mass flow rate controls the system?

[cindy1]

Is it correct that for a given refrigeration system (like a residential fridge), that has a known refrigerant, compressor, and line sizes, the only way to change the evaporating temperature is the mass flow rate? How does the system adjust with changes in thermal load? For example, does the flow rate automatically adjust if the fridge was just filled with room temp food versus after all the food is at the refrigerated temperature?

[RANGER1]

cindy1,
The short answer is yes.
With the warmer food put in the fridge, the refrigerant pressure will increase & compressor will be pumping more denser gas. The mass flow rate will increase.
The discharge pressure may also increase counteracting the higher suction pressure a bit (reducing volumetric efficiency).
At the end of the day it will slowly get on top of the load after a few hours.

[cindy1]

so while the fridge is working harder to get on top of the load, is the temperature constant at the evaporator or is it changing?

[Tesla]

Hi cindy1
Not to confuse you, if you look at it from a different angle - on a cooler day the condenser will be colder and more liquid will condense logging in the condenser with less space for condensing, this in turn will reduce the amount available in the evaporator for cooling. Both temperatures and pressures will change slightly as one effects the other with p1*v1/t1 = p2*v2/t2 relationship and in the closed system an increase in evaporator will have an increase in condensing. This is a dynamic cycle from tstat = on to tstat = off and from tstat = off to on, then there are three main variables being ambient temperature, fridge temperature and load in fridge.
So in short the liquid level in the condenser changes with loads - sort of self compensating load with capacity.

[cindy1]

Thanks so much for the help! I think I am slowly getting up to speed. So what I really need to understand is, 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 8F. 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?