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ayates · 25-02-2018, 15:15

It may be recalled that I posted some actual logged temperature data from a fridge, and questioned the cool down performance. Reference http://www.cruisersforum.com/forums/...on-191763.html

I have spent the past month or so running a number of performance tests on this fridge, utilising both the OEM thermostat and a Stainless Lobster unit.

Most fridge performance discussion is centered around power consumption and the desire for minimisation. While that is important, I was also interested in cool down performance, and the ability to recover to the set point after warm food had been placed in the unit.

The unit is a NovaKool RFU9000 with a BD35 compressor. The OEM thermostat runs the compressor at 3,500 RPM, and the Stainless Lobster has the option of 1,500 RPM or 3,500 RPM. The OEM thermostat is a bulb attached to the evaporator. The unit comes equipped with 1 1/2" poured insulation, and I added an addition 2 1/2" to most of the fridge compartment and some of the freezer compartment. Part of the freezer only had 1" added to due space constraints. The Stainless Lobster has a digital thermostat in the fridge compartment, and a fan to blow over the evaporator.

At the beginning of each test, I placed ~1 gallon of ~145F water in the fridge compartment to simulate the loading of food, and let the unit run untouched for 48 hours. The temperature of the fridge and freezer compartments was recorded with MadgeTech RHTemp101A devices, and the compressor current consumption with a MadgeTech Volt101A and a Bluesea 50A/50mV shunt. The ambient temperature for all tests was 20C.

The results summary table is one of the attachments. Key observations:

The OEM thermostat cycles the compressor during cool down, drastically extending the cool down duration.
The compressor running at 1,500 RPM has poorer cool down performance, but lower power consumption in the steady state.
The freezer temperature is very closely tied to the compressor duty cycle. Slight reductions in duty cycle can result in a large raise of the freezer temperature. This is probably exacerbated by not being able to insulate the freezer compartment as well as the fridge compartment.
My positioning of the Stainless Lobster thermostat results in it being slightly influenced by the evaporator temperature which is colder than the air temperature. i.e. it need to be set a bit lower than the desired cut in/out temperatures.

The optimum configuration appears to be the Stainless Lobster thermostat, configured for permanent boost mode (running at 3,500 RPM). Normally this mode is only triggered when a battery charging voltage is detected.

An ideal situation I think would be the Stainless Lobster thermostat, with a performance mode where it would run the compressor at high RPM when the compartment is well above the set point, but switch to low RPM for keeping it where desired. The owner of the company has provided great support, so I will be suggesting that to him.

Allan.