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Old 14-11-2013, 15:04   #16
Marine Service Provider

Join Date: Aug 2004
Location: Ft. Lauderdale
Posts: 702
Re: Understanding our Frigoboat system

Cris, this is a very unusual system that appears to have been designed by a cleaver engineer. The most important advice I can give is any changes to refrigeration system must be carefully planed and any change be simple enough to put back the way it was, if performance is not better.

One freezer picture shows both plates and two control valves side by side. After freezer expansion control valve refrigerant enterers bottom of right plate then leaves at top where it enterers top of left plate. My questions are:
Where is bottom return line going and what does it connect to? That control valve will have a temperature sensing tube where is it clamped to return line?

In conventional theory this system should not work with two off the shelf Thermo Expansion Valves because their full open orifice size is too large for compressor’s full output capacity. With two separate boxes and independent controls the problem is how to have one operate as a refrigerator and the other as a much lower temperature freezer.

In a typical refrigerant cycle there are two pressures High pressure liquid before expansion valve and Low pressure gas vapor after expansion valve. Heat is absorbed during the phase change from liquid to gas vapor and the lower the pressure inside plate’s refrigerant coils the colder that plate will get.

Now for the problem with a single compressor feeding two different temperature boxes with only one liquid source how can low pressures be different in each evaporator?. With the Refrigerator operating at 35 degrees F that plate’s low pressure when plate is starting to freeze will need to be 8 to 10 psi. Whit both thermostats calling for cold and frosted freezer plate’s suction pressure will need to be 4 to 5 psi. If both boxes holding plates return lines are connected to one line returning low pressure to compressor how is it possible to have a different suction in each box’s plates?

If the two different boxes are not operated at different low temperatures/pressures this single compressor will no be able to maintain desired plate freezing temperatures in each box. There are two methods used to avoid this two box one compressor dilemma; One installing and adjusting a low pressure by pass valve or resetting Refrigerator valve to a narrow super heat.

I recommend the following Performance tests using two inexpensive thermometers placing one in each box.:

Test one: Run ONLY refrigerator for more than two days and adjusting thermostat to achieve desired box temperature of 35 to 40 F and do not touch that thermostat again.

Test two with refrigerator turned OFF run freezer three or more days setting thermostat to stabilize and maintain +17 to + 21 degrees F.

Before running the third test I want you to try to change refrigerant flow conditions without making harmful valve adjustments. Find a 8 inch piece of moisture proof tube insulation like the black insulation on tubing in picture and a couple tie raps. I want to confuse the refrigerator expansion valve in refrigerator picture by covering its temperature sensing bulb and line it is clamped to. By increasing the heat transfer between this return line and valves sense tube this may reduce wasted energy and help reduce total running time by lowering both boxes low refrigerant pressure.

Last test is to run system for several days remembering it my take at least three days for system to stabilize with those holding plates frozen and compressor cycling a few times at the same thermostat setting proven to be correct in the first two tests.

After these tests and a close up picture of two valves together in freezer we should be able to comment on your systems performance with the information you collected.

Richard Kollmann is offline   Reply With Quote

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