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Old 28-06-2014, 20:01   #1
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Danfoss bd35 compressor economy mode

I'm currently looking at our power usage and although pretty good any savings are a good thing. I'm interested in what difference the danfoss bd35 economy mode makes. There is a switch on the outside of the fridge that will change the speed of the compressor from 3300 rpm down to as low as 2000. According to what I can find it drops the power usage from 44 watts to 35 watts or there abouts. Sounds good but if it runs longer at 35 watts to achieve the same cooling then where is the savings. My question is has anyone done a test to see if economy mode is worth it, or does it all even out? I can maybe understand when trying to pull the temp down or the outside temp is high then high speed looks the go, but if it's at temp and cool like overnight maybe economy mode may make sense, or is it better to just leave it on high or low all the time?

I'm probably over thinking this but before I go putting meters on things to see the difference has anyone done it already?


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Old 28-06-2014, 20:05   #2
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Re: Danfoss bd35 compressor economy mode

You may find the answer at Richard Kollman's web site.
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Old 29-06-2014, 01:06   #3
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Re: Danfoss bd35 compressor economy mode

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Originally Posted by Dod42 View Post
I'm currently looking at our power usage and although pretty good any savings are a good thing. I'm interested in what difference the danfoss bd35 economy mode makes. There is a switch on the outside of the fridge that will change the speed of the compressor from 3300 rpm down to as low as 2000. According to what I can find it drops the power usage from 44 watts to 35 watts or there abouts. Sounds good but if it runs longer at 35 watts to achieve the same cooling then where is the savings. My question is has anyone done a test to see if economy mode is worth it, or does it all even out? I can maybe understand when trying to pull the temp down or the outside temp is high then high speed looks the go, but if it's at temp and cool like overnight maybe economy mode may make sense, or is it better to just leave it on high or low all the time?

I'm probably over thinking this but before I go putting meters on things to see the difference has anyone done it already?


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I had a very brief look at that, though to be honest it wasn't really enough to be sure to any degree but looked like running at lower revs might save a touch of power.

Refrigerator Thermostat Replacement
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Old 29-06-2014, 04:45   #4
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Re: Danfoss bd35 compressor economy mode

I doubt if anyone has any hard data to prove that the lower rpm results in lower power. But it is a bit like global warming- the theory is sound and is supported by a little data.

Here is the theory: When a compressor runs at high speed it pumps a lot of Freon. This Freon must be condensed in the condenser, but with high volumes of Freon the condenser temperature goes up which causes the pressure to go up which uses more amps.

At slower speed the compressor is putting out less volume so the condenser operates at a lower temperature so it uses fewer amps.

So the theory is that yes the compressor might run a little longer at slow speed but the increased efficiency means it won't run as long as the ratio of rpms might indicate.

Some systems use a microprocessor control module to manage the rpms automatically.

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Old 29-06-2014, 09:22   #5
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Re: Danfoss bd35 compressor economy mode

Not to add to the unanswered question pile, But would the lower power setting also lower the startup power spike every time the compressor kicks in?

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Old 29-06-2014, 14:02   #6
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Re: Danfoss bd35 compressor economy mode

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Not to add to the unanswered question pile, But would the lower power setting also lower the startup power spike every time the compressor kicks in?

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NOT an expert on this, but from a problem I had I believe the answer is no; the startup spike would be the same.

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Old 29-06-2014, 14:12   #7
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Re: Danfoss bd35 compressor economy mode

There is no lower power setting on these compressors. There is the ability to change the speed the compressor runs at.

A lower speed will not reduce the starting power spike, but if you are noticing this happening, and it is a problem, you have either an electrical supply problem or a compressor problem.

The lower speeds use less current while running, but run for longer periods of time. The purpose of matching the compressor speed to the box and evaporator is to remove heat as efficiently as possible.

For any given box and evaporator, heat can only be removed at a set rate. If the compressor runs at too high of speed, you will be wasting capacity and power. Too low of speed, and you will not remove heat fast enough and the compressor will run continuously.

When the compressor speed is matched to the box and evaporator (and environmental and usage conditions), then the system will be as power efficient as possible. Running lower or higher speeds than this will not help.

Richard Kollman is very thorough about this in his books and website.

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Old 03-07-2014, 03:41   #8
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Re: Danfoss bd35 compressor economy mode

Ev45Econ.pdf

Ev45Full.pdf

Well the Jury is in. I put a meter on my EvaKool 45 fridge freezer. Not overly scientific but good enough for today. I ran it for about 21 hours each and the weather was the same both days. The batteries where full as we where plugged in at the marina. Temp was the same and the amount of food in the fridge was the same and the amount of times opened was the same. On normal mode 20.5 Ah against 15.4 Ah. The wierd part is that on eceonomy mode it seemed to run quite normally while on normal mode it seems to run very erratically. Certainly for this fridge it will stay on economy mode. I have now put the meter on the big fridge so it will be interesting to see how it goes.

I have attached the 2 graphs.
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Old 03-07-2014, 13:29   #9
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Re: Danfoss bd35 compressor economy mode

I Just had a phone conversation about this topic this morning with a client, and since things are a little different for holding plate systems I’ll throw our conversation into the mix. With a Holding plate system you are not running your compressor at a speed based on matching your evaporators ability to remove heat from the box for the most efficient compressor set-up. Unlike an evaporation plate that doesn’t have any hold-over capacity, the point of a holding plate is that the compressor runs and pulls down the holding plate at a faster rate and then shuts off for a longer period as the “energy” stored in the holding plate removes the heat from the box. So on a holding plate system, varying the compressor speeds like on evaporation plate system, besides being a nice sales/marketing gimmick, doesn’t really matter from a daily amp usage standpoint. We have ran tests at all the different RPM settings and have verified that the daily amp usage is within the margin of error (remember this is for a holding plate system). Sure a client may want to look at his amp meter and see the compressor using 2.5A rather than 5A, but if the compressor runs for 1hr at 5A and 2hrs at 2.5A….well you get it, the daily power usage will be the same.
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Old 03-07-2014, 15:33   #10
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Re: Danfoss bd35 compressor economy mode

Interesting, third day, and actually makes sense.

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Old 09-07-2014, 23:12   #11
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Re: Danfoss bd35 compressor economy mode

Very interesting few days. The biggest power user on our boat was the VitriFrigo DW180FR 2 drawer Fridge. It is a very nice fridge but eats power at about 31 AH per day. As it uses the Danfoss BD35 compressor I thought I would see what it was doing. I ran it with the meter attached for 2 days on normal and yes 31 AH per day. Turns out that from the factory it is set to run at 2500 rpm via a small resistor on a board. I purchased a speed board looks like this

Speed Board, manual

This just plugs into the back of the controller and allows for easy adjustment. I also put a logging thermometer in the fridge to make sure it kept temperature.

I then ran for 2 days set to 2000 rpm. Yes it used less power 28AH instead of the 31 but struggled to keep temperature and ran longer. I then tried 2 days at 3000 rpm. Much happier, only 24 Ah per day, ran for a shorter overall period and kept temperature very well. So as mentioned earlier in the thread tuning the compressor to the enclosure seems to be the trick. Yes it used 5 amps when running instead of 4.3 but it ran for less time and temp was more stable. I haven't tried 3500 as I don't think I'm going to get it better than what the 3000 provides so I will leave it set this way. If anyone wants all the pretty graphs I can provide them.

Yes marine refrigeration is black arts.
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Old 09-07-2014, 23:22   #12
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Re: Danfoss bd35 compressor economy mode

Not Black Arts...Science My Friend!

What a great post and data to help illustrate the value of tuning the compressor to your box configuration. What you have done is what all Cruisers SHOULD do to fine tune their refrigeration unit...but let's be honest here....not everyone out there cruising has the skills/patience/give-a-**** factor to take the time and do the testing you did to save 7AH/Day (heck have you seen the toys on some cruising boats these days?) but 7AH/day is 22% so that a damn good job!

Now here is the bad news.
I've been around chat rooms and cruisers long enough to know how this game will work.
The dock rumors will start by 6AM tomorrow and go like this:

Cruiser 1:
"everyone knows it's best to run a Danfoss BD35F compressor at 3000RPM, you save 22.6% in power usage"
Cruiser 2:
"but when I spoke to the owner/builder/Engineer that designed my system he told me it was best to.....(cut off by cruiser 1 with a hand gesture)
Cruiser 1:
"screw what the ower and builder of the equipment is telling you, he's a bozo. everyone knows you always run a BD35F at 3000RPM, now give me that resister and let me wire it in here for you. Hey grab me one of your beers while I do this for you, better yet...make it two."
Cruiser 2"
"don't you have any cold beer, you drank all of mine yesterday "helping me" work on the autopilot.
Cruiser 1:
"No bro...my refrigerator quit working last week, why do you think I've been helping you with your boat project for the last week, I'm outta cold beer. It's time for me to go anyway if you are outta beer."

And so the Cruiser "fact" will become written in stone rather than the truth that for YOUR box, of YOUR size, at YOUR temperature, with YOUR insulation, 3000RPM gave the best performance. "Your" will turn into "everyone" and so the Cruise Rumor games goes. As a seller of marine equipment I spend a lot of time correcting myths people hear around the dock and from online forums.
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Old 10-07-2014, 00:43   #13
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Re: Danfoss bd35 compressor economy mode

You have been hanging around our marina? That is what always happens here. Difference is I have had so much BS flung around that I try and look for the proof first and just because someone heard that someone else found ... Doesn't equal proof. It's like the old "of cause it's the best way. We have always done it that way"


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Old 10-07-2014, 00:53   #14
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Re: Danfoss bd35 compressor economy mode

I was of two minds about posting as it as you rightly said the results only apply to my setup. It was just that I asked the question in the first place hoping the someone said it was worth investigating. As it turned out for me it was however it could have turned out that it was tuned perfectly already.

I started looking as I was asked what our daily energy budget was and that was an easy answer as I had a battery monitor but when asked what was using what, it was harder to answer. This led to measuring and then of course to can I get it better. I know sad but I like to know what is going on.


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Old 10-07-2014, 07:18   #15
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Re: Danfoss bd35 compressor economy mode

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Originally Posted by Dod42 View Post
I then ran for 2 days set to 2000 rpm. Yes it used less power 28AH instead of the 31 but struggled to keep temperature and ran longer. I then tried 2 days at 3000 rpm. Much happier, only 24 Ah per day, ran for a shorter overall period and kept temperature very well. So as mentioned earlier in the thread tuning the compressor to the enclosure seems to be the trick. Yes it used 5 amps when running instead of 4.3 but it ran for less time and temp was more stable. I haven't tried 3500 as I don't think I'm going to get it better than what the 3000 provides so I will leave it set this way. If anyone wants all the pretty graphs I can provide them.

Yes marine refrigeration is black arts.
Danfoss makes an automated controller that continually adjusts the speed of the compressor to match the load on the box. We have it and it works very well.

RPARTS.COM - Danfoss 101N0320

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