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.
