Rules of Thumb?

[cindy1]

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!

[install monkey]

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

[cindy1]

@ 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.

[install monkey]

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

[mad fridgie]

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