How to calculate cooling capacity for fridge?

How do I determine the cooling capacity needed for a specific application?

I know the item I want to cool, it's two large bottles of water. Refrigerant is R-143a. The temperature I want in the fridge is 38F. So, based on the 3 variables of ambient temperature, thermal load and fridge temperature how do I determine the cooling capacity I need in Watts?

Is the thermal load simply Q=m*cp*deltaT of the bottles of water?

I know that to size a compressor, the cooling capacity is given for a specific temperature at the evaporator. I know that it needs to be less than the temperature I want in the cooling cabinet (38F), but how much less?

Is the calculation simply

Cooling Capacity (W) = Qthermal load/desire pull down time - Qheat gain through walls

where
Qthermal load = m*cp*deltaT

Qheat gain through walls = h*A*deltaT+k*A/d*deltaT

This doesn't account for the decrease in ability to cool at high ambient temperatures, so I don't think it's correct. help please!

Hi Cindy,

Might want to check our User Agreement on homework and also your course notes over the past few weeks, I am sure it was covered there, maybe in a slightly masked form, but there non the less. :)