Storage volume for air compressors: Reader feedback

This exchange took place between Chris Beals, president of Air Systems Management, Inc. and author of "Storage volume for air compressors," and Ron Marshall, CET for Manitoba Hydro and a loyal fan of Plant Services magazine. To read the article in question, visit Storage volume for air compressors.

Hi Ron,
 
Thanks for the kind comments. Assuming a rise to stop or unload pressure of 5-psi, your rule of thumb works well for a system with only one compressor and that compressor being a VSD; however, more storage may be required if the VSD compressor operates start/stop and blows down after it gets the signal to restart. 
 
You ask an excellent question concerning the size of the VSD compressor in relationship to the size of constant speed trim compressor in multiple compressor systems. Here’s an example of what we often see and what I was alluding to as a smaller VSD compressor – the end user installs one 100 hp constant speed compressors and a 100 hp VSD compressor. In this case, the VSD compressor typically has less capacity (is smaller) than the constant speed compressors; however, even if the capacity of the VSD compressor is the same as the constant speed compressors it can’t pick up the full capacity of the constant speed compressor when it unload. Therefore, the storage should be sufficient to prevent the constant speed compressor from short cycling and to allow it to operate efficiently at part load. As you know, the part load efficiency of a constant speed compressor is dependent upon its blow down time. In a system without a VSD compressor, we size the storage at 4.13 times the capacity of a constant speed trim compressor if it has a 40 second blowdown time; if the blowdown time is 80 seconds then the storage must be 8.26 times its capacity to achieve the same part load efficiency. 
 
In the multiple compressor system described above the VSD compressor’s capacity is 490 cfm and it has an 80 percent turndown and the capacity of the constant speed compressor is 500 scfm. The controls are set to:

  • Unload the constant speed compressor at a 3-psi rise in pressure above the VSD compressor’s target pressure,
  • Reload the constant speed compressor after a 3-psi dip below the VSD compressor’s target pressure
  • System demand is (500 cfm + ((490 cfm x 0.80)/2)) = 549 cfm

To prevent short cycling of the constant speed compressor its cycle time must be = 90 seconds; therefore, the storage must be = 1310-gallons.  1310-gallons of storage also provides a 41 second unload time so the compressor can blow down fully and maximize it part load efficiency.
 
If the constant speed compressor was larger – say 750 cfm and the demand was  (750 cfm + ((490 cfm x 0.80)/2)) = 799 cfm then 2000-gallons of storage is required to prevent short cycling of the constant speed compressor; the unload time is only 16 seconds so its part load efficiency is poor.  5200-gallons of storage is required to provide a 40 second unload time, which also provides a 233 second cycle time.
 
Again, if the constant speed compressor was larger – say 1000 cfm and the demand was (1000 cfm + ((490 cfm x 0.80)/2)) = 1049 cfm then 2150-gallons of storage is required to prevent short cycling of the constant speed compressor; the unload time is only 10 seconds so its part load efficiency is poor.  8700-gallons of storage is required to provide a 40 second unload time, which also provides a 364 second cycle time.
 
Not every VSD compressor has an 80 percent turndown so let’s see what happens if the VSD compressor only has a 50 percent turndown.  The unload and reload set points remain the same; the demand is changed to show the shortest cycle time that may occur.
 
In the first case, where the capacity of the constant speed compressor is 500 cfm and the demand is (500 cfm + ((490 cfm x 0.50)/2)) = 623 cfm, 2600-gallons of storage is require to prevent short cycling of the constant speed compressor and its unloaded time is 43 seconds so it has good part load efficiency.  Now let’s increase the size of the constant speed compressor.
 
If the constant speed compressor was 750 cfm and the demand was  (750 cfm + ((490 cfm x 0.50)/2)) = 873 cfm then 3600-gallons of storage is required to prevent short cycling of the constant speed compressor; the unload time is only 24 seconds so its part load efficiency is less than desired.  5800-gallons of storage is required to provide a 40 second unload time, which also provides a 144 second cycle time.
 
Again, if the constant speed compressor was 1000 cfm and the demand was (1000 cfm + ((490 cfm x 0.50)/2)) = 1123 cfm then 4025-gallons of storage is required to prevent short cycling of the constant speed compressor; the unload time is only 18 seconds so its part load efficiency is less than desired.  8900-gallons of storage is required to provide a 40 second unload time, which also provides a 199 second cycle time.
 
In the above cases we have looked at the worst case scenario, i.e., the demand rate that created the shortest cycle time.  If the demand profile is known the storage can be sized for that particular situation; however, if the demand profile changes the storage may be insufficient and short cycling could damage the constant speed compressor.  
 
As stated in my Plant Service Magazine articles titled, “Unwinding the Spin on VSD Compressors” and “Storage Volume for Air Compressors”, to minimize the storage requirement in multiple compressor system one should select a VSD compressor with a turndown near or exceeding the capacity of the constant speed trim compressor.           
 
I hope this addresses your question.
 
Best Regards, 
Chris

-------------------------------------------------------------------------------------------------------------------

Chris,

Quite a long explanation! You missed the point of my question though.

In the systems I deal with I would not try to operate in such a narrow band. I like to use 10 psi load unload range for VSD and wider for base and add a flow control to protect the load. If you ensure the vsd is equal to or larger than the base unit and the pressure band of the vsd is well within the base unit the fixed speed unit doesn't have to cycle at all.

In your scenario of the 500 cfm VSD and the 1000 cfm base what happens when the load is say 750?

Shouldn't the formula be (750 cfm + ((490 cfm x 0.20)/2)) = 799

Ron
 
-------------------------------------------------------------------------------------------------------------------

Ron,
 
I’m sorry I didn’t initially understand the point of your question. One of the reasons I find compressed air systems interesting is that there’s almost always more than one approach to an issue depending on what your trying to accomplish. In any case, after some reflection, you’ll find my response to your question and comment below:   
 
Comment - “In the systems I deal with I would not try to operate in such a narrow band. I like to use 10-psi load/unload range for VSD and wider for base and add a flow control to protect the load. If you ensure the VSD compressor’s capacity is equal to or larger than the base unit and the pressure band of the VSD compressor is well within the base units pressure band the fixed speed unit doesn't have to cycle at all.”
 
Response - First, I’d like to explain the perspective I’ve taken in my articles. The compressor manufacturers and their distributors promote VSD compressors on the basis that if you install one you can maintain the system pressure within ± 1.5-psi without installing a large receiver or a pressure/flow controller. As you know, this isn’t true unless the total variation in demand is less than or equal to the turndown of the VSD compressor and the VSD compressor is controlling off a remote pressure signal located downstream of the clean-up equipment. In my articles, I try to let the end user know what they have to do to come close to making the compressor manufacturer’s comments a reality; hence, the narrow control band and cascading the constant speed compressors’ unload set point between the VSD compressor’s target and stop/unload pressure. In fact, in my article titled, “Unwinding the Spin on VSD Compressors”, I show a multiple compressor system configuration that employs this approach to minimize the pressure variation during transition periods to 2-psi when the first constant speed compressor unloads or reloads and 4-psi when the second constant speed compressor unloads or reloads. The secret of this configuration is that the constant speed compressors control signal location is upstream of dedicated dryers. The differential across each dedicated dryer goes from 5-psi at full load to 0-psi when its upstream compressor unloads, which reduces the pressure variation by reducing the control band. 
 
The question I haven’t addressed in my articles is, “Is this the best way to configure a system containing a VSD Compressor?” I think the answer to the question is yes, if the end user just has a few compressors and they want to minimize the pressure variation, utilize the system storage, and don’t want to install a pressure/flow controller. On the other hand, when the pressure variation isn’t an issue and/or the end user is willing to install storage and pressure/flow controller then your approach of embedding the VSD compressor’s control band within the control bands of the constant speed compressors is best. In fact, the automation we often recommend uses that approach. 
 
Finally, if asked question, “What system configuration would I recommend for a system with multiple compressors?”, I would respond by saying “I recommend a system configuration where the VSD compressor and a backup compressor are located upstream of a pressure/flow controller and the rest of the base (constant speed) compressors are operating downstream of the pressure flow controller.” While this configuration increases the number of dryers and in many cases requires automation it’s always the most efficient system configuration.     
 
Question - “I am curious about one thing, the mention of having a VSD that is smaller than a base unit, in my experience this hardly ever works out. What strategies do you use to deal with this?”. 
 
Response - Just as you don’t, we don’t recommend having a VSD compressor that’s smaller than the largest trim constant speed compressor, but they get installed. Embedding the VSD compressor’s control band within the control bands of the constant speed compressors is an excellent way of beginning to address this issue because it utilizes the full capacity of the VSD compressor rather than just its turndown capacity; however, in most cases the end user will have to install additional storage.   
 
Thanks for your great questions,
 
Chris