VFD vs 2 speed fan on new ACC unit

[dpwatson]

We are currently in the design phase of a new construction project for a CC which will have an ACC. We do not have experience with an ACC and a discussion on fan controls is ongoing. This is specially discussion VFD vs 2 speed fans. Does anyone have experience or information that would lead us one direction or the other? We would like to share the information with our project so we can potentially head off any operational issues. Thanks for any help that you can provide, and you can send to me directly if preferred.

[Bhaumik Modi]

It really depends on how much process control you are looking for. For the most typical single speed motors, that is achieved by turning few fans off during cooler ambient days, but this is considered a big step change. 2-speed motors would provide a little more control, but the VFD will provide the ultimate precise control and provide the most efficiency. Since VFDs are getting cheaper, this option is utilized very often in refineries, LNG plants, etc.

The biggest operational concern is critical speed. Likely, there might 1 or 2 critical speed zones between 0-100% VFD speeds that might have a structural natural frequency coincide and create resonance. During commissioning, these critical speeds must be identified and locked out.

[dpwatson]

At the ACC user conference, that was held at Dominion, it seemed that VFD’s were favored as well when the discussion came up in the group. We will add the critical speed identification to the commissioning testing as well. Thank you for the reply.

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