Rules of Thumb?

I have been designing consumer products for a long time, but I am currently on a project that requires understanding refrigeration - and I am way out of my depth (think Junior year thermo with one chapter on cycles). So any and all help is appreciated!

The problem statement:
Designa n appliance to cool 2 one-gallon bottles of water. Assume a typical refrigeration system with R-143a. The evaporator is directly coupled to the cooler walls, so there is no fan/blower to circulate the cold air.

Are there rules of thumb for things like-

Setting the temperature at the evaporator? I know it needs to be less than the temperature I want inside the cooler cabinet, but how much less?

Leakage of heat from the environment to the cooler cabinet? I can calculate the heat transfer rate, but is there a typical margin that is used?

Margin on the cooling capacity required by the system? If I can determined the Watts I need for the compressor, do I really specify a compressor that is 1.5x or 2x that for margin?

Thanks!

Designa n appliance to cool 2 one-gallon bottles of water- but its already been designed,proved and patented, its called a fridge:o

@ install monkey.

well yes, captain obvious strikes again! haha

i guess, more specifically, an appliance that shaped and built to cool only the bottles. so like a small fridge, but with good cooling power.

http://www.hvacrinfo.com/loadcalcs.htm
heatcraft engineering manual
check out this manual
yours captain monkey

You will find that it is not the refrigeration that effects the speed of cooling, but the water! Your water needs to move!