Draft
SPECIFICATION FOR STANDARD SAVINGS
ESTIMATION CALCULATOR FOR FAN VFD
Submitted to
REGIONAL TECHNICAL FORUM
Submitted by
SBW CONSULTING, INC.
2820 Northup Way, Suite 230
Bellevue, WA 98004
December 16, 2011
Standard Savings Estimation Protocol - Fan VFD
TABLE OF CONTENTS
1. PURPOSE............................................................................................................ 1
2. DEFINITION
OF KEY TERMS ..................................................................................... 1
3. DATA ENTRY REQUIREMENTS .................................................................................. 1
3.1. General ............................................................................................................................... 1
3.2. System Curve ..................................................................................................................... 2
3.3. Machine Curve ................................................................................................................... 2
3.4. Data Import ........................................................................................................................ 2
3.4.1. Electrical measurements .................................................................................................... 2
3.4.2. Facility Control System Post-Period Trend Logs ................................................................. 3
3.4.3. Data Synchronization. ......................................................................................................... 3
3.5. Determinant of Fan Speed ................................................................................................. 3
3.6. Typical Meterological Year (TMY) Weather Data .............................................................. 3
4. SAVINGS ESTIMATION STEPS ................................................................................... 4
4.1. Derive Flow vs. Power Relationship for Post Period.......................................................... 4
4.2. Derive Flow vs. Power Relationship for Baseline Period ................................................... 4
4.3. Compute Savings for Trend Log Intervals .......................................................................... 4
4.4. Annualized Savings: Fan Speed Determined by Production Level .................................... 5
4.5. Annualized Savings: Fan Speed Determined by OAT ......................................................... 5
4.6. Annualized Savings: Fan Speed Determined by Facility Schedule ..................................... 5
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Standard Savings Estimation Protocol - Fan VFD
1. PURPOSE
The calculator described herein is used to implement a standard protocol by which annual electrical
energy savings (kWh) can be estimated for a fan unit that has been upgraded by the addition of a
Variable Frequency Drive (VFD).
2. DEFINITION OF KEY TERMS
Project. A VFD installed to control one fan unit and baseline control mechanism, e.g., Outlet Damper,
Inlet Vane, Variable Pitch, is either removed or disabled and set to the maximum flow condition.
Baseline. This modifier; as in baseline control strategy has one of two meanings. For fan units upgraded
in existing facilities, it refers to the period prior to the upgrade. For fan units in new construction, there
is no such period. For them, the term baseline refers to the design specification for the fan unit without
the efficiency upgrade.
Post. This modifier; as in post current trend log, refers to the period after the fan unit VFD upgrade is
installed and fully commissioned.
3. DATA ENTRY REQUIREMENTS
The following data must be collected and entered into the savings calculator. In some cases, alternative
procedures are allowed which would provide data of higher quality.
3.1. General

Baseline Fan Motor HP. Obtained from the motor name plate if available, or interviews with site
personnel.

Baseline Fan Motor RPM. Obtained from the motor name plate if available, or interviews with site
personnel.

Baseline Fan Motor Enclosure type. Obtained from the motor name plate if available, or interviews
with site personnel.

Baseline Fan Motor Efficiency. Obtained from the motor name plate if available, or interviews with
site personnel.

Post Fan Motor HP. Obtained from the motor name plate.

Post Fan Motor RPM. Obtained from the motor name plate.

Post Fan Motor Enclosure type. Obtained from the motor name plate.

Post Fan Motor Efficiency. Obtained from the motor name plate.
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Standard Savings Estimation Protocol - Fan VFD
3.2. System Curve
The system curve is assumed to be of the form:
h  h0  aQ C
where
-
h0 is the static head, or the head at zero flow
-
h is head
-
Q is flow
-
the exponent C defaults to 1.7, and goes no higher than 2.

Static head. By default the system curve is assumed to originate at the (0,0) point, i.e. have zero
head at zero flow. However, some systems have a static head which needs to be overcome to
generate any flow. This value may be specified. [Still needed here is a description of how that value
may be obtained.]

Fan Operating Point. This (flow, head) operating point must be with all dampers or valves removed
or wide open. The point may be taken from the equipment schedule on the facility’s mechanical
plan. Alternatively, this value may be determined by: (1) inspecting facility control system trend logs
of fan flow rate, if the system has a calibrated flow sensor and the log contains values at or near
100% speed; (2) taken from an air balance report; or (3) based on a pair of values – measured kW
along with corresponding VFD speed.
3.3. Machine Curve

Fan Curve. Data points from the manufacturer’s performance curve must be entered in to the
calculator. Flow, pressure, and efficiency points must be read from the appropriate fan curve and
entered. The fan curve needs to match the conditions at the site in terms of impeller size and speed
(RPM) and air temperature.
3.4. Data Import
Trend data listed below are captured with data loggers and copied by the user into a sheet in the
calculator. Where more than one trend is copied in, the user aligns the data by time stamp.
3.4.1. Electrical measurements

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One-Time Electrical Measurements. These measurements are recorded while the VFD is set to a
series of at least 3 levels of %speed, including one at 100% speed. The (amps, kW) pairs of points are
used to derive a power from current relationship.
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Standard Savings Estimation Protocol - Fan VFD

Trend Log of VFD Current or power. A trend of current or power input to the VFD is imported.
Current readings are converted to power using the relationship derived in the calculator.

Operating point. If the system curve is being calculated with a VFD %speed and power point (see
Section 3.2), the user enters this pair of values.
3.4.2. Facility Control System Post-Period Trend Logs

Trend Log of VFD Facility OAT. Data must be imported for the same period as the VFD current trend
log. These data are required only if fan speed is primarily a function of OAT (such as for a heating or
cooling units).
3.4.3. Data

Synchronization.
Start times for all trend log intervals are synchronized by moving each to the closest standard subhourly interval start times, e.g. 0, 15, 30 and 45 minutes after the start of each hour.
3.5. Determinant of Fan Speed
The user enters which of three methods is used to extrapolate from the measured period to the
remainder of the year. There are three possible determinants: (1) facility operations schedule, (2)
Outside air temperature combined with operations schedule, or (3) production level.
The following describes the data entry requirements for each option.

Facility Operations Schedule. The user enters dates, days, and hours defining each mode of
operation. The fan is assumed to not operate during hours which are not accounted for in the
entered schedule.

OAT. In addition to the operations schedule, the user imports a trend of OAT as described above.
(OR: If OAT is a significant determinant, identify the TMY (Typical Meteorological Year) weather
station which is closest to the project site.)

Production level. The user enters a base month and a % of full production level for each month of
the year. If kW data are entered for more than one complete month, the user still selects one base
month, but does not enter % of full production for other metered months.
3.6. Typical Meterological Year (TMY) Weather Data

TMY OAT. Typical Hourly OAT data for Pacific Northwest weather stations are contained in the
savings calculator to be used in estimating annual saving. Data from the station selected in Section
3.5 will be used.
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Standard Savings Estimation Protocol - Fan VFD
4. SAVINGS ESTIMATION STEPS
Savings are estimated as follows.
4.1. Derive Flow vs. Power Relationship for Post Period

Operating Point. If the system curve is being calculated from an entered (VFD %speed, kW) pair of
entered data, the point is calculated using the machine curve. The lower speed machine curve is
found using the affinity laws. Adding component efficiencies allows the development of a flow as a
function of kW relationship. The entered power value thereby determines the operating flow point,
and the lower speed machine curve equation is used to derive the operating heat point.

System Curve. The full-flow operating point is used to derive an equation for the system curve (flow
vs. head) as the parabola (or lower order equation) from the origin through the point on the fan
curve matching the operating point collected as described in Section 3.2.

Flow vs. Power Curve. The fan will operate along the system curve as the VFD changes fan speed.
Fan efficiency should remain nearly constant along this curve within the stable operating region of
the fan. The calculator derives an equation for flow as a function of power (kW) along this curve.
Motor efficiency and VFD efficiency are based on default relationships (DOE tables) according to
motor percent load.
4.2. Derive Flow vs. Power Relationship for Baseline Period

Flow vs. Power Curve. Assuming constant speed, the fan will operate along the fan curve. Using the
fan curve data points entered, the calculator derives an equation for power as a function of flow
along the fan curve.
4.3. Compute Savings for Trend Log Intervals

Power and Current Curve Fit. Spot measurements of true power and current (taken at least for 3
levels of speed, including 100%) are fit to a second order equation.

Post kW. Calculate kW from fitted equation for each current (amps) value.

Post flow. Calculate flow as a function of kW using the equation derived from the system curve.

Baseline flow. The assumption is that the baseline flow profile is identical to the post flow profile.

Baseline kW. Calculate kW as a function of flow using the equation derived from the fan curve.

Savings for Trend Log Period. Calculate the kW savings profile as the difference between baseline
kW and post kW.
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Standard Savings Estimation Protocol - Fan VFD
4.4. Annualized Savings: Fan Speed Determined by Production
Level

Savings Trend Log Month. Sum the savings for the full calendar months found in the current trend
log period.

Monthly Production Multiplier. Calculate monthly production multipliers by the ratio of each
month’s % of full production (see Section 3.5) to the trend log base month’s % of full production.
Savings for complete months which were metered but not selected as the base month are based on
metered data rather than the production multiplier.

Savings by Non-Trended Months. Calculate the savings for the non-trended months by multiplying
the trended month’s savings by each non-trended month’s production multiplier.

Annual Savings. Sum savings across all months.
4.5. Annualized Savings: Fan Speed Determined by OAT

Average Savings by Trend Log Bin. Average kW savings by two degree temperature bins for all trend
log intervals during operating hours, as defined by facility operations schedules (Section3.5). If the
facility has more than one operation mode (that determines fan speed), temperature bin averages
are separately computed for each operation mode.

Operating Hours by TMY Bin. Divide the 8,760 TMY OAT data into two degree bins and compute
frequency of annual operating hours for each bin, as defined by facility operations schedules
(Section 6.2).

Average Savings by TMY Bin. TMY bin average bin savings equal trend log average bin savings for
each matching bin. Extrapolate average savings for TMY bins that do not have trend log data. Higher
temperature bins are extrapolated by a linear equation fit to the trend log bins above 57 degrees
and lower temperature bins by a linear equation fit to the bins below 57 degrees. No bin value is
allowed to exceed the rated fan motor kW.

Saving by Bin. For each TMY bin, multiply the average bin savings by the number of operating hours
in each bin, to get kWh savings in each bin.

Annual Savings. Sum the kWh values across TMY bins.
4.6. Annualized Savings: Fan Speed Determined by Facility
Schedule

Average Savings for Trend Period. For the trend log period, average the savings for each operation
mode, as determined by facility operations schedule (Section 6.2).
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Standard Savings Estimation Protocol - Fan VFD

Annual Operating Hours. Determine the number of operating hours for each operating mode.

Savings by Operating Mode. Multiply the number of annual operating hours times the average
saving for each operating mode.

Annual Savings. Sum savings across operating modes.
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