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Old 28-02-2009, 05:19   #31
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Don, it sounds like you have a native superheat of 30 instead of a positive of 30. A positive superheat of 5 to 12 might be more in line to keep cooling process inside holding plate. You reported suction pressure in a vacuum and frost on return line past suction line accumulator. Assuming there is enough refrigerant in system with this condition at idle rpm what do you think will happen at cruise power. Lowering compressor rpm is not going to solve all these problems.
A normal ˝ to 1 ton compressor engine drive system will have 3/8 lines in liquid side of system and ˝ inch in low pressure side this also includes ˝ inch unrestricted tubing inside holding plates. Are these the line sizes in your system?
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Old 01-03-2009, 01:45   #32
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Richard, The superheat was a positive 30 degrees F, and that was with the TEV adjusted to as far open as it would go. That is why I thought the TEV was too small. The liquid lines are 3/8" and the low pressure lines are all 1/2".
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Old 01-03-2009, 05:17   #33
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Don, The problem I am having is with a superheat of 30 and suction pressure in a vacuum how could there be frost on return line? I would change expansion valve and if conditions do not improve look for a restriction at the point where frost on return line starts.
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Old 01-03-2009, 20:06   #34
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Richard, At about 5" Hg vacuum, R-12 boils at minus 30 degrees F. Add 30 degrees superheat and 0 degrees F will frost the return line. The boat is now out on charter and the captain says the box is colder than ever before but the glass is never clear of bubbles. There may be a leak, but after I fix that we may leave it alone. Still, it seems like it would work even better if the superheat were normal.
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Old 02-03-2009, 05:25   #35
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Don, you have to admit that the information you provided does not represent a system that performs the way it should. Suction pressure is reduces to an extremely low cooling capacity way before any real work is done. Superheat is out of the ballpark. Frost on return line indicates suction pressure reading and superheat reading are incorrect. Sight glass with the correct amount of refrigerant never clears of bubbles and it should with R12 pure refrigerant when plate is frozen.

If the captain is satisfied with the systems performance and boat remains in warm seawater you may luck out.
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Old 02-03-2009, 18:42   #36
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Richard, Yes, I agree with everything you said, especially if you mean by "suction pressure reading and superheat reading are incorrect" that they are not what they should be. I do believe they are accurate. And when the boat returns, I will recommend that all of those things are corrected. I think fixing any leaks and installing a larger TEV will do it.
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Old 08-03-2009, 03:22   #37
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Installing a large tx valve will NOT DO IT,is the compressor sized to the system as in speed capacity.And i will piont out that in oz to don and richard that you are telliing a somebody to brake the law to work on their own system,up to $250000 fine and 6 months in prison or both and in in oz you both would be guilty of of an offence.Greg
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Old 08-03-2009, 15:08   #38
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ozskip, installing a larger TXV will not be a good idea unless present valve's capacity is less than 1/6 ton.
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Old 09-03-2009, 20:59   #39
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Hi ozskip,
You may have missunderstood the train of this thread. The system was pulling into a vacuum with the engine at idle and with the TEV at the full open adjustment. Absent an obstruction in the evaporator or suction line, that indicates the TEV is too small. As I mentioned earlier, I was unable to read the plate on the top of the TEV, so I will need to remove it to see what size it is. I can then check its screen for obstructions.

I think both Richard and I are fully certified and qualified to work on boat refrigerators in our respective countries, but maybe not in grippsland/australia. I sure don't want to go to jail. Who knows if their refrigerators work at all!

Don
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Old 10-03-2009, 13:24   #40
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Hi mayball
I think that ozskip is a little bit worried that all that 12 is is going up in the atmosphere to wreck the ozone layer. We did a lot of that in OZ in the past. But I am sure that as a fully certified and qualified fridgie you would not allow that to happen.
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Old 10-03-2009, 20:14   #41
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Chala: Haven't vented any so far except that which was in 6" of hose. Still the hole doesn't seem to be getting any smaller.

Richard: The TEV is a Danfoss TR2 with a 01 orifice with a capacity of 3600 BTU/HR at 10 degrees F evap. The compressor is a York/Climate Control 10.35 cu. in. delivering 12600 BTU/HR at 2000 RPM. I know the valve is more than 1/6 ton but it does not approach the 1.05 ton output of the compressor, so where can that capacity go except to pull the suction into a vacuum? By the way, the cold box has a strong smell of napthalene. Do you know what that could be. The cold plate is installed upside down and is a FAST FREEZE made in South Hampton, England.
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Old 11-03-2009, 08:20   #42
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Don, If you are still reporting extremely low suction pressure, 5 inches of vacuum at idle Rpm, with frost on accumulator before plate is frozen the problem is caused by one or more of the following:

Not enough refrigerant in system.
Plugged TXV inlet screen..
Controlling gas in TXV sense tube leaked out..
Superheat adjusting screw turned to restrict flow.
A restriction somewhere in system.
Compressor's output is too large for eutectic solution capacity in holding plate.
Little or no liquid in holding plate.


I do not have information on a Danfoss TR2 valve. The valve I recommend for R12 refrigerant is a Danfoss TE2 with an N range -50 to +50 degrees F. For a holding plate under 40 pounds I recommend a 01 orifice in valve. Larger orifices up to a 03 can be used but will increase risk of compressor failure. As I mentioned before these compressors have demonstrated a high failure rate on engine driven refrigeration systems in the past because they were driven at too high an Rpm.
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Old 11-03-2009, 09:24   #43
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Richard,
I made a typo on the TEV: It is a Danfoss TF2, with a 01 orifice for R-12 and -40 to +50 degrees F. They do not sell Danfoss TEVs here, so I am substituting and equivalent Emerson/Alco valve.
Don
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Old 12-03-2009, 20:12   #44
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Richard,
From what you have posted previously, I understand that running an engine drive compressor near its capacity causes a high failure rate. What percentage of the total capacity should be the maximum designed into the system for reliability?
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Old 13-03-2009, 09:34   #45
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Don, The best guess I can give to answer your question is match the compressor output speed/capacity to the holding plate’s size and thermo efficiency say compressor limited to 1600 Rpm at max cruise engine Rpm with single plate under 40 pounds.

An engine drive refrigeration compressor depends on refrigerant and oil flow circulation for lubrication and compressor cooling. The higher the suction pressure (Back Pressure) the better compressor cooling and lubrication. Automotive air conditioning compressors like Climate control/York, Tecumseh, Sankyo and other swash plate models were designed for High Back Pressure (HBP) operation where suction pressures are always above 26 psi. These compressors as HBP units for air condition could deliver 12,000 to 24000 Btu per hour. The problem is when a HBP designed compressor is installed in a LBP system application with holding plate refrigerant and oil flow is at a minimum. Most pleasure boat engine drive refrigerators have thermally inefficient holding plates containing from 2.5 to 5 gallons of eutectic solution. When the thermo expansion valve senses compressor capacity several times greater than evaporator coil in plate can handle it will reduce refrigerant flow through plate coil by lowering suction pressure. Another problem that must be controlled is liquid slugging on compressor start ups, when refrigerant and oil liquid has migrated into holding plates during daily compressor off cycles.

Engine drive refrigeration became popular in the 1970s on large charter boat fleets using systems from the original Crosby and Grunert companies. Their good compressor reliability seems to have resulted from good mechanical engineering, use of cast iron Tecumseh compressor, more plate thermo capacity and slower engine Rpm. In the 80s and 90s with the absence of Mr. Crosby and Mr. Grunert along with a switch to the aluminum York compressor failures of compressors were on an increase. With the newer generation of higher Rpm lighter engines no one focused on reducing compressor speeds.
The feeling was that if the automobile could run these compressors at 6000 Rpm then 4000 to 5000 using a seven inch pulley on a Yanmar engine is OK. Charter boat companies soon learned to instruct boat captains to engage and run engine driven refrigeration compressors at slow engine speed helping to reduce compressor failures.

After tracking engine driven refrigeration compressor failures for 25 years I believe that a compressor can out last the boat’s serviceable life by limiting compressor speed and limiting TXV orifice size to match thermo capacity of holding plates. Suction line accumulators reduce liquid return to compressor but reduction of maximum TXV orifice size cuts back on start up mass liquid flow from plate to compressor doubling the safety factor.
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