Air handling units (AHUs) are critical components of heating, ventilation, and air conditioning (HVAC) systems for preserving indoor air quality and comfort. AHUs are in charge of filtering, chilling, and distributing conditioned air throughout a structure. AHUs are commonly configured in two ways: blow-through and draw-through. In this article, we will look at the distinctions between blow-through and draw-through AHUs.

Blow-through AHU:

Blow-through AHUs feature a unique design in which the supply fan is placed before the cooling coil or heating element. The fan takes air into the unit and pushes it past the coil or heating element before dispersing it to the conditioned environment in this arrangement. The air is subsequently expelled from the unit through the supply air ducts.

Draw-through AHU:

Draw-through AHUs are set up differently from blow-through units. The supply fan is placed after the cooling coil or heating element in this configuration. The coil or heating element draws the air, which is subsequently forced into the conditioned room.

Because air in draw-through AHUs goes through the cooling coil before reaching the fan, there is a lower chance of condensate carryover. This prevents moisture from entering the fan and causing damage or efficiency losses.

Diagramatic Representation of AHUs:

Major Differences between Draw-through and Blow-through AHU:

ParametersBlow-through AHUDraw-through AHU
ConstructionIn the Blow-through type of AHU, Supply fan is installed before the cooling or heating coils. Since the fan blows the air upon the coils, this system is referred to as Blow-through AHU.In the Draw-through type of AHU, Supply fan is installed after the cooling or heating coils. Since the fan draws the air from the coils, this system is referred to as Draw-through AHU.
Flow PatternTurbulence across the coil is more in case of Blow-through AHU. Due to more turbulence, you will get different temperatures on the overall surface of the coil.Flow pattern is evenly distributed in case of Draw-through AHU because air is drawn properly through the coil surface area. Even temperature on the coil surface.
Plenum LengthWe have to increase the length of the plenum;i.e. between the fan & the coil in order to provide a better airflow pattern across the coil.In the Draw-through type of AHU, there is no need to provide the gap after the fan. Therefore, AHU length doesn’t increase in this case.
AHU LengthAHU length will increase because of increased plenum lengthWhere there is a space constraint, Draw-through AHU will be the best choice.
Moisture Carryover IssueIn this system, air containing moisture can skip to space. This problem can be completely eliminated if we provide sufficient space between the fan & the coil.In the Draw-through system, the fan is after the cooling coil. This fan heat removes the moisture contained by air. So, this is a beneficial point in Draw-through AHU.
Filter Life
Filter life will be affected if there is a problem of carryover moisture.In this system, there is no issue of carryover moisture filter life will not be affected.
By-passWhere there is a need to by-pass some air, we can’t use Blow-through AHU. As there is positive pressure in the plenum after the blower, so it is not advisable to provide fresh air cutout in that plenum.In case of by-pass, Draw-through AHU is the only option.
U-Trap DesignIn Blow-through AHU, U-trap is on the positive pressure side. This will lead to exhaust of air from the AHU when U-trap is dried; especially in wintersIn Draw-through AHU, U-trap is on the negative pressure side. This will lead to inlet of outside air into the AHU when U-trap is dried
Application AreaDue to moisture carryover issue, Blow-through AHU’s are preferred in areas where RH is not an important parameter; like in CNC.Draw-through AHU can be used anywhere; to control the temperature & humidity of the space.
Cost   ComparisonInitial cost of AHU is high because of increased AHU length but operating cost will be less than Draw-through system.Initial cost of AHU is less compared to Blow-through.

In a Nutshell:

Making the choice between blow-through and draw-through AHUs is influenced by a variety of parameters, including cooling efficiency, filtration needs, maintenance concerns, and maintenance concerns. By carefully examining these aspects and consulting with HVAC specialists, one may choose the best AHU design to match the individual demands of their HVAC system while ensuring maximum performance and interior comfort.

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