Designing and procuring a ventilation system can be a daunting task. Everyone has their own ideas of how a ventilation system should look. But, as the saying goes, “there is more than one way to skin a cat.”
After a basic vent plan has been accepted and details such as the total volume of fresh air, duct type and diameter have been established, it is time to consider the “business” end of the system – the fan(s). During fan selection, several factors should be considered to ensure the end result is a system that meets performance goals as well as site specific environmental requirements. In addition to all of this, we are expected to design and install a system at the greatest value.
Below are factors to consider when purchasing ventilation fans for y our next underground construction project.
First, before any other criteria are met, the performance parameters must be met. This is a simple combination of volume (CFM or equivalent) and pressure (inches of water gauge or equivalent). Simple enough, Right?
Well, there is a little bit more to it than that. Once you have a handful of various performance curves from your “A” list suppliers, an evaluation is in order. There are a couple of important points to check. One condition that is critical to the fan’s longevity is where on the fan’s performance curve is the operating duty point. If the expected duty point is approaching the stall “zone” of the fan, one small change to the system, such as improperly installed ducting, can push the fan into a stall condition.
Also worth noting is the fan’s efficiency. Even within a single manufacturer’s range of offerings, there may be more than one model that can meet the required performance. However, there will be one model that will be more efficient than the others at your duty point.
2. Noise Sensitivity
With ever increasing limits on sound exposure, noise has become a major component of the modern ventilation system. When working in an urban environment, this issue is even more critical. Not only do we have to be concerned with our workforce, but our neighbors as well. To combat this issue, there are a couple of ways to minimize noise. The most common tactic is to add an inline silencer or two to the fan(s). This is an effective method.
However, to achieve some of today’s stringent requirements, simply adding silencers is not enough. Many fan manufacturers now offer fans built with integrated silencers. This virtually eliminates noise migrating through the fan casing. Another advantage of this design is that a silenced fan, compared to a standard fan with two silencers, will be shorter and easier to handle.
Also, fan speed affects noise levels. If excessive noise is an issue, choose the lowest rpm fan that will meet the required performance.
3. Motor Selection
Another important aspect is choosing a fan with an appropriate electric motor. Does the application call for and explosion proof (XP) equipment? If so, the fan must meet local regulations covering and gassy condition or possibly flammable dust. This can lead to another issue if the fan(s) will be operated with a variable frequency drive (VFD). Many electric motor companies offer XP motors that are not UL rated for operation with a VFD.
Ventilating underground excavations is quite different from ventilating a building. If the project proceeds as planned, the area to ventilate grows every day and the vent system should, in most cases, do the same. Moving fans and extending ducting is an essential part of the process. Unfortunately, this is not performed by an army of ventilation engineers, but miners. No matter how well designed a ventilation system may be, getting it implemented perfectly is easier said than done.
If a fan is specified with no safety factor and one of a million variables increases the load on the fan, a breaker will trip or worse, fan damage can occur. Make sure there is a reasonable cushion between the motor’s rating and the calculated load. Related to this subject is the motor service factor. This is the ability of the motor to operate above its rated load. A common service factor for ventilation fans is 1.15, meaning that the motor can operate at 115% of its rate load. Knowing that customers rarely ask for detailed motor info, some manufacturers cut costs by suppling motors with a 1.0 service factor, or in other words, no service factor.
One more potential pitfall regarding electric motors is the availability of replacement motors should a catastrophic failure occur. Some manufacturers use non-standard mounting and shaft designs that require replacement motors to be highly modified or special ordered resulting in long lead times.
While fans generally are not maintenance intensive, Murphy’s Law still applies. What do you do when there is a problem and the mining is delayed until the fan is back up and running? Do you call your supplier in Lower Slobbovia and wait patiently for him to return your call? Ideally, you can call someone in or close to your time zone and get things moving immediately. Along these same lines, a supplier should stand behind its product and manage warranty claims should the claim fall under the motor warranty. A supplier should also have parts in stock should a mysterious object destroy the fan rotor. The bottom line is, a supplier should make your problems a priority.
It is no coincidence that this consideration is listed last. While price is something that has to be taking into account when choosing a supplier, it certainly is not the most important. The cost of even a few hours of downtime on today’s construction projects far exceeds the savings you may see on a couple of fans.
Rob Pope is vice president at Mining Equipment LTD/Jetair, based in Durango, Colorado.