Regular technicalsSupport

By: Grant Laidlaw

Welcome to the Solutions page

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.

Ben sent in the following: Sir, can you please explain the evaporative coolers. How do these coolers work? Thank you.

Hi Ben, yes it is very important to understand how evaporative coolers operate as they are somewhat different to air conditioning units.

In essence we pass air over a pad that has been saturated with water into the space to be cooled. The water evaporates and in order to do so heat energy is absorbed from the air, this then cools the supply air.

An evaporative cooler works on what is known as adiabatic cooling;

• An adiabatic process is one in which there is no heat gained or lost in the process, such as the evaporative cooling process.
• In the process, heat is changed from one form to another (sensible heat to latent heat) but there is no overall change to the total heat in the process.
• Temperatures change but the total heat doesn’

A very important aspect when dealing with evaporative cooling is air flow. You must make sure that all of the air entering the space to be cooled must be able to exit freely. Failure to achieve this will result in an inefficient system with higher humidity levels in the space being cooled. Air flow is the quantity of air moving through the system and space to be conditioned. Typically air flow is measured in litres/sec (l/sec) or cubic meters /hour (cm/h or m3/h), or cubic meters/sec (m3/s)

Air flow should not be confused with air velocity, which is the rate at which the air is travelling through the system.

In the case of evaporative cooling human comfort factors are very important;

• A human being’s thermal sensation is mainly related to the thermal balance of his or her body as a whole. It is NOT only about temperature and humidity.

• This balance is influenced by physical activity and clothing, as well as the environmental parameters: air temperature, mean radiant temperature, air velocity and air humidity.
• When these factors have been estimated or measured, the thermal sensation for the body as a whole can be predicted by calculating the predicted mean.
• The predicted mean is an index that predicts the mean value of the votes of a large group of persons on the 7-point thermal sensation scale table based on the heat balance of the human body.

Seven-point thermal sensation scale

Let us consider what is meant by comfort

Climate control is largely about establishing a comfortable environment for people.  

When people are comfortable they are more productive, more tolerant, more healthy, more amiable, more reliable, etc.

This translates into a more pleasant social atmosphere in some cases, and more productivity (and hence profits) in other cases. 

Human COMFORT has 5 components;

• Temperature
• Relative humidity
• Air flow
• Personal activity
• Personal clothing

Conventional Air Conditioners create COMFORT by changing Temperature and Humidity

Evaporative Air Conditioners create COMFORT by changing Temperature and Air Flow.

Human beings “feel” the combination of temperature and humidity.  As the humidity increases they “feel” hotter when the temperature has not changed at all. We call this “felt” temperature, “Effective temperature”.

Effective temperature (or Apparent temperature);

• Apparent temperature is a measure of how “hot” we “feel” due to the combined
• Effects of temperature and humidity.
• The higher the relative humidity the hotter we “feel” when there is no change in actual temperature.

Cooling effect;

• This term applies to the cooling capacity of the equipment.
• Cooling effect is measured in kW.
• Che cooling effect is calculated after testing for the Saturation Efficiency and airflow according to standardised procedures and parameters.
• It is a measure of the ability of an Evaporative Air cooler to cool a hot space down to a standard exit temperature.

Types of evaporative coolers;

We find two main categories of evaporative coolers;

1) Direct evaporative air cooler.

An evaporative air cooler in which the primary supply air is cooled and delivered directly into the building

the cooled air has increased moisture content.

2) Indirect evaporative coolers

indirect evaporative air coolers have two air circuits, primary and secondary. The secondary air (ambient) is cooled and passed through a heat exchanger and exhausted to atmosphere. The primary air (from the room) is passed through the other side of the heat exchanger where it is cooled and delivered back to the room with no increase in moisture content.

Terms used in evaporative cooling;

Exit temperature;

• The dry-bulb temperature of the air exiting the building when an Evaporative Air cooling system is in operation.

• It is slightly higher than the indoor temperature at the centre of the conditioned space.
• The exit temperature is what is used to calculate the Cooling Effect (capacity) of Evaporative Air coolers.

Heat-load;

• The heat load of a zone within a building is the sum of the heat emanating from all sources including solar radiation, convection, conduction, machines, lighting and people.
• heat-load is measured in watts.
• heat-load calculations are complex. Various calculation methods and software are available.
• heat-load calculations for evaporative air cooling is simplified by ignoring latent heat.

Saturation Efficiency;

• The Saturation Efficiency of an evaporative air cooler is its ability to cool the incoming air down towards the wet-bulb temperature.
• typical Saturation Efficiency for high performance evaporative air coolers is 85% to 90%.

Spot cooling;

• Spot cooling is the method of cooling a target zone with high velocity cooled air that blankets the entire zone to a height of 2m so that people working within that zone will always feel cooled.
• Spot cooling pays no regard to the actual heat-load of the zone; it relies on a fixed air change rate of 45 ac/h to achieve its objective.
• Spot cooling is often the most cost effective method of cooling in large industrial premises where all that is required is that certain workers benefit from the cooling rather than cooling the whole factory area.

System Design

Method 1 – Air Change Rate method.

The air change rate method has been used for decades in the industry for sizing of air coolers. It is quick and satisfactory in many cases.

The Air Change Rate method suffers from two disadvantages:

1. It completely ignores the cooling efficiency of the air coolers.
2. It assumes an average building construction and heat load.

However it is important to know how the method is used.

To select an air cooler by this method:

• Measure and calculate the total volume of the area to be cooled. (Take the height from floor to air vent).
• Select the recommended air change rate for the area concerned. (air changes per hour)
• Multiply the volume by the rate to find the total air to be supplied per hour.
• Select an air cooler which has the desired air flow at zero static pressure.

Method 2 – Heat load method:

All air coolers have a rating in Kilowatts.

This allows the designer to apply well known air conditioning design procedures to calculate heat load, except that no calculation is done for Latent Heat load, and to then select air cooling equipment directly from the KW capacity ratings.

Method 3 – Spot Cooling method:

In many industrial and commercial projects it is either not practical, or it is not necessary, or it is too expensive to install full evaporative air cooling generally throughout the building.

In these cases the Spot Cooling method can provide an excellent Solution.

Spot Cooling systems provide cooled air into a specific “spot” within a building where people (or machines) require cooling due to the high heat load in that location. 

The design for Spot Cooling follows a different procedure from the other methods.

The object is to blanket the whole “spot” with high velocity cooled air, so that any person moving anywhere within that spot can feel the benefit of the cool air.

It is not necessary to erect walls around the spot. The air itself acts as a curtain.

Air distribution:

A poorly designed or poorly installed distribution system will create noise, cause excessive system energy consumption and deprive the customer of valuable air flow and cooling effect.

Always aim for the simplest air distribution layout.  The less bends and fittings you use the better.

Minimise duct resistance (and noise) with the use of large radius bends, splitters, expanding transitions to reduce velocity, swept branch junctions, minimum use of dampers.

Air coolers function efficiently up to 120 to 150 Pa. Air cooler air flow curves against system resistance are available. Best to aim for 80Pa system resistance.

Guide for duct velocities:

• Main ducts: 8 to 12 m/sec.
• Branch ducts: 5 to 8 m/sec.

Air flow:

Evaporative Air cooling systems employ 100% fresh air and the air is cooled once before being delivered to the room and then exhausted to outside. The air is NOT recirculated and re-cooled again and again.

In order to distribute the fresh cooled air to every corner of the building with minimum loss of temperature it is necessary to pump large amounts of the cooled air into the building. The air movement this creates contributes to the feeling of human  COMFORT since air movement is an essential element of feeling comfortable.  

Control of air movement velocity is determined by the selection of air outlet hardware. The air must be able to exit the conditioned space

Condensation:

In some circumstances condensation might occur when moist air strikes surfaces that are at Dew Point temperature or below. Evaporatively cooled air has a higher dew point than ambient air and will therefore cause condensation more readily.

Condensation will occur more readily if there are insufficient exhaust openings in the building.

Ben, I hope that this helps with your understanding around evaporative cooling, they are widely used especially in hot areas with low humidity.

Grant Laidlaw

REFERENCES:

ACRA

Breezair / Seeley International

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