OXYVAP® Technology – two stage evaporative cooling

Two-Stage Evaporative Cooling

 

Water that leaves a direct evaporative cooler at the bottom of the unit has a temperature close  to  that  of  the wet  bulb  temperature  of  the outgoing  airstream.  In  the  two-stage evaporative  cooling  setup,  that  chilled  water  is then led  through  an  air-to-water  heat exchanger  to  pre-cool  the  hot  air  before  it  is  cooled  down  further  by  the  direct
evaporative cooler.

In  terms  of  energy  balance,  nothing  has  changed  compared  to  the  standard  direct evaporative  cooling  principle;  overall,  it  is  still  an  adiabatic  cooling  process.  However, whereas the direct evaporative cooler has a uniformly distributed temperature profile at the side of the outlet air, the two-stage evaporative cooler shows a vertical temperature gradient: further down the air temperature gradually decreases.

Typically, the lower 60% of the outgoing air has a specific enthalpy lower than that of the inlet  air.  Whereas  the  temperature of  the  outlet  air  from  a  direct  evaporative  cooler  is restricted to the wet bulb temperature, in this case, when taking only the lower 50% of the outlet air from the two-stage evaporative cooler the temperature can even be lower than the initial wet bulb temperature. The corresponding wet bulb efficiency will then be higher than 100%.

 

Comparative calculations – supply temperature

 

The figure below shows the cooling process of a standard direct evaporative cooler.

 

Figure 1 – Cooling process of a standard direct evaporative cooler

Two Stage evaporative cooling fig.1


As described  above,  there  is  a  uniform  vertical  temperature profile.  Air  is  cooled adiabatically to 1.7 K above the initial wet bulb temperature, corresponding to a wet bulb efficiency of 90%, close to the physical limit of 100%.

 

The figure below shows the cooling process of the two-stage evaporative cooler in which the  exact  same  amount  of  air  is  flowing  through  the  system  as  in  the  configuration above.


Figure 2 – Cooling process of the two-stage evaporative cooler

 

Two Stage evaporative cooling fig.2


The  first  stage  (the  air-to-water  heat  exchanger)  pre-cools  the  hot  outdoor  air  to a uniform temperature  of 23.8 °C,  without  changing  its moisture content.  Due to  the fact that  there  is  a  large  temperature  difference  between  water  supplied  to  and  extracted from the direct evaporative cooler, contrary to the previous configuration, air leaving the second  stage (the  direct  evaporative  cooler) now  does  show  a  vertical  temperature gradient, as described above.

The lower 50% of the supply air has an average temperature of 18.3 °C, 2.7 K below the initial  wet  bulb  temperature,  corresponding  to  a  wet  bulb  efficiency  of  116%.
Furthermore, although achieving  a  lower  temperature  compared  to  the  previous configuration, less moisture has been added; the resulting absolute humidity is 11.4 g/kg instead of 15.0 g/kg.

The upper 50% of the supply air has an average temperature of 25.4 °C, which is higher than that of the supply air in the previous configuration, but may still be low enough to serve other purposes.

 

More information and specific details are available in the documents listed in our DOWNLOAD CENTER.

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