Why does it makes sense to replace Single Stage Evaporative Coolers with Two/Three Stage Coolers?

Single Stage Evaporative Cooling typically evaporates water directly into the primary airstream supplied into buildings. There are a number of limitations to Single Stage Cooling:

• Typically, only 90% of the possible wet bulb depression can be reached, even with 300mm thick evaporative media. So for Tdb/Twb of 30/20 °C you can achieve a Tsa of 21 °C.
• The primary supply air fan can typically not handle any static pressure requirements, so the units must discharge air directly into the area to be cooled.
• The units therefore are placed on top of a roof, which makes monthly maintenance a major issue (so it doesn’t get done).
• The vertical discharge from the unit also means that any water carryover and leakage will be directly into the space to be cooled.
• The units are made from inexpensive composites and thermosetting plastic and have a limited lifetime in South Africa of around 5 years.
• Single Stage units results in very humid air in the building. This limites secondary evaporation, especially from humans.
• The ducting requirements are typically pretty extensive and you need a lot of air to achieve a specific amount of cooling.

Two and Three Stage cooling on the other hand is a partial indirect evaporative process where water is cooled below wet bulb temperature using a cooling tower and dry cooling process. 

• Benefits of Two and Three stage Cooling are:120 and 130% dry-wet bulb depression is possible. So for Tdb/Twb of 30/20 °C you can achieve a Tsa of 18 and 17 °C.
• The units are manufactured from Stainless Steel and have lifetimes in the order of 20 years or more.
• The primary fan is a high quality high efficiency Backward Curved EC plug fan designed for 200Pa external pressure so can be connected to ducting.
• The additional cost of the hardware is typically immediately offset by the reduction in unit size for the same amount of cooling. So a 16 m3/s Single Stage unit offers as much cooling as a 8 m3/s Two Stage unit and they cost the same.
• Ducting cost is significantly cheaper due to lower air volumes compared to Single Stage.
• Units typically are placed on ground level and discharge horizontally so they are easy to maintain and no risk of product damage from leakage or water carryover.
• The final humidity in the room is typically around 60% RH compared to above 80% RH for Single Stage.

Differences between Single, Two and Three Stage Evaporative Cooling

Evaporative cooling today is mostly done with industrial machines where water is evaporated into the air, in a paper pack. This process is more robust than traditional nozzle sprays because blockage and fowling is less of a concern and the process is a very efficient (you can typically evaporate about 90-95% of the maximum amount of water possible, in a pack that is 300mm thick).

Single Stage

In Single Stage, water is evaporated into the air being pumped into a building. This means that for ambient conditions of 30 °C dry-bulb and 20 °C wet-bulb, can achieve a supply air temperature of 22 °C.

Rooms are typically designed to be below 24 °C , RH (Relative Humidity) 60% and if you have a supply temperature of 22 °C, you can do very little cooling before the air is too hot to use.

Two Stage

With Two Stage cooling, the air stream is split into a primary and secondary air stream. All  water is the paper pack is now cooled in the secondary stream (cooling tower) and that very humid air is discarded.

The primary air stream is dry cooled in a 6 row heat exchanger and now supply air temperatures of 18 °C  is possible for ambient conditions of 30/20 °C  dry-bulb/wet-bulb.

Three Stage

Three Stage cooling is a technical refinement of Two Stage cooling where the cooling tower water pack is disconnected from the primary air cooling pack. The result is that you can  lower the supply air temperature by another 1 °C ( so now you can supply at 17 °C  with 30/20 °C  dry-bulb/wet-bulb conditions).

This improved performance comes at a slight increase in cost as you now have more heat exchanger area, pumps and two sumps to maintain.