to an external motor module to protect. Do you know if the 6 thermistors have been used on the stators when re-wound or serviced?

Have you a Copeland drawing of this? For me, it's an unsafe situation, better 2 MCB on each PWM motor.
But if Copeland recommends this, the who am I to say that this isn't a good solution

US Iceman
26-04-2009, 02:52 PM
I said he was measuring total SH and not evaporator SH.


This is a very important point to remember. Evaporator superheat + Suction line superheat = total superheat @ the compressor.

You can can have the correct superheat at the evaporators and still have high superheat at the compressor.

Thanks for mentioning this Peter. A lot of people forget this.

US Iceman
26-04-2009, 02:56 PM
oops, found another thing I wanted to comment on...



I doubt the 1 element burnout filter for a capacity of 145 kW.


I agree Peter. I suspect someone selected this based on the pipe size and not the refrigeration capacity! This is another one of those components that should NOT be sized on pipe size, but rather the required capacity.

K.R.Iyer
27-04-2009, 07:58 PM
Peter_1,

Attaching one of the electrical diagrams shown in the manual copy that I have (4.5MB). This I downloaded from www.ecopeland.com (http://www.ecopeland.com). You could try there.

I agree with your and US Iceman's view on line sizing - that it should be beased on application capacity. I noticed that the single D8D60J model has an evaporating temperature envelope of (+)5 to (-)35C. The compressor line, as I havve noticed is usually sized for the high end capacity of the enevelope i.e. 5C in this case. As your system evp temp is 0C and as you ahve done exhaustive gas velocity calculations and line sizing, I quickly verified if it fits with the high end evp temp capacity line sizing of compressor. It matched here. However, I do agree with both of your views. Thanks!