blower controls for aeration efficiency · 2017-05-11 · common calculations for all blowers •...

Blower Controls for Aeration Efficiency

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Featured Speaker: Tom Jenkins, JenTech Inc.

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All materials presented are educational. Each system is unique and must be evaluated on its own merits.

Introduction by Rod Smith, Publisher

Blower & Vacuum Best Practices® Magazine

Tom Jenkins, JenTech Inc

• President of JenTech Inc.

• Over 30 years of experience with aeration blowers and blower controls

About the Speaker

Sponsored by

May 11, 2017

1:00 PM CST

Thomas E. Jenkins

President, JenTech Inc.

414-352-573

tom.jenkins.pe@gmail.com

Topics

• Common characteristics for all blowers

• Wire to air considerations for power evaluation

• PD blower control and evaluation

• Dynamic blower control and evaluation

Common Calculations for All Blowers

• Blowers are volumetric flow devices

– m3/hr, ACFM (Actual Cubic Feet per Minute)

– FAD (Free Air Delivery) = volumetric flow at ambient

conditions

• To convert mass flow rate to volumetric flow rate you

must correct for temperature and pressure

– SCFM (Standard Cubic Feet per Minute) typically 68°F 14.7

psia 36% RH is mass flow rate

– Relative humidity may be ignored for most applications

ACFM = SCFM ∙Ta

pa ∙ 35.92Ta = absolute temperature, °Rpa = absolute pressure, psia

• When relative humidity must be considered:

Note: ICFM = Inlet CFM, a special case corresponding

to the actual volumetric flow rate at the blower inlet.

ACFM = SCFM ∙14.58

pb − RH ∙ psat∙

528∙

Ta = actual absolute temperature, °Rpa = actual absolute pressure, psiapb = barometric pressure, psiapsat = saturation vapor pressure, psia

(a function of temperature)RH = relative humidity, decimal

• Pressure is the second parameter that determines

blower power

• There are several factors contributing to discharge

pressure:

– Static pressure from diffuser submergence

• This is typically the largest component of discharge pressure

• Static pressure is typically 80% to 90% of total discharge

pressure

– Friction losses through the diffusers

– Friction losses through pipe and fittings

– Pressure drop across flow control valves at basins

• Total discharge pressure and system curve can be

calculated from flow:

• The constant kf can be calculated from design or

operating data:

ptotal = d ∙ 0.433 + kf ∙ Q2

ptotal = total discharge pressure, psigd = depth of water at top of diffuser, feetkf = constant of proportionality for friction, psi/SCFM2

Q = flow rate, SCFM

kf =pdes − d ∙ 0.433

pdes = total pressure at design (or actual) flow, psigQdes = flow rate at design (or actual) demand, SCFM 6

• Blower power can be calculated from flow and

pressure: Pwa =Qs ∙ Ti

ηwa ∙ 3131.6∙ X

k−1k

k − 1

k≈ 0.283

Pwa = wire-to-air power, kWηwa = wire to air efficiency, decimal (includes blower , motor, and VFD)Ti = inlet air temperature, °Rpd, pi = discharge and inlet pressure, psiak = ratio of heat capacity = Cp/Cv, dimensionlessQs = flow rate, SCFM

Wire to Air Considerations

• The composite (average) power cost is often used to

estimate operating cost:

• The composite $/kWh is obtained by dividing total

annual power cost by total kWh consumed

• This may not accurately reflect actual operating cost

– On-Peak rate typically for 12 hours per weekday is higher

than Off-Peak rates

– Off-Peak rate for 12 hours per weekday and 24 hours

weekends

– Peak demand charge for highest 15 minute average per

month, is usually 1/3 of total power cost

Annual Cost = ൗ$kWh ∙ kWave ∙ 8760

• The diurnal flow pattern for wastewater flow rate and

process oxygen demand is similar to the electrical

power demand variations

• If QAve is the air flow rate required to meet process

demand at average daily flow (ADF):

• kW can be calculated from the flow rates and

pressures 9

QOnPeak = QAve ∙ 1.15

QOffPeak = QAve ∙ 0.85

QDemand = QAve ∙ 1.20

• Evaluating annual power cost should include time of

day rates and demand charges:

Note that the owner is billed for power and energy, not

efficiency

OnPeakCost = ൗ$kWhOnPeak

∙ kWOnPeak ∙60hr

week∙

52week

OffPeakCost = ൗ$kWhOffPeak

∙ kWOffPeak ∙10hr

week∙

52week

DemandCost = ൗ$kWDemand

∙ kWDemand ∙12month

• For the most accurate determination of performance

consult the blower manufacturers

• Need to specify:

– barometric pressure

– inlet pressure (barometric less filter and piping losses)

– inlet temperature

– relative humidity

– flow rate (SCFM is best)

– discharge pressure at flow rate

1:15 PM

• Positive displacement blowers move a fixed volume of

air for each revolution of the blower shaft

• Control is by variable speed, typically variable

frequency drives (VFDs)

• Discharge pressure will inherently rise to meet

restriction to flow

• Must have pressure relief valve to prevent damage

to blower and piping

• Max capacity limit, flow and pressure, is based on

motor power and mechanical limitations

• Min capacity limit (turndown) is based on motor and

blower temperature rise

Positive Displacement Blower Control

1:15 PM

• Positive displacement blowers are of two types

• Older lobe type

• Newer screw type

• Typically more efficient and more turndown, but

more expensive

Lobe Type PD Blower Screw Type PD Blower

1:15 PM

• Positive displacement performance is typically

presented in tabular form

• New CAGI (Compressed Air and Gas Institute) Data

Sheets are one example

1:15 PM

• Positive displacement performance is typically

presented in tabular form

• New CAGI (Compressed Air and Gas Institute) Data

Sheets are one example

1:15 PM

• Lobe type PD performance may be calculated from

manufacturer’s data if available

• Displacement

• Slip rpm (represents internal leakage)

• Slip increases with increasing discharge

pressure

• Friction hp

ICFM = Nactual − Nslip ∙CFR

bhp = 0.0044 ∙ Nactual ∙ CFR ∙ ∆p +FHP

N = rotational speed, rpmCFR = blower displacement, cubic feet per revolutionFHP = friction horsepower, bhp

1:15 PM

• PD performance is often presented as a family of

“curves”

• Performance is approximately linear

• Typical example for a Lobe Type PD:

1:15 PM

• PD performance is often presented as a family of

“curves”

• Typical example for a Lobe Type PD:

1:15 PM

• Screw performance is more complex, and

manufacturer’s calculations are usually proprietary

• Typical example for a Screw Type PD:

• Screw performance is more complex, and

manufacturer’s calculations are usually proprietary

• Typical example for a Screw Type PD:

1:15 PM

• If preliminary prediction of PD performance is needed

the simplest method is to enter the tabular data in a

spreadsheet and use the Excel “forecast” function

• Use forecast for each tabulated speed to create

“curve” data for the actual pressure

• Use forecast on the new curve data to determine

power for actual flow rate

1:15 PM

• Use it for each tabulated speed to create “curve”

data for the actual pressure

• Use it on the new curve data to determine power for

actual flow rate

1:15 PM

actual flow rate

1:15 PM

actual flow rate

Centrifugal (Dynamic) Blowers

• Centrifugal blower performance is more

complex than PD performance

• Centrifugal blowers convert impeller kinetic

energy to pressure and flow

• Performance is influenced by air density

and humidity (molecular weight)

• There are three types of centrifugal blowers

applied to wastewater aeration applications

– Multistage

– Geared single stage

– High speed gearless single stage (Turbo)

• There are three principal control techniques

for centrifugal blowers

– Inlet throttling: lease expensive, least efficient

– Guide vanes, inlet and discharge

– Variable speed with VFD - most efficient

• For any control technique determining

performance requires:

– Calculating the effect of the control on the

performance curve, flow vs. pressure and flow

vs. power

– Establishing the intersection of the system

curve with the flow vs. pressure performance

curve to establish flow rate

– Determining the power required at that flow

• Both the system curve and the performance

curve are required to determine the flow

rate 27

• Multistage blowers use successive compression

stages to obtain discharge pressure

• Generally controlled by inlet throttling or variable

• Example for inlet throttling:

– Calculate pressure drop across valve at several

flow rates

∆pv =Qs

22.66 ∙ Cv

∙SG ∙ Tu

Where:Δpv = pressure drop across the valve, psiQs = air flow rate, SCFMCv = valve flow coefficient from manufacturer’s data, dimensionlessSG = specific gravity, dimensionless, = 1.0 for airTu = upstream absolute air temperature, °Rpu = upstream absolute air pressure, psia

– Calculate new actual inlet pressures and

approximate flow vs. pressure performance at

several flow rates

pda=pc∙Tic

Tia∙

Where:pda = actual discharge pressure, psigpc = discharge pressure from curve, psigTic,ia = inlet temperature for curve and actual, °Rpic,ia = inlet pressure for curve and actual, psia

– Calculate approximate flow vs. power

performance at several flow rates

Pa=Pc∙Tic

Tia∙

Where:Pa = actual blower power, hpPc = blower power from curve, hp

– Plot new performance and system curves

• Geared single stage use gears and high impeller

speed to obtain required discharge pressure

• Generally controlled by guide vanes or variable speed

• The configuration and effect of guide vanes on geared

single stage blowers is usually proprietary

• Performance must be obtained by interpolating

manufacturer’s curves

• The configuration and effect of guide vanes on geared

single stage blowers is usually proprietary

• Performance must be obtained by interpolating

manufacturer’s curves

• Turbo blowers use impeller direct coupled to high

speed motors to obtain required discharge pressure

• Always variable speed with VFD in package, most of

efficiency gains are from variable speed

• For all centrifugal blowers, including turbos,

performance prediction for variable speed is based on

affinity laws: Qa=Qc∙Na

Xa=Xc∙Na

Pa=Pc∙Na

pd = pi ∙ Xa + 1k

Where:Qa,c = actual and curve volumetric flow rate, ICFMNa,c = actual and curve rotational speed, rpmPa,c = actual and curve blower power, hppi,d = inlet and discharge pressure, psia

See Slide 7

• For all centrifugal blowers, including turbos,

performance prediction for variable speed is based on

affinity laws

Summary

• All blower energy analysis depend on using correct

input data

• Analysis should always include blower performance

curves and system curves

• Calculations for determining energy demand depend

on the blower technology

• Variable speed is becoming common for all blower

types and often results in the lowest life cycle cost

Stephen Horne,Kaeser Compressors

•Blower Product Manager for Kaeser Compressors

About the Speaker

For your free subscription, please visit http://www.blowervacuumpractices.com/magazine/subscription.

Blower Master Controllers:

How the IIoT Can Optimize

Blower Station Performance

Stephen Horne

Product Manager—Blowers

How IIoT Can Optimize Blower Station Performance

WWTP Requirements

o Plant air requirements vary hourly, daily, and

yearly

o Plant design must accommodate the community

for 20-30 years

o Plant air demand and plant growth rate is not

linear

o Blowers must be sized to accommodate plant full

capacity

o Designers must consider both investment cost and

operational cost

Overview

o Specific Performance

o Levels of Controls

o Package vs. System Efficiency

o Satisfying Air Demand

o Evaluating and Selecting Blowers

o Dedicated vs. Centralized System Controls

The Industrial Internet of Things (IIoT)

“… the IoT promotes a heightened level of awareness about our

world, and a platform from which to monitor the reactions to the

changing conditions that said awareness exposes us to.”

— Brendan O’Brien, Chief Architect & Co-Founder, Aria Systems

Overview of WWTP Air Requirements

Variations in plant demand = Variations in blower output

o As the plant load varies, blower output must be

altered to meet plant needs.

o Too little air supply results in insufficient

oxygen levels in the basin

o Too much air supply results in excessive

energy cost

(producing air that is not required

Two Levels of Control

System Package

Blower Station Controls and Communications

Blower

ServicePackage

health

Flow required AlgorithmMaster

Controller

Think in Terms of System Efficiency

System efficiency includes:

o All blower packages

o Ancillary equipment

o Master controller

Factors in:

o How equipment meets

changing application demands

Closest match to your power bill

Thank you. Questions?

Thank you for attending!

The recording and slides of this webinar will be made available to attendees via email later today.

PDH Certificates will be e-mailed to Attendees within two days.

Sponsored by

Thursday, May 25, 2017 – 2:00 PM ESTRegister for free at: www.airbestpractices.com/magazine/webinars

Tim Dugan, Compression Engineering Corporation

Keynote Speaker

May 2017 Webinar:

Installation Guidelines for Flow Meters

Sponsored by

blower controls for aeration efficiency · 2017-05-11 · common calculations for all blowers •...

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