Blower motor distance from HX coil?

[ManCave]

Hi Guys,

I have a 3-core, 22" x 25" water-to-air heat exchanger coil which I'm using to build a heater for a pole building.

The case I made is large enough that I can mount the squirrel cage blower inside of the case with a filter on the back to filter in-coming air. However, this would result in the blower being about 5 inches away from the coil. Mounted that close, I'm concerned that only the center of the coil might be used.

Just wondering how critical the distance is between the heat exchanger coil and the blower motor for best efficiency?

 

[redsky49]

"Just wondering how critical the distance is between the heat exchanger coil and the blower motor for best efficiency? "

It is indeed critical for proper performance of the system. Based on the limited information provided, it is likely the system you describe will not perform anywhere near peak values.

Some general rules of thumb:

The discharge outlet of the fan should transition in a gradual manner to full size of the coil's flange connection.

Any duct transition should not exceed 45 degrees (30 degrees is optimum).

Velocity across the coil should be as low as possible, especially with a three row coil. For a hot water coil 600 fpm would be a reasonable limit. For your stated coil size, for example, a maximum cfm would be about 2290 cfm. [If used for cooling applications, a lower velocity would be recommended to avoid condensate blowing off the coil. In this type of application, I would have an upper limit of 400-450 fpm.] This is also somewhat dependent on HWS temperature, as well as coil design ( serpentine vs. single pass, coil fin style, cleanliness of the air, flow arrangement, etc.), etc., etc. Note that coils "age" over time as the typical aluminum fins become less aerodynamic over time (corrosion, dirt, etc.), and that the airside pressure drop across the coil will increase.

You should also have a flexible duct connection between the fan and the coil to avoid damage to the fan, the coil or both.

Avoid any dissimilar metal connections in hooking up your coil.

You may want to consider a draw-thru arrangement, rather than a blow thru.

The airside pressure drop across the coil, in addition to the filter/duct/register pressure drops should also be evaluated in order confirm the adequacy of the fan.

Hope the is helpful.

As always, offered only as opinion