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.

Cooling air with Ice

One of the biggest challenges for evaporative cooling is to demonstrate control in design. We have done that now by running simulations that can show you your historical room temperatures for a specific air conditioner.

As can be seen in the historical temperatures for Windhoek in Namibia, a Two Stage evaporative cooler would have produced air hotter than 15.7 °C for only 20 hours during the summer months of 2013/2014.

Figure 1: The Two Stage evaporative cooler temperatures off the unit during the summer 2013/2014

The question now is whether there is a cost effective way to manage the 20 hours in applications that are concerned about this issue.

We simulated a large hall (3000 m2) with 3000 people in it.

Outside temperature was assume to be 30/20 °C Tdb/Twb which would be typical for Pretoria/Johannesburg

The space  design condition was 22 °C with 60% RH.

Firstly, we simulated three hours in the morning each reflecting Tdb incrementally raising from 20, 25 to 30 °C (9:00, 10:00 and 11:00 hours)

Using a compressor

To size a DX system, we assume that each person must receive 5l of fresh air per second. It can be seen in Table 1 that the DX chiller size required to provide cooling for people in the room, would be 670 kW.

h outside air		kJ/kg	66
Room Air WB		°C	18
h room air		kJ/kg	57
Air from the outside	5 l per person per second	kg/s	15
Outside air total heat	OATH	kW	133
DX Chiller size	RTH+OATH	kW	670

Table 1: DX Chiller size calculation

Using ice storage to lower the DX chiller size

The problem here is that the cooling load is very focused on a few hours and if one could store the energy to cool the room, then a smaller compressor can be used to store the energy over time.

The next part of the simulation first calculated what the load in the room would be for the 3 hours. The RSH increases from 93, 101 to 109 w/m^2 for the hours in question.

Hours of the day			09:00	10:00	11:00
RSH		Total	93	101	109
Facade		w/m2	8	16	24
Lights		w/m2	15	15	15
People sensible		w/m2	70	70	70
People latent		w/m2	70	70	70
RTH		w/m2			179
RSHF		RSH/RTOTAL	0.57	0.59	0.61

The evaporative cooler will still do the bulk of the work, with the ice coil just filling in when required.

Hours of the day			09:00	10:00	11:00
Tdb		°C	20	25	30
Twb	Constant dewpoint	°C	17.0	18.5	20.0
T sa db	118% total cooling of db-wb depression	°C	16.5	17.3	18.2
Tsa wb	altitude correction for wb (80% dry and 90% wet cooling)	°C	16.3	16.9	17.6

The specific size of the Two Stage evaporative cooler is given below.

Q=RSH		kW	279.0	303.0	327.0
Cp		kW/kg.C	1	1	1
dT	Troom - Tsupply air	°C	9	9	9
m (Q=mCPdT)	Q=mCpdT	kg/s	31	34	36

And now comes the coil performance requirements to augment the two stage evaporative cooler.

Energy removed by ice coil
h_2S_off		kJ/kg	52.6	54.9	57.1
h_coil_off		kJ/kg	39.2	39.2	39.2
dh		kJ/kg	13.4	15.6	17.9
Energy removed	Airflow * dh	kW.h	416	527	652
Hours chiller operation		hrs	20
Total energy removed in 3 hours	Subtotal of energy removed over 3 hours	kW.h	1594
Energy per hour required for 20 hours	Compressor size	kW	80

Comparing the DX system with the ice storage alternative

It can be seen that the DX Chiller must be 8 times larger in capacity to supply the 670 kW vs the 80 kW required by the ice coil.

DX Chiller size	RTH+OATH	kW	670
Glycol Chiller Size	operating the no off hours in 37	kW	80
Chiller size increase		X	8

The amount of ice required to store the energy.

Size of ice tanks
latent heat of melting		kJ/kg	334
Kg ice required to supply 1355 kJ		kg	14601

The electrical connection size for the two installations.

Electrical connection GLYCOL	Air flow@600pa+Chiller@COP=3	kW	58
Electrical connection DX		kW	254

Sensitivity analysis

The initial study was done for ambient temperatures of 30/20. WHat will happen in drier climates where the ambient is 30/19 or even 30/18?

At 30/19, the chiller needs to be even smaller than before and a 10 times size reduction can be achieved.

DX Chiller size	RTH+OATH	kW	613
Glycol Chiller Size		kW	59
Chiller size increase		X	10

At 30/18, the solution is even more favourable with a 15 times size reduction in the ice storage chiller size.

DX Chiller size	RTH+OATH	kW	558
Glycol Chiller Size		kW	38
Chiller size increase		X	15

Conclusion

The conclusion of this analysis is that by installing ice storage in this case, the operating energy requirements, specifically the chiller size can be reduced by between 8 and 15 times the traditional chiller size.