Cold Plate/Hold Over Refrigeration

March 15, 2010 Facebook Twitter LinkedIn Google+ Maintence Tips,Refrigeration

crosby cold plate

For many years Hold Over/Cold plate refrigeration was the way to go for long distance cruising yachts, and over the road refrigerated trucks...The function is Just as the Phrase Implies...Hold Over...For extended periods with a minimum of refrigeration system run time.

The Advantages, and Disadvantages of a Hold Over system are similar to a block of ice because that is essentially what is done...

The hold over plate/s in the box is/are approx 3" thick, and have copper tubing run through them which is then surrounded by a liquid that freezes (and re-freezes) when the refrigeration system is run... a Block of Ice really...The idea is to not to have to run the system but once or twice a day to maintain a box's design temp.

The the solution/liquid is generally a glycol solution (or brine water in older plates) that is mixed for a lower freezing point than water...A refrigerator plate solution can be mixed to around 26 degrees F...A freezer plate can be 0 degrees or even lower depending on the temp you want the box maintained, and how much insulation is surrounding the box (more on that later)

Other than what I have stated above...There really aren't that many advantages to a hold over plate system...Yes they can work well for the intended purpose, and have for many years for that purpose...Thing is, they really are nothing like what most folks are used to at home, and that in itself  can be one big disadvantage...

If you have, or are considering a hold over system I will try to list some of the lower points when compared to a traditional refrigeration system...

  • Expense...Custom Hold Over Systems & Boxes are expensive to design & build...Proper insulation & door gaskets are key factors too...Insulation should be around 4" thick everywhere (even against the hull) with no air pockets.
  • Given Space...The insulation requirements and the space the plates take up, really tend to limit internal box size in the space allocated for a box.
  • Weight...A plate full of liquid is quite heavy.
  • Power...Big plates full of liquid require a large refrigeration unit to pull them down to temp, and Initial pull down time is also much longer to get the box temp to stabilize.
  • Recovery...Holding Plates don't recover temp very fast after opening the box...No more leaving the door open at all...In fact you really should only open them once a day...But what about my beers ?
  • Defrost...Any ice build up acts like a insulation blanket over the plate, and thus raises box temp...By design, hold over plates can't utilize auto defrost cycles effectively, so you end up scraping the ice off all the time to maintain temp, and the more you open the door...The more the ice/frost forms...Break out that windshield scraper you came south to get away from. (but at least up there you could put the beer outside)
  • Safety...Last but not least is the possibility of a Glycol leak into your food without knowing it...Yes it's rare but does happen...Glycol is like antifreeze in your car and is Very toxic.

I have found one of the biggest misunderstandings about Holding Plate systems is...Hold Over Period Box Temp/s...I'll try to explain...

Hold Over Period is the time it takes the plates to thaw from a solid back to a liquid...Hold Over Period Temp is the box temp during that period...This is where the box/system was designed/intended to operate....

Any additional run time below the freezing point of the solution is pretty much wasted energy because the plates will rise in temp very rapidly back up to the change over (thawing ) point anyway...Then the box temp will again rise rapidly when the solution has thawed (if the compressor is not run)

This is where insulation, door gaskets, drains etc... Have the most effect...Ideally with all things at their best, you would like to maintain a box temp that is 15 degrees above the plate temp when the plate is frozen...That means with a 26 degree refrigerator plate you might expect a 41 degree hold over period, and with a 0 degree freezer plate you might expect +15 degree hold over temp  (not good for ice cream)....Yes you can go lower with the solution temps (mixture to attain lower freezing point) but to effectively do that you will need to use a system that is set up for a lower temp refrigerant than the old standard R-12 or it's replacement R134A systems that we have seen for years...Grunert offers a R-404A system and plates mixed to do it...

Different than a conventional system (non hold over) that senses ambient air box temp, and cycles the compressor (if power is available) to maintain that temp...A good Hold Over system only senses Plate temp (to keep the plate frozen) in order to save energy, and thus the box temp...Is what it is...Depending on all the factors I have mentioned...

Thing is...Boxes, doors, gaskets & insulation are almost never perfect...Especially as they age...So as you might see...Maintaining these marginal temps for food storage is very critical, and a chore for the owner of such systems...Most never utilise the real design of the system as a hold over system so to speak...They generally end up adjusting the thermostats to get the box temp they want and leave it at that...Thus running it like a conventional system...Which it is not as good at as one designed to operate that way.

Knowing all the advantages & disadvantages is key...Yes Hold Over systems can still provide a long distance cruising option, but with things like today's high efficiency DC to AC power inverters, or the little DC powered flat plate systems...A more standard system can offer the refrigeration you may be more accustomed to at less expense.