# Air-Cooled Heat Exchanger Steam Saturation Temperature Question

1. Sep 28, 2012

### sanka

Hi All,

I am currently undertaking some research involving condensation phenomena in ACC's. The current ACC which I am working on has a variable speed fan. So my question is this; how does increasing the fan speed reduce the steam saturation temperature? Specifically, I am interested in how this happens physically, i.e. how phenomena in the boundary layers affect the steam temp.

My current understanding is as follows; mathematically if we apply Newtons Law of Cooling Q=hA(Ts-T∞) (where Ts is the steam temp. and T∞ is the air temp.) to the system, we know that as the fan speed increases (h increases) Ts must decrease for a fixed mass flow rate. So, as h→∞, Ts→T∞, and therefore the steam saturation temperature decreases. I understand that much.

What I am intrigued about, however, is how the physical mechanisims at the interface of the air-flow and heat exchanger tube wall determine the decrease in steam temp? I was thinking that as you increase fan speed, (thus increasing air velocity), the temperature gradient at the tube wall decreases and the wall temperature also decreases. If we assume negligible steam-side resistance and that the the wall temp.=steam sat temp., any decrease in wall temp. will also cause a decrease in steam sat. temp?
Am I completely wrong here or on the right track?

Thanks in advance for any help/criticisms!