Optimum condenser fan speed

[star882]

If a condenser is speeded up, airflow will increase and the condenser temperature will decrease, making the system more efficient (ignoring the power used by the condenser fan). But at some point, the energy used by the fan motor will undo the savings from lowered condenser temperature.
Is there a "fast and simple" way to calculate the optimum fan speed or does it vary too much depending on the design? (This is all just for curiousity, BTW.)

[US Iceman]

But at some point, the energy used by the fan motor will undo the savings from lowered condenser temperature.

Exactly right.

I have seen this happen on large ammonia systems also. In the attempt to reduce the discharge pressure to some lower pressure you can expend considerable fan energy to save energy on the compressor motors.

You need to develop some sort of a model that accounts for fan energy and compressor motor energy compared to ambient conditions and condenser capacity to see the results.

Depending on the size of the system and the costs you are able to spend, you can use a VFD to incrementally decrease the fan speed. This will provide energy savings on the fan power, close discharge pressure control, and allow you to minimize the problem you have stated.

[Peter_1]

You can simulate this in Microsoft Excel with the function "iteration"
I'm plotting it out in a graph, is much easier for me.

[Lc_shi]

You can simulate this in Microsoft Excel with the function "iteration"

I've not found "iteration" in my Excel. Maybe it need to install add-ins ?

There is one thing I want to add, increasing air volume will enhance the condenser heat rejection,but it's without obvious effect while reaching one point. Only lifting air side heat transfer coefficient can't increase total K value. So the optimum value should think both the power consuming and K value improvement.

rgds
LC

[wambat]

I've not found "iteration" in my Excel. Maybe it need to install add-ins ?

There is one thing I want to add, increasing air volume will enhance the condenser heat rejection,but it's without obvious effect while reaching one point. Only lifting air side heat transfer coefficient can't increase total K value. So the optimum value should think both the power consuming and K value improvement.

rgds
LC

Of course You are correct in that there is a point on deminishing returns where increasing the air flow will not increase the heat transfer to any apprechible amount with out increasing the delta T