ASHRAE Journal - November 2014 - 9

LETTERS

Overestimating Savings
In July's article "Overestimating Energy and Cost Savings
of Installing VFDs" by Adam K. Wolfe, Ph.D., P.E., the
software used for the study uses fan efficiency, motor efficiency, percent of rated flow and percent time at flows,
annual operating time and cost of electrical energy. There
is no mention of static pressure required at various flows.
I assume the fan is a backward inclined centrifugal or
a variation. Some systems have a static pressure requirement that changes more slowly than flow requirement. If
a fan is operating at 80% or 90% of its peak pressure, speed
reduction will be driven by static pressure, rather than
flow requirement. VAV boxes may have a minimum inlet
static pressure requirement such that the system requires
80% of full speed static pressure when operating at 30%
flow. Since static pressure varies as the square of fan speed,
a 10% speed reduction is the most the system will tolerate
(given the energy used by the VFD, the input energy to the
VFD may exceed what the fan previously required).
Dust exhaust systems should provide a constant flow
through each open damper. That means the pressure
where the branch connects to the main duct must remain
constant. Thus, even when the required flow has dropped
by 50%, the required static pressure might have dropped
only 20%. Fan speed can only be reduced by 10%.
An inlet vane damper may offer more savings than a
VFD. A partly closed inlet vane damper reduces flow and
power without reducing peak static pressure.

analysis takes static pressure into consideration (Figure 2,
Bubble 1), via use of fan curve data. This article does not
provide a detailed discussion on fan design and operation.
Adam K. Wolfe, Ph.D., P.E., Member ASHRAE, Washington, D.C.

Don't Take Tools on a Plane
In a recent conversation with an FAA official, I discovered
companies that service equipment and take tools on planes
are increasingly being fined for rules violations. One small
mistake can result in a large fine. We found out the hard
way with a small cylinder of MAP gas forgotten in a tool bag
checked as luggage. That mistake led to 11 infractions in 11
areas in the Code of Federal Regulations. Fines were levied
for each infraction, adding up to a considerable sum.
A lot of valuable information on the FAA website can
aid in training. We fell into the trap of being comfortable with air travel regulations and made a preventable
mistake. I hope learning about our mistake can help my
fellow ASHRAE members avoid a similar error.
Jon J. Cohen, Member ASHRAE, Palatine, Ill.

Ed Chessor, P.Eng., Life Member ASHRAE, Vancouver, BC, Canada

The Author Responds
From your letter, it appears we are on the same page
with respect to VFD operations and savings. However,
the boiler-burner fan environment and operation are
different, force-draft fan versus VAV boxes and dust
exhaust systems. In general, the designer has designed
the fan-motor set for the proper boiler-burner, controls,
and ductwork systems. Although the design is adequate,
the operational personnel were not operating the system
per normal operating procedures.
The boiler-burner system in a central heating system
operates at a steady modulating load and near the equipment capacity to match the demand. Therefore, one
should ensure proper operations prior to installing additional equipment such as VFDs or any other energy and
water saving equipment to keep ownership cost down.
With respect to static pressure, the software used for the
www.info.hotims.com/49810-2
N OVEM BER 2014

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ASHRAE Journal - November 2014

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