Author Topic: Condenser fans, CFM sweetspot ?  (Read 4993 times)


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Condenser fans, CFM sweetspot ?
« on: October 19, 2012, 01:51:21 am »
I did a bunch of searches and have learned a lot, but not to my particular question.

I recently purchased a 1.8 cubic foot fridge with a BD 35f for my class B RV.  It came with a 270 ohm resistor and a 120mm/4.75 fan rated at 71.8 CFM on the condenser.

To hold sub 40f box temps, I found it was running about a ~25% duty cycle in 65 degree ambients.  Then I removed the resistor and it is about a 33% duty cycle at 75f ambients and around 40% in 85f ambients.  The vehicle interior rarely exceeds ambient.

I have since added 3/4" more insulation all around the fridge, and 9/16" more  to the fridge cabinet itselfand 1/8 more on the door behind a stainless steel dorfae replacing theblack plastic..  The condenser fan pulls air from the floor under the fridge, cannot recycle any heated air, and the air is funneled across the compressor and controller before exiting the compartment.

The 71 cfm fan is loud at nearly 35db.  It is a sleeve bearing fan that draws 0.24 amps. 
I saw one Chinese manufactured fridge of the same internal dimensions that come with a 92mm fan rated at 38cfm.

I think 71 CFM might be excessive in my application, especially since I removed the resistor.  I found a 54 CFM fan which draws 0.05 amps at 22 db with a high static pressure capability which interests me greatly.  For general  RV ventilation I use a variable speed computer muffin fan.  33 to 110 cfm, 0.1 amp to 0.4 amps, 17 to 39db which I also have considered for use on the condenser to dial in an acceptable fan noise when attempting to sleep, and dial it up when noise is less important.

While I have enough solar that .2 amps is not going to break the bank, the fan noise is bothersome.

I would enjoy knowing what Condenser fan ratings are for other size fridges/ compressor speeds, and if you agree 71 CFM seems excessive on my small, seeming well performing fridge.



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Re: Condenser fans, CFM sweetspot ?
« Reply #1 on: October 20, 2012, 12:13:06 pm »
Compressor speed and correct fan CFM size will determined System Coefficient Of Performance (SCOP) for the day. The only way to monitor SCOP is in daily amp-hrs consumed. Yes, the higher the fan Cfm the higher the noise DB.


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Re: Condenser fans, CFM sweetspot ?
« Reply #2 on: October 20, 2012, 03:32:15 pm »
I do have an amp hour monitor but I believe there are too many variables  in my situation to determine exactly what Cfm fan and compressor rpm works best without extensive logging and strictly controlled variables. 

Lots of companies now make these computer muffin fans.  It seems to be a very competitive arena for computer nerds and gamers.  The amps consumed for the air moved at what DB level and price point vary very widely, and I personally feel improvements in this area are overlooked.

I was hoping to find a general range of CFM fan speeds vs compressor rpm and box size.  I know the Danfoss controllers can only handle .5 amps output and double that for a second or 2

 I was surprised to see a 71 CFM condenser fan on a 1.8 cubic foot Fridge.  My IR thermometer shows that during box cool down, the copper pipe exiting the compressor is 92.5 degrees.  It then travels in a square in front of the fan and loses ~10 degrees before entering the condenser, and exits the condenser at ambient temperature which was 72.

  I have ordered the 54 CFM fan.  It's ratings are 0.05amp and 22 DB. It has a much narrower exit airflow, and will direct the airflow more  onto the compressor instead of off to the sides.  I  first emailed the manufacturer asking if the compressor vibrations would be detrimental to fan life and they believed it would not.

I'll monitor duty cycle after installation to see if the loss of 17 cfm is negatively effecting performance.  Since the  54 CFM fan only draws 1/5 the amperage of the 71 CFM  fan that will have to be taken into account as well.

I can provide links to fans I've researched but do not know if they are allowed on this forum.


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Re: Condenser fans, CFM sweetspot ?
« Reply #3 on: October 24, 2012, 03:06:42 pm »

I received my new 120 mm/54 CFM / 0.05amp muffin fan yesterday.

  While it was noticeably quieter when running in free air with no restriction on flow, once I mounted it to pull air through the condenser as the original fan was mounted, it got nearly as loud as the  120mm/ 71 CFM/0.24 amp fan that came with the fridge.

I then placed it so it was pushing air into the condenser then across the compressor and controller and it remained  nearly as quiet as when in free air with no restriction, so I mounted it in this orientation.

I then  closely monitored duty cycle with a stopwatch as before and was at first somewhat disappointed to find the compressor was running it's ON Cycle ~ 30 seconds to 1 minute longer than normal at the same ambient temps, but then the OFF cycle lasted significantly longer.

Ultimately the average duty cycle went from 4:20 ON/ 9 to 12 minutes off to ~5:00 ON/  16 to 20 minutes OFF.

I was hoping for no increase in duty cycle with a lower CFM fan, but instead got a  fairly dramatic decrease in duty cycle in the same ambient temps, all  with sub 38f box temperatures and am frankly perplexed by these results.

I keep taking ambient temp readings around the fridge, and then in the fridge with an IR thermometer gun, and timing  the on/ off cycles looking for an error, but the results are consistent.  Somehow a lower/slower  CFM fan mounted to push air through the condenser instead of pull it has reduced  Noise AND the duty cycle.  These results are consistent across all ambient temperatures I have measured duty cycle in so far.

This makes no sense to me.    Even if the fan somehow improved condenser cooling, I would have expected the ON cycle to decrease, and the OFF cycle to remain the same.

 The New fan is quieter, moves less air, and draws significantly less amperage, yet somehow when pushing air into the condenser  has lowered duty cycle from ~33% to ~  25% and as low as 12.5% when ambient temps fell to 58f. 

I am struggling to understand how these results are possible and am doubting my sanity.