Tuesday, 6 May 2014

Evap cooling vs. air conditioning

Comparing evaporative cooling with traditional air conditioning

Why do we evaporate water?

The fact that the energy derived from the evaporation of water is the driving force between the cooling potential of evaporative cooling establishes beyond doubt that evap cooling uses much less electrical energy to accomplish an energy reduction in the air, required to remove a heat load from any occupied space.
The efficiency of evaporation is however directly proportional to the moisture content in the air per se which then confirms that the more arid the region, the more cooling can be required.
With the development of two stage evaporative cooling (or dry/wet cooling), the geographical areas suitable to evaporative cooling can be vastly expanded, as two stage evaporative cooling cools their air through 20-25 °C in arid areas and 10-15 °C for the wetter areas. This then also establishes the limitations of the system from a cost point of view.
Generally in South Africa the two stage evaporative cooling developed by Protek cools the air to 19 °C for the worst climate of Polokwane and Rustenburg and down to 14 °C for the arid areas of Kimberley and Bloemfontein.
In general the overall temperature drop is 120% of wet bulb depression (Ambient dry bulb - Ambient wet bulb temperature).
Due to the upper comfort limit of 25 °C and 26-27 °C for industrial applications, the economic feasibility then limits two stage evaporative cooling to a supply air of below 20 °C. Fortunately this is possible for the complete South Africa excluding Mpumalanga, Limpopo lowveld regions and the KZN coastal area.
Single stage evaporative systems can only achieve between 80-90% of the wet bulb depression which generates air at 4-6 °C higher than two stage evaporative cooling and is thus only feasible as an alternative for human comfort in the more arid areas. Provided that the increased humidity is acceptable. The humidity of single stage is 10% higher over two stage evaporative cooling due to the process which sprays water into hot air as opposed to two stage evaporative cooling that sprays water into cooler precooled air.

What is the total cost of owning and operating evaporative coolers?

How feasible is two stage evaporative cooling compare to traditional compressor based air conditioning?
The running cost of cooling for two stage evaporative cooling is between 25-30% lower than packaged air conditioners. Capital expenditure comparison depends on the following params:
  • Geography: The more arid, the more cost effective two stage evaporative cooling
  • Application: The higher the floor temp the more cost effective two stage evaporative cooling.
  • Building construction: The less airtight the building, the more cost effective two stage evaporative cooling.
  • Fresh air requirements: The higher the requirement, the more beneficial two stage evaporative cooling.
  • Other factors:
    • Limited availability of electrical power may compel the use of evaporative cooling.
    • Unavailability of water might favour traditional air-conditioning.
    • Limitation on RH, the higher the limit, the more AC units are favoured.
    • Space on ducting: Less ducting favours aircon
    • Heating: Two stage evaporative cooling can be equipped with heating like any aircon. With gas, electricity, water. With high air volume floor distribution is better with two stage evaporative cooling provided the fresh air is properly limited.

Cost study to establish the capex and running cost

This study focused on a moderate climate such as Pretoria.
  • Tambient - 25 °C
  • Floor area - 1000 m2
  • Climate - Pta, Schoemans street WB40.
  • Tinside - 24 °C /60 % RH
  • Fresh air package: 1.5 l/m2

Control modes:

Evap cooling - 3 steps control
  • Ventilation
  • Single Stage
  • Two stage
Package unit - 3 steps control
  • Ventilation
  • 50% cooling
  • 100% cooling
COP overall including air movement air conditioner 2.5.
Study done for constant volume and variable volume as alternatives:
Two stage evaporative cooling
Packaged AC
GTH - EC (kW)
Fresh air
1.5 l/s.m2
35/20 °C
35/20 °C
24 °C/60% RH
23 °C/50% RH
Tsupply air (Db/Wb)
18/17.2 °C
13/12 °C
Airflow m3/s
kWH/annum Constant Volume
27118 (57%)
kWH/annum Variable Volume
Capex Constant Volume

R220 000
R270 000
R220 000
R170 000
Total cost
R440 000
R440 000


The saving of approx 20 000 kWH/annum would favour two stage evaporative cooling considerably.
Sensitivity analysis in respect to capex shows the following results in R/m2 for equipment and ducting only.

Tambient (°C)
Two stage evaporative cooling
Pretoria East
Johannesburg East
Oliver Thambo Airport

Capacity has been increased to the warmer climates to allow for the increased facade loads.
In general CAPEX for two stage evaporative cooling is equivalent or lower by up to 10% compared to AC.

Who else makes use of Evap cooling?

Industrial areas

They tend to favour two stage evaporative cooling more due to the nature of the building structure which are normally not airtight and operations requiring roller shutter doors and good entering and leaving the building.
Floor temp also tend to be on the higher side and 27 °C/50RH is not uncommon to achieve for two stage evaporative cooling.
For Rosselyn with a 30% fresh air infiltration load and RSH 110W/m2 the capex as follows:
  • two stage evaporative cooling industrial plant R410/m2
  • Packaged AC R450/m2
Running costs will again be 40-50% lower for two stage evaporative cooling.

Office blocks

We would refer you by Toon Herman for extensive analysis that has been done on office blocks.


Agricultural products normally needs a higher RH to preserve product life and prevent dehydration and two stage evaporative cooling has proven in terms of stage evaporative cooling lf to be ideally suitable for export grape cooling.