Cold slabs/shelves

[Goober]

I need to work out the capacity of a refrigerated space that utelises utectic plates or more to the point, where the evaporator pipe work is attached directly to the inside skin of the compartment, aka a cold slab or shelf. I've been told to use a formula designed to work out the capacity of a cold shelf in that I work out the total area of the internal surfaces of the compartment and apply the cold shelf formula. I have a formula,
"Square feet x 2.5 BTU/FT squared x 25F = BTU's x 1.2 Safety = Total BTU's" I will need to work out the capacity for chiller and freezer compartments of various sizes, how do I utilise the above formula for chillers and/or freezer.

And......does anyone have a formula in kilowatts, degrees C and metres?

The compartments will be relatively small, small undersole freezers, upright fridges etc, that would be found on a typical 50m super yacht, if there is such a thing.

[Peter_1]

Have you done already a search on "eutectic" on this forum? There were a lot of posts the last months about it.
There's no such a formula,
You will install pipes in or on the wall or slab,ok, but how will you manage the eutectic solution?
Desired temperatures?
If there should be a formula - which there isn't - end you need SI instead of IP units, you simply could convert the formula from IP to SI.

[Goober]

It's not euctectic plates. The evap pipes are soldered direct to the inner lining of the compartment, not inside compartment but ouside, then covered in insulation, same as a domestic chest freezer set up. Not surprised about the formula not existing, I have used it then converted it to required scales.

Temperature required, is freezer -18 or below and for chillers of various sizes ant where between 0 and 4 degrees C

[Peter_1]

Those aren't then eutectic plates
There's no such a rule for this, it's based on trial and error.
What will be the material of the inside panel?

[Goober]

Stainless steel, and yep, there has been plenty of trial and error to date, with mixed results.

To clarify the formula I was given

Total square feet of walls refrigerated x Heat rejection being 2.5BTU/feet squared x 25 degrees F = BTUS x by safety margin of 1.2 then = total BTU's.

If the above formula makes no sense to anyone, is there a formula to work out the heat load of a cold slab?

[Peter_1]

Is the shape somehow rectangular like a box or more a non regular shape?
How thick is the stainless steel (SS) ?
Look once to my pictures I posted here how we improved the very bad thermal conductivity of the SS.
And how we attached the tubes to the SS.
You can also let flow the refrigerant between a double wall of the SS, like milktanks. The refrigerant is then in direct contact with the SS.
Just some ideas.
To calculate capacity, how I should do it and not re-inventing the wheel: i should take a household freezer, measure inside area , look what capacity they've installed and calculate what capacity they use per m²./
I should count on +/- 125 W/m² inner surface, evaporating at -30°C for a inside temperature of -20°C.
This will be close enough to make your system.

[Goober]

Thanks Peter_1, some of the compartments are an iregular shape and some are just a rectangular box. But your idea of measuring a domestic unit is not a bad suggestion. We have already installed all the compartments and the system is operational, I copied the designs from a previous boat a previous employee did, there was little change in the overall size of the various compartments. So I'm kind of reverse checking the original design.

The issue is not with the refrigerant capacity but with the brazed plate HX used and the Glycol system supplied by the boat yard to cool them, Bit of an imbalance now that the boat has gone to the tropics. NOt enough cool glycol to condensers, coupled with the fact that the HX may be a bit small. Hence my reason to work out the capacity of compartment to more closely work out the heat of rejection.