Wednesday, 15 October 2014

Indirect evaporative cooling

Cooling air with water can be done in a number of ways. This article explores the traditional single and two stage evaporative cooling and contrasts it with newer methods and products.

The basic principle of evaporative cooling evaporates water into air, thereby cooling the air.
There are a number of critical issues to consider in cooling:
  1. The thermodynamic drive for cooling. How cool can the air be when it leaves the unit, influenced mainly by the humidity of the air (as measured by the wet bulb temperature).
  2. The thermodynamic performance with increase velocity/volume. Often processes perform well at low velocities because there is enough time for heat exchange to take place, but when velocity increases, the size of heat exchangers increase and consequently the unit costs.
  3. The pressure drop through the unit. Heat exchangers typically perform better with turbulent flow, but the result is also that pressure drops increase non-linearly at increased velocities, again, driving  capital costs up.
  4. Evaporation media performance is typically measured of how much water you can evaporate into the air at a specific design velocity. The higher the velocity of the air, the lower the fraction of water that can be evaporated.
  5. Wet bulb depression. Cooling of evaporation coolers is typically measured as a fraction of the difference between the dry bulb and wet bulb temperatures of the ambient air. Single stage coolers achieve in order of 60 to 90% of the wet bulb depression, while more modern two stage processes can achieve in the order of 120% of wet bulb depression.
  6. Human comfort typically requires room conditions of 24 °C and an RH of 60%.

Single stage evaporative cooling

Figure 1: Single stage evaporative cooling
With single stage, water is evaporated into air using paper media. The air can only be cooled by about 90% of the wet bulb depression (300mm thick media). When the media is thinner (100mm), 60% wet bulb depression can be expected.
This means in practice that during summer when temperatures are Tdb/Twb 30/20, that a single stage unit can only produce air at 22 °C.
Most imported single stage units however, have very thin 100mm media which means they only produce air at about 24 °C.
The RH of single stage units is also high, in the order of 70-90 %.
The reality is that single stage cooling is sufficient for most of the summer when wet bulb temperatures are in the order of 17 to 18 °C. The problem days when the temperatures go above this, result in humid, muggy air.
Figure 2: Psychrometric performance of single stage evaporative cooling

Dry/Wet two stage cooling

Figure 3: Two stage evaporative cooling
Two stage cooling first pre-cools the air using a dry water-to-air heat exchanger, thereby not only lower the temperature of the air, but also increasing the capacity of the air to absorb water (lowering its wet bulb temperature) resulting in overall drop in temperature in the air off the unit.
The water leaving the dry coil is now hot and put through a cooling tower. The air that evaporates the water in the cooling tower, is discarded, while the cool water (now close to the wet bulb temperature of the air), is used to adiabatically cool the primary air even further.
This process can typically reach 120% of the wet bulb depression.
In the psychrometric chart below, air is first cooled dry from A to B after which water is sprayed into the air, cooling it further to C.

Figure 4: Psychrometric performance of two stage evaporative cooling

Indirect evaporative cooling

Two stage evaporative cooling is limited by the efficiency/cost of dry cooling. Normally the water temperature is limited to the wet bulb temperature of the air. With two stage cooling, lower temperatures would be reached if the pre-cooled dry air (primary air) is used to evaporate water into where the water will now approach the new wet bulb temperature of the dry air (Point E in Figure 5)

Figure 5: Indirect evaporative cooling psychrometric performance

The technical design

There are a number of global manufacturers who currently manufacture the integrated air to air, cross flow heat exchanger that does both dry and wet cooling in the same heat exchanger.
The primary concept rests on 2 types of channels in the plate heat exchanger that isolates the primary and secondary air streams from each other.
Air is cooled in the primary stream to as close as possible to the dewpoint temperature of the air. Some of this air is diverted back into the secondary channels where the dry air is now used to evaporate water also introduced into this channel.

Figure 6: Indirect evaporative cooler design

Figure 7: Heat exchanger design
Calculations show for 1 m3/s design, with a heat transfer coefficient of 50 w/m2/K,  that approximate 80 m2 of plate will be required to supply air at approximate 130% of wet bulb depression.


Currently the traditional Two stage evaporative cooler manufactured by Protek is still lower than 50% of the cost of the indirect evaporative coolers supplied in China.
None of the indirect coolers can provide air flow capacities higher than 12 m3/s. This is primarily a result of the large capital cost for producing these coolers at large sizes.
Pressure drops in the indirect coolers limit the size of the heat exchanger and requires a modular design, again, something that negatively affects scaling to industrial volumes.

Global heating trends not only increase the dry bulb temperature of air, but also the wet bulb temperatures, which renders single stage cooling even less effective.