Defining Cooling Power: What Does it Really Mean?

[freddie_mclair]

Hi everybody!

Well, these are just two basic questions that are bothering me.

1.When it is said that, for example, a Cryocooler has 1W of cooling power at 4.2K, what does it really mean?
To me, the action of "cooling" depends on the material that it's being cooled down. So, for different materials, what will happen is that they will take more or less time to cool down. Is that it?

2. Using a gas/liquid for cooling down some material, can we define it's cooling power as being: P=q*cp*ΔT, being q the mass flow (kg/s), cp the gas/liquid specific heat and ΔT the difference in temperature between the temperatures at the inlet and outlet of the system to be cooled down?

So, for you, what's the meaning of cooling power?

Thanks in advance!

 

[mfb]

Materials of different thermal capacitance will need a different time to cool down, sure. Even the final temperature depends on the setup. The cooler can cool away 1W (at 4.2K), but if your material gets warmed by 5W from the environment it will heat up until both power values are equal.

2. Using a gas/liquid for cooling down some material, can we define it's cooling power as being: P=q*cp*ΔT, being q the mass flow (kg/s), cp the gas/liquid specific heat and ΔT the difference in temperature between the temperatures at the inlet and outlet of the system to be cooled down?

If the whole temperature difference comes from cooling the sample, cp is independent enough of the temperature to justify a constant value, and if there are no phase transitions: sure. You might want to reduce this value by heat conducted in the cooling pipes and other parts of the setup.

 

[freddie_mclair]

Thanks mfb for your reply.

if your material gets warmed by 5W from the environment it will heat up until both power values are equal..

I didn't understand this... what do you mean by "both powers"?

 

[mfb]

The temperature is constant if "heat flowing in" is equal to "heat flowing out" (due to the cooler).