Comfort Cooling Using Fans

In last month’s newsletter, we discussed cooling products using fans.  Almost immediately, an application came up that pointed out the weaknesses of that process. You must be able to contain the cooling air to effectively use it (the product has to be a large portion of the cross sectional area of the cooling air blast zone), and there must be an adequate difference between the cooling air temperature and the final part temperature.
 
This month, we will look at providing comfort cooling.  One unique subset of those applications is the ability to provide a high velocity blast zone at a long distance from the fan.  In our terminology, we call that, "spread and throw."
 
 
Let’s take the example of a situation where there are light heat loads and activity levels.  In order to provide comfort cooling, we will need somewhere between 1000 fpm and 1500 fpm of air blowing on the worker.  For one reference source on different situations and required velocities for comfort cooling, you can look in the Industrial Ventilation Manual.

One additional requirement in our example is that the only spot we can install the cooling fan is 15 feet away from the worker.  Many fan designs be found, but one of the most important features to look for is to have the air focused by some type of shroud.  If the fan blades are surrounded by a basket guard, it is unlikely that they will provide high velocity at long distances.

The Hartzell HartKool fan was designed specifically for this purpose.  The discharge of the fan is a smaller diameter than the prop, which provides what we call a tip seal.  This feature not only helps to keep the air from re-circulating back over the blade tips to the low pressure side, but also increases the amount of throw from the fan.  Let’s look at a couple examples.

We set up a large grid in the test lab where we could measure the velocity profiles of various fans, over an area that was 20 feet wide and 35 feet long, leaving an extra 10 feet of clearance all the way around so that the walls wouldn’t affect the readings.  We then tested several sizes, makes, and models of cooling fans, and plotted the profiles.  We found that fans with basket guards could not provide as much throw as those with a shroud, just like we had suspected.

For illustration purposes, let’s say that we had selected a 24” diameter fan.  The fans analyzed had similar catalog ratings.  The results of a basket guard type fan is shown in Figure 1. 

                                      
 
Figure 1

You will note that if we were looking for 1000 fpm, this fan could only be placed about 10 feet away from the worker.  Also note that if you had a situation where the closest you could place the fan way 20 feet away, the velocity drops all the way down to 400 fpm.
 
Here is another basket guarded type fan with slightly better throw performance, shown in Figure 2:
 
                                   
 
Figure 2

The HartKool fan, using our tip seal design, has the velocity pattern shown in Figure 3, below.  Note that the zone of 1000 fpm velocity reaches out to 10 feet, but more significantly, it still provides 600 fpm all the way to 25 feet, and 400 fpm all the way to 35 feet.
 
                                      
 
Figure 3

You can also see how the tip seal design concentrates these medium velocity airstreams and provides a much higher throw.
 
Formulas can be derived that describe this spread and throw phenomena.  They are shown in our CoolBlast and HartKool catalogs.  The factors involved are the catalog airflow rating, the fan diameter, and the distance from the fan discharge.
 
Comfort cooling can provide the environment where a worker can work for longer periods of time without a rest break, can work harder, and will be a more satisfied worker.  Providing the comfort cooling at a long distance is almost totally dependent on the type of fan selected.