Author Topic: Archive 12  (Read 3076 times)

Richard

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Archive 12
« on: May 03, 2012, 10:08:44 am »






Tying it all together    Archive 12
Several years ago when I started my building project, I designed my own refrigeration systems, using Do it Yourself Boat Refrigeration, and Calders book. Due to the size and location of the freezer and refrigerator within the hull, the boxes and holding plates were fabricated and installed prior to completing the interior. The refrigerator has a volume of 3 1/3 cu ft. with a design loss of 1475 Btu per 24 hours. Computations were made using an actual requirement of 2500 Btu. The plate has an effective size of 0.5 cu ft, to hold about 2.5 gal of solution. The evaporator coil is 46 ft of 5/8 tubing. The freezer also has a volume of about 3 1/3 cu ft with a design loss of 2675 Btu per 24 hours. Computations were made using an actual requirement of 4000 Btu. The plate has an effective size of 0.8 cu ft, holding about 4 gal of solution. The evaporator coil is two parallel lengths of 5/8 tubing, each 46 ft long. Both units have identical tube within a tube water cooled condensers 12 ft long. Both units have the large Tecumpsi HC 1000 compressors. Both units have there own low freon, high pressure, and low suction safety switches. Both units have 10 lb. valved receivers for freon containment during system maintenance. All systems will be R12 as more than enough was purchased during initial construction. Secondary, small evaporator coils were included in both plates. The refrigerator has 20 ft of tubing. The freezer has two parallel lengths of tubing, each 16 ft long. The AC sealed compressor is rated at 360 Btu. There will be an accumulator in the suction line. Most of my boat systems are completed: steering, exhaust, plumbing, and hydraulic. The interior is almost complete. Before I start wiring, I want to complete the refrigeration and charge it with nitrogen. I have several questions and know I will have more in the future. 1) On the high side of the compressor, the fitting is flair. Inlet and outlet of the condenser and the receiver are 3/8. Should one reduction be done at the compressor and run 3/8 all the way to the expansion valve? Or reduce where necessary and run as much as possible? 2) Are your recommendations still to use propylene glycol for the plate solution? Still using 14% refrigerator and 40% freezer? If so, can automotive Sierra brand antifreeze be used? It is advertised as being 100% PG with corrosion inhibitors added. 3) Is the AC secondary system going to work properly? Tubing lengths in the plates were designed for them to run in series. Is this going to over cool the refrigerator plate? Will the lettuce freeze? Could a 3-way solenoid be used to bypass the refrigerator when a thermostat said the box was cool enough?

Thermostat
If the thermostat cap tube is touching the plate or tubing it will cause the compressor to cycle too often. I do not know what the solution in your plate is set to freeze at but if it is set to freeze at +10 degrees you wasting energy to drive plate temperature below zero. Holdover eutectic plates are for storing surplus energy and are not as efficient as thin plate evaporator in maintaining a refrigerator box temperature. If the thermostat control is set to 1 on the dial and the plate is still to cold I would replace it.

Calder's Book
Your tubing sizes and compressor are oversized for heat load of the boxes. Holding Plate tubing of 5/8 inch will cause poor oil return on those small plates and the expansion device will cut back on compressor flow so much it will lose its cooling. This seems to be the common mistake when designing a system yourself, copying a proven design is always best. Be sure to install a suction line accumulator and never turn the compressor over 1200 Rpm. Tubing sizes; 13/32 in. hose gas pressure line then 1/2 in. OD line to condenser , 3/8 in. OD line from condenser to expansion valve, line sizes through the rest of system are normally , in. OD, suction hose to compressor ID. Yes, Food Grade Propylene Glycol because it is non toxic. There is a design in the Do It Yourself book for the 110 system; Two boxes, two expansion valves, two thermostats and one Solenoid.

Super Cold Machine
I have an old, but unused SCM-15, S/N 3548, R-12, 7.5 amp compressor unit. Thinking about installing it as a backup machine. Want to convert to MP-39 or R409A. How do I change the oil in the compressor? Read somewhere these compressors have brushes, is this true and if so what kind of lifetime can one expect. Any recommendation on evaporators or anything else? Thanks, Kevin

Update
Richard, Been gone for a few day, here's an update. Four more "bleeds" and symptoms still the same (that's about 12 bleeds total). Still compressor will not start, audible beep. Some more notes: Checked voltage at the control module - 12.3 at startup. In the cycle of no startup, both fans (I have an aux fan to help ventilate the compressor space) still operate, drawing about .5A. When the compressor tries to start both fans kick off. Startup amp draw is about 6A (I disconnected the fans and the amp draw was about the same). When the unit has started, amp draw is 6A with the fans. In a pretty hot box (80F+) the evaporator frosts up about halfway in an hour or so. Doesn't seem to be getting as frosted as before my bleeds. Put my hand atop the compressor in the startup event and there appears to be no mechanical motion in the compressor. This is in contrast to when I had a faulty electronic unit last year and the compressor did actually have some motion before kicking out. Now it appears it is dead still. Is there a way to check power to the actually compressor thru the power cord when the startup is attempted? Or a way to test the electronic module outside of process of elimination? I suspect the module might have seen some pretty high temperatures recently. Thanks Peter


R. Kohlman wrote: [color=blue:c678d35d60][i:c678d35d60] 1a. Problem as you describe it : The WAECO AC/DC BD35 refrigeration runs on both power supplies AC/DC for a while then it will not restart for a long period of time. 1b. You believe it restarts after something cools off. [/color:c678d35d60][/i:c678d35d60] The above questions may be on the right track! I ran this system over the past 3 days. We started with the AC power supply for about 4 hours while at dock and system cooled down to close to desired temperature (4C) I then disconnected from AC (we were heading out cruising), but unit would not start back up on DC and the temperature started to rise. The air surrounding the compressor/module was relatively cool (about 36C on a 31C day), but the compressor itself was quite hot to the touch. I have a fan that helps ventilate the compressor enclosure. I ran this for about 10 minutes, and then the compressor started. The unit ran perfectly on DC for the next 3 days having no trouble keeping the 4 cu.ft box at just above freezing when set about mid way on thermostat. THEORY - After observing this several times, I believe that it may be that heat radiated from the compressor causes the module to overheat and cut out and prevent a restart after one source of power is [b:c678d35d60]switched off[/b:c678d35d60] for a few minutes. The compressor is very close to the module and even although the surrounding area is relatively cool, the fans are no longer running and the module likely heats up. We can probably live with this, but perhaps there should be a thin heat shield bewteen the compressor and the module?

Can not follow instructions.
It does not appear I am going to be able to help you any more, as troubleshooting is a process of elimination and you want to try theories. If Compressor is truly overheating there is a reason and unless corrected the compressor eventually will fail. Did the LED indicate an overload indicating overheating? Did eliminating the AC/DC unit correct an over voltage or under voltage problem? Sorry, but I give up call a Qualified repair person.

Old Compressor
If the compressor does not have a separate electronic module it is not a Danfoss BD brushless compressor. The old SCMs did use a brush type motor and the life of brushes will determine the compressor's life. I have no idea of how many hours these units last but most of them are long gone. Hermetic compressors designed for mineral oil and Freon had their field coil windings protected with a varnish coating. The new oils and refrigerants do attack some coatings. My policy is never change refrigerant or oil type without the compressor manufacturers approval. Using substitute blended refrigerants like 409a is being done on mineral oil compressors but I only recommend it in cool climates where evaporator is higher than compressor.

BD high current draw and Fan question
Now that the refrigerant level has been reduced below the normal level and there is a start problem the Question is, are the fans drawing too much amperage? Disconnect them and connect them to another power source. When there are two fans it normally requires a relay. Checking voltage at module will not discver a boat wiring problem. The only good voltage check is to connect the compressors electronic module direct to a full charged battery. With the refrigerant low as is indicated now by reduced frost the amperage draw should be at 4 amps after unit runs for at least 15 minutes. If amperage is now above 4 there is a good chance the refrigerant is contaminated, requiring it to be evacuated with a good refrigerant vacuum pump. Be sure to use the correct refrigerant, BD2.5 Freon R12, BD3-BD35-BD50 all use 134a.

Fan Amperage
Richard, I've been monitoring the fan amp draw during the "down" cycle between attempted starts. It is pretty steady at .5A. I have also disconnected the fans and the startup amp draw is about the same. Also, the fans both "stop" during the attempted start cycle. Unit amp draw when it has been able to start last time was at 6A (including the fans). In a refrigerant vacuum process, does the dryer have to be replaced? You seem to be pretty clear that the compressor is not the issue here. Should I pursue the control module fix options?


Please disconnect the fans. Eliminate the boats wiring as a problem. Last test, send the module off to Miller to be checked. The filter/dryer does not need to be changed because there if frost present on evaporator when unit is running.