Introduction

An important detail to consider when installing an Air Handling Unit (AHU) is the condensate drains. This consideration is not always followed and it can cause significant operational problems. Mainly in cleanrooms AHUs.

Why shall we consider the Air Handling Unit condensate drains?

The image below shows the pressure profile in a typical Air Handling Unit:

As we can see from the graph, the fan provides a high pressure to compensate for the pressure drop due to the AHU elements (coils, prefilters, etc.). Also provides for the ductwork system pressure drop. As we know well, the static pressure is higher than other systems than the pharmaceuticals, due to the presence of HEPA filters.

Air leakage through a drain can occur. These drains are typically located in cooling coils or wherever there is a possibility of condensation.

This video from Practical HVAC shows clearly the problem:

The solution

It is for that reason that we have to provide a siphon.

This siphon should contain a non-return valve and a self-filling device. The diameter should match the unit’s connection.

The height of the siphon depends on the pressure of the ventilation units. If the drain is located before the fan, it could happen a suction of water if the height is lower than the pressure. Conversely, if located in after the fan, the draining could not be done properly.

Another consideration is that the water must flow directly from the siphon into a tray or a funnel. Do not under any circumstances connect the siphon directly to the sewage network.

To this point is recommended to follow the manufacturer specifications.

Example. Air Handling Units condensate tray calculation

For this example, we are going to consider a Trox unit (link here)  Other manufacturers give similar directions as well (Johnson Controls, Robatherm, Rosenberg, Flakt, etc.)

• Negative Pressure

For negative pressure, they recommend the following values:

H1 = P/10 + 50 (mm)

H2 = H1 – (GR-60) (mm)

Where P is the pressure inside the Air Handling Unit (in Pa), and GR is the base frame height (in mm).

• Positive Pressure

For positive pressure (drain after the fan), they recommend the following values:

H1 = 1.5 x P/10 (mm)

H2 = H1 + (GR+110) (mm)

H3 ≥ 50 mm

Where P is the pressure inside the Air Handling Unit (in Pa) (positive value), and GR is the base frame height (in mm).