Author Topic: Archive 14  (Read 3795 times)

Richard

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Archive 14
« on: May 03, 2012, 10:01:04 am »










Cold machine evaporator tubing   Archive 14
I have a new cold machine with a large vertical evaporator. I'm getting ready to thread the tubing through the box wall. There is a small tube ( 1 mm) wrapped around a larger tube (4 mm). The small tube then connects to a larger tube (6mm) which is soldered to the other tube. The larger (6mm) tube reduces down to about 3 mm after about 1 1/2 inches. My question is: for best efficiency what should be inside the freezer box and what should be outside the box. The run to the compressor is very short so most of the tubing will be coiled outside of the box . Are there any issues that I should be aware of in taking care of this excess tubing. Thank-you. Max

Refrigerator to Freezer
Changing the thermostat in a built in cabinet refrigerator is not as simple as it sounds and another problem will be condenser cooling. Norcold first installs the thermostat and its temperature sensing tube then they fill in the area with foam insulation making it extremely difficult to replace the thermostat assembly. These cabinet refrigerators were intended to be installed in RVs where the process heat is exhausted outside the vehicle. When cabinet refrigerators are installed in boats rarely do the boat builders provide adequate air flow for condensers. In converting a 35 degree refrigerator to a 10 degree freezer not only is the power consumption doubled but so is the condenser heat removal doubled. Before going any further I would recommend you do a couple of tests while the weather is still warm. Put an inexpensive thermometer in bottom of refrigerator for 24 hrs and set thermostat to full cold, then record temperature. Before attempting this test turn off A/C power and be sure there are only two wires connected to the thermostat and do not power up 110 volt system till test is complete. Norcold has used over twenty different electrical wiring layouts on their systems so it is important to know that this thermostat is just a switch. Second test, remove the two wires from thermostat and connect them together and turn on refrigerator for another 24 hrs, and again record temperature. With the two thermostat wires connected the compressor will run continuously for 24 hrs and the box temperature will never get colder than this unless it is running on a cooler ambient temperature day. If you do determine that you can and want to change the thermostat you will need one that has a wide operating range say -15 to + 40 Degree F in order to cover the convertible range you are looking for. When operated as a freezer you may find that the exterior condensation is a problem do to the lack of insulation.

Lowest temp for a small 12 V system?
I have read a few postings on this forum about folks getting their freezer temps as low as -10 F with a 12 volt system. Seems really cold, and I never got that in my own freezer box with the Cool Blue system run by a Danfoss BD-35 compressor, about 2001 vintage. My lowest temp in the freezer box, with the spill-over set-up has been -18 C or about 0 F. The other day I decided to try for the lowest ever: Plugged the 4 spill-over holes with balled up paper towels, set the thermostat to the highest (lowest temp) setting, scraped the 2.5" holding plate of all frost, and let the whole thing run for 24 hours. My recording thermometer indicated -5.6 F for the lowest temp obatined over night: Roughly 78 F cabin temp in the boat. During the day the lowest was -4.0 F in the freezer with a cabin temp of 90 F or so. (36 C) The fridge box was still fairly cool at about 50 F despite the clogged up spill-over holes. The insulation between the boxes are 3/4" foam material with a plastic cover on each side. The total insulation around both boxes are 4 and 3/4" on the sides and 7" to 10" in the bottoms. The lids are factory molded plastic with foam inside, probably 5" thick. Based on the above results, recorded in Florida a few days ago during the hot month of August: I should be able to get a lower temp in January when the ambient temp occasionally goes down to 40 F, but not sure how "cold" I could get the freezer. Question: Is my insulation poor since I can not get the -10 F that other guys are recording? Or is this "normal" and I should be happy with the results? Rime ice, or frost starts forming on the holding plate fairly soon. Is there a food-grade product that will keep the frost at bay?. (Not counting highly poisenous aircraft de-icing fluid.)

System too Small
Ability to reach a low temperature in a freezer depends on two things condensing units capacity at a given evaporator temperature and evaporators size and efficiency. Holding plates are for storing surplus energy and are not efficient heat conductors especially at low temperatures. The BD 35 compressor at 3500 Rpm can deliver 321 Btu of energy an hour with an evaporator temperature of +10 degrees F, this is what the standard thin aluminum plate BD evaporator and capillary tube systems is designed to operate at. It is possible for a BD35 condensing unit to lower the temperature below zero but the compressors output will be much less. Again a BD35 at 3500 Rpm with evaporator temperature at -13 degrees F can only produce 123 Btu per hour. A holdover plate is an evaporator and is very slow to absorb and transfer heat during the phase change of its solution to a solid block of ice. The phase change starts inside the plate around the evaporator coil and slowly progresses through the plate. This slow progression of ice becomes an insulator dropping the capacity (Btu output) of the compressor. In order to freeze a holding plate solid the evaporator coil temperature needs to be 20 degrees colder than the solutions eutectic freeze point. This should explain why you dont have the refrigeration capacity for a low temp freezer unit based on the present daily heat-load of your size box. As to frost on holding plate, you must live with it and when it gets inch thick defrost the plate. Aircraft deicing means removing ice after it forms which is what you do when you defrost your evaporator once a month. Aircraft today are deiced the same way a refrigerator is deiced with a warm solution, the solution they use has a ant-icing effect that lasts until takeoff. In flight all Jet aircraft use anti-icing hot air to transform snow and ice crystals to vapor preventing ice formation, windshields and other areas are anti-iced electrically.


Perhaps this would be a good time to ask about defrosting a thin plate bin type evaperator. I have heard that it should be done slowly as in , turn off reefer, leave door open. Obviously that would work, but why not warm water poured over it so food will stay cold? Is there a proper way to defrost one of these units?


Thanks for the reply Mr. Kollman. Do I understand you correctly that a holding plate freezer will never get as cold as a freezer with an evaporator ? In that case, is my system working well when I can obtain a box temp of -5.6 F? (-20 C) Is the thermostat at MAX (lowest temo) setting trying to reach a low temp, OR is it just ON continously? Been wondering about that because my system never stops when the knob is all the way up. Thanks again for the information.

Frost Removal
billandbeaufort, Methods of evaporator defrosting need to be slow enough to prevent rapid thermal expansion of the metal and refrigerant, a fan or only warm water I believe is safest way to get rid of it. Never try to remove ice or frost with any type scraper.


[quote:5992796cf5] Never try to remove ice or frost with any type scraper. [/quote:5992796cf5] Oops, been scraping my cold plate many, many times with a plastic ice-scraper... :shock: Won't do it again, I promise.. :mrgreen:

Wrong Application
CSY Mon, The Cool Blue with BD35 compressor and large holding plate is an excellent ice box conversion unit but was not the right unit if you wanted a low temperature freezer or even a good high temperature freezer. There is probable only one out of 500 pleasure boat icebox conversion units where the freezing compartments will maintain subzero temperatures. Low temp freezers need a different system designed for a low temperature refrigerant like R502 or one of the new blended replacements, and a holding plate set to freeze a 20 degrees F. It is not possible to have a holding plate maintain a temperature below its solution freeze point. If compressor does not stop when thermostat is set to full cold either the thermostat is not installed correctly or the refrigeration unit can not overcome the boxs heat load at that low of a temperature.


Thanks for the reply. No, I don't need a really low temp freezer, I was just curious to see how low it could get...At least in the summer the answer is -5.6 F. Will check again in six months when cool nights come-a-visiting. My freezer needs are fairly basic, ice cubes, frozen meats for up to 4 weeks, small amounts of ice cream. We do limited cruising these days, 5 or 6 Bahamas trips a year and some local FL sailing. I am very happy with my boxes and compressor/plate combo. If I don't demand much from the system it will only draw 30 to 45 amps every 24 hours. I keep learning stuff everey time I check in to this forum: Thanks again for your answers and comments..

new supercold machine and power plate
I installed a new adler barbour supercold machine with power plate. I can not lower the temperature below 33' F and the unit run continuiously with the analog thermostat set on 3.5. The box is about 8 cubic feet with 2" of foam on the sides and top and 6" of foam on the bottom. I have installed a auxilary fan in the floor to bring cool from the bilge to the compressor and vent the warm air from the compressor out of its compartment. We are located in Jacksonville Fl. Any suggestions on what to look for to lower the temperature would be appreciated.

AB Super Cold Machine
We are the second owners of this older unit that has been working well for the past several years. Now it is running continuous and only lowering the freezer temperature to 23 F max. The unit does not have a electronic module or any LED light. It does have a shroud covered cooling fan on the condenser, a small relay attached to the compressor, a small terminal strip with 6 wires on it and a power plate in the freezer. I have cleaned the condenser by removing the fan and vacuuming, defrosted the unit etc. At 70 F ambient temperature, the compressor had 12.5 volts on it, it was drawing 7.8 amps and the lines were iced up to within two feet of the compressor. The compressor is about 15 feet away from the freezer. With all wires removed, the compressor is showing .6 ohms resistance when measured with a meter. If the compressor is at fault , would it make sense to replace it as opposed to replacing the whole unit? Also what size and make compressor would you recommend ? Do you have any recommendation where to purchase it? Any information you could provide would be much appreciated. Thank you.