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.

Three Stage Evaporative Cooling

What is Three Stage Evaporative Cooling?

Toon Herman recently proposed an improvement on our Two Stage evaporative cooling process that would result in a supply air dropping by another 1 deg C.



The process splits the sumps between our primary and secondary air sections.

Advantages of this design include:

1. Up to 7 steps of cooling/heating. This allows for a very flexible and cost effective control strategy because there are so many control options.
2. Low supply air temperatures. The unit can now approach dewpoint much more closely because of the lack of air reheat in the primary water pack (exposure of our primary circuit to the conditions in the cooling tower).
3. A complete integrated design. Compressors are located inside the unit and the outdoor coil, located efficiently in a cooling tower with improved oncoil conditions.
4. Compact footprint. The air intakes are now on three sides of the unit, making the unit very compact to install.
5. Modular design with hinging doors. Two of the side panels now conveniently hinge open, thereby simplifying the maintenance of the unit.
6. Most importantly for our customers, our units have been reduced to two configurations: A 6.5 m^3/s and a 3.25 m^3/s configuration. The main benefit of this commoditization  is an effective reduction in cost of about 20% per m^3/s (or kW cooling) of the units.

Building a prototype

Ecoaire and Protek built the first version of this unit in August in China.

Modular side panel can hinge open for maintenance

Integrated Three Stage coil

Integrated dome compressors and outdoor coil. No more connecting pipes and gassing between indoor and outdoor units.

The complete 6.5 m^3/s unit, ready to ship to South Africa

Reduce operating costs by 50% when moving to Evaporative Cooling

Normal air-conditioning is primarily driven by a building design, solar radiation and then lastly by the local climate (cloud cover etc) at a specific location.

Evaporative cooling is additionally also dependent on the humidity at a location.

Design day

Most consultancies approach this problem by selecting a design day, something that broadly corresponds to a day when it is very hot and pretty humid. You cannot select the worse day of the year to size your air conditioner because this will result in a very expensive oversized solution. It is normal practice to accept that you will be 1 or 2 °C over specification for up to 40 hours a year (5 days a year).

The design day approach is problematic in that it can easily result in an over design. Imagine a location that is pretty cold but have a very hot summer month (high altitudes)? Ok, so you over designed the solution by 20%. No biggie.

But when you try and work out the operating cost of this solution, knowing the exact daily/hourly temperature distribution is very important.

ProtekCooling has developed a simulation package that will simulate the performance of your evaporative cooler for historical temperature and humidity data.

So now, for the first time, you can get an idea of how much it would have cost to operate a DX only system vs. an evaporative cooler.


Water as a scarce resource

A major complaint against evaporative cooling is that what you save in electricity, you lose in paying for water.

This argument proves to be invalid for two reasons:

1. It turns out that the total cost of water consumption for an evaporative cooler per year, even at elevated rates, is only a 1/5 of the cost of electricity for the year. When you look at your electricity savings, typically 50% of the comparable cost of DX electricity, then the water cost less than 1/5 of the savings you make in electricity.
2. In many of the locations for industrial cooling, the annual rainfall exceeds the water consumption requirements. So by collecting rain water, which is Ph neutral and relatively easy to use in evaporative cooling, you can remove your dependency on external water supplies.

Example simulation

Here are the results for a real world application. It is a new mall in Transvaal, with approx 20000 m^2 floor space (15 000m^2 under evaporative cooling). The client required 220 m^3/s of Two Stage air to condition the mall to 24 °C.


Cost of electricity

To do the whole mall with DX, would have resulted in operating costs of approx R1,100,000 per year.

Moving to Two Stage Evaporative cooling, reduced that cost to about R500,000, less than 50% of DX.


Cost of water

If water was purchased from the municipality at R25/kL, the annual costs would have been about R110,000


Availability of water


Rainfall in this region is in the order of 600mm per year. Collected from the roofs of 20 000m^2, this would provide 12 kL of water per year at zero cost (you have to pay for the storage tanks).

The units in this simulation consumed 4 kL per annum.


Total cost of operation

The total cost of operation for Two Stage cooling is about R610,000 per year compared to R1,100,000 for DX. This is about 55% of the DX installation.

If the Evaporative Coolers are assumed to have cost R8,000,000 capital layout, assuming the cost of electricity stays constant, the whole plant would have been paid off after 16 years. Note that this is not the payback time (which is the time it requires to pay for the additional cost of evaporative cooling vs DX) because the capital costs for both plants are about the same. This means that after 16 years, you could have paid for the complete evaporative cooling plant from the operational savings compared to a DX plant.


Other observations

The unit state through the year

The number of hours that the unit would be above specification (24 deg C) and how much it would be over specification.

The unit supply air temperature vs ambient conditions. The red and purple lines are the ones that show the supply air temperature for single and two stage units.

The energy consumption of the different types of units. It is clearly visible that nearly at all temperatures, the evaporative cooler requires significantly less energy that the DX system.

Monte Vista evaporative cooler performance

Histogram of temperatures over specification (red bars) 

Two Stage Evaporative Cooler Temperatures for Summer 2013/2014 Monte Vista

Windhoek evaporative cooler performance

Histogram of temperatures over specification (red bars) 

Two Stage Evaporative Cooler Temperatures for Summer 2013/2014 Windhoek

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.

Conclusions

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.