Evaporative Cooling and Humidity

Evaporative Cooling and Humidity

July 23 2020

People often ask me how effective evaporative cooling is in different weather environments or climates, especially those areas that have high relative humidity.

This blog examines some of the issues regarding using evaporative cooling in application. 

First of all I would like to take a look at how the cooling works in principle before then moving on to how we as a manufacturer embrace the principles of evaporative cooling and apply it in practice.

So How Does Evaporative Cooling Work?

Picture yourself on a very warm day when on vacation and you're standing by a large expanse of water, maybe a lake or by the seaside. The ambient temperature may be  95°F (35°C) (that’s what is called the Dry Bulb temperature) and although it may be very warm inland, when we stand by the body of water, we feel cooler.  There is a natural cooling effect that is occurring. Simply put, water evaporates from the surface and is suspended as water vapor in the air. Air movement across the surface of the body of water concerned moves the suspended water vapor to engulf you, displacing heat and reducing the air temperature.

In physical terms this cooling effect is being produced by a combination of humidification and air volume. The increased humidity in the air suppresses the Dry Bulb temperature to close to the Wet Bulb temperature. This temperature drop is felt without the feeling of wetness as the air volume is sufficient to dissipate the water vapor to achieve a pleasant cooling affect.

How Does COOL-SPACE Embrace the Concept?

The fan draws in hot dry air which is then drawn through the thick moist media pads that are within the cooler. As water evaporates through the passing air, it displaces heat. The result is a mist-less discharge calling people in spaces.

So how much does it cool? If the air temperature is 91°F (33°C) with a relative humidity of 30% then the cooling effect will be a drop in temperature of 10°C (18°F). The similar temperature of 91°F (33°C) but with a relative humidity of 60% RH then the cooling drop will only be around 10°F (6℃). So, in general terms the lower the relative humidity the greater the cooling effect from an evaporative cooler. 

However, there are a few things to be said about the perception of relative humidity and how is measured across the conditions of the day. Relative humidity is not a constant throughout the day and generally is measured when the day is at its coolest in the morning. Therefore, the relative humidity is going to be a higher rate than it would be in the afternoon when the temperature has reached its peak. For this reason, it is always recommended to look at the weather data in your region very specifically across the day, because it is in the peak afternoon temperature when generally the greatest level of cooling within a factory or a commercial space is needed. It has been my experience that when customers think they have a relative humidity of say 60%, when the weather data is analyzed in detail for peak summer afternoon temperatures it is quite often as low as 25 to 30%, therefore the cooler will be operating far more effectively in those periods and providing higher levels of cooling – when it’s really needed - than in the morning. 

This is a precise science based on the properties of a psychrometric chart (see below): so, using the Dry Bulb, Wet Bulb actual readings, the resultant suppression of temperature can be calculated with some certainty. In other words, this is not a matter of promotional company fiction, but a matter of fact based on physics.

COOL-SPACE in Application

As can be seen on our website the applications for COOL-SPACE are varied. The coolers are particularly effective in large industrial and commercial spaces and can also be used in restaurants, cafés, patio areas and bars.

In an enclosed space it is important to consider the amount of ventilation that is being provided into the space that requires cooling.  Any evaporative cooling system needs fresh air in order for it to thrive and work more effectively. This is generally calculated by the amount of air change rate per hour (ACH). In a factory environment an air change rate of between five and 8 ACH should be achieved in order to provide adequate ventilation for effective cooling. This may vary depending on any industrial processes that are occurring within the factory that may require a higher rate of fresh air ventilation.

For portable evaporative cooling it is not always necessary to consider the whole volume of the building when calculating the ventilation rate. As generally portable coolers are used to provide spot cooling for people or machinery. However, it is important to ensure that all doors and windows are open when the coolers are in operation and that any mechanical ventilation system is also turned on to derive maximum benefit. It is the opposite thinking to running an air conditioning system that relies on a sealed space to be effective.

This is why in our guidance we refer to the area that each of our coolers can cool as a rule of thumb guide. It saves the customer a lot of calculation and it has been based on years of experience.  The largest of the coolers the Blizzard 50 is recommended to cover an area of around 4850 sq ft (450m2). The customer should also look at the amount of physical obstructions within the space, if the area is packed with machinery and racking then allowance has to be made. As by way of example if the area to be cooled is 50% obstructed, these rules of thumb area guidelines should be adjusted accordingly. So, instead of the 4850 sq ft (450m2) for the Blizzard 50 it should be assumed each unit can cool 2425 sq ft (225m2).

So, in summary, COOL-SPACE portable coolers are easy to use, very cost effective for providing cooling in large spaces and even in open environments where air conditioning would not be applicable. Sure, they are more effective in lower humidity but also look at your region and see how the humidity changes through the day in peak summer conditions. In addition, the natural air movement from the cooler will always give an additional few degrees of cooling sensation.

COOL-SPACE - Nature’s cooling Solution.

About the Author:

Simon Hartles is the International Export Director for COOL-SPACE. Simon has been with COOL-SPACE for nearly a decade and has worked in the heating and cooling industry for at least twice that. He resides in the UK with his family and supports all international sales efforts.