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Old 19-03-2009, 20:55   #46
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Final Disposition of Engine Drive System

Today the boat was in port so I removed the old Danfoss TF2 expansion valve with a #1 orifice and replaced it with an Emerson equivilent valve with a #3 orifice. When the plate was cold and equilibrium was reached, the suction pressure was 5.4 inches Hg and the discharge was 106.5 psig. The superheat at the bulb was 20 degrees F and the total superheat at the accumulator outlet was 64 degrees F. The engine was idling with a 6 inch dia. pulley driving a 7 inch pully on the compressor. The expansion valve was at midrange so it could be opened, but I plan to leave it that way for a while to keep from overloading the compressor.

Don
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Old 20-03-2009, 05:53   #47
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Don it sounds like you are back where you started from and nothing has changed there is extremely poor performance but you did not report any frost on return line this time. Suction pressure would indicate evaporator coil inside plate is about -25 degrees F. and plate is frozen solid. New expansion valve now confirms a major restriction in low pressure side of system after TXV or contaminated refrigerant. When you do get system working I would reinstall the old Danfoss valve with #1 orifice and install a smaller engine drive pulley limiting compressor speed.
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Old 20-03-2009, 16:40   #48
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You are right, Richard, I am back about where I started. There is a pressure tap at the inlet to the accumulator, where I can check for a high pressure drop in the accumulator and the suction hose. Perhaps I'll find a restriction there. I cannot see where there could be a restriction in the cold plate or in the uninterupted 1/2" line from evaporator to the accumulator. Do you think there could be an oil dam in the evaporator?
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Old 20-03-2009, 19:06   #49
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Very low suction pressure, very high superheat and no frost on suction line would indicate TXV problem or restriction in plate close to valve, you already changed valve.

Low suction pressure, high superheat and frost on return line there is a restriction somewhere in low pressure side of system or contaminated refrigerant. There should be more frosting after a restriction point.

The Expansion valve is supposed to be the only major restriction in a properly functioning system. I do not see how oil in a standard suction line accumulator could cause a major restriction. Bad solder joints and kinked tubing cause this type of a restrictions.
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Old 02-04-2009, 08:43   #50
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The boat has been out on charter until two days ago and the captain has been happy with the refrigerator performance. Yesterday, I was able to go aboard and found the following:
5" Hg suction, 94 psig discharge, suction line frosting to the accumulator outlet, 0.69 psig pressure drop from accumulator inlet to the compressor suction. I looked for any restrictions in the suction line and found none. The bends in the 1/2" tubing were all long radius and there were no kinks. The only part of the suction line that I could not inspect, of course, was the tubing in the cold plate. The sight glass was full, and the engine was at idle speed (the tach was not functioning). By the way the refrigerant was new and the system was thoroughly vacuumed.
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Old 02-04-2009, 14:39   #51
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A vacuum of 5 inches indicates plate temperature below zero F. or system low on refrigerant or restriction in low pressure side of system.

Frost on suction line accumulator can indicate normal on many engine driven systems but unusual when suction pressure is in a vacuum.

High pressure low at 95 psi unless plates are frozen below zero or seawater is below 70 degrees F or system low on refrigerant.

Are you sure your gauges are correctly calibrated and sight glass is full?
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Old 08-04-2009, 07:35   #52
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Suction Pressure and Measurements

I've checked my gages and they are accurate. The sight glass is full. I have done some research on engine drive systems: This system is closest to the Grunert, but their literature doesn't give normal/abnormal pressures. The Sea Frost engine drive literature does have a series of charts (their system uses a swash plate compressor and no accumulator) and the chart for a properly charged system in warm water with a cold plate shows the suction pressure running in a vacuum after about 45 minutes of operation.

http://www.seafrost.com/ED-R12.pdf
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Old 09-04-2009, 10:20   #53
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ANSWER:

Until you understand refrigerants and how pressure and temperatures affect them you should not tamper with other peoples systems. When a well designed and properly serviced engine driven refrigeration system plateís solution is completely frozen suction pressure may reach a vacuum. For holding plate systems to be efficient its expansion device needs to have a very wide control range like -40 to +50. The exchange of heat energy (BTU) is directly related to refrigerant flow, the higher the suction pressure the higher the flow. As the suction pressure drops to achieve lower temperatures less Btu are exchanged resulting in less work accomplished extending compressor running time.

Seafrostís statement ďsuction pressure running in a vacuum after about 45 minutesĒ is advertising there plate is frozen or almost frozen solid in 45 minutes. Each engine drive system is a one of a kind system because they are icebox conversion units. Experienced service persons or boat operators with gauges needs to understand the high and low pressure ranges for refrigerant used. These pressures relate to systemís performance, if pressures are not where they should be performance will suffer. Suction pressure on standard evaporator systems stabilizes after several minutes to the desired temperature and suction pressure. Suction pressure on engine driven refrigeration will never be stable when lowering plateís solution temperature even after solution is frozen solid, this is why a timer or low pressure safety switch is required. Mayball I believe you have my DIY book go to section on fine tuning engine drive systems, page 157 in latest DIY book. This chart provides temps, pressures and frost line every 10 minutes for a 2 hour of compressor run time on a typical system. Condition and readings will vary with your test but watch all perimeters decrease and suction pressure did not reach zero for 50 minutes.



Refrigerator pre cruise check list for the average boater and no tools required.
  • Donít for get to clean the condenser coil annually on air cooled refrigerators.
  • You donít need a mechanic to check out your boatís 12 volt refrigerator before your cruise. Turn the refrigerator on and after 30 minutes there should be frost covering the evaporator. The system is functioning correctly if there is a thin layer, or more of frost covering the complete surface form top to bottom. An area not covered with frost indicates a loss of refrigerant or a performance problem.
  • Large refrigeration systems in boats generally have holding plates inside the refrigerated box that may take more than an hour to freeze. A touch of a finger on different parts of the system can tell if the system is working OK. On the line just before the plate is an expansion valve, by touching the outlet of this valve performance of the system can be determined. After about ten minutes the temperature of the outlet of the valve should be below +10 degrees F., If you place a finger on the valve outlet at this time your finger will almost stick to the valve indicating the system is probably OK.
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