Airflow through condensor

[Peter_1]

Suppose you have 6 small fans on a condensor.
They're rated each at 800 m³/h at 0 pressure rop.
Once mounted in the condensor, you measure a flow of 1.3 m/sec.
Given the are of the condensor, you find that the flow/fan is 400 m³/h.
Lookin now in the fan chart, you find an actual pressure drop of 100 Pa for the coil.
Suppose we install now 12 identical fans, what will be the flow through the condensor? It will be not the double because pressure drop will increase due to the increased flow.
This is not a riddle but a serious question.

Suppose we install 2 other fans with a free flow of 2.500 m³/h and which gives each 1.800 m³/h at 100 Pa pressure drop. How can you predict the flow because the 2 will give 3.600 m³/h at 100 Pa.

I'm confused a litlle bit.

We made a condensor and flow is too low (.3 m/sec through the condensor)
How will the codnensor capacity adapt to this increased flow? It will be anyway not a proportional function.

[US Iceman]

The fan ratings are based on laboratory testing methods. The air flows are usually rated with free air delivery and no static pressure loss. Certain types of fans are rated differently.

The fan volume can be affected by the installation significantly. Anything that causes an increase in the static pressure loss reduces the volume of air delivered.

These reductions are called "system effects". Please look at the links below for additional information.

http://www.greenheck.com/technical/files/engineering/00apr_eng.pdf

http://www.hpac.com/member/archive/pdf/2004/0404/brooks.pdf

http://www.feta.co.uk/downloads/fma-guide.pdf

A general recommendation for propeller fans is to have the fan at least 1 diameter from the coil. The fan cowling can have a significant negative impact, if the fan tip clearance is too large. Also, the throat depth of the cowling can also reduce the CFM.

A draw-thru fan will have slightly better performance, since the air velocity over the coil is more uniform. A blow-thru unit is different, since you will have highly localized air profiles over the coil. With a blow-thru unit the air velocity is less uniform.

I believe if you look at the fan affinity laws, you can use the established pressure loss (100 Pa) through the coil at the measured velocity. This should allow you to reasonably select the new fans.

However, the system effects should be reviewed to see if they can be reduced.