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Old 03-01-2012, 22:16   #1
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HELP ! Fridge Question !

Howdy,

OK, we've just moved aboard today - it is HOT. About 35degC.

The major hassle was moving the contents of our home fridge.

Our approach was to put everything into large tupperware containers and use these as dividers in the fridge - (which is deep like a chest freezer).

The system is a Frigoboat with a Danfoss 50 compressor and the largest evaporator plate they sell.

The fridge is now PACKED.

It has been running for the past 4 hours - there's frost on the plate and it's cold in there... BUT the thermostat hasn't switched it off yet.

The compressor is hot enough to fry an egg on, as is one of the copper tubes, so I switched the "Smart Speed Control" to "Manual" and have the compressor set at a low speed now just in case.

It's pulling about 4A at the moment.

Questions:

* How hot is the compressor supposed to get?

* I mounted the temperature sensor for the digital thermostat near the TOP of the fridge box, on the opposite side of the evaporator plate, reasoning that it is the furthest place away from the cooling source and the warmest place, so if the temp sensor is at the right temp, then the rest of the fridge will be even cooler. Although having just done some googling, it appears that a lot of people put their sensors on the BOTTOM of their fridge box... ? Which is correct?

* Having thought about it - the tupperware around the food is probably acting like a HUGE chunk of insulation and is restricting cool air to move around (and get to the temp sensor too?). Should I put a whole pile of wholes in all our tupperware to facilitate the cold air getting in and around?

Thank you muchly!
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Old 03-01-2012, 23:24   #2
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Re: HELP! Fridge Question!

Quote:
Originally Posted by akio.kanemoto View Post
Howdy,

OK, we've just moved aboard today - it is HOT. About 35degC.


It has been running for the past 4 hours - there's frost on the plate and it's cold in there... BUT the thermostat hasn't switched it off yet.


Questions:

* How hot is the compressor supposed to get?

Can't say for your more modern Danfoss, but our antique one gets too hot to hold your hand on. Never have checked it with our IR thermometer... should do that one day!

* I mounted the temperature sensor for the digital thermostat near the TOP of the fridge box, on the opposite side of the evaporator plate, reasoning that it is the furthest place away from the cooling source and the warmest place, so if the temp sensor is at the right temp, then the rest of the fridge will be even cooler. Although having just done some googling, it appears that a lot of people put their sensors on the BOTTOM of their fridge box... ? Which is correct?

Well, ours is about in the mid-level! I don't think that it really matters... you will learn by experience how the thermostat setting and the temperatures at various places in the fridge relate.

* Having thought about it - the tupperware around the food is probably acting like a HUGE chunk of insulation and is restricting cool air to move around (and get to the temp sensor too?). Should I put a whole pile of wholes in all our tupperware to facilitate the cold air getting in and around?

Yes, allowing good air circulation through the contents will speed up the cooling process. On the other hand, once the cooling has reached the set temps, and the compressor has stopped, having that good circulation will allow convective circulation which tends to speed up warming, especially if the lid is not as well insulated as the other surfaces (a common failing).
Dang... nothing is simple!
Thank you muchly!
AK, I think that 4 hours to cool down a big box jammed full of warmish food isn't out of reason. These units are not killer cold sources, and it takes a while to slowly pump all the heat out.

Cheers,

Jim
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Old 04-01-2012, 10:49   #3
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Re: HELP! Fridge Question!

The small 12V compressors remove about 300 BTU's an hour, give or take, which is not that much. A BTU (british thermal unit) is the energy it takes to drop (or raise) the temperature of one pound mass by 1 degree F (or 1.8 degree C). So if you put say 50 pounds (22.7 kilo) of goods in the refrigerator that are at 45 degrees F and the thermostat is set at 35, then your going to use 500 btu's to cool it down.

The compressor will get hot, its quite normal. Plus the efficiency drops as the temperature rises so more Power is used. 35 degrees C is toasty for sure.

Me I might move the thermometer/bulb down to 1/3 the distance from the top. This as you want a location of average temperature in the box.
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Old 04-01-2012, 10:56   #4
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Re: HELP! Fridge Question!

This thread is a great summary of why I ditched the builtin icebox/norcold unit in favor of an Engel.
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Old 04-01-2012, 13:47   #5
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Re: HELP! Fridge Question!

akio.kanemoto, Performance and energy efficiency of your unit will depend on proper thermostat temperature control of evaporator, in box thermo efficiency and compressor cooling.
Another point to remember is at 3500 rpm maximum capacity of BD50 compressor will only deliver its maximum 500 Btu when evaporator temperature is at plus 10 degrees F (-12 C). Btu output is greatly reduced at lower evaporator temperatures. To put 500 Btu of energy into perspective think of the amount of energy produced by flame of a candle per hour. Lowering temperature of a box full of warm food may take 24 hours before all product and box temperature is stabilized at desired temperature. Add insulation of plastic storage containers may add a few minutes to temperature pull down.

In order to normalize temperature of all product to be cooled in a refrigerator there needs to be air movement around product. Air cooled by evaporator will naturally tumble down and warm air will rise circulating heat back to evaporator. Upright refrigerators always have open shelves permitting air movement. Top loading boxes need provisions for air movement like side and bottom stand off rails.

Yes these compressors do run hot at times because of condenserís cooling mediumís temperature and the amount of work they are attempting to do. Overheated compressor burnout is a common failure on most hermetically sealed electrical compressors but not so with these Danfoss BD compressors. As compressor temperature rises so will amperage and pressure increase signaling control module to stop compressor before it is damaged. Cooling of this BD compressor is accomplished by return temperature of refrigerant and supplemental air cooling from condenser fan.. Returning suction gas flow is controlled by evaporatorís low pressure. On Frigoboatís new water cooled units they have added a fin type heat conductor to outside of compressor but it is too small dissipate much of compressorís heat.

In your system if the thermostatís temperature sense tube is not attached to evaporator it will reduce refrigerant flow and may be overpowering evaporator by not cycling compressor when desired evaporator temperature is reached. Most all small heat absorption by conduction evaporator systems like yours use thermostats that control evaporator temperature instead of box temperature to achieve energy efficiency and best performance. If heat transfer at evaporator were equal to or greater then capacity of compressor then a box temperature sensing thermostat could be used.

My recommendations are:
Donít worry about compressor heat or insulation affect of Tupper wear at this time. Install thermostatís sense tube where Frigoboat has designed it to be attached to evaporator plate. If you have one of the nonstandard electronic thermostats not intended for refrigerators set it for an evaporator temp of +10 F (-12C) and set differential off cycle for 8 (13C) to 10 (12C) temperature rise before compressor restarts.
After box temperature is stable with thermostat set to your desired box temperature set compressor speed. If your unit is equipped with an automatic speed control, place it in automatic. The best compressor speed will be when compressor runs at its lowest speed to maintain desired box temperature yet runs less than 50% of the time.
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Old 04-01-2012, 19:00   #6
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Re: HELP! Fridge Question!

Richard,

While I'm not the Original Poster, I'd like to take this opportunity to compliment you on your book.

Regards,
Brad
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Old 04-01-2012, 21:34   #7
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Re: HELP! Fridge Question!

Quote:
Originally Posted by Jim Cate View Post

AK, I think that 4 hours to cool down a big box jammed full of warmish food isn't out of reason. These units are not killer cold sources, and it takes a while to slowly pump all the heat out.

Cheers,

Jim
Hi Jim,


Thanks for that - I will point my IR gun at the compressor and find out how hot - but yes, placing my hand on it - I can't keep it there for more than a few seconds. Likewise with one of the copper tubes, it is HOTTT!


Quote:
Originally Posted by sailorchic34 View Post
The small 12V compressors remove about 300 BTU's an hour, give or take, which is not that much. A BTU (british thermal unit) is the energy it takes to drop (or raise) the temperature of one pound mass by 1 degree F (or 1.8 degree C). So if you put say 50 pounds (22.7 kilo) of goods in the refrigerator that are at 45 degrees F and the thermostat is set at 35, then your going to use 500 btu's to cool it down.

The compressor will get hot, its quite normal. Plus the efficiency drops as the temperature rises so more Power is used. 35 degrees C is toasty for sure.

Me I might move the thermometer/bulb down to 1/3 the distance from the top. This as you want a location of average temperature in the box.
Thank you. I've since snipped the cable ties around the temp sensor and moved it down to around 1/3 from the top as well and this is making a lot more sense. It's still working hard, but it's actually down to 5degC or so now and has started cycling.

Actually including yesterday's initial nightmare, we've used 85A in 24 hours (!) - I'm hoping the next 24 hours will be a lot less!


Quote:
Originally Posted by Richard Kollmann View Post
akio.kanemoto, Performance and energy efficiency of your unit will depend on proper thermostat temperature control of evaporator, in box thermo efficiency and compressor cooling.
Another point to remember is at 3500 rpm maximum capacity of BD50 compressor will only deliver its maximum 500 Btu when evaporator temperature is at plus 10 degrees F (-12 C). Btu output is greatly reduced at lower evaporator temperatures. To put 500 Btu of energy into perspective think of the amount of energy produced by flame of a candle per hour.
Firstly, thank you very much for the detailed reply. Wow, 1 candle per hour is not much is it?

Quote:
Lowering temperature of a box full of warm food may take 24 hours before all product and box temperature is stabilized at desired temperature. Add insulation of plastic storage containers may add a few minutes to temperature pull down.
The other thing I've read is that empty air spaces are excellent insulators, and the tupperware is full of empty air spaces, hence my suspicion in that direction as well.

Quote:
In order to normalize temperature of all product to be cooled in a refrigerator there needs to be air movement around product. Air cooled by evaporator will naturally tumble down and warm air will rise circulating heat back to evaporator. Upright refrigerators always have open shelves permitting air movement. Top loading boxes need provisions for air movement like side and bottom stand off rails.
OK, I have stand offs behind the evaporator plate, but that's pretty much it - they're LARGE tupperware containers (20L/5gal) stacked on top of each other about 3 high by 2 wide and there is definitely no airflow between them.

There is some space along one edge as well and that's packed with loose items.

Due to discovering just how inconvenient getting anything out of the fridge was over the last 24 hours - I have just purchased a new set of storage containers, which fit a little better and will leave more space around the edges - however I'm still more than happy to drill a bunch of holes in these as well if it helps airflow.

Quote:
Yes these compressors do run hot at times because of condenserís cooling mediumís temperature and the amount of work they are attempting to do. Overheated compressor burnout is a common failure on most hermetically sealed electrical compressors but not so with these Danfoss BD compressors. As compressor temperature rises so will amperage and pressure increase signaling control module to stop compressor before it is damaged. Cooling of this BD compressor is accomplished by return temperature of refrigerant and supplemental air cooling from condenser fan.. Returning suction gas flow is controlled by
This is a Frigoboat keel-cooled system and there are no fans on the compressor at all, although there are two heatsinks (HOTTT!!!) - one on the control/circuitry side, and another which I'm assuming is the oil heatsink.

Is it worth installing a pair of computer/CPU fans on there heatsinks?


Quote:
In your system if the thermostatís temperature sense tube is not attached to evaporator it will reduce refrigerant flow and may be overpowering evaporator by not cycling compressor when desired evaporator temperature is reached. Most all small heat absorption by conduction evaporator systems like yours use thermostats that control evaporator temperature instead of box temperature to achieve energy efficiency and best performance. If heat transfer at evaporator were equal to or greater then capacity of compressor then a box temperature sensing thermostat could be used.
The unit I bought came with a Carel IR32 (?) digital thermostat and the Frigoboat "Smart Speed Control" option, there is no other mechanical thermostat or otherwise. Likewise, there were no instructions on where to place the temp sensor from the Carel, hence my non-ideal placement...

Quote:
My recommendations are:
Donít worry about compressor heat or insulation affect of Tupper wear at this time. Install thermostatís sense tube where Frigoboat has designed it to be attached to evaporator plate. If you have one of the nonstandard electronic thermostats not intended for refrigerators set it for an evaporator temp of +10 F (-12C) and set differential off cycle for 8 (13C) to 10 (12C) temperature rise before compressor restarts.
So just to clarify - I don't think I have a "sense tube" - just a Carel digital thermostat supplied with the unit. Are you suggesting that I move the temperature sensor from the "wall" of the box to being directly on the evaporator?

Quote:
After box temperature is stable with thermostat set to your desired box temperature set compressor speed. If your unit is equipped with an automatic speed control, place it in automatic. The best compressor speed will be when compressor runs at its lowest speed to maintain desired box temperature yet runs less than 50% of the time.
If the digital temp sensor is on the evaporator plate - then it will always be below the desired box temperature right?

Thank you again!
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Old 05-01-2012, 14:06   #8
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Re: HELP! Fridge Question!

Just a quick addendum to this - I pointed the IR gun at the compressor (black dome part - top) - and it was 70 deg C!!!

It this still acceptable?
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Old 07-01-2012, 07:22   #9
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Re: HELP ! Fridge Question !

Akio.kanemoto, my responce
YOUR REPLY The other thing I've read is that empty air spaces are excellent insulators, and the tupperware is full of empty air spaces, hence my suspicion in that direction as well.
ANSWER, As I said before forget Tupperware the air inside is not dry desert air.

YOUR REPLY I'm still more than happy to drill a bunch of holes in these as well if it helps airflow.
ANSWER Yes, many holes would help reduce time to lower temperature of contents inside containers. Also adding a very small fan to tumble air around and over evaporator will lower temperature inside containers faster. All modern refrigerators use air to conduct heat to evaporators where it is absorbed and disposed of by condenser.
YOUR REPLY
This is a Frigoboat keel-cooled system and there are no fans on the compressor at all, although there are two heatsinks (HOTTT!!!) - one on the control/circuitry side, and another which I'm assuming is the oil heatsink.

Is it worth installing a pair of computer/CPU fans on there heatsinks?


ANSWER
Danfoss provides only limited application engineering information and appears to agree with me that their units are not recommended to have water used as a cooling medium. The only information I have found on Danfoss BD compressor/condenser unit maximum temperatures are:
BD50 Machine max temperature of 131 degrees F (55 C).
BD80 runs at higher rpm than BD50 maximum motor wire temp Short term 135 C, long term 125 C
BD80 condenser temp maximum 60C short term 70 C long term.

Experience from the number of control module failures and Danfossís requirement to place a cooling fan on heavily loaded BD50 modules says Yes to module fan, I would add a module fan. After seeing hundreds of these compressors too hot to touch and never a failure from running hot I can not say they must have fan cooling. The only reason you would have purchased a keel cooler unit was to eliminate the 3 to 6 amp-hours of fan current per day. Proper thermostat control over evaporatorís low pressure and gas flow should increase return gas flow helping to reduce compressorís high temperature and improve daily energy efficiency.

YOUR REPLY
The unit I bought came with a Carel IR32 (?) digital thermostat and the Frigoboat "Smart Speed Control" option, there is no other mechanical thermostat or otherwise. Likewise, there were no instructions on where to place the temp sensor from the Carel, hence my non-ideal placement...
ANSWER
Again thermostat must control evaporator on this system not box temperature. When thermostat is controlling box temperature it will overpower evaporator causing its temperature to drop and cause low pressure to drop. This pressure drop then reduces refrigerant cooling flow to compressor. Again cool refrigerant returning to compressor is the only source of compressor cooling on your system.
Digital thermostats are Mickey Mouse gadgets that are not designed for marine environments. I would carry an inexpensive standard snap action spare thermostat, if you are off shore long enough you will need it.
These thermostats whether electronic or manual have either an electrical or a gas probe the senses temperature. The temperature at this probe will cause thermostat to start and stop compressor. The electronic thermostats with a temperature gauge will generally have two probes one for each function, temperature reading and the other for compressor ON or OFF control.
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Old 07-01-2012, 17:10   #10
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Re: HELP ! Fridge Question !

Hi Richard,

Thank you very much again for your reply. Greatly appreciated - my responses are inline:

Quote:
Originally Posted by Richard Kollmann View Post
ANSWER Yes, many holes would help reduce time to lower temperature of contents inside containers. Also adding a very small fan to tumble air around and over evaporator will lower temperature inside containers faster. All modern refrigerators use air to conduct heat to evaporators where it is absorbed and disposed of by condenser.
I have just drilled holes through all of the containers and indeed it has made a significant difference in pull-down time, so definitely heading in the right direction!

Quote:
Danfoss provides only limited application engineering information and appears to agree with me that their units are not recommended to have water used as a cooling medium. The only information I have found on Danfoss BD compressor/condenser unit maximum temperatures are:
BD50 Machine max temperature of 131 degrees F (55 C).
BD80 runs at higher rpm than BD50 maximum motor wire temp Short term 135 C, long term 125 C
BD80 condenser temp maximum 60C short term 70 C long term.

Experience from the number of control module failures and Danfossís requirement to place a cooling fan on heavily loaded BD50 modules says Yes to module fan, I would add a module fan. After seeing hundreds of these compressors too hot to touch and never a failure from running hot I can not say they must have fan cooling. The only reason you would have purchased a keel cooler unit was to eliminate the 3 to 6 amp-hours of fan current per day. Proper thermostat control over evaporatorís low pressure and gas flow should increase return gas flow helping to reduce compressorís high temperature and improve daily energy efficiency.
OK, so I will add a small fan to the control module (on the heatsink behind the "box" where all the wires connect), but won't add one to the heatsink on the compressor itself then.

Quote:
Again thermostat must control evaporator on this system not box temperature. When thermostat is controlling box temperature it will overpower evaporator causing its temperature to drop and cause low pressure to drop. This pressure drop then reduces refrigerant cooling flow to compressor. Again cool refrigerant returning to compressor is the only source of compressor cooling on your system.
Digital thermostats are Mickey Mouse gadgets that are not designed for marine environments. I would carry an inexpensive standard snap action spare thermostat, if you are off shore long enough you will need it.
These thermostats whether electronic or manual have either an electrical or a gas probe the senses temperature. The temperature at this probe will cause thermostat to start and stop compressor. The electronic thermostats with a temperature gauge will generally have two probes one for each function, temperature reading and the other for compressor ON or OFF control.
OK, I'm still a little bit confused on this point and apologize. I googled "sense tube" and the images that I found have mechanical thermostats with a coil of what appears to be a copper wire attached.

I don't have a mechanical thermostat, but I do have a coil of copper on the evaporator plate. I've attached a picture of this - is this the sense tube or something else?

If the attached photo isn't a sense tube, then I have attached another photo of the temperature sensor (there is only one) from the Carel digital thermostat - and to confirm, I should attach this directly to the evaporator and set the thermostat to -12degC with a hysteresis value of 8degC?

You also mention in your previous reply that once the box temperature has stabilized - then I can set the thermostat to the desired box temperature - I'm assuming that at this point, I remove the temp sensor from the evaporator and reattach to the side wall about a third of the way down from the top?

Thank you very much again for your help!
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Old 07-01-2012, 18:03   #11
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Re: HELP ! Fridge Question !

The picture on the right is your electronic temperature sensor.
It's a resistor which changes value with temperature.
I believe the coil on the left is the capillary tube where the refrigerant expands and gets cold.
That's why it's where the ice starts.
Richard, feel free to correct me if I'm wrong.
PS, I've got your book.
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Old 07-01-2012, 18:57   #12
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Re: HELP ! Fridge Question !

i Know nothing about frigdes really, But I do know that we pre-freeze a lot of food/drinks etc to accelerate and minimize the cooling process. Same works for eskies (I think you north Americans call them coolers) etc.
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Old 07-01-2012, 20:23   #13
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The temp sensor of the Carel thermostat (right picture) must be mounted opposite the evaporator, alfway up the side wall. They sell these nylon clips to hold cable ties which is perfect for mounting these sensors.



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