When considering whether to have multiple variable frequency drive (VFD) (also referred to as variable speed drive) air compressors the main question we usually ask is “why or why not?”
We can all agree that putting in multiple VFDs will be expensive. However, if there are significant rebates from a local utility to go all VFD, then it’s hard to argue against putting all VFDs in. If your company has a mandate to put in as many VFD motors as possible, again, it’s a hard sell against using multiple VFDs. From a maintenance standpoint, the difference between a fixed speed and a variable speed compressor is the drive components/electrics. These can be very expensive to replace within a compressor lifetime, whereas a fixed speed unit doesn’t have these components.
We did a five-year analysis comparing multiple options. It’s not a multi-VFD system but you can see that the five-year cost is lower for the two systems with VFDs but not by much. Keep in mind these simulations include a master controller. Without a master controller, the energy costs could be completely different. Plus the cost of a back-up compressor is significantly lower for the fixed speed option (Option 2). Running various flow simulations between fixed speed compressed air systems and systems with one or more VFDs the energy consumption will be close. It all depends on what system, what mixture of machines, master controller included or not, and how many fixed speeds of what type/efficiency, plus what your demand profile looks like. What we do best is using a flow profile, and simulating such a system to find the best solution. This often returns systems with one or more VFD units. The best news? Kaeser SFC (Sigma Frequency Control) and SAM 4.0 (Kaeser master controller) can do it all (including multiple SFCs of different sizes even).
When you consider a single VFD versus multiple fixed speed units, back-up is important. If you put in a fixed speed back-up, then when will it run? With a system controller like the SAM 4.0, the fixed speed can be actuated to exercise a few hours a week. Typically the impetus is that we don’t really want to sacrifice energy, but for a few hours it shouldn’t make a difference. However, now there’s a very expensive compressor sitting there doing very little each week. It could be very efficient at making water, which could mean it will need a major overhaul more quickly. Another consideration is demand; what if the demand increases? Now that “back-up” compressor will load more often. If the VFD and fixed compressor are the same size, you have…you got it…a Control Gap! This means that you’ll need to be quite a bit higher in demand to avoid short cycling both compressors or keeping the VFD at minimum speed too long. Remember VFDs at minimum speed too long lead to a potential major overhaul much quicker than one running between 40% and 85%. So that SAM 4.0 graphic above really sells the VFD maintenance costs short, assuming that the VFD is running lower than 40%.
On the other hand, if you spend a bit more money (maybe quite a bit more), and purchase exactly the same VFD as a back-up instead of that fixed speed, this could theoretically split the 8000 hour load between the two compressors and run them at 4000 hours per year each. This means that you’d potentially be pushing out major services, saving some money in the long run by splitting the load, keeping the system reliable, and potentially increasing service life on both units.
Another issue with putting in a VFD is they are typically more sensitive to electrical issues versus those fixed speed counterparts. Plus if there is a problem with one VFD (electrical or environmental), there will likely be a problem with the others, making the system potentially more unreliable.
Having multiple VFDs is a good idea if the system requires a large trim capacity or is highly dynamic with multiple load profiles. For example, if the first shift is several thousand CFM and the swing is several hundred, but the third shift is only a few hundred CFM and the swing is +/- 50 CFM, then you’d need two different sized VFDs for these differing load profiles.
Example 1: 3x 250 hp oil-flooded VFDs operating together with a master controller
In this example, there are periods where only two VFDs are required, however, there are also periods where demand is high enough where all three compressors are running and loaded to keep demand and maintain system efficiency. As seen below in the comparison graphics, all three VFDs have close to a 100% duty cycle, and all operating hours are very similar. Meaning there is little to no idling costs and the master controller is ensuring that the compressors equalize operating hours to maximize maintenance visits.
Example 2: 2x 200 hp oil-free VFDs operating together with a master controller
In this example, only one VFD is required at any given time, with the second as a ready back-up. With the master controller, the two VFDs are rotated evenly so the operating hours are within a few hundred hours of one another. As with the previous example, there is little to no idling which is saving energy, and the balancing of the operating hours allows for ease of maintenance scheduling.
While the text above may not be overwhelmingly supportive of multiple VFDs, there are a lot of folks in the compressed air industry (including Kaeser) and customers who love these machines.
It’s hard to make the investment case for multiple VFD sizes in general unless you have a 24/7 system, one that has significantly different seasonal demands, or there is a rebate or company initiative as mentioned earlier. Often we see where systems grow significantly over time and perhaps they have smaller equipment to start with, but then need bigger compressors later on. Those “starter” compressors could be used for overnight or weekend demands, and that could justify multiple VFD sizes.
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