Author Topic: Selecting a new Refrigeration system  (Read 4136 times)

UJAMn

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Selecting a new Refrigeration system
« on: December 11, 2014, 10:06:23 pm »
I have two 5.6 cf well insulated purpose built cold boxes each with top and side access. I plan on extensive live aboard long term cruising and the system will see continuous use. I have the ability to power the cooling systems using AC or 12vdc. I have considered three options:
1) Use existing Grunert cold plates, a single Grunert AC475 115VAC Passagemaker and R404A refrigerant
2) Use existing Grunert cold plates, a single Seafrost BG-1000 115VAC condenser and 134a refrigerant
3) Use Frigoboat 130F evaporator with keel cooled 12vdc Danfoss 35 for the refrigerator box and Frigoboat 200F evaporator with keel cooled 12vdc Danfoss 50 for the freezer box. Each with 134a refrigerant.
Which system would you select and why?

Richard

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Re: Selecting a new Refrigeration system
« Reply #1 on: December 14, 2014, 01:57:58 pm »
If you mean by well insulated boxes insulation would be R valve of at least R30, you may still require in worst case under tropical conditions as much as 9,000 Btu of  cooling per day. This Btu projection is based on hot climates, two people on board and heat gain in boxes with front opening doors. I have seen where even two BD 50 12 volt Danfoss  compressor systems have trouble handling boat box temperatures equal to a home refrigerator’s desired temperatures.

I do not like any of your choices number 1 and 2 requiring a generator running at least two to four hours per day plus water cooling creates an unfriendly system. Also compressor cooling in tropical weather conditions shortens life of these large compressors. Large capacity AC compressor will produce higher Btu output needed and permit one compressor to cool both refrigerator and freezer boxes but there will be no refrigeration redundancy on board. There is also a risk involved in using old eutectic holding plates.

Number 3 Frigoboat keel cooler system with two BD 50 systems should including their optional add on air fan cooled condenser to each system for warm climate operation.

You may want to view the slide show on my web site as the area it reviews can help you with decisions you must make regarding on board power grid, how to reach desired box temperatures and affects of ambient temperature conditions on mobile refrigeration. 



UJAMn

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Re: Selecting a new Refrigeration system
« Reply #2 on: December 15, 2014, 08:29:50 pm »
Thanks for your reply Richard.
I agree with your comments on my first two options. I do not know the R-factor for my boxes but I suspect the 3" foam is less than R30. That said I need to learn more in order to select wisely the new system. I went looking for the slides you mention but was not able to locate them... Can you advise on where I can find them?
Thanks, Jeff

UJAMn

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Re: Selecting a new Refrigeration system
« Reply #3 on: December 26, 2014, 09:35:31 am »
Hi Richard,
All the best to you!  I have reviewed your slides and ran numbers based on your shortcuts. For my 5.7sqft refrigerator in the tropics, 2 extra crew, and allowance for a front loader (though we will use the top load routinely) I get 6320 Btu/day heat removal required.  I have selected the Frigoboat keel cooled BD35 with their 130F evaporator. A separate equal size freezer box requires 9740 But/day heat removal and so I selected Frigoboat keel cooled BD50 with their 160F evaporator. Note that to fit a larger evaporator (200F) in the freezer would require blocking the front door access.
I have three questions:
1)Do you agree with the compressor selections?
2)Do you think the 160F in the freezer will be adequate or would you opt for the 200F evaporator and scrap the door?

I acknowledge your insights on the simplicity of the air cooled systems. In my boat the air cooled units will not be easily accommodated. So:
3)What do you recommend to optimize the reliability of the keel cooled systems? Please do comment if you feel this is a bad system choice.


Thanks,
Jeff

Richard

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Re: Selecting a new Refrigeration system
« Reply #4 on: December 26, 2014, 04:53:50 pm »
Jeff, There is no way we can accurately project your real world, worst case, tropical daily energy consumption. With a 5.7cu ft refrigerator and a similar size freezer maintaining box temperatures equal to a home refrigerator I would guess daily 12 volt average amperage consumption to be 250 amp-hrs. When operating in cool air and seawater climates 65 to 70 degrees F daily amp-hrs would be reduced to 125 per day. Instead of operating both boxes at normal home refrigerator temps of 33 to 40 degrees for refrigerator and +4 to 10 degrees in freezer you can reduce amp-hrs and compressor heat stress again by 2% for each degree increase in box temperature.

Two BD50 compressors is the correct way to go. After reports from other live aboard boaters with box insulation of no greater than R20 the automatic speed controllers are not necessary and can cause higher than desired compressor temperatures in tropical conditions with keel cooler systems. In your application I prefer a means to manually changing compressor speed when changing major ambient temperature climate zones.

No one knows for sure what causes the major problems with Frigoboat keel cooler systems so I quit recommending them. The main problems with these keel cooler systems I see are; line connector refrigerant  leaks, failure to filter liquid refrigerant interring capillary tube and poor compressor cooling. Reducing high pressure and related compressor temperatures will help reduce these design problems by using two compressors operating at lower power and operating at higher evaporator box temperatures.

The design of your freezer box is important if you expect to keep frozen food items longer. Evaporator needs to surround product as much as possible covering two or more walls. I always recommend that freezer evaporator be sized to handle the maximum capacity of compressor if a large evaporator can be made to fit in the box. .All good freezers need a way for air to naturally tumble around inside the box. Evaporators are generally mounted high in the box so warm air rises and heat is absorbed  in evaporator high in box. Air that is cool settles from top to bottom creating the tumbling action. To assist the air tumbling it is a good idea to fasten ½ inch rails down one side of freezer across bottom and up the other side to assist in heat removal and normalizing the freezing of product far away from evaporator.

UJAMn

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Re: Selecting a new Refrigeration system
« Reply #5 on: December 27, 2014, 07:21:41 am »
Hi Richard,
Thanks for reviewing the energy requirements, a reality that needs to be attended. I have been focused on the Frigoboat keel cooled system and would like to believe I can solve the problems to achieve a reliable system however I must admit you have given me reason to doubt this course. What would be your first choice of system and manufacturer for this application? I would like to consider how the recommended solution could be accommodated in my boat.
Regards,
Jeff

Richard

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Re: Selecting a new Refrigeration system
« Reply #6 on: December 31, 2014, 03:38:44 pm »
I am disappointed with Frigoboat and a few other 12/24 volt ice box conversion refrigeration companies. Frigoboat US knows what design problems they have and they say they are going to correct them on new models sold in the US. This Danfoss compressor is a great compressor when operated within its intended design performance envelope. Many of these companies seem to be diversifying their product lines so trouble units can be dropped from production without affecting their bottom line.

There is a good possibility that in your application two keel cooler systems when in tropical climates running at lower than normal; speeds can avoid problems other experience.  Using manually adjusted compressor speeds will prevent the high compressor heat generated by an automatic increase in speed when not necessary. 

UJAMn

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Re: Selecting a new Refrigeration system
« Reply #7 on: January 01, 2015, 12:11:21 pm »
Humm so is the intention to keep the compressor from getting too hot or to assist refrigerant cooling/condensing? Assuming the later and if we configure the system with the air add-on condenser in series with the keel condenser will problems arise from using the air assist when the boat is in the water?

Richard

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Re: Selecting a new Refrigeration system
« Reply #8 on: January 01, 2015, 10:30:06 pm »
USA has suggested fixes and helped with refrigerant cooling flow troubles but they alone can not correct these failures.

If you approach this without tacking some preventative action and operate this system for extended time in warm seawaters the risk potential exists for major system down tine. The options are do not purchase Frigoboats  keel cooler system, Purchase two systems and operate them unloaded to a degree and running them maybe longer hours as I suggested when in tropical waters. The third option Frigoboat has made available is adding their inline fan air cooled second condenser to each unit.

The add on air cooled condenser can offer additional refrigerant cooling capacity and at the same time provide air circulation that Danfoss design I believe would have call for in this type application.
As compressor gets hot the volume of liquid refrigerant in any capillary tube system reduces. In this keel cooler system there is normally less refrigerant than in other systems reducing refrigerant flow and return gas cooling if liquid side temp/pressure is too high.  Tests of keel coolers efficiency with high temperature/ pressure tend to indicate causes of failures can not be linked to keel cooler itself. I believe the serious problem is excessively high temperatures over time causing oil and or motor wiring insulation to break down and eventually cause refrigerant flow restrictions in capillary tube. Low flow complicated by adding excess refrigerant to achieve a good operation pressure differential across cap tube raises high pressure even higher and eventually complete system must be replaced. Because this problem does not appear to show up in non tropical climates these failures do not seem to get the attention of the Frigoboat company. Frigoboat

UJAMn

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Re: Selecting a new Refrigeration system
« Reply #9 on: February 02, 2015, 10:40:45 am »
Hi Richard,
I am closing in on design choices thanks to your inputs. I have opted for keel cooled K50 compressors for the freezer and refrigerator boxes and plan to include the add-on air condensers. So it comes down to evaporators... With my 5.6cf boxes the Frigoboat evaporator selection chart suggests their 130F would work well for the refrigerator and their 200F evaporator for the freezer. Is bigger better? That is, would you opt for the 160F in the refrigerator? Is there any down side to oversized evaporators?