How to determine the correct air over an evaporator coil

[kengineering]

Hi everyone,
I work for a small custom Display Case Mfr. Last week, we had an issue in the field with too much air overloading a unit and it has me thinking that in some instances, fan choice may have been just a guess based on a previous design. Simply put, an educated guess. We haven’t made that particular sized case since 2010 so I don’t have recent history to refer to.

In determining the proper airflow over the evaporator is there a way to come up with the best fan choices? When I was in in the field as a service tech I would look for a 10°f differential across the coil and call it a day.

I realize that airflow is probably the most complex issue in a display case especially open type air-curtains. However I believe this site has the brightest minds around and I just may find some little thing that leads to learning something new. (and I'm not just blowing smoke)

As always thanks for your input.

[Magoo]

Hi Ken
I watched with interest your earlier post, and the resolve.
Evaporator coil selection and air flow are complex ed. Most coil manufacturers publish performance based on evaporator TD ( SST versus air on temperature ) Comes down to air dwell time in coil, with staggered pitch tubes and fin spacing. High air flow equals high sensible load and low air flow equals high latent load.

[kengineering]

You're a smart man Magoo. My coil manufacturer said pretty much the same thing..

What happened to you avatar! Kinda miss it.

[Magoo]

Hi Ken,
Basically high air flows screws the diffusion factor and increases the off coil temperature which in turn increases the Sensible heat ratio of coil. Results in high suction pressures which in turn overloads compressor selection and high discharge pressures, that compounds the problem even further.
I sent a PM if you are interested.
cheers magoo

[Goober]

Had this exact problem quite a few years ago. Local manufacturer supplied a display case, that could never get to temperature, condemned compressors, replaced refrigerant and multiple service calls later, we finally, by pure luck found the problem!

The supplier had installed a very small and non nondescript evap fan speed controller (one small 10watt fan motor) buried in the back of junction box. Seems someone, either supplier or service tech had knocked the setting and increased fan speed. Once we adjusted back to a slower fan speed, case worked beautifully.

But most definitely a case of trial and error or suck it and see on the part of the supplier as to what would be the best airflow...

[Grizzly]

Nice explanation Goober.
Not dissimilar to too much product in the cabinet disrupting air flow.
You continue to impress with the breadth of your knowledge Magoo!
(Cannot award rep points as I have to spread the love (STL.))
Grizzly