1. Introduction

Blow Thru or Draw Thru fans?. That’s the question I was discussing a few days ago with a colleague about the position of the fan or set of fans in an Air Handling Unit (AHU).

There are two possibilities:

• Blow thru. The fan is located before the coils.

• Draw thru. In this case, the fan is located after the coils.

Choosing one option or another has energy implications, so let’s try to figure out how to select the optimal arrangement.

2. Draw thru advantages

We know that the flow is pretty even in the negative section (suction) of the air handling unit, and less even in the discharge section (supply). That means that the face velocity through the coil will be more uniform. That is the reason why typically this is the most common arrangement.

One important thing to consider is the temperature increase due to the fan (you can see the explanation in our post Cooling and dehumidification in Excel section 3.4) so it should be considered an additional room heat gain.

3. Blow thru

As discussed above, this arrangement needs to add supplementary distance from the fan to the coil to achieve a uniform face velocity.

Nevertheless, there is a situation where we can use the heat generated by the fan to our benefit: in dehumidification processes. Dehumidification requires cooling and reheating, and we could save heating capacity by using the heat released by the fan.

Let’s see an example consisting of a make-up air handling unit supplying 12,800 m3/h from 32 °C / 15.14 g/Kg humidity to 18 °C / 7.20 g/Kg:

So in conclusion, by using the draw thru arrangement, in this particular case, we can get substantial energy savings. Ask your favourite engineering office for analysis in order to figure out the possible benefits of one arrangement or another.

Blow thru or Draw thru fans? The answer is: it depends!