Cooling tower - Two-speed vs VFD fan, bug?

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Hi all,
Has anyone tried to see the savings associated with upgrading two-speed fan
to VFD in a cooling tower? I see increased energy use due to the vfd. on
further digging, it looks to me that both two-speed and vfd fans use the
same fan performance curve. Is this the case or am I missing something?
Thanks in advance,
-Rohini

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R B
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Joined: 2011-09-30
Reputation: 4

The default on vfd is 40%. I see savings compared to every other option -
one-speed, discharge damper etc. None of them use the fan-speed vs power
fan curve except two-speed and vfd options. So, basically both are working
in the same way with a penalty of vfd losses in the vfd case? I would think
that the two-speed option will not need a curve.
Thanks,
-Rohini

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R B
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Joined: 2011-09-30
Reputation: 4

Energy savings in cooling tower fans are a result of the ability for
turndown when heat loads are less than the full capacity of the cooling
tower. You will not see any savings if the heat load in the condenser water
loop does not vary. In a similar manner, if the heat load does decrease but
the fans remain running at full speed in order to drive down the condenser
water temperature, there will not be any savings. You may want to check if
there is a condenser water temperature reset which is not being utilized.

Jason Millheim, P.E., LEED AP

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Joined: 2011-09-30
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There are generally three ways to control heat rejection fans. In order of least efficient to most efficient they include cycling the fan(s), using a two-speed fan, or using a variable frequency drive. Theoretically, a fan running at half speed only requires 12.5% of full load shaft power. The two-speed and variable speed options use virtually the same energy as shown in the figure. Variable speed fans use slightly less because they can precisely control speed and power to match the load.

carol wayne

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Joined: 2013-07-07
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