Author Topic: Cold plate design questions  (Read 3296 times)

mashenden@comcast.net

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Cold plate design questions
« on: February 17, 2014, 07:57:33 am »
This weekend I started doing some in-depth research on a cold plate design that I want to build. My goal is to take advantage of shore power while at port. I found good info both on this site and the Rparts site (which is a water cooled design :( ).

I plan to make mine 120 VAC only, air cooled, TEV controlled, and hope to use a holding plate I was given. It is 21" x 12.5" x 4.5" two zone - I plan to use the smaller diameter loop with 1/4" conn on one end and 3/8" on the other. The icebox is about 8 cu ft with 3"+ insulation and I want to use it as a refrigerator, not freezer.

4 questions come to mind as I reviewed the Kollmann information vs the  Rparts design:

1) The Rparts design uses 3/8" line on the suction side, whereas the Kollmann design has 1/4" for everything. Is this critical to have one way or the other?

2) I plan to include a receiver in my design, but noticed that the Rparts design does not have one. Do the larger diameter line and the C-032 dryer serve the purpose of storing refrigerant instead?

3) Is a high pressure switch a good idea?

4) Then finally, any suggestions on a house fridge model that would be ideal to scavenge for the compressor and evaporator (edit: Ooops, I meant condenser)? I can find these for a fraction of the cost of the traditional DC compressors, which I am sure are well suited for systems needing DC, but mine will be 120 VAC only so I hope to avoid that cost.

Thanks in advance for any assistance on my questions.
« Last Edit: February 17, 2014, 03:59:42 pm by mashenden@comcast.net »

Richard

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Re: Cold plate design questions
« Reply #1 on: February 17, 2014, 12:41:33 pm »
If more boaters would ask questions like yours before buying or building a boat refrigeration system major mistakes could be avoided. Eutectic holding plates are only for storing energy when surplus energy is available. The volume and phase change temperature of eutectic material inside plate will determine how long low temperature will be maintained and at what temperature. Holding plate’s surface area will determine rate of heat transfer from box to evaporator coil inside plate. Inside holding plate is an evaporator coil connectly sized to handle capacity and flow velocity of refrigerant produced by a complete condensing unit. For a dual energy hybrid holding plate there are two separate evaporator coils inside holding plate permitting two totally independent energy sources to lower temperature inside plate.

My holding plates over 200 of them were manufactured from 304 stainless steel. The evaporator coils were refrigerant copper tubing. The eutectic solution was food grade Propylene Glycol. For refrigerator plates ½ gallon of 17 degree solution was required per cu ft of refrigerator. Freezer plates held one gallon of +10 degree solution per cu ft of freezer.


YOUR QUESTIONS

1)   The Rparts design uses 3/8" line on the suction side, whereas the Kollmann design has 1/4" for everything. Is this critical to have one way or the other?

Answer: stay with size lines and evaporator coil line compressor manufacturer recommends.

A.   Compressor discharges high pressure gas vapor and small amounts of oil through a small line to condenser.  Refrigerant flow is separated between high pressure side and low pressure side by the expansion valve or capillary tube. High pressure 134a systems will range from 90 to 250 depending on condenser and its cooling medium’s efficiency. The Danfoss BD50 compressor air cooled compressor system’s high pressure will range from 105 to 115 psi while a ½ HP AC compressor with ambient boat air will range from 120 to a high of 250 psi. Water cooled condensing on Danfoss BD compressors are not necessary and generally are unfriendly.  In most cases compressors on boats larger than 1/6 HP will require water cooled condensing units in order to dispose of heat inside a boat.
B.   Pressures on suction side of system are very low with 134a or Freon R12 refrigerant. When low pressure side tubing anywhere in system can not maintain flow velocity sufficient enough to return oil to lubricate compressor it will fail. The size fitting for suction low pressure vapor return at compressor will define all tubing size to maintain correct velocity. Danfoss BD compressor low side fitting is ¼ inch OD or Metric size. The risk of compressor failure is not worth taking a chance with larger tubing when you know what size tubing compressor manufacture plans for suction side of system.

2)   I plan to include a receiver in my design, but noticed that the Rparts design does not have one. Do the larger diameter line and the C-032 dryer serve the purpose of storing refrigerant instead?

Answer: Wrong, Holding plate to be efficient needs a thermo expansion valve and a receiver to store excess refrigerant as valve reduces flow. Without a receiver liquid will back up into condenser increasing energy consumption. I do not know about capacity in 032 filters, but I would stay with a larger capacity small $20 receiver.

3)   Is a high pressure switch a good idea?

    Answer: High pressure switch is not required on Danfoss 12/24 volt compressors because they are protected from high pressure by electronic control module amperage monitoring. They are also protected from fan cooling problems and low and high voltage spikes. The only Danfoss BD  compressor problems I am aware of are on water cooled units do to poor compressor cooling.
Large water cooled systems must have a High Pressure Switch to prevent explosive problems.

 
4) Then finally, any suggestions on a house fridge model that would be ideal to scavenge for the compressor and evaporator? I can find these for a fraction of the cost of the traditional DC compressors, which I am sure are well suited for systems needing DC, but mine will be 120 VAC only so I hope to avoid that cost.

Answer: You can take any refrigerator apart and use compressor and maybe condenser coil. Freezer holding plates though will require a Low Back Pressure (LBP) compressor not normally found in home refrigerators. With the exception of BD3 AND BD80 that are LBP only all other Danfoss BD compressors are rated LBP,MBP and HBP.

mashenden@comcast.net

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Re: Cold plate design questions
« Reply #2 on: February 17, 2014, 04:25:12 pm »
Lots of good info. Thank you, Richard.

I agree on the need for a receiver - I will include one but was just curious if I was missing why the other design did not need one. Maybe because it was a water cooled design or maybe the c-032 filter serves that purpose - dunno?? Regardless, your advice is well taken.

Another valued nuggets of knowledge that you provided: "stay with size lines and evaporator coil line compressor manufacturer recommends... The size fitting for suction low pressure vapor return at compressor will define all tubing size to maintain correct velocity."

Just for clarification, when you mentioned Freezer holding plates require a LBP compressor, that only applies to plates mixed to be freezers (10 degree), not refrigerators, correct? I should be OK using a compressor off a small 120 VAC fridge if I am going for a "refrigerator" design??

Do you think a mini-fridge compressor would do the job?