More on Hold Over/Cold Plates

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In another forum I visit…I answered a post http://www.boatered.com/forum/topic.asp?TOPIC_ID=130856  from a gentleman that was considering replacing his 12v standard plate system with a like system utilizing a Hold Over/Cold Plate in his factory (boat builder) built top loading box…The system he was considering is one of the package type systems offered today, and below is a copy of my post of considerations…(I had linked to my previous article here on Marine-AC)  http://marine-ac.com/2009/03/15/cold-platehold-over-refrigeration/

Let me say first that I’m not against holding plate systems but…Some of the points I’m trying to make in the article are…

Holding plates are a much different animal…In order to get the performance/temp desired, and holding time at that temp…Many factors come into play such as insulation, size of box vs size of plate, door seals, top or bottom loading, plate solution temp etc…

A holding plate systems main design purpose is to keep a given box at a desired temp for a period of time, without running the refrigeration unit to keep it there for that period of time…

Basically…The temp of the box while running the unit has little to do with how a holding plate system was intended to be used…and given two plates of the same size (area used in the box) a holding plate will not offer anywhere near the BTU of heat absorption that a standard plate will offer…

• Running…Plate for Plate, a holding plate needs to be much larger to do that same job of heat absorption.
• A typical 134a standard plate system will run that plate surface at -10 F.
• A 134a holding plate system can run the tubing inside the plate…and solution (liquid) at that same -10 F….But the solution is a limiting factor in some ways…
• 134a system plate liquid is typically mixed to change states from a liquid to a solid somewhere between 0 F. to 26 F. Depending on if the plate was meant for a freezer or refrigerator box…
• It is also a larger mass to change the temp of, so running it down to -10 as you might imagine takes more time & energy…and Likely never getting the surface of the plate quite down to -10 F.

If a holding plate/box is not designed/sized as such (To hold a desired box temp when off) it can be much less efficient than a standard plate system that can recover the box temp much faster and may provide for less overall run time in a 24 hr period.

As you see…It’s hard to explain, but installing just any holding plate into a box because it will fit is almost never the answer to the most efficient or ideal box.

Holding plates can be over sized too, and thus undesirable if they keep the box too cold during that hold over period…Freezing lettuce etc…

What I’m saying is…A Hold over plate can and will continue cooling long after the t-stat has cycled the unit off if it has been run long enough to freeze the solution in the plate…and If it is enough BTU to compensate for the amount of heat leaking into the box…
If Not…Then you will need to run it longer to attain the desired temp in that box…
The thing is it will warm very rapidly back up to the point that the solution begins to thaw…and This is where the holding period is (during the thaw, or change of states of the solution) and the box temp during that period is related to all the factors I discussed.

If that holding period box temp is not what is desired…Then a holding plate has not gained you a thing…It has hindered things somewhat.

Am I making sense ?

The OP responded that I had…So I felt it might help here also…

Steve~