5H86 Problems - Unloader Forks and Pins

Hi folks,

I'm a new user, and have recurring issues with Carrier 5H86 HVAC compressors.

In my case, we have 8 cylinder units, and have encountered problems with the unloader forks breaking and/or the cotter pins which hold the pin in place which secures the unloader fork breaking, with the subseqent failure of that connection.

The units are operated in 30 day intervals - we have two trains, A which runs for 30 days, then we shift to the B system and run it for 30 days. We believe we have some slugging going on, and have taken steps to pump down the compressors on shutdown to help reduce the potential.

We have also installed the Carrier oil cooler modification. We are running with Mobil Gargoyle 300 oil.

Questions:
1. Has anyone encountered this issue? (cotter pins falling out / breaking on unloaders)
2. Do you have any suggestions on what we can do to eliminate this issue?

Please advise. We have tried to solve this one without success, and we keep having problems.

5H86User

I think you have gotten the wrong forum, this is an Air Conditioning forum.

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Originally Posted by back2space

I think you have gotten the wrong forum, this is an Air Conditioning forum.

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A carrier 5H86 is an air conditioning compressorsomething like 100 tons. ;)

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You don't provide much detail of the exact application, but from my distant memory of working on these compressors, the unloader yoke and fork assembly is operated by oil from a sliding hydraulic relay. It really is just an arm on a pivot and should not be subject to undue stress in normal operation.

Two things occur to me:

First the obvious one.... have you talked to Carrier? There is a tremendous amount of experience there as this compressor design has been in production in one form or another for over 50 years.

Second, a clue is in the fact that you mention flooding back. If liquid refrigerant is entrained in oil that serves the unloader calliper, don't overlook the hydraulic stress that can result. Any increase in pressure can easily snap a metal connecting rod.

It's just a theory, but I would solve the problems in your evaporator first.



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Although Carriers are not my speciality I have overhauled a few over the years.
I would look at the quality of the cotter pins you are using.
I do not understand how the unloader forks could break. Seeing a picture might be helpful.
Carrier has had some problems with parts quality the last few years. I have seen bad pistons and rings. I assume you are using OEM parts

First off, thanks for the reply. Yes, we are using OEM parts, and we have installed factory new compressors.
When the compressors arrive, we open the heads and reorient the clevis pins so that the heads of the pins are up, and the washer and the cotter pin for retaining the clevis pin are down.

I'm thinking at this point that we have several issues which are beating up the machines. I'd like to get the collective team's view.

Issue 1: low refrigerant velocity. We have a two-stage coil system that is pressure operated rather than temperature operated. Right now, the local REs are suggesting that we move the setpoint up, since there may be fighting between the unloader and the second stage coils (basically causing the machine to load and unload a lot more than it otherwise might. Due to the low velocities, when the second stage coil stages on, we can get a slug of oil / refrigerant back to the unit. Moving up the setpoint of the second stage coil would increase the velocity in the first stage coil to some extent, but it appears that our lines may be oversized.

2. Our heatloads are overstated in our calculations due to conservatisms. As a result, the unit needs to throttle down with the diurnal cycle as the heat load drops off in the room.

3. We think we have leaking valves in the system, which is allowing oil/refrigerant to flood back and cause slugging on the start.

To get around this, here is what we are thinking of doing:

1. Raise the second stage coil setpoint to around 90 pounds. That will keep it out until the air approach temperature hits around 55 F.

2. Manually isolate the compressor refrigerant side. We do pump it down on shutdown, but do not have a pump down circuit (and given the current valve leaks, until they are fixed, a pumpdown circuit would probably hurt our 50 hp driver motor.

3. Try to address long term, the pressure relationship between the first and second stage coils and the compressors by changing from a pressure driven system to a temperature driven system.

We are also considering bringing in a top notch RE who is in the Illinois area to come in as a hired gun, take a clean sheet look at our systems, and provide some recommendations on the actions we need to take to make these systems run properly. This would be a short term contract position, with a fixed cost.

In any case, any suggestions would be appreciated.

5H86User

Your memory is correct - that is how they work.

We have talked extensively with Carrier. They have provided some helpful tips, but we are continuing to have issues, which is why I am here, polling the field technicians who have seen these things, and who hopefully can provide some insights as to suggested results.

I owe you more detail on the exact application. I'll get one of my experts to provide more detailed information. Is there anything in particular that you need?

5H86User

From your description of the operational issue it seems the unloader mechanisms are being worn out due to excessive cycling. If your cooling loads are that erratic or require that precise of temperature control a better option may be to use variable frequency drive on the compressor motors. Then use a pressure transducer to provide a control signal to the VFD.

Start the compressor and let it ramp up to it's minimum speed. Then, as the load increases, speed up the motor. Since the Carlyle unloaders (as standard) are hydraulically operated off of pressure you may need to convert these to electric solenoids, which would then provide additional steps of capacity control in addition to the speed variation. With the VFD and the electric solenoids you have almost limiless steps of capacity control between minimum and maximum speed.

Carlyle has an example of this electric unloading in the compressor manual. And the VFD's are relatively cheap and easy to install.

If the compressor soperate at exceptionally low loads for long periods of time the oil (and some refrigerant) can accumulate in the evaporators due to the low gas velocities exhibited during low load conditions. Then when the compressor suddenly loads up (capacity increases) a combination of lower pressure and higher gas velocity suddenly pull back the content of the evaporators right back to the compressors.

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Originally Posted by 5H86User

...a top notch RE who is in the Illinois area...

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Who might that be, if I can ask?

Know nothing about these machines but do they have / need dampening orifice in oil line to unloaders .
Even if overused due to poor setup , thought they should still last .
If oil on unloaders is not metered it could hammer unload mechanism .

On 5H compressors Carrier recommends 150 viscocity oil

Our application is for office air conditioning. LAT design is 55°F. Two cooling coil stages, 1st stage on at start, 2nd stage on at 63 pisg. On shutdown LLS valves close and compressor continues to operate to pump down to 35 psig and then isolate. During the ideal period the suction and discharge pressures equalize and subsequently on startup we experinece a slug of *****/oil in the first minute (sounds like marbles going through). Our instrumentation apparently does not capture this as it shows 10 -12°F superheatand the SP 55 psig. The machines' condenser is air cooled with pressure control valves to set to maintain a minium 180 psig.

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From the term 'ideal periods' I suppose that you mean the idle or off cycle?

I suspect that you have liquid accumualting in the coils when they are shut down caused by migration when the unit is off and this may explain your liquid dilution problem.

The standard solution to this, if you have not already done so, is to fit a solenoid valve in each evaporator circuit in front of the TEV.
Each of these solenoids should close as the load is satisfied and the final circuit should close its valve allowing the compressor to pump down and clear out the circuit before the compressors shuts down on a low pressure switch, typically set at about 2 – 2.5 bar.
In this fashion you will avoid any liquid refrigerant being able to migrate to the evaporator on the off-cycle.
Liquid slugging is a common problem and this is the standard solution - many packaged chillers come with this feature as standard.
It is necessary to adjust the unloader set points as each evaporator cycles down. It is not an operation that can be hurried.

Next, oil will always circulate through the system and it is a feature of the pipe work design that velocities are maintained at the lowest capacities that will be encountered in operation so that oil is returned to the sump.
If the system runs for prolonged periods at low capacity you may find that the oil cannot return to the compressor.
The symptoms will be a loss of oil pressure at the delivery pump and possible periodic trips of the oil pressure switch. As this unit is unloaded through oil pressure a loss of pressure may give you erratic or poor temperature control.


Your local expert should pick this up if it is a pronlem.

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Quote:

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Originally Posted by 5H86User

...on startup we experinece a slug of *****/oil in the first minute (sounds like marbles going through).

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This can also present a problem with the compressor wear & tear.

It is common to see systems just like yours. The main component of the normal design process is to ensure sufficient capacity is installed to meet the worse case scenario cooling load. Then when the system experiences something other than full load, it has trouble responding to those changes.

Based on what I've read here it seems like you have two issues.

1) poor part load capacity control
2) liquid migration into the evaporators (by some means)

Hello 5H-86 USER the pin lifting forks break very rarely , ive changed the sleeves many a times because the pins wear it away or the end of forks get worn away, if there is liquid being pumped in the hydrulics cyclinders it shouldnt have an affect on centre pin or the forks,because with hydrulic pressure the forks drop at the front and liner becomes active, the only time the forks are under stress is when the compressor is unloaded and the forks are raised pushing the suction reed up. I think may be the aliumium gaskets under the hydrulic mechanism are too thick or the pins and the springs in the the liner are long, putting stress on forks and cotter pin,when you put the unloading liner in the body it is proud but press it down with your hand and see if its flush with top of the crankcase, if not then check the ali gasket and liner pin and springs