Small Refrigerator Calculations

I'd just buy one myself but since you asked I would say going cap tube will work but an expansion valve will work much nicer over a wider pressure range.. it isn't just tubing length and diameter, you have to feed them to a proper sized coil to maintain the proper velocity of your refrigerant and oil.

It will compete with one of these:

with some of these

placed inside it that were chilled in a larger freezer earlier.

That volume is about the size of a standard 'mini-fridge'. Here in the US you can buy a new one for around $100. They're popular on college campuses all over and used ones can be gotten a lot cheaper. If you can't just buy one and fit it in, you might considering buying one and ripping out the parts you need for your project.

This is way more complicated than you seem to think....and would require an enormous amount of number crunching...Save yourself the frustration, if you can't do the calculations involved, just buy one....You'll come out cheaper in the end....

There is no point to build, when you can buy one cheaper.

A small one for $100 with 110V (or 220V) AC feed.

Or you can buy a 12VDC fed Peltier module or two. Easy to use, easy to calculate, widely used in drink cooling boxes for cars. See them on Internet or sporting goods store.

the box I have is a stainless steel cooler that I got from my previous company. It was a prototype for one of those 4x4 coolers that run off a 12 compressor. It was designed for the frigoboat 12v compressor but I never managed to get one of those (way too expensive.

to get a second hand bar fridge would cost me around the equivalent of $100 where as I can get the 1/12th hp compressor for around $30 and I already have all the copper pipe and gas

Another approach is to buy an off the shelf device for a few $'s and then proceed to strip it and rebuild it. This way you get into the practical stuff very rapidly without getting bogged down by heavy theory & complex calcs. Also, you will be sure that the device works. Make sure the supplier demonstrates the working of the device before investing your $'s. I have, in the past, done this with even complex devices (The GAs Chromatograph for one) - and with scant knowledge of electronics, etc. I was lucky to have a good technician show me the basics though. While I am trained as a chemical engineer, I often follow the age old approach of simply hacking away and tinkering around. Do note : Innovation more often than not comes from straight-forward tinkering around and less so from high powered formulae.

Not to be knocking the formulae method. One will find that those that tinker at the workbench level are more involved in developing equipment that the academics fiddling with formulae on the blackboard or in spreadsheets as they do these days. Through Cr4 I have encountered half dozen or so close contacts who are into invention & innovation. Out of these all have workshops and all are involved in tinkering around. One is an artist (painter and sculptor).

On the other hand it is worthwhile browsing through old practical manuals on refrigeration. The principles are outlined in these without necessarily delving into complex calcs. I will try to search out the reference later if I can finds it. At the least one should be able to understand the 4-step refrigeration cycle. And, of course, do remember that this covers only conventional refrigeration and Chilling. There are other methods as well.

Your Original Post states: "I have a well insulated box 400mmx400mmx500mm that I wish to refigerate", and here you add: "...designed for the frigoboat 12v compressor".

Nowhere has it been mentioned WHAT the "end-use" is, or WHERE it will be utilized (stationary/portable, what have you). I'm willing to bet SOME people already "assume" that you want it to keep the "fish-catch" cool while still offshore.

Simply from that old "For-What-Its-Worth" category , IF you intend for it to keep the beers cold under the umbrella up on the roof, Durban's subtropical climate might be perfect for an absorptive refrigerator system. There have been numerous threads here on the subject over the years, with some nice plans either posted or linked-to.

Hoping you find exactly what you *need*...

Sounds like its time for a static condenser and evaporator.

the pland use is for keeping food fresh and drinks cold when we go on dive trips where ice is not redily available. It would be transported to our camp site pluged into the mains power and left there for up to 2 weeks at a time no need to run while being transported just an easely transported fridge. Top opening to keep the cold in even if opened and with a rugged stailess steel shell with carry handels to take the abuse of transporting and camping also has a locking lid to keep the wildlife out. one one of out trips we had some monkeys steel redbull cans out of the tent they just bite through the can and drink it that way (can you say flying monkeys)