AIR CHANGES AND RELIEF AIR

An evaporative air conditioner operates on one very important principle, COOL AIR IN - WARM AIR OUT. In other words large amounts of cool, fresh air are introduced into the home or building whilst displacing warm, stale air. It is for this reason that when designing an evaporative system we use a high number of air changes. Generally, for local conditions, a figure of 30 AIR CHANGES PER HOUR is used. In applications where there may be a large number of people or where other influences may cause excessive conditions, a figure of 35 AIR CHANGES PER HOUR is usually used. A figure of 25 AIR CHANGES PER HOUR can be used where there is a very big area to cool with few people in it. Using the figure of 30 AIR CHANGES PER HOUR it means that for every hour that passes, the volume of air in the room or building will be replaced with new air 30 times. In other words the amount of air that the evaporative cooler can produce every hour is 30 times the volume of air in the room or building. From this we can use the following formulae to calculate the quantity of air required to cool a particular area :

QUANTITY OF AIR REQUIRED  =  VOLUME OF ROOM  X  30 AIR CHANGES PER HOUR
       
         =  CUBIC METRES PER HOUR

OR

QUANTITY OF AIR REQUIRED  =  VOLUME OF ROOM  X  30 AIR CHANGES PER HOUR
                                                 3600
       
          = CUBIC METES PER SECOND

This figure can be used to compare against the figures of design air flow on the COOL BREEZE UNITS AND SPECIFICATIONS PAGE to select the size of evaporative cooler required.

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This leads to another topic of discussion which is RELIEF AIR. Because evaporative cooling is based on a high quantity of air changes it is very important to make sure that there is some form of outlet for the relief air. The most common manner of letting air out is through windows and doors. It is for this reason that evaporative cooling supports "open living" by the way of the fact that windows and doors can be left open whilst cool air is continuously introduced into the room. For the security conscious, relief air vents can be placed in ceilings or doors so that the system can be run with windows and doors closed. If the system is not able to expel the large volumes of introduced air, then the area will become pressurised, the fan motor will automatically begin to "coast" and the effectiveness of the system will be reduced.

Opening sizes can be calculated for relief air outlets in the following way. It is recommended that the maximum speed of air leaving the room should be in the region of 3 m/s for airvents and a lot slower for full size doors and windows. We can take the amount of air that is entering the room, from the specifications of the unit being used, and divide it by 3 m/s to get the minimum area required for the opening as follows:

AREA OF OPENING  =  QUANTITY OF AIR ENTERING(CUBIC METRES PER SECOND)
                                  3 M/S

  =  SQUARE METRES                                    

If an increase in fan speed becomes apparent when a door is opened, then this is usually a good indication of insufficient relief air facility.