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frank
15-07-2002, 06:24 PM
I have been asked to take on the service and maintenance of a large Dunham Bush chiller (circa 400kw) that is fairly new - 4/5 years - and operates on R407c.

Had a look at the beast today and noticed one or 2 things that didn't seem right.

The discharge pressure under load was 229psig (about 45C saturation) but the discharge pipe was measured at 84C - some superheat or what! and the customer says that the unit keeps tripping but they don't know why?

The second stage control circuit stat was set at 87C and the sensor is connected to the discharge line about 3" from the pot. The pot is a Hartford vertical screw with part winding start and unloader valves on a sort of sliding solenoid arrangement.

The adjacent machine (same size) is currently out of service because the sliding unloader has released into the screws and seized the pot!!!

Oil is not apparent in the sight level - where has it gone in a sealed machine with no sign of leaks.

I had the machine running and cycling for over 3 hours today and the pressures, temperatures and current draw were all OK. Any one know why (before I do the PH chart) the machine has such a high superheat?? and why it is running so close to the limit of the control stat??:confused:

Andy
15-07-2002, 09:47 PM
Hi, Frank screws normaly have some means of cooling the oil or the discharge gas to cool the oil. i.e liquid injection in the case of small screws or possibly a heat exchanger fed with a cooling fluid in their larger brothers. Normal liquid injection starts at about 55 to 65c dependant on circuit design and the oil used. I personally like to see the oil at less than 75c, lower on a high temp chiller say 55c. If the oil circuit is internal, the oil which cools, lubricates and seals the rotor tips can be checked for temperature from the discharge temp, normally this follows the oil temp or can be slightly higher.
I would say you oil is too hot, this may be aproduct of a cooling fault or simply you are low in oil. Also over-heated oil will carry-over to the low side of the system. Again high oil carry-over and high discharge as recips indicate mechanical wear. I would proceed with caution, what is the run hours on the compressors, do they require their bearing tolerances checked. Also for my information are these screws totaly sealed or semi's where condition can be checked.
Also 229 psig is about 41c on 407c, a least according to my chart.
Regards. Andy.

Frosty
17-07-2002, 03:40 PM
Franky boy

Long time - no speak! R u still taking the pills? Hope so, cos you don't want another stint in there, do you? LOL

Anyway, jokin apart, the discharge superheat you have seen is completely normal, in fact any reading up to perhaps 10% of the HT cut out would be considered normal (pending operating conditions)

The oil level sight glass, is it on the oil seperator or on the body of the screw (bitzer screws have a sight glass on the thrust housing presumably for checking thrust lubrication!)

Speak to you soon Frank

Frosty

Andy
17-07-2002, 07:11 PM
Hi Frank, tell Frosty to keep taking those tablets if he's running his screws with a 85c discharge temp he will need them.
I suppose what Frosty is pointing out screws run warmer than recips due to the oil cooling load being rejected to the discharge gas, if you have not got addtional cooling to cool the discharge/oil you will have high discharge temps. I would personaly not run a screw compressor of that size without suplimentary oil cooling i.e at least liquid injection, it's life will not be prolonged by operating at these temps. With regards to semi bitzer screws on R22 I would consider on a coldstore aplication I had a problem if the discharge was above 75c and on blast freezing if the discharge was above 85c, you application is water chilling, in my book you discharge should not be above 65c. Bitzer fit a INT 69VS to their screws cutting out at 105-110c and this is for aplications using synthetic oil.
I would say it's safe to assume that if additional cooling is not fitted the compressors will have a reduced life.
Regards. Andy.

Frosty
18-07-2002, 02:13 PM
Andy

65 deg c discharge temps???? way to low!! Read the bitzer screw manuals....they recommend a minimum discharge superheat of 30 K to avoid condensation of vapour within the oil tank.

With todays oils, you are better off running the screws warmer rather than colder. Its a common misconception that the life of a screw will be greatly enhanced if you operate with lowish discharge temps...in fact the opposite normally applies!!

To back up my claim - heres a story for you. NH3 screwpack, chilling IMS, utilising liquid injection as the method of oil cooling. System originally set to inject at 50 deg c with a 5 deg diff. We had all sorts of problems....premature wear, low oil level faults (even though the level was ok) Anyway, this went on for a while until we upped the oil injection set-point to 80 deg c (discharge gas temp now 90 deg c) Anyway, to cut a long story short, the low temps we were trying to achieve adversly affected the plant by changing the density of the oil (too much liquid NH3 in the sump!!!) this caused the level floats to sink in the mix...even though the level was fine!! Upping the discharge superheat completely changed the operating charecteristics of the plant. And guess what? no more LL faults and touch wood, we're still on the same machine!

Frosty

Andy
18-07-2002, 07:40 PM
Hi, Frosty, doctors differ patient die!
Would that plant have had an XRV Howden fitted to it with castrol 2293 oil, I think I have it's twin brother over my side of the water. The use of sythetic oil is usually associated with the higher discharge temps, some of this oil is simply not suited to running at low discharge temps. Again with oils, that Castrol 2293 can cause other problems associated with the o-ring material, I will shortly be adding a seal swell additive to a plant fitted with this oil to stop oil and refrigerant leaks caused by the oil re-acting with the o-ring material.
Back to the Bitzer screws, Bitzer will allow their compressors to run at a 140c discharge temp on special aplications provided that a recommended oil mix is used, sounds more like a cookerthan a fridge to me. 30K above condensing rings a bell, this would mean at a 45c condensing at least 75c discharge temp, this would still be a bit lower than a running temp of 85c. I would be inclined to set my HT cut-out at this temp, not run my screw every day at this discharge. Frank what type of oil is in this plant anyway and what do you mean by bring on the second stage at 87c, is this the liquid injection coming in?
Regards. Andy.

frank
19-07-2002, 09:33 PM
Just an update

Managed to get a print out from the Trend control system and it showd<sp> that the reason for the fault was not the high discharge temp sensor but the part load start!

With the gauges attached while introducing some oil into the pot the sysem switched off due to the being satisfied. As the pot came to a standstill the low side gauge went ballistic up to maximum ( gauge f*****- more expense) and I figured that the discharge pressure went diredtly to the low side posibly through the unloader valves?

The sound of the pot trying to start on part winding usually lasted about 3 seconds before the contactor kicked in but this time the pot just "tried and died" - reset required!

As the pot is a vertical hermetic screw it is difficult to determine what is happening.

frank
19-07-2002, 09:41 PM
Andy, Yes the compressor was running hot due to a shortage of oil but I have fixed that and the discharge temp has come down a little (circa 74C).

The machine does not have an oil separater but is equiped with a liquid injection valve for comp cooling.

Andy
20-07-2002, 10:58 AM
Hi, Frank, with screws a non-return is fitted on either the compresor inlet or the compressor outlet, if fitted to the inlet you would have the high pressure in the compressor at cut-out. This would not expalin the failure on part-wind start. Easy way of determining that you have a faulty outet check valve is to fit a gauge on the suction line prior to the compressor pot it'self (might mean fitting a shreider valve). A faulty outlet check valve could easily cause the failed start, it would also cause the compressor to cycle on L.P if it is pumpdown control instead of off at the stat control.
The lack of oil worries me, has the oil been carried over due to mechanical damage or has the system never been topped up after initial commisioning (probably the latter).
Good luck with your fault finding, keep us posted please.
Regards. Andy.

frank
20-07-2002, 06:49 PM
Here's a link to the type of compressor I'm working on http://www.hartfordcompressors.com/medium.htm

It's the bottom picture

D.Evans
23-07-2002, 11:21 PM
Hi Frank.You don't say if the machine is air or water cooled, but most Dunham Bush chillers have liquid injection which is set to come in at around 76C. If the unit is short of charge then you will not get sufficient refrigerant to cool the unit resulting in high discharge temps.
Oil levels are usually not visible in the compressor sight glass, however there only needs to be around two inches of oil in the bottom of the compressor to make the oil sensor or float which is fitted in the older compressors. Vertical screw compressors do not have oil pumps.
I don't know what you mean by running to the limit of the control stat, unless you mean it is drawing max. amps. This could be due to the high head pressure conditions you are experiencing.

frank
24-07-2002, 08:22 PM
Just an update

Been back to site today as I've been in Scotland for 3 days and checked on the operation of the chiller. The second chiller in the circuit has been repaired by the controls man as the fan inverter was kaput! so some pressure was taken of the main unit.

Stopped the chiller via software close to set point and then timed the restart period - 6 minutes! no wonder the pressures were still high.

Had the controls man alter the run on time for the condenser fans the help reduce pressures and to increase the anti recycle timer - will see tomorrow how this has affected the unit.

No sign of an external one way valve on the discharge line

Andy
27-07-2002, 02:05 PM
:) Hi Frank,
The check valve would be a built in thing on a screw such as this, possibly servicible by removing the discharge flange assembly. Also on the built-in thing the bottom of the pot would be the oil sep and oil accumulator built into one.
When retro-fitting a semi-bitzer onto an existing system we usualy fit a 1/4" line between discharge and suction (prior to the oil sep) this allows the oil to boil off any disolved refrigerant and more importantly in your case relieves the discharge pressure load off the screw for current reduced starting.
:) Regards. Andy.

frank
10-08-2002, 04:33 PM
I've spent quite a few hours on this machine these past 2 weeks and finally, it looks like we have solved the problem. Had to get a controls specialist in to help and between us we figured that the soft start module was faulty.

Changed the pcb on the soft start and hey presto! the machine is humming!!

Can't seem to get more that 3deg C across the machine but I guess that is how it was designed.

Anyone want to buy some spare headache pills?

Andy
11-08-2002, 12:13 AM
Hi, Frank:)
Congratulations are in order, worked through the system and isolated the fault, no better feeling.
Regards. Andy:D