Danfoss compressor BD35 RPM? low or high?

[vas]

evening all,

still in lockdown not much I can do, so I'm tackling (silly) details.
Two fridges 80lt each onboard, one in salon (Vertifrigo) with compressor assembly not directly attached to the fridge, so better venting, runs generally better and cooler.
Second in a v.tight spot in the galley (Waeco), much hotter place, struggled to bring fresh air from outside and struggled even more to extract it from the compressor.
Second recently serviced with filter replaced (partially blocked)
Both fridges run on chinese 10euro a pop digital thermostats with thermistor placed at the back of the fridge 1/3 from the freezer compartment.
Both fridges also have a digital thermostat (DS18D20) wired to an arduino logging temp every minute.
VenusOS (Victron) running raspberry pi communicates with the BMV700 battery monitor so power/battery state/amps/etc are also monitored.
Following graph shows all data together:



As you can see on the DC System Power & SOC the Waeco runs at higher amperage for shorter periods of time (and more often due to spot/vent) compared to the Vertifrigo.
Waeco is fitted with a 200Ohm resistor in parallel with a 1kOhm potentiometer so I can alter the RPM of the compressor. This setting now is at say 2000rpm, compared to the Vertifrigo running slower and for longer.
Now, visually it looks like the area "under" each curve is more or less the same, so consumption should be similar which sort of makes sense.

Question time: is it better to run a fridge at higher compressor speed for shorter periods at higher stress, or slower speed for longer periods more relaxed. OK, short is 5min, long is around 10... And all that with 10-12C ambient, I can tell you things change dramatically at 25-30ambient

any scientific opinions or plain experience?

cheers

V.

 

[superheat6k]

Vas If you are using a potential divider circuit to control the speed by voltage drop then you are wasting a considerable amount of battery power through the PD circuit. Your PD resistance is 0 - 167 ohms, so at the lowest setting PD losses = 0, but at say 150 ohms your compressor current needs to be squared then multiplied by the residual resistance to work out the PD loss.

For this reason I would suggest a higher speed run with less PD loss might be more efficient, although generally slower running comprressors do suffer less internal losses.

The key to overall efficiency is keeping the condensing temperature down. Condensing temperature determines the compressor discharge pressure, which in turn determines the compressor amps. Generally a low current fan supplying cooler air over the condenser wil be paid back by a factor of 3 or more from the power saved at the compressor. Introducing a form of seawater cooling will have a tremendous difference, but it is the condenser that needs cooling most rather than the compressor.

 

[vas]

Trevor,
lost you on the first part!
I'm following Danfoss instructions [see attached PDF, p2]
I'm using 101N0210 Danfoss digital controller (I think it's the most common one for all BD35/50 fridges)
So I can use R1 between C and T terminals to alter compressor speed from 2000-3500.
with the 200Ohm in series with a 1K pot (don't think there are 1.5K pots) I'm running between circa 2400-3200rpm.
BTW, I can hear the change of rpm in the compressor note when I crank it up and I can also see the extra 0.8Amp@24V when doing so.
So consumption is in proportion to rpm, nothing strange there and no loss of efficiency

Re cooling the condenser I've seen great difference by channelling fresh air from outside, force feeding it to the compressor/condenser area (they are together at the rear top of the fridge) and a second fan to extract the air from the condenser to the salon (unfortunately) Sea cooling would be nice, but frigoboat seacoolling kit is inxs of 400euro!
May have a look at how water-cooling on gaming PCs work and see if I can modify it for my ask (to cool air before reaching the condenser as even ambient in the summer is inxs 30C at boat deck where the intake is)

cheers

V.

 

[Roberto]

Hello,
have you seen these modules? They look interesting, I am slowly reviewing my boat refrigeration system I wonder if anyone used them; they are sometimes mentioned on US forum by people using their fridges in very high temperature environments, not sure if these boards would better things in milder climates.

http://coastalclimatecontrol.com/images/PDF/Refer/Merlin-II-Instructions-v3.pdf

 

[vas]

yes, I'm aware of that, would be interesting to know how the ramping up and down is done, don't seem too intelligent tbh, but not 125USD worth it for me. I've got all the bits it does (minus the intelligence) for 15euro (digital thermostat, speed variation, led for warning codes)