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Energy and Buildings 128 (2016) 503–510

Contents lists available at ScienceDirect

Energy and Buildings

j ourna l ho me page: www.elsev ier .com/ locate /enbui ld

onstant design air flow industrial ventilation systems withegenerative dust filters: Economic comparison of fanpeed-controlled, air damper controlled and uncontrolled operation

hristof Lanzerstorfera,∗, Florian Nedera, Rainer Schmiedb

School of Engineering/Environmental Sciences, University of Applied Sciences Upper Austria, Stelzhamerstraße 23, A-4600 Wels, AustriaKappa Filter Systems GmbH, Im Stadtgut A1, A-4407 Steyr-Gleink, Austria

r t i c l e i n f o

rticle history:eceived 11 September 2015eceived in revised form 14 May 2016ccepted 13 July 2016vailable online 15 July 2016

eywords:entilation system

a b s t r a c t

Variable speed drives for fans in small and medium-size ventilation systems with a constant design airflow, including a regenerative filter for dust separation, are evaluated in this paper. In the past, vari-able speed drives were considered for systems with varying air flow. It was found that this is also anappropriate measure to reduce the operating costs of such systems. With the chosen approach the calcu-lated payback period based on 24 h operation is between 0.7 and 1.7 years, depending on the size of thesystem. For systems in a two-shift operation the payback periods are still in the range of 1.4–3.4 years.The profitability of the installation of fan speed-control based on 12,000 service hours is in the range of

abric filteran operationlow controlariable-speed drive

80–150%. A new operation mode, in which the filter cleaning is triggered at a fixed pressure drop of thefilter cake, would further reduce the payback period and improve profitability. Systems with automaticdamper flow control are much less economical. However, for the assumed conditions such systems arestill better than uncontrolled systems. The installation of speed-control for fans in ventilation systemswith a constant design air flow is therefore highly recommended for an economical design.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

Most of the power consumption of ventilation systems is causedy the motor of the suction fan which is required to provide theecessary draught and to overcome the pressure drop of the sys-em caused by ducts, dampers, dust filters and other equipment.herefore, measures for the reduction of the operating costs of suchystems often aim to reduce the pressure drop of the system [1].nother energy-saving strategy in ventilation systems is the use ofighly efficient electric motors [2,3]. In many ventilation systemshe required air flow is not constant. To save energy the air flowan be reduced to a level that still delivers the required air qualityventilation on demand). Therefore, the ventilation air volume isontrolled by the use of variable speed drives for the fans to matchoad requirements [2,4,5]. Depending on the variation of the ven-ilation demand, electrical energy savings by the use of variable

peed drives are typically in the range of 25–50% [6]. The otherperating alternatives are operation at the maximum capacity ofhe fan or flow control with a control damper increasing the pres-

∗ Corresponding author.E-mail address: c.lanzerstorfer@fh-wels.at (C. Lanzerstorfer).

ttp://dx.doi.org/10.1016/j.enbuild.2016.07.032378-7788/© 2016 Elsevier B.V. All rights reserved.

sure loss in the suction line. However, the power consumption ofthe fan motor is higher with both alternatives [7,8].

Numerous studies deal with the energy optimization of heating,ventilation and air-conditioning systems (HVAC) for buildings. Inthese systems the main purpose of ventilation is the control of theroom atmosphere with respect to temperature and carbon diox-ide concentration by replacing the air with fresh air. Ventilationon demand by the use of variable speed drives for the ventila-tion fans was investigated for various kinds of buildings such asdetached family houses [9], schools [10–12], university buildings[13] and office buildings [14,15]. This operation enabled a reduc-tion in power consumption of the fan in the range of 35–50% [9,13].The reduction in the total energy consumption of the fan and theheating were reported to be in the same range [10,16].

Papers on the use of variable speed drives for fans in industrialapplications concentrate on two areas: cement kiln fans [17–22]and mine ventilation fans [23–25]. In mine ventilation the controlof the atmosphere is essential, whereas cement kiln fans are a partof the kiln off-gas de-dusting system. For cement kiln fans elec-

trical energy savings of 25–40% are reported for speed-controlledoperation [17,19]. The reported electrical energy savings in mineventilation are up to 50% or 60% [25]. Other industrial applicationsof variable speed drive fans are described for cooler fans [26], fans

504 C. Lanzerstorfer et al. / Energy and B

Nomenclature

�pS Pressure drop of the air collection system, Pa�pFM Pressure drop of the filter medium, Pa�pFC Pressure drop of the filter cake, PaPsh Shaft power of the fan, WPel Electric power demand of the fan, WPR Rated power of the fan motor, kW�fan Efficiency of the fan�drive Efficiency of the electric driveV̇ Volumetric flow rate, m3/s (at fan inlet conditions)�CAI Additional investment costs, D�pst Difference in static pressure, PapA Ambient pressure, Papst Static pressure, Pan Polytropic exponent� Isentropic exponentf Factor according VDI 2044k0–k3 Coefficients in fan volumetric flow calculationh0–h3 Coefficients in fan efficiency calculationA–D Coefficients in motor efficiency calculation accord-

ing ÖVE/ÖNORM EN 60034-30R(t) Resistance of the system

Indices1 At fan inletM At design point of the fan (maximum pressure drop)C Ventilation system with volumetric air flow con-

trolled by a speed-controlled fanU Ventilation system with uncontrolled volumetric air

irpqtc

svfmwvaosottttttcfittatfias

flow

n a boiler house [27] and for laboratory exhaust fans [28]. Theeported electrical energy savings are about 50% [28]. The paybackeriod of the additional investment cost for the installation of a fre-uency converter to enable speed-controlled operation depends onhe variation of the required ventilation air flow and the electricityost per unit. It is generally less than two years [26,27,29,30].

Common to all the above-mentioned applications of variablepeed drives for ventilation fans is the variable demand for theentilation air volume flow. However, in many ventilation systemsor processing units e.g. for gas, laser and plasma-cutting, blast

achines, grinding, brushing and polishing operations in metal-orking or mechanical sand processing in foundries, the required

entilation air flow is constant. However, in these systems the oper-ting conditions for the fan are not constant because the resistancef the installed filter for de-dusting increases gradually due to theeparated dust, thus reducing the air flow. The total pressure dropf such a system is the sum of the pressure drop of the air collec-ion system and the pressure drop of the filter, which results fromhe pressure drop of the filter medium and the pressure drop ofhe filter cake formed by the separated dust. The pressure drop ofhe filter medium (residual pressure drop) increases slightly whenhe filter medium is in use because some dust particles in or onhe filter are not removed during the regenerative cleaning pro-ess. The pressure drop of the filter cake is zero for the cleanedlter medium and increases during the filtration cycle. The rate ofhe increase depends on the dust concentration, the dust charac-eristics (particle size, shape of the particles) and on the specificir flow rate (air to cloth ratio). At the end of each filtration cycle

he filter is regenerated by a compressed air pulse detaching thelter cake. After regeneration the pressure drop of the filter cake isgain zero. Thus the pressure drop of the filter system is not con-tant [31,32]. Fig. 1 shows the course of the pressure drop which is

uildings 128 (2016) 503–510

usually found in small and medium-sized fabric filters. During eachfiltration cycle the pressure drop increases from the residual pres-sure drop of the filter medium to the chosen maximum pressuredrop, when the regenerative cleaning is started.

In large filters the cleaning is spread more evenly over timebecause only a part of the large filter area is cleaned at once. Inthe cleaned part of the filter area the resistance is reduced. Hence,the pressure drop has to be the same for the whole filter area, theair flow through the cleaned filter area increases, and the flow inthe un-cleaned area decreases until the pressure drops are equal inboth areas. Thus, the reduction in the pressure drop is smaller inthis case.

The aim of this study is to investigate the economic feasibility ofthe use of frequency converters for speed-controlled operation ofthe fan and air damper control in small and medium size ventilationsystems with a constant design air flow. The evaluation is made onthe premise that a new ventilation system is built. The basic designof the ventilation system investigated does not have flow control.It consists of an air collection system, a regenerative fabric filter fordust separation and a centrifugal fan. The additional investmentcosts for both controlled systems and the energy savings of con-trolled operation are estimated for both systems. From these resultsthe payback periods for the required additional investments arecalculated.

2. Ventilation system design

Ventilation systems are usually designed for operation at themaximum pressure drop of the system which is reached just beforeregeneration of the filter medium is triggered (shown in Fig. 2 aspoint M). When the filter medium is clean, the pressure drop issmaller. In an uncontrolled system the starting point of each fil-tration cycle also has to be on the performance curve of the fan.Therefore, the air flow is higher. The corresponding operation pointis shown in Fig. 2 as point U. During the build-up of the filter cakethe operation point moves gradually from U to M, the regenera-tion of the filter medium brings the operation point back to U. Inthe course of the time of use the operation point at cleaned filtermedium U is shifted slowly in the direction of M. This is caused bythe limited efficiency of the filter regeneration. The efficiency of afan is not constant either. Usually, the fan is selected to have thehighest efficiency at the design point. The more the operation pointdiffers from the design point, the lower the efficiency of the fan.Depending on the gradient of the performance curve and the fanefficiency, it often happens that the power consumption is higherfor operation points with a reduced pressure drop.

With the installation of a control damper in the suction linethe fan can always be operated at the design operation point (M)because the controlled pressure drop of the damper compensatesfor the variable pressure drop of the filter cake. In this case thepower consumption is constant.

The installation of a frequency converter in the power supplyline of the fan motor enables constant air flow operation by speedcontrol. The fan performance curve is shifted towards less capacity(flow and pressure) when the speed is reduced. In this case theoperation point with a clean filter medium would be C. In the courseof the filtration cycle it moves gradually from C to M as a filter cakeis formed by the separated dust and is shifted back to C when thefilter is regenerated. With increasing time of use the starting point

of the filtration cycle C moves slowly in the direction of M. Theefficiency of the fan is also lower at C, but due to the remarkablereduction in pressure drop the fan’s power consumption is muchlower.

C. Lanzerstorfer et al. / Energy and Buildings 128 (2016) 503–510 505

Fig. 1. Total pressure drop curve of a regenerative fabric filter.

opera

3

3

sasc

•••

Fs

Fig. 2. Typical

. Calculation

.1. System designs considered

The ventilation systems investigated consist of an air collectionystem with fixed settings, a fabric filter which is regenerated when

maximum pressure drop is reached and a centrifugal suction fanupplying the required draught. The following designs for air flowontrol are considered:

uncontrolled air flow (UC),constant air flow controlled by a control damper (CD),constant air flow controlled by a speed-controlled fan (SCF).

The flow sheets for the different system designs are shown inig. 3. For the system with uncontrolled air flow (Fig. 3a) a motoroft starter and a manual damper upstream of the fan are consid-

tion diagram.

ered. For the system with constant air flow controlled by a damper(Fig. 3b) an actuator for the damper and flow measurement for thevolumetric air flow are additionally required. When the air flowis controlled by a speed-controlled fan (Fig. 3c) a frequency con-verter replaces the soft starter and shielded cables are required. Aflow measurement device is also needed in this system.

3.2. Power consumption of fans

The calculation of the power consumption was programmed inVisual Studio. Assuming equal inlet and outlet air velocity the fanshaft power can be calculated by Eq. (1) as follows:

Psh = 1�fan

· V̇1 · pst,1 · n

n − 1·[(

1 + �pst

pst,1

) n−1n

− 1

](1)

506 C. Lanzerstorfer et al. / Energy and B

Ffl

wiooe

P

if

f

P

oms

�

tt

p

ig. 3. Flow sheets of the different system designs (a: air flow uncontrolled; b: airow controlled by a control damper; c: air flow controlled by speed-controlled fan).

here the index 1 relates to the inlet conditions of the fan. Accord-ng to VDI 2044 [33] in case of fans with a relative pressure increasef �pst/pst,1 ≤ 0.1 and equal air velocity at the inlet and the outletf the fan, the fan shaft power can be calculated by a simplifiedquation Eq. (2):

sh = V̇1 · �pst

�fan· f (2)

For fan efficiencies > 0.5, the isentropic exponent (�) can be usednstead of the polytropic exponent (n). Then, for air (� = 1.4), theactor f can be calculated by Eq. (3):

= 1 − 0.36 · �pst

pst,1(3)

The electric power demand is given by Eq. (4):

el = Psh

�drive(4)

The required pressure increase of the fan results from the sumf the pressure drops of the air collecting system (�pS), of the filteredium (�pFM) and the filter cake (�pFC). It can be written as

hown in Eq. (5):

pst = �pS + �pFM + �pFC (5)

In typical ventilation systems the fan is situated downstream of

he filter close to the air discharge. Therefore, the static pressure athe fan inlet can be approximated by Eq. (6):

st,1 = pA − �pst, (6)

uildings 128 (2016) 503–510

where pA is the ambient pressure. Thus, the electrical powerdemand of the fan can be calculated by Eq. (7):

Pel = V̇1 · �pst

�fan · �drive·(

1 − 0.36 · �pst

pA − �pst

)(7)

3.3. Performance curves of fans

For the calculation of the performance curves of the centrifugalfans the computer program RKVent® of Reitz Ventilatoren GmbH& Co. KG was used. With this program different types of fans witha power range of up to 1000 kW can be designed. The use of theprogram is free of charge and it can be downloaded from the com-pany’s website [34]. For each plant size the fan performance andefficiency curves were calculated for optimum fan efficiency at thedesign operation point. The resulting curves were then digitalizedby using the program DataLab® and expressed by polynomial func-tions Eqs. (8) and (9) which were found through linear regression[35]. The specific constants k0–k3 and h0–h3 for each fan size wereincluded in the calculation model:

V̇1 = k0 + k1 · �pst + k2 · �p2st + k3 · �p3

st (8)

�fan = h0 + h1 · V̇1 + h2 · V̇21 + h3 · V̇3

1 (9)

3.4. Efficiency of motors

The efficiency of a grid-connected motor operated close toits design point can be calculated by the following approxima-tion equation Eq. (10), published in the standard ÖVE/ÖNORM EN60034-30 [36]:

�N = A ∗ [log10 (PN)]3 + B ∗ [log10 (PN)]2 + C ∗ [log10 (PN)] + D (10)

The coefficients A–D depend on the nominal power of the motorPN (in kW), the efficiency class and number of pole pairs. The coef-ficient can also be found in this standard.

The efficiency of a drive system consisting of a motor and a fre-quency converter (FC) can also be taken as a constant. The efficiencyof a combined motor-FC is constantly 3% less than the efficiency ofa grid connected motor of the same size [37,38].

3.5. Calculation models for the fan power consumption

In the calculation of the ventilation system with constant airflow controlled by a damper the pressure increase of the fan (�pst)is equal to the maximum pressure drop (�pM). Due to the constantpressure increase also the volumetric air flow is constant.

For a ventilation system design with a speed-controlled fanoperated at constant flow the pressure drop starts in each filtra-tion cycle at �pC(t) which is the sum of the pressure drop of theair collection system and the residual pressure drop of the filtermedium (Eq. (11)):

�pC (t) = (�pS + �pFM(t)) (11)

At the end of a filtration cycle the maximum pressure drop of thesystem is reached. Assuming, on average, a linear increase in thepressure drop, the sawtooth-shaped curve of the actual pressuredrop was replaced in the calculation model by a curve resultingfrom the average of both (Eq. (12)):

�pst(t) = �pM + �pC (t)2

(12)

Measured functions of the pressure drop are slightly concave

[32]. Therefore, the above assumption slightly over-estimates thepressure drop. The development of the residual pressure drop ofthe filter medium (�pFM(t)) was approximated by two functionsbased on measured results of the residual pressure drop at a filter

C. Lanzerstorfer et al. / Energy and Buildings 128 (2016) 503–510 507

Fig. 4. Cost functions for o

Table 1Selected design data of the ventilation systems.

Air flow rate inNm3/h

Pressure increasein Pa

Fan motor sizerated power inkW

Dimensions ofdamper in m * m

9500 3250 15 0.5 × 0.620,000 3250 30 0.6 × 1.0

uachtet

cartca

R

wofc

tfitfiti

3.7. Chosen parameters for the calculation

37,500 3250 55 1.2 × 1.063,000 3250 90 1.4 × 1.4

nit in the ventilation system for a rolling mill that was operatedt a constant air flow of 25,000 Nm3/h and an average inlet dustoncentration of about 7 mg/Nm3) [35]. For the first 200 operationours the linear function �pFM(t) = 1.16 · t + 250 was used. For theime beyond 200 h the function �pFM(t) = 322 · ln(t) − 1225 deliv-rs a good approximation of the measured results. In both functionshe unit of the operation time t is hours.

For the uncontrolled system the starting point of the filtrationycle is the intersection of the fan performance curve and the char-cteristic curve of the system. The latter curve is defined by theesistance of the system (R) which depends on the actual status ofhe filter material. Thus, the pressure drop �pU(t) and the flow V̇U

an be calculated by solving the system of the equations Eqs. (13)nd (8) numerically:

(t) = �pC (t)

V̇2M

= �pU(t)

V̇2U(t)

(13)

For uncontrolled operation a linear increase in the pressure dropas assumed. In the calculation model the sawtooth-shaped curve

f the actual pressure drop again was replaced by a curve resultingrom the average pressure drop. The corresponding air flow wasalculated by Eq. (8).

For the speed-controlled fan system (SCF) an additional opera-ion mode (SCF-CPD) was investigated. In this mode the end of altration cycle is reached and filter regeneration is started whenhe pressure drop has increased by a certain value �p within this

FCltration cycle. In this operation mode the average pressure drop ofhe system is lower and the filter cleaning is more frequent, whichncreases the consumption of compressed air.

ptional equipment.

In the ventilation system with the constant operation point alsothe efficiency of the fan �fan,M is constant. It can be calculated usingEq. (9). For the system with the speed-controlled fan and for theuncontrolled system, the average of the fan efficiency at the startand the end of the filtration cycle was used as fan efficiency in thecalculation:

�̄fan(t) = �fan,M + �fan(t)2

(14)

As the efficiency of a centrifugal fan is nearly constant at vari-able speed [39] the fan efficiency at the start of the filtration cycle(�fan(t)) is the same for both cases of speed controlled fan opera-tion. It can be calculated in Eq. (8) using the actual air flow (V̇U).The efficiency of the drive was assumed as constant in each case.

3.6. Cost functions

The selected design data for the ventilation systems are summa-rized in Table 1. The air flow rate is based on the fan inlet conditions.The nominal power of the fan motors was selected by experience.

In the calculation of the investment costs only items whichare not required in all three configurations were considered. Theassembly cost for the different configurations was assumed to beequal for all systems. The typical prices of the optional componentsare summarized in Fig. 4. The prices for the frequency convert-ers (standard type with B6U rectifier including a built-in DC or ACchoke for current smoothing and potential standard EMV) wereobtained from ABB and Vacon [40,41]. The prices for soft startersand the additional costs for shielded cables were obtained fromHainzl Industriesysteme [42]. For the dampers the prices wereobtained from Aumayr [43]. The price for the flow measurement[44] and the control loop for flow control were estimated at D 1200independent of the size of the ventilation system. For the cost calcu-lation the linearized cost functions were derived from the availabledata.

For the economic evaluation static methods were selectedbecause of the simple use and the short payback periods expected.

508 C. Lanzerstorfer et al. / Energy and B

Table 2Payback periods.

Air flow rate in Nm3/h PR in KW Payback period in operation hours

CDa SCFb SCF-CPDc

9500 15 39,000 13,400 900020,000 30 25,000 11,800 520037,500 55 11,300 5600 300063,000 90 7600 6400 3200

AtIsstCthmowcioc

4

4

tftoAtits

4

tisofi

TC

a Constant air flow controlled by a control damper.b Constant air flow controlled by a speed-controlled fan.c SCF with a fixed max. pressure drop of filter cake.

ll equations were implemented in an Excel calculation tool, wherehe payback period for the additional investment was calculated.n the calculation twenty-four-seven operation of the ventilationystem was assumed. For a reduced monthly operation time of theystem the resulting payback periods have to be adapted by mul-iplying them by the ratio of 720 to the monthly operation hours.alculations were carried out for a pressure drop of the air collec-ion system of 1050 Pa and a filter service life of 12,000 operationours. The ambient pressure was defined as 101,325 Pa. The fanotors were chosen with a motor efficiency class IE2 and a number

f pole pairs of two. For the cost of power consumption 0.10 D /kWhas used in the calculations. For the calculation of the increased

ompressed air consumption of the SCF-CPD operation the follow-ng assumptions were used: specific compressed air consumptionf filter regeneration: 0.021 Nm3/m2; air to cloth ratio: 1.7 m/min;osts of compressed air: 0.03 D /Nm3.

. Results and discussion

.1. Additional investment cost

The resulting costs of additional investment (CAI in D ) in the sys-em with a speed-controlled fan can be calculated by the linearizedunction CAISCF = 46.6 · PR + 1200, where PR is the rated power ofhe motor in kW. For the system with a control damper the costsf the additional investment result from CAIDC = 0.87 · PR + 1330.s expected, with increasing capacity of the ventilation system

he additional cost for the system with the speed-controlled fanncreases. In contrast to this, for the system with the control damperhe additional investment costs do not depend very much on theize of the system.

.2. Payback period calculation

The results for amortisation of the additional investment costs inhe different ventilation system sizes and designs are summarizedn Table 2. Generally the payback period decreases with increasing

ystem size. This corresponds with the results for other applicationsf speed controlled drives [30]. Compared to an automatic damper,or all system sizes the payback period of the speed-controlled fans shorter. In speed-controlled fan systems the payback period can

able 3ost savings in 12,000 service hours and profitability for this period.

Air flow rate in Nm3/h Fan motor size rated power in KW CDa

Cost savings Pr

9500 15 400 D 220,000 30 630 D 437,500 55 1400 D 163,000 90 1800 D 1

a Constant air flow controlled by a control damper.b Constant air flow controlled by a speed-controlled fan.c SCF with a fixed max. pressure drop of filter cake.

uildings 128 (2016) 503–510

be cut by approximately half when a controlled maximum pres-sure drop of the filter cake of 400 Pa is applied. The frequency offilter cleaning calculated as an average for the assumed service lifeof the filter material of 12,000 h will increase by 80% through suchoperation. For the calculation of additional compressed air costs,an average cleaning frequency of 0.4 1/h was assumed for the stan-dard speed controlled fan system. Depending on the system size,the payback period for the additional investment costs of a sys-tem with a speed-controlled fan are in the range of 0.7–1.7 years,assuming continuous operation (8000 operating hours per year).This is in a similar range as reported for applications of speed con-trolled drives in ventilation systems with variable air flow [26]. Fortwo shift operations (4000 operating hours per year), the paybackperiods are doubled.

The development of the accumulated savings (operating costsavings minus additional investment costs) for a ventilation systemwith an air flow of 37,500 m3/h and a 55 kW fan motor is shown inFig. 5.

4.3. Comparison of profitability

The profitability, defined as the net profit in a certain perioddivided by the average tied-up capital, was calculated for the periodof the assumed service life of the filter material. A period of 12,000 his quite unusual as a time basis for the calculation of the prof-itability. However, it was chosen because the cost savings are notuniformly distributed over the operation time, but are higher for thenew filter material and decrease towards the end of the service life.The results are summarized in Table 3. Generally, the profitabilityincreases with increasing system size. The profitability is higherfor the speed-controlled system than for the automatic dampercontrolled system. The highest profitability can be achieved withthe speed-controlled system when a controlled maximum pressuredrop of the filter cake is applied.

5. Conclusions

The use of speed-control systems for fans in small and medium-size constant design air flow ventilation systems which include aregenerative dust filter was investigated. It was shown that theinstallation of a speed-control system for fans is also an appropriatemeasure for cost reduction. The use of static methods for the eco-nomic evaluation proved to be adequate because of the resultingshort payback periods. With the chosen approach, the calculatedpayback period is between 0.7 and 1.7 years for 24 h operation,depending on the size of the system. Generally, the payback periodis shorter for larger systems. For systems in two-shift operations,the payback periods are still in the range of 1.4–3.4 years. The prof-itability of the installation of a fan speed-control, based on 12,000

service hours, is in the range of 80% to 150%. An operation modewhere filter cleaning is triggered by a fixed pressure drop of thefilter cake can help to reduce the payback period and improve theprofitability further. The system with flow control by an automatic

SCFb SCF-CPDc

ofitability Cost savings Profitability Cost savings Profitability

9% 1500 D 80% 2200 D 120%7% 2600 D 100% 4000 D 155%00% 5600 D 150% 8200 D 220%30% 8000 D 150% 11,500 D 210%

C. Lanzerstorfer et al. / Energy and Buildings 128 (2016) 503–510 509

r vent

dftft

A

USpa

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

Fig. 5. Accumulated savings and break-even fo

amper is much less economical compared to the speed-controlledan system. However, for the assumed conditions it is still bet-er than the uncontrolled system. The installation of speed-controlor fans in ventilation systems with a constant design air flow isherefore highly recommended for economic design.

cknowledgements

The project was funded by the University of Applied Sciencespper Austria, Project no. 8813 and by Kappa Filter Systems GmbH,teyr-Gleink, Austria. English proofreading by P. Orgill and sup-ort with the program Visual Studio by C. Derndorfer are gratefullycknowledged.

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